Blood pressure-monitoring system with alarm/alert system that accounts for patient motion

Abstract

The invention provides a system and method for measuring vital signs (e.g. SYS, DIA, SpO2, heart rate, and respiratory rate) and motion (e.g. activity level, posture, degree of motion, and arm height) from a patient. The system features: (i) first and second sensors configured to independently generate time-dependent waveforms indicative of one or more contractile properties of the patient's heart; and (ii) at least three motion-detecting sensors positioned on the forearm, upper arm, and a body location other than the forearm or upper arm of the patient. Each motion-detecting sensor generates at least one time-dependent motion waveform indicative of motion of the location on the patient's body to which it is affixed. A processing component, typically worn on the patient's body and featuring a microprocessor, receives the time-dependent waveforms generated by the different sensors and processes them to determine: (i) a pulse transit time calculated using a time difference between features in two separate time-dependent waveforms, (ii) a blood pressure value calculated from the time difference, and (iii) a motion parameter calculated from at least one motion waveform.

Claims

1. A system for measuring vital signs from a patient, comprising: (a) a first sensor configured to generate a first time-dependent waveform indicative of one or more contractile properties of the patient's heart; (b) a second sensor configured to generate a second time-dependent waveform indicative of one or more contractile properties of the patient's heart; (c) at least two motion-detecting sensors each configured to be worn on a location selected from a forearm, upper arm, and a body location other than the forearm or upper arm of the patient, each of said motion-detecting sensors generating at least one time-dependent motion waveform indicative of motion of the location on the patient's body to which the motion-detecting sensor is affixed; and, (d) a processing component configured to be worn on the patient's body and comprising a microprocessor, said processing component configured to receive the first time-dependent waveform, the second time-dependent waveform, and the at least one time-dependent motion waveform generated by each motion-detecting sensor and determine therefrom: (i) one or more vital signs calculated using the first and second time-dependent waveforms; (ii) the patient's posture, activity level, arm height, and degree of motion calculated using at least one motion waveform generated by each of said motion-detecting sensors; and (iii) at least one alarm condition indicative of the occurrence of a variance of one or more of the vital signs calculated using one or more of the vital signs, wherein the at least one alarm condition is regulated according to the patient's posture, activity level, arm height, and degree of motion determined by the processing component.

2. A system according to claim 1, wherein: said first time-dependent sensor is an optical sensor comprising: a source of electromagnetic radiation configured to irradiate tissue of the patient with radiation emitted therefrom, and a detector configured to detect one or more properties of the electromagnetic radiation after it irradiates said tissue, wherein the first time-dependent waveform is an optical waveform indicative of volumetric changes in the irradiated tissue caused by ventricular contraction of the patient's heart; and said second time-dependent sensor is an electrical sensor comprising: at least two electrodes configured to detect electrical signals from the patient's body, and an electrical circuit operably connected to the electrodes and configured to process the detected electrical signals, wherein said second time-dependent waveform is an electrical waveform indicative of ventricular depolarization of the patient's heart.

3. A system according to claim 2, wherein the optical waveform is generated by a pressure waveform resulting from ejection of blood from the left ventricle.

4. A system according to claim 3, wherein the pulse transit time is a time difference between an electrocardiogram QRS complex and a corresponding inflection point in said optical waveform.

5. A system according to claim 1, wherein: said first time-dependent sensor is an optical sensor comprising: a source of electromagnetic radiation configured to irradiate tissue of the patient with radiation emitted therefrom at a first location on an extremity, and a detector configured to detect one or more properties of the electromagnetic radiation after it irradiates said tissue, wherein the first time-dependent waveform is an optical waveform indicative of volumetric changes in the irradiated tissue at said first location caused by ventricular contraction of the patient's heart; and said second time-dependent sensor is an optical sensor comprising: a source of electromagnetic radiation configured to irradiate tissue of the patient with radiation emitted therefrom at a second position on said extremity, and a detector configured to detect one or more properties of the electromagnetic radiation after it irradiates said tissue, wherein the second time-dependent waveform is an optical waveform indicative of volumetric changes in the irradiated tissue at said second location caused by ventricular contraction of the patient's heart.

6. A system according to claim 1, wherein the at least one alarm condition is suppressed when the patient's posture, activity level, arm height, and degree of motion determined by the processing component indicates that said patient is ambulatory.

7. A system according to claim 1, wherein each of said three motion-detecting sensors are accelerometers.

8. A system according to claim 1, wherein the motion-detecting sensor configured to be worn on the forearm is positioned on the wrist.

9. A system according to claim 8, wherein the processing component is configured to be worn on the wrist proximate to the motion-detecting sensor.

10. A system according to claim 9, wherein the motion-detecting sensor configured to be worn on the upper arm is electrically connected to the processing component through a cable.

11. A system according to claim 10, wherein the motion detecting sensor configured to be worn on a body location other than the forearm or upper arm is positioned on the chest and is electrically connected to the processing component through said cable.

12. A system according to claim 11, wherein the motion waveform generated by the motion-detecting sensor configured to be worn on the upper arm is transmitted to the processing component through said cable as a first digital data stream and the motion waveform generated by the motion-detecting sensor configured to be worn on a body location other than the forearm or upper arm is transmitted to the processing component through said cable as a second digital data stream, wherein the first digital data stream is separately resolvable from the second digital data stream by the processing component.

13. A system according to claim 12, wherein the electrical waveform is transmitted to the processing component through said cable as a third digital data stream separately resolvable from each of the first and second digital data streams.

14. A system according to claim 13, wherein said cable terminates at a connector in electrical communication therewith, said connector configured for reversible attachment of one or more electrodes, wherein said connector comprises an electrical circuit configured to receive electrical signals from said one or more electrodes and to determine one or more electrical waveforms therefrom, and an analog-to-digital converter configured to convert said one or more electrical waveforms into said third digital data stream.

15. A system according to claim 14, wherein the connector is positioned proximal to the sensor configured to be worn on a body location other than the forearm or upper arm.

16. A method of measuring vital signs from a patient, comprising: attaching a system according to one of claims 1-15 to a patient; determining a pulse transit time, blood pressure value, and the patient's posture, activity level, arm height, and degree of motion using said processing component; and transmitting said blood pressure value, posture, activity level, arm height, and degree of motion to said remote receiver using a wireless communication system, wherein said method comprises generating an alarm condition based on the blood pressure value and the posture, activity level, arm height, and degree of motion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic drawing of a body-worn vital sign monitor featuring three accelerometers for detecting motion, along with ECG, optical, and pneumatic systems for measuring vital signs;

(2) FIG. 2 shows a graph of time-dependent waveforms (ECG, PPG, and ACC.sub.1-9) generated by, respectively, the ECG system, the optical system, and the accelerometer system of FIG. 1;

(3) FIGS. 3A and 3B show an image of the body-worn vital sign monitor of FIG. 1 attached to a patient with and without, respectively, a cuff-based pneumatic system used for an indexing measurement;

(4) FIG. 4 shows an image of the wrist-worn transceiver featured in the body-worn vital sign monitor of FIGS. 3A and 3B;

(5) FIG. 5 shows a schematic drawing of a coordinate system used to calibrate accelerometers used in the body-worn vital sign monitor of FIGS. 3A and 3B;

(6) FIG. 6 shows a schematic drawing of the three accelerometers, attached to a patient's arm and connected to the body-worn vital sign monitor of FIGS. 3A, 3B, and 4;

(7) FIG. 7 shows a schematic drawing of a coordinate system representing an accelerometer coordinate space superimposed on a patient's torso;

(8) FIG. 8 shows the accelerometer coordinate space of FIG. 7 and a vector representing the direction and magnitude of gravity, along with angles separating the vector from each axis of the coordinate system;

(9) FIG. 9A is a graph showing time-dependent motion waveforms corresponding to different posture states and measured with an accelerometer positioned on a patient's chest;

(10) FIG. 9B is a graph showing posture states calculated using the time-dependent motion waveforms of FIG. 9A and a mathematical model for determining a patient's posture;

(11) FIG. 10 is a graph of time-dependent waveforms indicating a patient's elbow height and corresponding PTT;

(12) FIGS. 11A and 11C are graphs of time-dependent waveforms generated with an accelerometer that represent, respectively, a patient walking and convulsing;

(13) FIGS. 11B and 11D are graphs of frequency-dependent waveforms representing the power spectra of the time-dependent waveforms shown, respectively, in FIGS. 11A and 11C;

(14) FIG. 12 is a schematic drawing of a calculation used to determine a type of activity state exhibited by a patient;

(15) FIGS. 13A and 13B are receiver operating characteristic (ROC) curves that show results from the calculation of FIG. 12 for, respectively, a patient that is walking and resting; and

(16) FIG. 14 is a flow chart describing an algorithm that processes information from the body-worn vital sign monitor of FIGS. 3A, 3B, and 4 to continuously determine a patient's vital signs and motion.

DETAILED DESCRIPTION OF THE INVENTION

System Overview

(17) FIG. 1 shows a schematic drawing of a body-worn vital sign monitor 10 according to the invention featuring a wrist-worn transceiver 12 that continuously determines vital signs (e.g. SYS, DIA, SpO2, heart rate, respiratory rate, and temperature) and motion (e.g. posture, arm height, activity level, and degree of motion) for, e.g., an ambulatory patient in a hospital. The transceiver 12 connects to three separate accelerometers 14a, 14b, 14c distributed along a patient's arm and torso and connected to a single cable. Each of these sensors measures three unique ACC waveforms, each corresponding to a separate axis (x, y, or z), which are digitized internally and sent to a computer processing unit (CPU) 22 within the transceiver 12 for processing. The transceiver 12 also connects to an ECG system 16 that measures an ECG waveform, an optical system 18 that measures a PPG waveform, and a pneumatic system 20 for making cuff-based ‘indexing’ blood pressure measurements according to the composite technique. Collectively, these systems 14a-c, 16, 18, and 20 continuously measure the patient's vital signs and motion.

(18) The ECG 16 and pneumatic 20 systems are stand-alone systems that include a separate microprocessor and analog-to-digital converter. During a measurement, they connect to the transceiver 12 through connectors 28, 30 and supply digital inputs using a communication protocol that runs on a controller-area network (CAN) bus. The CAN bus is a serial interface, typically used in the automotive industry, which allows different electronic systems to effectively communicate with each other, even in the presence of electrically noisy environments. A third connector 32 also supports the CAN bus and is used for ancillary medical devices (e.g. a glucometer) that is either worn by the patient or present in their hospital room.

(19) The optical system 18 features an LED and photodetector and, unlike the ECG 16 and pneumatic 20 systems, generates an analog electrical signal that connects through a cable 36 and connector 26 to the transceiver 12. As is described in detail below, the optical 18 and ECG 16 systems generate synchronized time-dependent waveforms that are processed with the composite technique to determine a PTT-based blood pressure along with motion information. The body-worn vital sign monitor 10 measures these parameters continuously and non-invasively characterize the hospitalized patient.

(20) The first accelerometer 14a is surface-mounted on a printed circuited board within the transceiver 12, which is typically worn on the patient's wrist like a watch. The second 14b accelerometer is typically disposed on the upper portion of the patient's arm and attaches to a cable 40 that connects the ECG system 16 to the transceiver 12. The third accelerometer 14c is typically worn on the patient's chest proximal to the ECG system 16. The second 14b and third 14c accelerometers integrate with the ECG system 16 into a single cable 40, as is described in more detail below, which extends from the patient's wrist to their chest and supplies digitized signals over the CAN bus. In total, the cable 40 connects to the ECG system 16, two accelerometers 14b, 14c, and at least three ECG electrodes (shown in FIGS. 3A and 3B, and described in more detail below). The cable typically includes 5 separate wires bundled together with a single protective cladding: the wires supply power and ground to the ECG system 16 and accelerometers 14b, 14c, provide high signal and low signal transmission lines for data transmitted over the CAN protocol, and provide a grounded electrical shield for each of these four wires. It is held in place by the ECG electrodes, which are typically disposable and feature an adhesive backing, and a series of bandaid-like disposable adhesive strips. This simplifies application of the system and reduces the number of sensing components attached to the patient.

(21) To determine posture, arm height, activity level, and degree of motion, the transceiver's CPU 22 processes signals from each accelerometer 14a-c with a series of algorithms, described in detail below. In total, the CPU can process nine unique, time-dependent signals (ACC.sub.1-9) corresponding to the three axes measured by the three separate accelerometers. Specifically, the algorithms determine parameters such as the patient's posture (e.g., sitting, standing, walking, resting, convulsing, falling), the degree of motion, the specific orientation of the patient's arm and how this affects vital signs (particularly blood pressure), and whether or not time-dependent signals measured by the ECG 16, optical 18, or pneumatic 20 systems are corrupted by motion. Once this is complete, the transceiver 12 uses an internal wireless transmitter 24 to send information in a series of packets, as indicated by arrow 34, to a central nursing station within a hospital. The wireless transmitter 24 typically operates on a protocol based on 802.11 and communicates with an existing network within the hospital. This information alerts a medical professional, such as a nurse or doctor, if the patient begins to decompensate. A server connected to the hospital network typically generates this alarm/alert once it receives the patient's vital signs, motion parameters, ECG, PPG, and ACC waveforms, and information describing their posture, and compares these parameters to preprogrammed threshold values. As described in detail below, this information, particularly vital signs and motion parameters, is closely coupled together. Alarm conditions corresponding to mobile and stationary patients are typically different, as motion can corrupt the accuracy of vital signs (e.g., by adding noise), and induce changes in them (e.g., through acceleration of the patient's heart and respiratory rates).

(22) FIG. 2 shows time-dependent ECG 50, PPG 52, and ACC.sub.1-9 54 waveforms that the body-worn vital sign monitor continuously collects and analyzes to determine the patient's vital signs and motion. The ECG waveform 50, generated by the ECG system and three electrodes attached to the patient's chest, features a sharply peaked QRS complex 56. This complex 56 marks the peak of ventricular depolarization and informally indicates the beginning of each cardiac cycle. For a normal rhythm it occurs for each heartbeat. The optical system generates a PPG 52 using an infrared LED and matched photodetector incorporated into an optical sensor that attaches to the base of the patient's thumb. A pulsatile feature 58 in the PPG 52 follows the QRS complex 56, typically by about one to two hundred milliseconds, and indicates a volumetric expansion in arteries and capillaries disposed underneath the optical sensor. The temporal difference between the peak of the QRS complex 56 and the foot 60 of the pulsatile feature 58 in the PPG waveform is the PTT, which as described in detail below is used to determine blood pressure according to the composite technique.

(23) Each accelerometer generates three time-dependent ACC waveforms 54 (ACC.sub.1-9) that, collectively, indicate the patient's motion. Each waveform is digitized within the accelerometer using an internal analog-to-digital circuit. In general, the frequency and magnitude of change in the shape of the ACC waveform indicate the type of motion that the patient is undergoing. For example, a typical waveform 54 features a relatively time-invariant component 53 indicating a period of time when the patient is relatively still, and a time-variant component 62 when the patient's activity level increases. As described in detail below, a frequency-dependent analysis of these components yields the type and degree of patient motion. During operation, an algorithm running on the CPU within the wrist-worn transceiver operates an algorithm that performs this analysis so that the patient's activity level can be characterized in real time.

(24) FIGS. 3A and 3B show how the body-worn system 10 described with respect to FIG. 1 attaches to a patient 70. These figures show two configurations of the system: FIG. 3A shows the system used during the indexing portion of the composite technique, and includes a pneumatic, cuff-based system 85, while FIG. 3B shows the system used for subsequent continuous monitoring of the patient featuring a non-invasive blood pressure (cNIBP) measurement. The indexing measurement typically takes about 60 seconds, and is typically performed once every 4 hours. Once the indexing measurement is complete the cuff-based system is typically removed from the patient. The remainder of the time the system 10 performs the cNIBP measurement.

(25) The body-worn system 10 features a wrist-worn transceiver 72, described in more detail in FIG. 5, featuring a touch panel interface 73 that displays blood pressure values and other vital signs. A wrist strap 90 affixes the transceiver 72 to the patient's wrist like a conventional wristwatch. A cable 92 connects an optical sensor 94 that wraps around the base of the patient's thumb to the transceiver 72. During a measurement, the optical sensor 94 generates a time-dependent PPG, similar to the waveform 52 shown in FIG. 2, which is processed along with an ECG to measure blood pressure. As described in detail in the above-referenced patent application, this provides an accurate representation of blood pressure in the central regions of the patient's body.

(26) To determine ACC waveforms, such as the time-dependent waveform 54 shown in FIG. 2, the body-worn vital sign monitor 10 features three separate accelerometers located at different portions on the patient's arm. The first accelerometer is surface-mounted on a circuit board in the wrist-worn transceiver 72 and measures signals associated with movement of the patient's wrist. The second accelerometer is included in a small bulkhead portion 96 included along the span of the cable 86. During a measurement, a small piece of disposable tape, similar in size to a conventional bandaid, affixes the bulkhead portion 96 to the patient's arm. In this way the bulkhead portion 96 serves two purposes: 1) it measures a time-dependent ACC waveform from the mid-portion of the patient's arm, thereby allowing their posture and arm height to be determined as described in detail below; and 2) it secures the cable 86 to the patient's arm to increase comfort and performance of the body-worn vital sign monitor 10.

(27) The cuff-based module 85 features a pneumatic system 76 that includes a pump, valve, pressure fittings, pressure sensor, analog-to-digital converter, microcontroller, and rechargeable battery. During an indexing measurement, it inflates a disposable cuff 84 and performs two measurements according to the composite technique: 1) it performs an inflation-based measurement of oscillometry to determine values for SYS, DIA, and MAP; and 2) it determines a patient-specific relationship between PTT and MAP. These measurements are performed according to the composite technique, and are described in detail in the above-referenced patent application entitled: ‘VITAL SIGN MONITOR FOR MEASURING BLOOD PRESSURE USING OPTICAL, ELECTRICAL, AND PRESSURE WAVEFORMS’ (U.S. Ser. No. 12/138,194; filed Jun. 12, 2008), the contents of which have been previously incorporated herein by reference.

(28) The cuff 84 within the cuff-based pneumatic system 85 is typically disposable and features an internal, airtight bladder that wraps around the patient's bicep to deliver a uniform pressure field. During the indexing measurement, pressure values are digitized by the internal analog-to-digital converter, and sent through a cable 86 according to the CAN protocol, along with SYS, DIA, and MAP blood pressures, to the wrist-worn transceiver 72 for processing as described above. Once the cuff-based measurement is complete, the cuff-based module 85 is removed from the patient's arm and the cable 86 is disconnected from the wrist-worn transceiver 72. cNIBP is then determined using PTT, as described in detail below.

(29) To determine an ECG, similar to waveform 50 shown in FIG. 2, the body-worn vital sign monitor 10 features a small-scale, three-lead ECG circuit integrated directly into a bulkhead 74 that terminates an ECG cable 82. The ECG circuit features an integrated circuit that collects electrical signals from three chest-worn ECG electrodes 78a-c connected through cables 80a-c. The ECG electrodes 78a-c are typically disposed in a conventional ‘Einthoven's Triangle’ configuration which is a triangle-like orientation of the electrodes 78a-c on the patient's chest that features three unique ECG vectors. From these electrical signals the ECG circuit determines up to three ECG waveforms, which are digitized using an analog-to-digital converter mounted proximal to the ECG circuit, and sent through a cable 82 to the wrist-worn transceiver 72 according to the CAN protocol. There, the ECG is processed with the PPG to determine the patient's blood pressure. Heart rate and respiratory rate are determined directly from the ECG waveform using known algorithms, such as those described in the following reference, the contents of which are incorporated herein by reference: ‘ECG Beat Detection Using Filter Banks’, Afonso et al., IEEE Trans. Biomed Eng., 46:192-202 (1999). The cable bulkhead 74 also includes an accelerometer that measures motion associated with the patient's chest as described above.

(30) There are several advantages of digitizing ECG and ACC waveforms prior to transmitting them through the cable 82. First, a single transmission line in the cable 82 can transmit multiple digital waveforms, each generated by different sensors. This includes multiple ECG waveforms (corresponding, e.g., to vectors associated with three, five, and twelve-lead ECG systems) from the ECG circuit mounted in the bulkhead 74, along with waveforms associated with the x, y, and z axes of accelerometers mounted in the bulkheads 75, 96. Limiting the transmission line to a single cable reduces the number of wires attached to the patient, thereby decreasing the weight and cable-related clutter of the body-worn monitor. Second, cable motion induced by an ambulatory patient can change the electrical properties (e.g. electrical impendence) of its internal wires. This, in turn, can add noise to an analog signal and ultimately the vital sign calculated from it. A digital signal, in contrast, is relatively immune to such motion-induced artifacts.

(31) More sophisticated ECG circuits can plug into the wrist-worn transceiver to replace the three-lead system shown in FIGS. 3A and 3B.

(32) FIG. 4 shows a close-up view of the wrist-worn transceiver 72. As described above, it attaches to the patient's wrist using a flexible strap 90 which threads through two D-ring openings in a plastic housing 106. The transceiver 72 features a touchpanel display 100 that renders a graphical user interface 73 which is altered depending on the viewer (typically the patient or a medical professional). Specifically, the transceiver 72 includes a small-scale infrared barcode scanner 102 that, during use, can scan a barcode worn on a badge of a medical professional. The barcode indicates to the transceiver's software that, for example, a nurse or doctor is viewing the user interface. In response, the user interface 73 displays vital sign data and other medical diagnostic information appropriate for medical professionals. Using this interface 73, the nurse or doctor, for example, can view the vital sign information, set alarm parameters, and enter information about the patient (e.g. their demographic information, medication, or medical condition). The nurse can press a button on the user interface 73 indicating that these operations are complete. At this point, the display 100 renders an interface that is more appropriate to the patient, e.g. it displays parameters similar to those from a conventional wristwatch, such as time of day and battery power.

(33) As described above, the transceiver 72 features three CAN connectors 104a-c on the side of its upper portion, each which supports the CAN protocol and wiring schematics, and relays digitized data to the internal CPU. Digital signals that pass through the CAN connectors include a header that indicates the specific signal (e.g. ECG, ACC, or pressure waveform from the cuff-based module) and the sensor from which the signal originated. This allows the CPU to easily interpret signals that arrive through the CAN connectors 104a-c, and means that these connectors are not associated with a specific cable. Any cable connecting to the transceiver can be plugged into any connector 104a-c. As shown in FIG. 3A, the first connector 104a receives the cable 82 that transports a digitized ECG waveform determined from the ECG circuit and electrodes, and digitized ACC waveforms measured by accelerometers in the cable bulkhead 74 and the bulkhead portion 96 associated with the ECG cable 82.

(34) The second CAN connector 104b shown in FIG. 4 receives the cable 86 that connects to the pneumatic cuff-based system 85 and used for the pressure-dependent indexing measurement. This connector is used to receive a time-dependent pressure waveform delivered by the pneumatic system 85 to the patient's arm, along with values for SYS, DIA, and MAP values determined during the indexing measurement. A user unplugs the cable 86 from the connector 104b once the indexing measurement is complete, and plugs it back in after approximately four hours for another indexing measurement.

(35) The final CAN connector 104c can be used for an ancillary device, e.g. a glucometer, infusion pump, body-worn insulin pump, ventilator, or end-tidal CO.sub.2 delivery system. As described above, digital information generated by these systems will include a header that indicates their origin so that the CPU can process them accordingly.

(36) The transceiver includes a speaker 101 that allows a medical professional to communicate with the patient using a voice over Internet protocol (VOIP). For example, using the speaker 101 the medical professional could query the patient from a central nursing station or mobile phone connected to a wireless, Internet-based network within the hospital. Or the medical professional could wear a separate transceiver similar to the shown in FIG. 4, and use this as a communication device. In this application, the transceiver 72 worn by the patient functions much like a conventional cellular telephone or ‘walkie talkie’: it can be used for voice communications with the medical professional and can additionally relay information describing the patient's vital signs and motion.

(37) Algorithms for Determining Patient Motion, Posture, Arm Height, Activity Level and the Effect of these Properties on Blood Pressure

(38) Described below is an algorithm for using the three accelerometers featured in the above-described body-worn vital sign monitor to calculate a patient's motion, posture, arm height, activity level. Each of these parameters affects both blood pressure and PTT, and thus inclusion of them in an algorithm can improve the accuracy of these measurements and the alarms and calibration procedures associated with them.

(39) Calculating Arm Height

(40) To calculate a patient's arm height it is necessary to build a mathematical model representing the geometric orientation of the patient's arm, as detected with signals from the three accelerometers. FIG. 5 shows a schematic image of a coordinate system 109 centered around a plane 110 used to build this model for determining patient motion and activity level, and arm height. Each of these parameters, as discussed in detail below, has an impact on the patient's vital signs, and particularly blood pressure.

(41) The algorithm for estimating a patient's motion and activity level begins with a calculation to determine their arm height. This is done using signals from accelerometers attached to the patient's bicep (i.e., with reference to FIG. 3A, an accelerometer included in the bulkhead portion 96 of cable 86) and wrist (i.e. the accelerometer surface-mounted to the circuit board within the wrist-worn transceiver 72). The mathematical model used for this algorithm features a calibration procedure used to identify the alignment of an axis associated with a vector R.sub.A, which extends along the patient's arm. Typically this is done by assuming the body-worn vital sign monitor is attached to the patient's arm in a consistent manner, i.e. that shown in FIGS. 3A and 3B, and by using preprogrammed constants stored in memory associated with the CPU. Alternatively this can be done by prompting the patient (using, e.g., the wrist-worn transceiver 72) to assume a known and consistent position with respect to gravity (e.g., hanging their arm down in a vertical configuration). The axis of their arm is determined by sampling a DC portion of time-dependent ACC waveforms along the x, y, and z axes associated with the two above-mentioned accelerometers (i.e. ACC.sub.1-6; the resultant values have units of g's) during the calibration procedure, and storing these numerical values as a vector in memory accessible with the CPU within the wrist-worn transceiver.

(42) The algorithm determines a gravitational vector R.sub.GA at a later time by again sampling DC portions of ACC.sub.1-6. Once this is complete, the algorithm determines the angle □.sub.GA between the fixed arm vector R.sub.A and the gravitational vector R.sub.GA by calculating a dot product of the two vectors. As the patient moves their arm, signals measured by the two accelerometers vary, and are analyzed to determine a change in the gravitational vector R.sub.GA and, subsequently, a change in the angle □.sub.GA. The angle □.sub.GA can then be combined with an assumed, approximate length of the patient's arm (typically 0.8 m) to determine its height relative to a proximal joint, e.g. the elbow.

(43) FIG. 6 indicates how this model and approach can be extended to determine the relative heights of the upper 117 and lower 116 segments of a patient's arm 115. In this derivation, described below, i, j, k represent the vector directions of, respectively, the x, y, and z axes of the coordinate system 109 shown in FIG. 5. Three accelerometers 102a-c are disposed, respectively, on the patient's chest just above their armpit, near their bicep, and near their wrist; this is consistent with positioning within the body-worn vital sign monitor, as described in FIGS. 3A and 3B. The vector R.sub.B extending along the upper portion 117 of the patient's arm is defined in this coordinate system as:
{right arrow over (R)}.sub.B=r.sub.Bxî+r.sub.Byĵ+r.sub.Bz{circumflex over (k)}  (1)
At any given time, the gravitational vector R.sub.GB is determined from ACC waveforms (ACC.sub.1-3) using signals from the accelerometer 102b located near the patient's bicep, and is represented by equation (2) below:
{right arrow over (R)}.sub.GB[n]=y.sub.Bx[n]î+y.sub.By[n]ĵ+y.sub.Bz[n]{circumflex over (k)}  (2)
Specifically, the CPU in the wrist-worn transceiver receives digitized signals representing the DC portion (e.g. component 53 in FIG. 2) of the ACC.sub.1-3 signals measured with accelerometer 102b, as represented by equation (3) below, where the parameter n is the value (having units of g's) sampled directly from the DC portion of the ACC waveform:
y.sub.Bx[n]=y.sub.DC,Bicep,x[n]; y.sub.By[n]=y.sub.DC,Bicep,y[n]; y.sub.Bz[n]=y.sub.DC,Bicep,z[n]  (3)
The cosine of the angle □.sub.GB separating the vector R.sub.B and the gravitational vector R.sub.GB is determined using equation (4):

(44) $\begin{array}{cc}\cos \left({\theta }_{\mathrm{GB}}\left[n\right]\right)=\frac{{\stackrel{.Math.}{R}}_{\mathrm{GB}}\left[n\right].Math.{\stackrel{.Math.}{R}}_B}{.Math.{\stackrel{.Math.}{R}}_{\mathrm{GB}}\left[n\right].Math..Math.{\stackrel{.Math.}{R}}_B.Math.}& \left(4\right)\end{array}$
The definition of the dot product of the two vectors R.sub.B and R.sub.GB is:
{right arrow over (R)}.sub.GB[n].Math.{right arrow over (R)}.sub.B=(y.sub.Bx[n]×r.sub.Bx)+(y.sub.By[n]×r.sub.By)+(y.sub.Bz[n]×r.sub.Bz)  (5)
and the definitions of the norms or magnitude of the vectors R.sub.B and R.sub.GB are:
∥{right arrow over (R)}.sub.GB[n]∥=√{square root over ((y.sub.Bx[n]).sup.2+(y.sub.By[n]).sup.2+(y.sub.Bz[n]).sup.2)}  (6)
and
∥{right arrow over (R)}.sub.B∥=√{square root over ((r.sub.Bx).sup.2+(r.sub.By).sup.2+(r.sub.Bz).sup.2)}  (7)
Using the norm values for these vectors and the angle □.sub.GB separating them, as defined in equation (4), the height of the patient's elbow relative to their shoulder joint, as characterized by the accelerometer on their chest (h.sub.E). is determined using equation (8), where the length of the upper arm is estimated as L.sub.B:
h.sub.E[n]=−L.sub.B×cos(θ.sub.GB[n])  (8)
As is described in more detail below, equation (8) estimates the height of the patient's arm relative to their heart. And this, in turn, can be used to further improve the accuracy of PTT-based blood pressure measurements.

(45) The height of the patient's wrist joint h.sub.W is calculated in a similar manner using DC components from the time-domain waveforms (ACC.sub.4-6) collected from the accelerometer 102a mounted within the wrist-worn transceiver. Specifically, the wrist vector R.sub.W is given by equation (9):
{right arrow over (R)}.sub.W=r.sub.Wxî+r.sub.Wyĵ+r.sub.Wz{circumflex over (k)}  (9)
and the corresponding gravitational vector R.sub.GW is given by equation (10):
{right arrow over (R)}.sub.GW[n]=y.sub.Wx[n]î+y.sub.Wy[n]ĵ+y.sub.Wz[n]{circumflex over (k)}  (10)
The specific values used in equation (10) are measured directly from the accelerometer 102a; they are represented as n and have units of g's, as defined below:
y.sub.Wx[n]=y.sub.DC,Wrist,x[n]; y.sub.Wy[n]=y.sub.DC,Wrist,y[n]; y.sub.Wz[n]=y.sub.DC,Wrist,z[n]  (11)
The vectors R.sub.W and R.sub.GW described above are used to determine the cosine of the angle □.sub.GW separating them using equation (12):

(46) $\begin{array}{cc}\cos \left({\theta }_{\mathrm{GW}}\left[n\right]\right)=\frac{{\stackrel{.Math.}{R}}_{\mathrm{GW}}\left[n\right].Math.{\stackrel{.Math.}{R}}_W}{.Math.{\stackrel{.Math.}{R}}_{\mathrm{WB}}\left[n\right].Math..Math.{\stackrel{.Math.}{R}}_W.Math.}& \left(12\right)\end{array}$
The definition of the dot product between the vectors R.sub.W and R.sub.GW is:
{right arrow over (R)}.sub.GW[n].Math.{right arrow over (R)}.sub.W=(y.sub.Wx[n]×r.sub.Wx)+(y.sub.Wy[n]×r.sub.Wy)+(y.sub.Wz[n]×r.sub.Wz)  (13)
and the definitions of the norm or magnitude of both the vectors R.sub.W and R.sub.GW are:
∥{right arrow over (R)}.sub.GW[n]∥=√{square root over ((y.sub.Wx[n]).sup.2+(y.sub.Wy[n]).sup.2+(y.sub.Wz[n]).sup.2)}  (14)
and
∥{right arrow over (R)}.sub.W∥=√{square root over ((r.sub.Wx).sup.2+(r.sub.Wy).sup.2+(r.sub.Wz).sup.2)}  (15)
The height of the patient's wrist h.sub.W can be calculated using the norm values described above in equations (14) and (15), the cosine value described in equation (12), and the height of the patient's elbow determined in equation (8):
h.sub.W[n]=h.sub.E[n]−L.sub.W×cos(θ.sub.GW[n])  (16)
In summary, the algorithm can use digitized signals from the accelerometers mounted on the patient's bicep and wrist, along with equations (8) and (16), to accurately determine the patient's arm height and position. As described below, these parameters can then be used to correct the PTT and provide a blood pressure calibration, similar to the cuff-based indexing measurement described above, that can further improve the accuracy of this measurement.
Calculating the Influence of Arm Height on Blood Pressure

(47) A patient's blood pressure, as measured near the brachial artery, will vary with their arm height due to hydrostatic forces and gravity. This relationship between arm height and blood pressure enables two measurements: 1) a blood pressure ‘correction factor’, determined from slight changes in the patient's arm height, can be calculated and used to improve accuracy of the base blood pressure measurement; and 2) the relationship between PTT and blood pressure can be determined (like it is currently done using the indexing measurement) by measuring PTT at different arm heights, and calculating the change in PTT corresponding to the resultant change in height-dependent blood pressure. Specifically, using equations (8) and (16) above, and (21) below, an algorithm can calculate a change in a patient's blood pressure (□BP) simply by using data from two accelerometers disposed on the wrist and bicep. The □BP can be used as the correction factor. Exact blood pressure values can be estimated directly from arm height using an initial blood pressure value (determined, e.g., using the cuff-based module during an initial indexing measurement), the relative change in arm height, and the correction factor. This measurement can be performed, for example, when the patient is first admitted to the hospital. PTT determined at different arm heights provides multiple data points, each corresponding to a unique pair of blood pressure values determined as described above. The change in PTT values (□PTT) corresponds to changes in arm height.

(48) From these data, the algorithm can calculate for each patient how blood pressure changes with PTT, i.e. □BP/□PTT. This relationship relates to features of the patient's cardiovascular system, and will evolve over time due to changes, e.g., in the patient's arterial tone and vascular compliance. Accuracy of the body-worn vital sign monitor's blood pressure measurement can therefore be improved by periodically calculating □BP/□PTT. This is best done by: 1) combining a cuff-based initial indexing measurement to set baseline values for SYS, DIA, and MAP, and then determining □BP/□PTT as described above; and 2) continually calculating □BP/□PTT by using the patient's natural motion, or alternatively using well-defined motions (e.g., raising and lower the arm to specific positions) as prompted at specific times by monitor's user interface.

(49) Going forward, the body-worn vital sign monitor measures PTT, and can use this value and the relationship determined from the above-described calibration to convert this to blood pressure. All future indexing measurements can be performed on command (e.g., using audio or visual instructions delivered by the wrist-worn transceiver) using changes in arm height, or as the patient naturally raises and lowers their arm as they move about the hospital.

(50) To determine the relationship between PIT, arm height, and blood pressure, the algorithm running on the wrist-worn transceiver is derived from a standard linear model shown in equation (17):

(51) $\begin{array}{cc}\mathrm{PTT}=\left(\frac{1}{m_{\mathrm{BP}}}\right)\times P_{\mathrm{MAP}}+\tilde{B}& \left(17\right)\end{array}$
Assuming a constant velocity of the arterial pulse along an arterial pathway (e.g., the pathway extending from the heart, through the arm, to the base of the thumb):

(52) $\begin{array}{cc}\frac{\partial \left(\mathrm{PWV}\right)}{\partial r}=0& \left(18\right)\end{array}$
the linear PTT model described in equation (17) becomes:

(53) $\begin{array}{cc}\frac{\partial \left(\mathrm{PTT}\right)}{\partial r}=\left(\frac{1}{L}\right)\left(\frac{1}{m_{\mathrm{BP}}}\times \mathrm{MAP}+\tilde{B}\right)& \left(19\right)\end{array}$
Equation (19) can be solved using piecewise integration along the upper 117 and lower 116 segments of the arm to yield the following equation for height-dependent PTT:

(54) $\begin{array}{cc}\mathrm{PTT}=\left(\frac{1}{m_{\mathrm{BP}}}\times \mathrm{MAP}+B\right)-\frac{1}{m_{\mathrm{BP}}}\times \left[\begin{array}{c}\left(\frac{L_1}{L}\right)\left(\frac{\rho {\mathrm{Gh}}_E}{2}\right)+\\ \left(\frac{L_2}{L}\right)\left(\frac{\rho G}{2}\left(h_W+h_E\right)\right)\end{array}\right]& \left(20\right)\end{array}$
From equation (20) it is possible to determine a relative pressure change P.sub.rel induced in a cNIBP measurement using the height of the patient's wrist (h.sub.W) and elbow (h.sub.E):

(55) $\begin{array}{cc}{P}_{\mathrm{rel}}\left[n\right]=\left(\frac{L_1}{L}\right)\left(\frac{\rho {\mathrm{Gh}}_E\left[n\right]}{2}\right)+\left(\frac{L_2}{L}\right)\left(\frac{\rho G}{2}\left(h_W\left[n\right]+h_E\left[n\right]\right)\right)& \left(21\right)\end{array}$
As described above, P.sub.rel can be used to both calibrate the cNIBP measurement deployed by the body-worn vital sign monitor, or supply a height-dependent correction factor that reduces or eliminates the effect of posture and arm height on a PTT-based blood pressure measurement.

(56) FIG. 10 shows actual experimental data that illustrate how PTT changes with arm height. Data for this experiment were collected as the subject periodically raised and lowered their arm using a body-worn vital sign monitor similar to that shown in FIGS. 3A and 3B. Such motion would occur, for example, if the patient was walking. As shown in FIG. 10, changes in the patient's elbow height are represented by time-dependent changes in the DC portion of an ACC waveform, indicated by trace 160. These data are measured directly from an accelerometer positioned near the patient's bicep, as described above. PTT is measured from the same arm using the PPG and ECG waveforms, and is indicated by trace 162. As the patient raises and lowers their arm their PTT rises and falls accordingly, albeit with some delay due to the reaction time of the patient's cardiovascular system.

(57) Calculating a Patient's Posture

(58) As described above, a patient's posture can influence how the above-described system generates alarms/alerts. The body-worn monitor can determine a patient's posture using time-dependent ACC waveforms continuously generated from the three patient-worn accelerometers, as shown in FIGS. 3A, B. In embodiments, the accelerometer worn on the patient's chest can be exclusively used to simplify this calculation. An algorithm operating on the wrist-worn transceiver extracts DC values from waveforms measured from this accelerometer and processes them with an algorithm described below to determine posture. Specifically, referring to FIG. 7, torso posture is determined for a patient 145 using angles determined between the measured gravitational vector and the axes of a torso coordinate space 146. The axes of this space 146 are defined in a three-dimensional Euclidean space where {right arrow over (R)}.sub.CV is the vertical axis, {right arrow over (R)}.sub.CH is the horizontal axis, and {right arrow over (R)}.sub.CN is the normal axis. These axes must be identified relative to a ‘chest accelerometer coordinate space’ before the patient's posture can be determined.

(59) The first step in this procedure is to identify alignment of {right arrow over (R)}.sub.CV in the chest accelerometer coordinate space. This can be determined in either of two approaches. In the first approach, {right arrow over (R)}.sub.CV is assumed based on a typical alignment of the body-worn monitor relative to the patient. During manufacturing, these parameters are then preprogrammed into firmware operating on the wrist-worn transceiver. In this procedure it is assumed that accelerometers within the body-worn monitor are applied to each patient with essentially the same configuration. In the second approach, {right arrow over (R)}.sub.CV is identified on a patient-specific basis. Here, an algorithm operating on the wrist-worn transceiver prompts the patient (using, e.g., video instruction operating on the display, or audio instructions transmitted through the speaker) to assume a known position with respect to gravity (e.g., standing up with arms pointed straight down). The algorithm then calculates R.sub.CV from DC values corresponding to the x, y, and z axes of the chest accelerometer while the patient is in this position. This case, however, still requires knowledge of which arm (left or right) the monitor is worn on, as the chest accelerometer coordinate space can be rotated by 180 degrees depending on this orientation. A medical professional applying the monitor can enter this information using the GUI, described above. This potential for dual-arm attachment requires a set of two pre-determined vertical and normal vectors which are interchangeable depending on the monitor's location. Instead of manually entering this information, the arm on which the monitor is worn can be easily determined following attachment using measured values from the chest accelerometer values, with the assumption that {right arrow over (R)}.sub.CV is not orthogonal to the gravity vector.

(60) The second step in the procedure is to identify the alignment of {right arrow over (R)}.sub.CN in the chest accelerometer coordinate space. The monitor can determine this vector, similar to the way it determines {right arrow over (R)}.sub.CV, with one of two approaches. In the first approach the monitor assumes a typical alignment of the chest-worn accelerometer on the patient. In the second approach, the alignment is identified by prompting the patient to assume a known position with respect to gravity. The monitor then calculates {right arrow over (R)}.sub.CN from the DC values of the time-dependent ACC waveform.

(61) The third step in the procedure is to identify the alignment of {right arrow over (R)}.sub.CH in the chest accelerometer coordinate space. This vector is typically determined from the vector cross product of {right arrow over (R)}.sub.CV and {right arrow over (R)}.sub.CN, or it can be assumed based on the typical alignment of the accelerometer on the patient, as described above.

(62) FIG. 8 shows the geometrical relationship between {right arrow over (R)}.sub.CV 140, {right arrow over (R)}.sub.CN 141, and {right arrow over (R)}.sub.CH 142 and a gravitational vector {right arrow over (R)}.sub.G 143 measured from a moving patient in a chest accelerometer coordinate space 139. The body-worn monitor continually determines a patient's posture from the angles separating these vectors. Specifically, the monitor continually calculates {right arrow over (R)}.sub.G 143 for the patient using DC values from the ACC waveform measured by the chest accelerometer. From this vector, the body-worn monitor identifies angles (θ.sub.VG,θ.sub.NG, and θ.sub.HG) separating it from {right arrow over (R)}.sub.CV 140, {right arrow over (R)}.sub.CN 141, and {right arrow over (R)}.sub.CH 142. The body-worn monitor then compares these three angles to a set of predetermine posture thresholds to classify the patient's posture.

(63) The derivation of this algorithm is as follows. Based on either an assumed orientation or a patient-specific calibration procedure described above, the alignment of {right arrow over (R)}.sub.CV in the chest accelerometer coordinate space is given by:
{right arrow over (R)}.sub.CV=r.sub.CVxî+r.sub.CVyĵ+r.sub.CVz{circumflex over (k)}  (22)
At any given moment, {right arrow over (R)}.sub.G is constructed from DC values of the ACC waveform from the chest accelerometer along the x, y, and z axes:
{right arrow over (R)}.sub.G[n]=y.sub.Cx[n]î+y.sub.Cy[n]ĵ+y.sub.Cz[n]{circumflex over (k)}  (23)
Equation (24) shows specific components of the ACC waveform used for this calculation:
y.sub.Cx[n]=y.sub.DC,chest,x[n]; y.sub.Cy[n]=y.sub.DC,chest,y[n]; y.sub.Cz[n]+y.sub.DC,chest,z[n]  (24)
The angle between {right arrow over (R)}.sub.CV and {right arrow over (R)}.sub.G is given by equation (25):

(64) $\begin{array}{cc}{\theta }_{\mathrm{VG}}\left[n\right]=\arccos \left(\frac{{\stackrel{.Math.}{R}}_G\left[n\right].Math.{\stackrel{.Math.}{R}}_{\mathrm{CV}}}{.Math.{\stackrel{.Math.}{R}}_G\left[n\right].Math..Math.{\stackrel{.Math.}{R}}_{\mathrm{CV}}.Math.}\right)& \left(25\right)\end{array}$
where the dot product of the two vectors is defined as:
{right arrow over (R)}.sub.G[n].Math.{right arrow over (R)}.sub.CV=(y.sub.Cx[n]×r.sub.CVx)+(y.sub.Cy[n]×r.sub.CVy)+(y.sub.Cz[n]×r.sub.CVz)  (26)
The definition of the norms of {right arrow over (R)}.sub.G and {right arrow over (R)}.sub.CV are given by equations (27) and (28):
∥{right arrow over (R)}.sub.G[n]{right arrow over (R)}=√{square root over ((y.sub.Cx[n]).sup.2+(y.sub.Cy[n]).sup.2+(y.sub.Cz[n]).sup.2)}  (27)
∥{right arrow over (R)}.sub.CV∥=√{square root over ((r.sub.CVx).sup.2+(r.sub.CVy).sup.2+(r.sub.CVz).sup.2)}  (28)

(65) As shown in equation (29), the monitor compares the vertical angle θ.sub.VG to a threshold angle to determine whether the patient is vertical (i.e. standing upright) or lying down:
if θ.sub.VG≤45° then Torso State=0, the patient is upright  (29)
If the condition in equation (29) is met the patient is assumed to be upright, and their torso state, which is a numerical value equated to the patient's posture, is equal to 0. The torso state is processed by the body-worn monitor to indicate, e.g., a specific icon corresponding to this state. The patient is assumed to be lying down if the condition in equation (8) is not met, i.e. θ.sub.VG>45 degrees. Their lying position is then determined from angles separating the two remaining vectors, as defined below.

(66) The angle θ.sub.NG between {right arrow over (R)}.sub.CN and {right arrow over (R)}.sub.G determines if the patient is lying in the supine position (chest up), prone position (chest down), or on their side. Based on either an assumed orientation or a patient-specific calibration procedure, as described above, the alignment of {right arrow over (R)}.sub.CN is given by equation (30), where i, j, k represent the unit vectors of the x, y, and z axes of the chest accelerometer coordinate space respectively:
{right arrow over (R)}.sub.CN=r.sub.CNxî+r.sub.CNyĵ+r.sub.CNz{circumflex over (k)}  (30)
The angle between {right arrow over (R)}.sub.CN and {right arrow over (R)}.sub.G determined from DC values extracted from the chest accelerometer ACC waveform is given by equation (31):

(67) 0$\begin{array}{cc}{\theta }_{\mathrm{NG}}\left[n\right]=\arccos \left(\frac{{\stackrel{.Math.}{R}}_G\left[n\right].Math.{\stackrel{.Math.}{R}}_{\mathrm{CN}}}{.Math.{\stackrel{.Math.}{R}}_G\left[n\right].Math..Math.{\stackrel{.Math.}{R}}_{\mathrm{CN}}.Math.}\right)& \left(31\right)\end{array}$
The body-worn monitor determines the normal angle θ.sub.NG and then compares it to a set of predetermined threshold angles to determine which position the patient is lying in, as shown in equation (32):
if θ.sub.NG≤35° then Torso State=1, the patient is supine
if θ.sub.NG≥135° then Torso State=2, the patient is prone  (32)
If the conditions in equation (32) are not met then the patient is assumed to be lying on their side. Whether they are lying on their right or left side is determined from the angle calculated between the horizontal torso vector and measured gravitational vectors, as described above.

(68) The alignment of {right arrow over (R)}.sub.CH is determined using either an assumed orientation, or from the vector cross-product of {right arrow over (R)}.sub.CV and {right arrow over (R)}.sub.CN as given by equation (33), where i, j, k represent the unit vectors of the x, y, and z axes of the accelerometer coordinate space respectively. Note that the orientation of the calculated vector is dependent on the order of the vectors in the operation. The order below defines the horizontal axis as positive towards the right side of the patient's body.
{right arrow over (R)}.sub.CH=r.sub.CVxî+r.sub.CVyĵ+r.sub.CVz{circumflex over (k)}={right arrow over (R)}.sub.CV×{right arrow over (R)}.sub.CN  (33)
The angle θ.sub.HG between {right arrow over (R)}.sub.CH and {right arrow over (R)}.sub.G is determined using equation (34):

(69) $\begin{array}{cc}{\theta }_{\mathrm{HG}}\left[n\right]=\arccos \left(\frac{{\stackrel{.Math.}{R}}_G\left[n\right].Math.{\stackrel{.Math.}{R}}_{\mathrm{CH}}}{.Math.{\stackrel{.Math.}{R}}_G\left[n\right].Math..Math.{\stackrel{.Math.}{R}}_{\mathrm{CH}}.Math.}\right)& \left(34\right)\end{array}$
The monitor compares this angle to a set of predetermined threshold angles to determine if the patient is lying on their right or left side, as given by equation (35):
if θ.sub.HG≥90° then Torso State=3, the patient is on their right side
if θ.sub.NG<90° then Torso State=4, the patient is on their left side  (35)
Table 1 describes each of the above-described postures, along with a corresponding numerical torso state used to render, e.g., a particular icon:

(70) TABLE-US-00001 Posture Torso State Upright 0 supine: lying on back 1 prone: lying on chest 2 lying on right side 3 lying on left side 4 undetermined posture 5
Table 1—Postures and their Corresponding Torso States

(71) FIGS. 9A and 9B show, respectively, graphs of time-dependent ACC waveforms 150 measured along the x, y, and z-axes, and the torso states (i.e. postures) 151 determined from these waveforms for a moving patient. As the patient moves, the DC values of the ACC waveforms measured by the chest accelerometer vary accordingly, as shown by the graph 150 in FIG. 9A. The body-worn monitor processes these values as described above to continually determine {right arrow over (R)}.sub.G and the various quantized torso states for the patient, as shown in the graph 151 in FIG. 9B. The torso states yield the patient's posture as defined in Table 1. For this study the patient rapidly alternated between standing, lying on their back, chest, right side, and left side within about 150 seconds. As described above, different alarm/alert conditions (e.g. threshold values) for vital signs can be assigned to each of these postures, or the specific posture itself may result in an alarm/alert. Additionally, the time-dependent properties of the graph 151 can be analyzed (e.g. changes in the torso states can be counted) to determine, for example, how often the patient moves in their hospital bed. This number can then be equated to various metrics, such as a ‘bed sore index’ indicating a patient that is so stationary in their bed that lesions may result. Such a state could then be used to trigger an alarm/alert to the supervising medical professional.

(72) Calculating a Patient's Activity

(73) An algorithm can process information generated by the accelerometers described above to determine a patient's specific activity (e.g., walking, resting, convulsing), which is then used to reduce the occurrence of false alarms. This classification is done using a ‘logistic regression model classifier’, which is a type of classifier that processes continuous data values and maps them to an output that lies on the interval between 0 and 1. A classification ‘threshold’ is then set as a fractional value within this interval. If the model output is greater than or equal to this threshold, the classification is declared ‘true’, and a specific activity state can be assumed for the patient. If the model output falls below the threshold, then the specific activity is assumed not to take place.

(74) This type of classification model offers several advantages. First, it provides the ability to combine multiple input variables into a single model, and map them to a single probability ranging between 0 and 1. Second, the threshold that allows the maximum true positive outcomes and the minimum false positive outcomes can be easily determined from a ROC curve, which in turn can be determined using empirical experimentation and data. Third, this technique requires minimal computation.

(75) The formula for the logistic regression model is given by equation (36) and is used to determine the outcome, P, for a set of buffered data:

(76) $\begin{array}{cc}P=\frac{1}{1-\exp \left(-z\right)}& \left(36\right)\end{array}$
The logit variable z is defined in terms of a series of predictors (x.sub.i), each affected by a specific type of activity, and determined by the three accelerometers worn by the patient, as shown in equation (37):
z=b.sub.0+b.sub.1x.sub.1+b.sub.2x.sub.2+ . . . +b.sub.mx.sub.m  (37)
In this model, the regression coefficients (b.sub.i, i=0, 1, . . . , m) and the threshold (P.sub.th) used in the patient motion classifier and signal corruption classifiers are determined empirically from data collected on actual subjects. The classifier results in a positive outcome as given in equation (38) if the logistic model output, P, is greater than the predetermined threshold, P.sub.th:
If P≥P.sub.th then Classifier State=1  (38)

(77) FIGS. 11A-D indicate how the above-described predictors can be processed to determine a patient's specific activity level. As shown in FIG. 11A, a time-dependent ACC waveform 130 measured with an accelerometer from a walking patient typically features DC portions before 129a and after 129b the patient finishes walking, and a periodic AC portion 131 measured during walking. A Fourier Transform of the waveform, as shown by the frequency-dependent waveform 132 in FIG. 11B, yields a power spectrum featuring a well-defined frequency component 133 (typically near 1-3 Hz, with an average of about 1.5 Hz), corresponding to the frequency of the patient's stride. FIGS. 11C and 11D indicate another activity state. Here, the patient is convulsing, and the time-dependent ACC waveform 134 shown in FIG. 11C features two oscillating ‘bursts’ 135a, 135b separated by periods of inactivity. A Fourier Transform of this waveform, as shown by the power spectrum 136 in FIG. 11D, indicates two frequency components 137a, 137b, both having relatively large values (typically 4-8 Hz, depending on the type of convulsion) compared to that shown in FIG. 11B. These frequency components 137a, 137b correspond to the frequency-dependent content of the oscillating bursts 135a, 135b.

(78) FIG. 12 shows a block diagram 180 indicating the mathematical model used to determine the above-described logistic regression model classifier. In this model, the series of predictor variables (x.sub.i) are determined from statistical properties of the time-dependent ACC waveforms, along with specific frequency components contained in the power spectra of these waveforms. The frequency components are determined in a low-frequency region (0-20 Hz) of these spectra that corresponds to human motion. Specifically, the predictor variables can be categorized by first taking a power spectrum of a time-dependent ACC waveform generated by an accelerometer, normalizing it, and then separating the fractional power into frequency bands according to Table 2, below:

(79) TABLE-US-00002 predictor variable Description x.sub.1 normalized power of the AC component of the time-dependent accelerometer signal x.sub.2 average arm angle measured while time- dependent accelerometer signal is collected x.sub.3 standard deviation of the arm angle while time-dependent accelerometer signal is collected x.sub.4 fractional power of the AC component of the frequency-dependent accelerometer signal between 0.5-1.0 Hz x.sub.5 fractional power of the AC component of the frequency-dependent accelerometer signal between 1.0-2.0 Hz x.sub.6 fractional power of the AC component of the frequency-dependent accelerometer signal between 2.0-3.0 Hz x.sub.7 fractional power of the AC component of the frequency-dependent accelerometer signal between 3.0-4.0 Hz x.sub.8 fractional power of the AC component of the frequency-dependent accelerometer signal between 4.0-5.0 Hz x.sub.9 fractional power of the AC component of the frequency-dependent accelerometer signal between 5.0-6.0 Hz .sub. x.sub.10 fractional power of the AC component of the frequency-dependent accelerometer signal between 6.0-7.0 Hz
Table 2—Predictor Variables and their Relationship to the Accelerometer Signal
The predictor variables described in Table 2 are typically determined from ACC signals generated by accelerometers deployed in locations that are most affected by patient motion. Such accelerometers are typically mounted directly on the wrist-worn transceiver, and on the bulkhead connector attached to the patient's arm. The normalized signal power (x.sub.i) for the AC components (y.sub.W,i, i=x,y,z) calculated from the ACC is shown in equation (39), where F.sub.s denotes the signal sampling frequency, N is the size of the data buffer, and x.sub.norm is a predetermined power value:

(80) $\begin{array}{cc}{x}_1=\frac{1}{x_{\mathrm{norm}}}\left(\frac{F_S}{N}\right)\underset{n=1}{\overset{N}{.Math.}}\left[{\left(y_{W,x}\left[n\right]\right)}^2+{\left(y_{W,y}\left[n\right]\right)}^2+{\left(y_{W,z}\left[n\right]\right)}^2\right]& \left(39\right)\end{array}$
The average arm angle predictor value (x.sub.2) was determined using equation (40):

(81) $\begin{array}{cc}{x}_2=\left(\frac{1}{N}\right)\underset{n=1}{\overset{N}{.Math.}}\cos \left({\theta }_{\mathrm{GW}}\left[n\right]\right)& \left(40\right)\end{array}$
Note that, for this predictor value, it is unnecessary to explicitly determine the angle □.sub.GW using an arccosine function, and the readily available cosine value calculated in equation (12) acts as a surrogate parameter indicating the mean arm angle. The predictor value indicating the standard deviation of the arm angle (x.sub.3) was determined using equation (41) using the same assumptions for the angle □.sub.GW as described above:

(82) $\begin{array}{cc}{x}_3=\sqrt{\left(\frac{1}{N}\right)\underset{n=1}{\overset{N}{.Math.}}{\left(\cos \left({\theta }_{\mathrm{GW}}\left[n\right]\right)-x_2\right)}^2}& \left(41\right)\end{array}$

(83) The remaining predictor variables (x.sub.4-x.sub.10) are determined from the frequency content of the patient's motion, determined from the power spectrum of the time-dependent accelerometer signals, as indicated in FIGS. 11B and 11D. To simplify implementation of this methodology, it is typically only necessary to process a single channel of the ACC waveform. Typically, the single channel that is most affected by patient motion is y.sub.W, which represents motion along the long axis of the patient's lower arm, determined from the accelerometer mounted directly in the wrist-worn transceiver. Determining the power requires taking an N-point Fast Fourier Transform (FFT) of the accelerometer data (X.sub.W[m]); a sample FFT data point is indicated by equation (42):
X.sub.W[m]=a.sub.m+ib.sub.m  (42)
Once the FFT is determined from the entire time-domain ACC waveform, the fractional power in the designated frequency band is given by equation (43), which is based on Parseval's theorem. The term mStart refers to the FFT coefficient index at the start of the frequency band of interest, and the term mEnd refers to the FFT coefficient index at the end of the frequency band of interest:

(84) $\begin{array}{cc}{x}_k=\left(\frac{1}{P_T}\right)\underset{m=\mathrm{mStart}}{\overset{\mathrm{mEnd}}{.Math.}}\left(a_m+{\mathrm{ib}}_m\right)\left(a_m-{\mathrm{ib}}_m\right)& \left(43\right)\end{array}$
Finally, the formula for the total signal power, P.sub.T, is given in equation (44):

(85) $\begin{array}{cc}{P}_T=\underset{m=0}{\overset{N/2}{.Math.}}\left(a_m+{\mathrm{ib}}_m\right)\left(a_m-{\mathrm{ib}}_m\right)& \left(44\right)\end{array}$

(86) As described above, to accurately estimate a patient's activity level, predictor values x.sub.1-x.sub.10 defined above are measured from a variety of subjects selected from a range of demographic criteria (e.g., age, gender, height, weight), and then processed using predetermined regression coefficients (b.sub.j) to calculate a logit variable (defined in equation (37)) and the corresponding probability outcome (defined in equation (36)). A threshold value is then determined empirically from an ROC curve. The classification is declared true if the model output is greater than or equal to the threshold value. During an actual measurement, an accelerometer signal is measured and then processed as described above to determine the predictor values. These parameters are used to determine the logit and corresponding probability, which is then compared to a threshold value to estimate the patient's activity level.

(87) FIGS. 13A,B show actual ROC curves, determined using accelerometers placed on the upper and lower arms of a collection of patients. An ideal ROC curve indicates a high true positive rate (shown on the y-axis) and a low false positive rate (shown on the x-axis), and thus has a shape closely representing a 90 degree angle. From such a curve a relatively high threshold can be easily determined and used as described above to determine a patient's activity level. Ultimately this results in a measurement that yields a high percentage of ‘true positives’, and a low percentage of ‘false positives’. FIG. 13A shows, for example, a ROC curve generated from the patients' upper 192 and lower 190 arms during walking. Data points on the curves 190, 192 were generated with accelerometers and processed with algorithms as described above. The distribution of these data indicates that this approach yields a high selectivity for determining whether or not a patient is walking.

(88) FIG. 13B shows data measured during resting. The ACC waveforms measured for this activity state feature fewer well-defined frequency components compared to those measured for FIG. 13A, mostly because the act of ‘resting’ is not as well defined as that of ‘walking’. That is why the ROC curves measured from the upper 194 and lower 196 arms have less of an idealized shape. Still, from these data threshold values can be determined that can be used for future measurements to accurately characterize whether or not the patient is resting.

(89) ROC curves similar to those shown in FIGS. 13A,B can be generated empirically from a set of patients undergoing a variety of different activity states. These states include, among others, falling, convulsing, running, eating, and undergoing a bowel movement. A threshold value for each activity state is determined once the ROC curve is generated, and going forward this information can be incorporated in an algorithm for estimating the patient's activity. Such an algorithm, e.g., can be uploaded wirelessly to the wrist-worn transceiver.

(90) FIG. 14 is a flow chart 200 showing the various algorithms described above, along with other supporting algorithms described in co-pending patent application, the contents of which are fully incorporated herein by reference.

(91) The initiation phase of the algorithm begins with collection of time-dependent PPG, ECG, ACC, and pressure waveforms using analog and digital circuitry within the body-worn vital sign monitor described above (step 201). An optical sensor attached to the patient's thumb measures PPG waveforms, while an ECG circuit attached to three electrodes on the patient's chest measures ECG waveforms. Once collected, these waveforms are digitally filtered according to step 202 using standard frequency-domain techniques to remove any out-of-band noise. The pressure waveform, generated during an indexing measurement during step 204 using a pneumatic system and cuff wrapped around the patient's bicep, is measured during inflation and processed using oscillometry, as described in the above-referenced patient application entitled: ‘VITAL SIGN MONITOR FOR MEASURING BLOOD PRESSURE USING OPTICAL, ELECTRICAL, AND PRESSURE WAVEFORMS’ (U.S. Ser. No. 12/138,194; filed Jun. 12, 2008), the contents of which have been previously incorporated herein by reference. This yields an indirect measurement of SYS, DIA, and MAP values. Alternatively, SYS can be determined directly by processing the PPG in the presence of applied pressure according to step 206 (as described in patent application referenced immediately above). PTT is measured as a function of applied pressure during the indexing measurement, and is processed during step 208 according to determine a personal, patient-specific slope (as described in patent application referenced immediately above). This slope, along with blood pressure values determined with oscillometry during the indexing measurement, is used along with PTT values measured from a temporal separation between the ECG and PPG to determine cNIBP according to step 220 (as described in patent application referenced immediately above).

(92) Motion, as described in detail above, can complicate measurement of the above-described parameters, and is determined by processing time-dependent ACC signals from multiple accelerometers attached to the patient and connected to the body-worn vital sign monitor. These signals are processed according to steps 210, 212, and 214, as described in detail above, to determine the degree of motion-based signal corruption, and according to step 218 to determine posture, arm height, and activity level. If motion is determined to be present, cNIBP can be estimated according to step 216 using a read-through technique.

(93) SpO2 is measured according to step 222 with the body-worn vital sign monitor using an integrated reference hardware design, algorithm, and code base provided by OSI of Hawthorne, Calif. Conventional algorithms for SpO2 are optimized for measurements made at the tip of the patient's index finger.

(94) The above-described measurements for PTT-based cNIBP are performed according to step 220 by collecting data for 20-second periods, and then processing these data with a variety of statistical averaging techniques. Pressure-dependent indexing measurements according to steps 204 and 206 are performed every 4 hours. In the algorithm described above, a technique for rolling averages can be deployed, allowing values for cNIBP (step 220), HR and TEMP (step 226), RR (step 224), and SpO2 (step 222) to be displayed every second. The interval for pressure-dependent indexing measurements may be extended past four hours.

(95) In addition to those methods described above, a number of additional methods can be used to calculate blood pressure from the optical and electrical waveforms. These are described in the following co-pending patent applications, the contents of which are incorporated herein by reference: 1) CUFFLESS BLOOD-PRESSURE MONITOR AND ACCOMPANYING WIRELESS, INTERNET-BASED SYSTEM (U.S. Ser. No. 10/709,015; filed Apr. 7, 2004); 2) CUFFLESS SYSTEM FOR MEASURING BLOOD PRESSURE (U.S. Ser. No. 10/709,014; filed Apr. 7, 2004); 3) CUFFLESS BLOOD PRESSURE MONITOR AND ACCOMPANYING WEB SERVICES INTERFACE (U.S. Ser. No. 10/810,237; filed Mar. 26, 2004); 4) CUFFLESS BLOOD PRESSURE MONITOR AND ACCOMPANYING WIRELESS MOBILE DEVICE (U.S. Ser. No. 10/967,511; filed Oct. 18, 2004); 5) BLOOD PRESSURE MONITORING DEVICE FEATURING A CALIBRATION-BASED ANALYSIS (U.S. Ser. No. 10/967,610; filed Oct. 18, 2004); 6) PERSONAL COMPUTER-BASED VITAL SIGN MONITOR (U.S. Ser. No. 10/906,342; filed Feb. 15, 2005); 7) PATCH SENSOR FOR MEASURING BLOOD PRESSURE WITHOUT A CUFF (U.S. Ser. No. 10/906,315; filed Feb. 14, 2005); 8) PATCH SENSOR FOR MEASURING VITAL SIGNS (U.S. Ser. No. 11/160,957; filed Jul. 18, 2005); 9) WIRELESS, INTERNET-BASED SYSTEM FOR MEASURING VITAL SIGNS FROM A PLURALITY OF PATIENTS IN A HOSPITAL OR MEDICAL CLINIC (U.S. Ser. No. 11/162,719; filed Sep. 9, 2005); 10) HAND-HELD MONITOR FOR MEASURING VITAL SIGNS (U.S. Ser. No. 11/162,742; filed Sep. 21, 2005); 11) CHEST STRAP FOR MEASURING VITAL SIGNS (U.S. Ser. No. 11/306,243; filed Dec. 20, 2005); 12) SYSTEM FOR MEASURING VITAL SIGNS USING AN OPTICAL MODULE FEATURING A GREEN LIGHT SOURCE (U.S. Ser. No. 11/307,375; filed Feb. 3, 2006); 13) BILATERAL DEVICE, SYSTEM AND METHOD FOR MONITORING VITAL SIGNS (U.S. Ser. No. 11/420,281; filed May 25, 2006); 14) SYSTEM FOR MEASURING VITAL SIGNS USING BILATERAL PULSE TRANSIT TIME (U.S. Ser. No. 11/420,652; filed May 26, 2006); 15) BLOOD PRESSURE MONITOR (U.S. Ser. No. 11/530,076; filed Sep. 8, 2006); 16) TWO-PART PATCH SENSOR FOR MONITORING VITAL SIGNS (U.S. Ser. No. 11/558,538; filed Nov. 10, 2006); and, 17) MONITOR FOR MEASURING VITAL SIGNS AND RENDERING VIDEO IMAGES (U.S. Ser. No. 11/682,177; filed Mar. 5, 2007).

(96) Other embodiments are also within the scope of the invention. For example, other techniques, such as conventional oscillometry measured during deflation, can be used to determine SYS for the above-described algorithms. In another embodiment, ‘vascular transit time’ (VTT) measured from two PPG waveforms can be used in place of PTT, as described above.

(97) Still other embodiments are within the scope of the following claims.