Electrocardiogram sensing and processing

Abstract

Real-time electrocardiogram (ECG) monitoring system for wearable devices. Embodiments of the invention are based on parallel Delta modulator architecture with local maximum point and local minimum point algorithms to detect QRS and PT waves. The parallel Delta modulators preferably convert ECG signals to two channels of three-state Delta modulated bitstreams. Using embodiments of the invention, real-time PR and RT intervals, as well as ST segment measurements, can be achieved in long-term wearable ECG recording devices.

Claims

1. An electrocardiogram sensing and processing apparatus comprising: a second-order Delta modulator circuit configured to subtract an integrated feedback voltage from an electrocardiogram signal and to generate a delta voltage, wherein the second-order Delta modulator circuit comprises a parallel Delta modulator circuit which comprises a pair of second-order Delta modulator circuits arranged in parallel with one another such that a first Delta modulator circuit of said pair of second-order Delta modulator circuits is configured to process a bit stream from a QRS wave and a second of said pair of second-order Delta modulator circuits is configured to process a bit stream from a PT wave; and non-transitory computer-readable media comprising a digital logic algorithm stored thereon, said digital logic algorithm configured to provide delineation to extract at least a plurality of fiducial points from the electrocardiogram signal during analog to digital conversion comprising Delta modulated bitstreams.

2. The electrocardiogram sensing and processing apparatus of claim 1, wherein said second-order Delta modulator circuit is configured to convert the electrocardiogram signal into two digital bitstreams, wherein a first of the two digital bitstreams represents a rising slope of the electrocardiogram signal and wherein a second of the two digital bitstreams represents a falling slope of the electrocardiogram signal.

3. The electrocardiogram sensing and processing apparatus of claim 2, wherein the two digital bitstreams control a feedback voltage switch and wherein the feedback voltage switch is configured to control a feedback voltage and wherein the feedback voltage is integrated in an integrator.

4. The electrocardiogram sensing and processing apparatus of claim 1, wherein said second-order Delta modulator circuit further comprises a tri-state comparator.

5. The electrocardiogram sensing and processing apparatus of claim 1, wherein said electrocardiogram sensing and processing apparatus is configured to record at least two fiducial points selected from the list consisting of an onset, peak, and end points of any one or more items selected from a list consisting of P, Q, and R waves of the electrocardiogram signal.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The accompanying drawings, which are incorporated into and form a part of the specification, illustrate one or more embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating one or more embodiments of the invention and are not to be construed as limiting the invention. In the drawings:

(2) FIG. 1 is a drawing which illustrates an example of an ECG waveform delineation highlighting the fiducial points and intervals according to an embodiment of the present invention;

(3) FIG. 2A is a system block diagram of an ECG monitoring system that uses parallel Delta modulators according to an embodiment of the present invention;

(4) FIG. 2B is a schematic diagram of a system block of a first-order ternary Delta modulator according to an embodiment of the present invention;

(5) FIG. 2C is a schematic diagram of a system block of a second-order ternary Delta modulator according to an embodiment of the present invention;

(6) FIG. 3 is a drawing which illustrates an example of a three-state delta modulated ECG waveform according to an embodiment of the present invention;

(7) FIG. 4 is a flowchart illustrating P and PR interval detection according to an embodiment of the present invention;

(8) FIG. 5A is a series of graphs which correspond to a first-order Delta modulator with ramp input signals;

(9) FIG. 5B is a series of graphs which correspond to a second-order Delta modulator with inputs of up-turning points;

(10) FIG. 5C illustrates graphs corresponding to a second-order Delta modulator with inputs of down-turning points;

(11) FIG. 6 is a drawing which illustrates a wearable ECG monitor using a parallel second-order Delta modulator for delineation;

(12) FIG. 7 is a flowchart that depicts an example of a digital fiducial points localization system using the bitstream from the parallel second-order Delta modulators according to an embodiment of the present invention;

(13) FIG. 8 is a drawing of an example waveform resulting from a second-order Delta modulator, which identifies onset, peak and end points of the P and T waveforms according to an embodiment of the present invention;

(14) FIG. 9 is a flowchart which illustrates operational flow of a delineation algorithm using both a first and a second Delta modulator according to an embodiment of the present invention;

(15) FIG. 10 is a collection of graphs that depict ECG waveform delineation with baseline wandering on the left and categorization of the morphology of P waves on the right according to an embodiment of the present invention;

(16) FIG. 11 is a schematic diagram of a Delta modulator according to an embodiment of the present invention;

(17) FIG. 12 is a schematic diagram of an operational transconductance amplifier (OTA) used in the Delta modulator according to an embodiment of the present invention;

(18) FIG. 13 is a schematic diagram of a comparator based on a 1-bit fourth-order delta sigma modulator in accordance with an embodiment of the present invention

(19) FIG. 14 is a QRS detection load maximum point (LMaP) algorithm flowchart according to an embodiment of the present invention;

(20) FIG. 15 is a graph which illustrates timing and register values according to an embodiment of the present invention;

(21) FIG. 16 is a flowchart of T wave and RT interval, ST segment detection according to an embodiment of the present invention;

(22) FIG. 17 is a table that illustrates simulation results of P and T wave detection for records in the QT database according to an embodiment of the present invention;

(23) FIG. 18 is a table which illustrates performance of PR, RT interval and ST segment detections according to an embodiment of the present invention;

(24) FIG. 19 is a graph depicting measured results of an experiment of an embodiment of the present invention;

(25) FIG. 20 is a table that illustrates a comparison of low-power QRS detectors and P and T wave detectors and a detector of an embodiment of the present invention; and

(26) FIG. 21 is a table which illustrates performance of QRS detectors and a detector of an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(27) Embodiments of the present invention relate to a system for QRS detection and P and T wave detection using parallel Delta modulators. The Delta modulation-based ECG monitoring system preferably includes two parts. The first part is the ternary first and second-order Delta modulators that derive Delta modulated bitstreams from analog signals. The second part is the ECG delineation circuits and methods that interpret the resulting Delta modulated bitstreams. The Local Maximum Point (LMaP) and Local Minimum Point (LMiP) algorithms are preferably provided, which can process bit streams from the Delta modulators to identify QRS and PT waves, as well as measure PR/RT intervals and ST segments. ECG delineation detects the timing information of the peak, onset, and end points of different ECG waves, including P, Q, R, S, and T waves, in order to measure the intervals and segments between these waves (see for example FIG. 1). During delineation, the detection of the QRS complex is one of the most important tasks because the QRS complex indicates the contraction process of the ventricles. Besides the QRS complex, the P wave and T wave detection also play very important roles in extracting the features. This is because the P wave indicates that if the heartbeat is initiated by the sinus node while the end of the T wave shows the repolarization of the ventricles. Moreover, the PR/RT/QT intervals and the ST segment are helpful with diagnosing some arrhythmia.

(28) Embodiments of the present invention were verified using 48 modified limb lead II (MLII) ECG records from the MIT-BIH Arrhythmia Database. The P and T wave detection algorithms were verified by 103 MLII or modified chest lead V5 records from the QT Database. The accuracy of embodiments of the present invention in QRS, P wave, and T wave detection was found to achieve above 99%, 91%, and 98%, respectively, in both sensitivity and positive prediction. A parallel Delta modulator of an embodiment of the present invention was fabricated in IBM 0.13 m CMOS technology with 720 nW power consumption at a 1 kHz sampling rate. The algorithm was implemented in a Xil- inx Spartan-6 FPGA. The measurement result of the prototype system illustrates that embodiments of the present invention provide desirable results in the field of wearable ECG sensors.

(29) System Architecture

(30) A parallel Delta modulator system according to an embodiment of the present invention is illustrated in FIG. 2A, As illustrated therein, two first-order three-state (sometimes referred to as ternary) Delta modulators (Delta-qrs and Delta-pt) are followed by digital circuits that implement QRS and PT detection algorithms used to detect QRS and PT waves separately. In this embodiment, the PT detection circuit uses the information of the detected R peak from the QRS detection circuit in order to locate P and T waves and measure PR/RT intervals and ST segments.

(31) Delta-qrs and Delta-pt share the same circuit structure of a three-state Delta modulator, as illustrated in FIG. 2B. In the Delta modulator circuit, the input ECG signal V.sub.in preferably subtracts the integrated feedback voltage V.sub.fb to generate the Delta voltage V.sub.d. Then, V.sub.d is compared with the trigger reference voltages +V.sub.ref and V.sub.ref. The comparison has three potential results, i.e. three states: S.sub.+1, S.sub.0, and S.sub.1. Here S.sub.+1 means V.sub.d is higher than +V.sub.ref, which indicates the input signal is rising, which is labeled as ecgR. Similarly, S.sub.1 means V.sub.d is lower than V.sub.ref, which means the input signal is falling, which is labeled as ecgF. So means V.sub.d is between the window formed by +V.sub.ref, and V.sub.ref, which is labeled as ecgS. In one embodiment, the output of the Delta modulator is a three-state bitstream representing the slope information of the input signal. A denser ecgR means the input signal is rising with a higher slope, and vice versa. A long period of time of ecgS means the input signal stays in the range of Vref and V.sub.ref. Examples of Delta modulator output with ECG signal inputs is illustrated in FIG. 3.

(32) In one embodiment, the outputs of the parallel Delta modulators are two Delta modulated bitstreams from both Delta-qrs and Delta-pt, which are then processed in real time using the LMaP and LMiP algorithms of an embodiment of the present invention to detect QRS and PT waves.

(33) The second-order Delta modulator is preferably similar to the first-order Delta modulator, while the residue voltage V.sub.d is now generated by V in subtracting the sum of the outputs from both the integrators. A ternary second-order Delta modulator preferably performs a pulse density modulation in which the pulse density is proportional to the input slope variation. FIG. 2C illustrates the second-order Delta modulator circuit. A ternary quantizer is applied so that no output pulse from S.sub.+1 and S.sub.1 is triggered when the input signal has a constant slope. Two integrators are preferably applied in the feedback loop. Non-overlapping clocks are preferably used to control the switching phases of sampling, integration and comparison. The positive or negative output pulses of the ternary comparator are preferably generated only when the slope of input signal has an up or down variation. Regardless of the initial input slope, an upward-turning point (UTP) of the analog input preferably results in positive output pulses (S.sub.+1) while a downward-turning point (DTP) preferably results in negative output pulses (S.sub.1).

(34) A simulation of exemplary behavior of the ternary first-order Delta modulator and the second-order Delta modulator is illustrated in FIGS. 5A-5C. In this embodiment, the first-order Delta modulator preferably detects the slope of the input waveform. If the input has a rising slope, the output preferably generates a positive bitstream and vice versa. While the second-order Delta modulator detects the turning point of the input waveform, when there is an upward turning point, the output generates a positive bitstream where the first positive bit represents the moment of the turning point. Thus, the second-order Delta modulator can be used to detect the onset, peak and end point of the input waveform. FIG. 5A illustrates the first-order Delta modulator's outputs for ramp signals with different slopes. A larger slope in the input signal generates a higher density in the output pulses and vice versa. FIGS. 5B and C illustrates the second-order Delta modulator's output signals with different upward and downward turning angles of the input waveform. A larger upward or downward turning angle () produce a higher pulse density at the positive or negative output, respectively.

(35) Embodiments of the present invention provide delineation using a first-order Delta modulator, for example as illustrated in FIG. 2A. In one embodiment, two first-order three-state Delta modulators (Delta-qrs and Delta-pt), with the following digital circuits implementing QRS and PT detection algorithms, are preferably used to detect QRS and PT waves separately. In the system, the PT detection circuit preferably uses the information of the detected R peak from the QRS detection circuit in order to locate P and T waves and measure PR/RT intervals and ST segments. Because QRS complex has a much higher amplitude and slope compared to P and T waves, the Delta modulators Delta-qrs and Delta-pt preferably have different trigger reference voltages and integration gains. A higher integration gain in Delta-qrs avoids an overload problem of the Delta modulator so the R peak can be accurately detected without time domain distortion. Also, a higher trigger reference voltage can help Delta-qrs to only detect the R wave and ignore low amplitude variations of the ECG signal, which includes P, T, and U waves, DC offsets, and other noises. However, this causes Delta-qrs to ignore the low amplitude waves, such as P and T waves. Meanwhile, Delta-pt is preferably optimized for P and T waves with lower integration gain and trigger reference voltage. However, due to overloading, Delta-pt experiences saturation when R peak arrives, which introduces time domain inaccuracy in R wave detection. Therefore, in order to optimize the integration gain and reference voltages in both the Delta modulators, parameter analysis is preferably applied during circuit design. The parallel Delta modulators can also be implemented using the Delta modulators with the same parameter, but preferably with different analog gain in the previous amplifiers. In choosing the optimized parameters, it is desirable to (1) to avoid saturation in the Delta modulators, (2) to have Delta-qrs only response to QRS complex, and (3) have a clear Delta-pt response to P waves and T waves. As Delta modulators have a low power consumption, the parallel Delta modulators do not require much additional power compared to the single Delta modulator structure. The outputs of the parallel Delta modulators are two Delta modulated bitstreams from both Delta-qrs and Delta-pt, which is then processed in real time to find maximum and minimum points of the waveforms, in order to detect the QRS and PT waves.

(36) Embodiments of the present invention preferably provide delineation using a second-order Delta modulator as illustrated in FIG. 6. To avoid the overloading problem, a parallel structure is preferably applied with two channels of the second-order Delta modulator (DM2) for ECG delineation. Circuit DM2_pt preferably has an extra amplifier with a gain of about 4 for detecting the P wave and T wave that have lower slope variation than the QRS complex, while circuit DM2_qrs preferably detects the QRS complex. Output pulses of the parallel DM2 are preferably then processed digitally to extract timing information of the fiducial points and the P wave morphology for arrhythmia detection.

(37) Because the QRS complexes are the most distinct marks in ECG signals, in one embodiment, the first step of the algorithm is to find the QRS complex using a predefined threshold of UTP/DTP pulse density in a timing window. As illustrated in FIG. 7, the algorithm is preferably based on detecting the R wave. The detection of a positive/negative R wave is defined as the detection of UTP-DTP-UTP/DTP-UTP-DTP with independent pulse density threshold values from DM2_qrs. Once the R wave is detected, the algorithm searches back to locate the Qonset and Rpeak in a two-channel data-cache that stores past data from DM2_qrs. In one embodiment, the positive Rpeak is preferably located by the first pulse (FP) of the detected DTP pulse cluster and vice versa. An FP is preferably identified if there is no pulse within a predetermined number of prior clock cycles, and a pulse cluster is preferably defined as all pulses in a fixed timing window followed by an FP. Other pulse clusters, before and after Rpeak, are preferably identified from the S-1 of DM2-qrs as the onset and offset of the QRS complex.

(38) The P/T wave detection algorithms preferably use similar strategies. The T wave detection algorithm is preferably not activated until QRS complex is detected. The P wave detection algorithm is preferably parallel with that of R wave. When the first adequate UTP/DTP is identified in positive/negative R wave detection, the P wave detection algorithm is preferably paused. The algorithm preferably resumes once T wave is recognized. Exact timing information of fiducial points of P wave is then extracted from a two-channel data-cache of DM2_pt data. The P wave morphology information is also extracted in this process. Moreover, some protection mechanisms are preferably applied to avoid interference of noise or other disturbances. For example, individual noise pulse in DM2 output can be identified and removed if it is the single pulse in its pulse cluster and located on an unexpected site. Further, constraints are preferably applied for the durations of each wave and the intervals between each pulse cluster. Finally, the timing information of the interval or segment can be calculated. Example waveforms for detecting the fiducial points of the P wave and T wave is illustrated in FIG. 8.

(39) Delineation can also be achieved using both the first and second-order Delta modulator. In one embodiment, parallel first-order and second-order Delta modulators can be used. The second-order Delta Modulator, Delta-qspt2nd, the first-order Delta modulator, Delta-qrs1st, and Delta-pt1st with different integration gains can be provided for detecting the QRS and the P/T waves, respectively. An embodiment of a delineation algorithm according to an embodiment of the present invention is illustrated in FIG. 9. Most preferably, in a first step, the QRS complexes are detected by the Delta modulator, Delta-qrs1st, with the QRS detection algorithm. The objective is to find the waves that meet the defined requirement of the slope. In a second step, the Delta modulator, Delta-pt1st, preferably starts to search back to find the rough P wave location and wait for detecting the T wave location. Meanwhile, a moving average value register preferably records the maximum point in the QRS detection area from the bitstream of the Delta-Sigman modulator, Delta-sigma1st. Then, the onset, peak, and the end point of the P/T wave, as well as the onset of Q wave, and the end point of S wave are preferably detected by the second-order Delta modulator, Delta-qspt2nd, because the second-order Delta modulator is good at detecting the turning point of the input waveform. The parallel Delta modulators can perform delineation with baseline wandering, as is illustrated in the results of ECG delineation and P wave morphology detection in FIG. 10.

(40) Delta Modulator Circuit Design

(41) The design parameters of the Delta modulators preferably include the sampling rate, the bandwidth, the integration gain, the threshold voltages, and the reference voltage. Because the oversampling modulators generate only one bit at a time, the total power consumption is much lower than a conventional multi-bit ADC. Given the fact that biomedical signal acquisition and processing usually require oversampling, the Delta modulators can extract digital features directly from the analog waveform during the analog to digital conversion, which uses less power than the combined power of the conventional ADC and its following digital signal processing circuits.

(42) The Delta modulator circuit preferably performs two jobs in a system according to an embodiment of the present invention. The first job is to convert the analog input of the ECG signal into a digital format for the next processing step. The second job is to work as a feature extractor that illustrates whether the input signal has a positive or negative slope, or stays in a defined range. Delta modulators or Sigma-Delta modulators used as a part of an analog to digital converter (ADC), may require a very high oversampling rate (OSR) to achieve a high signal to noise ratio (SNR) in order to recover the analog input signal. A high OSR introduces a lot of data which then becomes a heavy burden on the following QRS and PT wave detection circuits. Therefore, in an embodiment of the present invention, because the recovery of the analog input is not the goal, the requirement of OSR can be alleviated in order to save circuit power. While the targeted ECG signal is concentrated in the bandwidth 0.5 Hz-40 Hz, in the Delta modulators of an embodiment of the present invention, the sampling clock is preferably set at a predetermined frequency, most preferably about 1 kHz, and the system input bandwidth is about 0.05 Hz to about 150 Hz for design margins.

(43) In one embodiment, a switched-capacitor-based Delta modulator can be used and will provide desirable results. FIG. 11 illustrates a schematic diagram of the switched-capacitor-based Delta modulator of an embodiment of the present invention. The capacitor C.sub.sub is the subtraction capacitor that generates V.sub.d. Capacitors C.sub.s and C.sub.i are the sampling and integrating capacitors of the feedback integrator, respectively. Non-overlapping clocks control the switch phases 1 and 2. The three-state comparator is preferably clocked by 2ec which has a positive edge a little prior to the negative edge of 2e. As the Delta modulator works at a relatively low speed, the subthreshold-leakage suppression switches are preferably applied. The leakage current of the switch SWL in FIG. 11 can be expressed in the following relationship:

(44) $\begin{array}{cc}{I}_{\mathrm{leak}}\frac{W}{L}.Math.e^{\frac{v_{\mathrm{GS}}-v_{\mathrm{TH}}}{m-v_T}}.Math.\left(1-e^{\frac{-v_{\mathrm{DS}}}{v_T}}\right)& \left(1\right)\end{array}$
where m and v.sub.T denote the body effect coefficient and the thermal voltage, respectively. During 1, SW1, SW2, and SW5 are turned on, which makes node N1, N2 and N3 have the same voltage. During 2, SW3, SW4, and SW6 are turned on and the rest of the switches are turned off, then V.sub.DS of SW2 is reduced to zero to remove the leakage current according to equation (1).

(45) A schematic diagram of an operational transconductance amplifier (OTA) that can be used in the Delta modulator is illustrated in FIG. 12. The recycling folded cascode architecture enhances the transconductance, gain and slew rate of this one stage amplifier, which makes it a good choice for integrators with low power consumption and low design complexity. In phase 2, the gain-bandwidth (GBW) product of the OTA is GBW=G.sub.m/C.sub.L. Here, G.sub.m is the transconductance of the OTA; =C.sub.i(C.sub.i+C.sub.s+C.sub.p+C.sub.sub) is the feedback factor. C.sub.p (less than 0.1 pF) is the OTA input parasitic capacitance; C.sub.L=C.sub.sub+(1)C.sub.i+C.sub.pl is the total load capacitance; C.sub.pl (less than 0.1 pF) is the load parasitic capacitance.

(46) FIG. 13 illustrates a schematic diagram of a comparator design based on a 1-bit fourth-order Delta-Sigma modulator. The comparator has a dynamic latch and an SR latch. Very low static power is consumed because of the pseudo-differential topology. When the positive edge of 2ec comes, Mn3 and Mn4 are turned on. With the help of pre-charged voltage at node N and node P, the dynamic latch (Mp1, Mn5, Mp2, Mn6) pushes node N and node P to the rails based on the input. When 2ec is low, node N and node P are both charged to V DD. The SR latch secures the previous decision of the comparator.

(47) QRS and PT Wave Detection Algorithms

(48) A. QRS Complex Detection Algorithm

(49) In one embodiment, the bitstream processing algorithm is preferably based on counting the number of pulses in a moving window, without performing addition and multiplication, such as in the wavelet transform algorithm. The features extracted from the counting results can be used in machine learning algorithms like support vector machines. The digital counting circuits use much less power than the multiply-and-accumulate circuits at the same clock frequency. The arrhythmia classification algorithm can use the location of the fiducial points and the pulse density modulation from the Delta modulators to detect arrhythmias.

(50) In one embodiment, a Local Maximum Point (LMaP) algorithm is preferably used to detect QRS complex directly from the Delta-modulated bitstream. An algorithm flow chart is illustrated in FIG. 14. The algorithm preferably has two steps. Step one detects the rising edge of the R wave while step two detects the falling edge of the R wave to complete the QRS detection.

(51) In a preferred first step, the trigger reference voltage and integration gain of the Delta modulator, Delta-qrs, is preferably optimized so that Delta-qrs is sensitive to a large rising slope, for example that of the Q-R segment. Also, the Q-R slope preferably does not overload the Delta modulator, in order to allow accurate detection of Rpeak in the time domain. The LMaP algorithm starts by detecting an ecgR label, which indicates a potential beginning of a QRS complex. Then, a counter starts counting the number of consecutive ecgR labels, which is preferably marked as P.sub.rise, and that number is then compared with a threshold value for a rising slope (THR), which can be from observation of simulation results. Once the number of consecutive ecgR labels is higher than THR, the algorithm preferably makes an assumption that a Q-R segment is detected (QRpredicted). During this process, if an ecgF label is detected, then the detection is preferably marked as failed, and the algorithm is preferably reset to the original state. Moreover, if the duration between the first and second ecgR, or between the second and third ecgR is more than a pre-defined value, the algorithm also preferably resets the process to the original state. This process is preferably the first ecgR protection mechanism (FEPM). FEPM preferably protects the algorithm so that it is sensitive to only large rising slopes. By doing so, sparse ecgR sequences, which indicates small rising slopes, are ignored by the algorithm to prevent false detection.

(52) In a preferred second step, based on the ECG morphology, around the peak of the R wave, the slope is reduced to a small value. Thus, a sequence of ecgS labels appears if there are enough sampling points, which are preferably marked as P.sub.stay. Then, the following Q-R segment contains a sequence of ecgF labels, which is preferably marked as P.sub.F all. Similar to step one, the algorithm preferably counts the number of the consecutive ecgF labels and compares the number to a threshold value obtained from simulation of the falling edge (THF). Once the number of consecutive ecgF labels reaches THF, the complete QRS complex is detected. Once P.sub.F all happens after a QR predicted label, a predicted R peak label (Rp predict) is preferably marked on the first ecgF location. After the complete falling edge of the R wave, a complete R peak label Rp complete is preferably marked to declare the full detection of the QRS complex. Otherwise, if there is no Rp complete label within a certain time after the Rp predict label, the Rp predict label is preferably removed. This means the detection has failed, in which case, the algorithm is preferably returned to its original status. To prevent large baseline drifts of the EGG wave form triggering the QRS detection process, a P.sub.stay state protection mechanism (PSPM) is preferably provided. With PSPM, when the algorithm is entered in the P.sub.stay state, a counter preferably starts to count with the sampling clock up to a pre-defined period. If no P.sub.F all happens during the period, the algorithm also preferably resets to its original state.

(53) B. P and T Wave Detection Algorithm

(54) P Wave Detection Algorithm: The P wave detection circuit preferably implements the P wave detection algorithm to process the Delta modulated bitstream from the Delta modulator Delta pt. Because the Delta pt is sensitive to low-amplitude and slow-variation of the input signal, compared to the QRS detection algorithms, the P wave detection algorithm handles more local maximum points. A flowchart of the P wave detection algorithm of an embodiment of the present invention is illustrated in FIG. 4. In one embodiment, the LMaP detection algorithm used in the P wave detection is similar to the QRS complex detection except that different threshold values THR_p and THF_p are applied instead of THR and THF.

(55) Another important task of the P wave detection circuit is to measure the PR interval. The PR interval measurement, in an embodiment of the present invention, preferably records three timing information aspects: P wave peak (P.sub.peak) to R wave peak (R.sub.peak), P wave onset (P.sub.onset) to P.sub.peak, and Q wave onset (Q.sub.onset) to R.sub.peak. For P.sub.peak-R.sub.peak and P.sub.onset-P.sub.peak segments, the input data from Delta-pt is preferably delayed about 60 ms to avoid the influence from QR S complex in the P wave detection algorithm. Because the QRS complex can also be detected as LMaP from Delta-pt, the 60 ms delay acts as a protection window so that QRS complex will not trigger LMaP in the P wave detection. After the delay, a counter (PR counter) preferably starts counting when the algorithm meets the first LMaP and restarts counting when it meets the next LMaP. A P.sub.peak-R.sub.peak Interval Register (PRIR) preferably records the counted value at the appearance of the Rp predict label. A wave onset detection (WOD) block can be applied to detect the P.sub.onset-P.sub.peak segment. A P.sub.onset, counter (Po counter, the negative P wave uses a Pno counter) preferably starts counting when it meets the first ecgR until the P.sub.peak is detected. Then, the data is recorded in the P.sub.onset-P.sub.peak Register (POPR). Similar to QRS complex detection, a FEPM is preferably applied to limit the first and second ecgR. The value of the Po/Pno counter (#cntr) is compared to a programmable threshold (THpp) value, so that if the recorded P.sub.onset-P.sub.peak is too large, the algorithm resets to its original state to avoid a false detection. Similarly, negative P wave onset (Pno) detection can also be achieved using a Pno counter. For Q.sub.onset-R.sub.peak detection, the long ones register (LOR) and the long negative ones register (LnOR) are preferably used to detect the starting point of the Q-R segment, which results in continuous ecgRs or ecgFs without interruption by the easy-to-saturation feature of Delta-pt. Once the Q-R segment onset is detected, Q.sub.onset and negative Q.sub.onset counter (Qo/Qno counter) preferably starts to count until Rp_complete comes, and data is then recorded in Q.sub.onset-R.sub.peakRegister (QORR). Finally, the PR interval register (PR_interval) preferably obtains the value of the PR interval from computing an equation PR_interval=PRIR+POPR-QORR+60 ms+Calibration_pr. Here, Calibration_pr is preferably used to compensate the timing error between Q wave onset and the starting point of the Q-R segment, as illustrated in FIG. 15. Then, the value of PR_interval is preferably sent to the following PR interval warning generation block (WGB) when Rp_complete is identified.

(56) A special case in P wave is that the P wave has an opposite polarity. This can happen due to atrial tachycardia. In such cases, LMaP is not able to detect such P waves. Therefore, a complementary Local Minimum Points (LMiP) algorithm is preferably introduced to detect the opposite polarity P waves. LMiP detects P.sub.F all first, and then P.sub.stay and P.sub.Rise with corresponding THF_p and THR_p values. The LMiP algorithm is preferably run in parallel with the LMaP algorithm in different circuits. The LMiP circuit has its own PR interval register PRIR_N measuring the PR interval and sending the value to the WGB.

(57) In addition, atrial flutter or other arrhythmic symptoms may introduce fast P waves, and sinus exit block or other arrhythmia may cause missing P waves, which make the PRIR value out of PR interval limit. To handle such cases, when Rp complete comes, WGB preferably first compares the PRIR value with the pre-defined PR interval limit values. If the measured PR interval is within the range of the pre-defined limits, WGB preferably keeps the value, Otherwise, WGB preferably sends the value to a temporal register and then fetches the value from PRIR_N, and compares the value with the limit range again. If the value from PRIR_N is in the range, WGB uses the value from PRIR_N. Otherwise, WGB takes the value back from the temporal register. In summary, in P wave detection, normal or fast P waves are detected first. If no normal or fast P waves are detected, the algorithm preferably seeks an opposite polarity P wave. If still no P waves are found, the WGB preferably marks the result as long distance P wave or missing P wave. In each case, WGB generates different warning signals according to the detection result.

(58) T Wave Detection Algorithm: T waves are more recognizable than P waves because T waves have relatively larger amplitude and longer duration compared to P waves. Therefore, the T wave detection algorithm is much simpler. As illustrated in FIG. 16, after the detection of QRS complex, a counter preferably starts counting until a pre-defined number of ecgR or ecgF labels is reached, which indicates that there is a rising or descending slope, and then sends data to RT interval register (RTIR). This can be seen as an appearance of the T wave (T_detect), and according to the polarity of the label, T wave polarity information (T_polarity) is sent out. Because the ECG morphology may have patient-to-patient variation, a programmable calibration register Calibration_rt is preferably applied to compensate for the difference between the detected T wave and the real T wave locations. Finally, the RT interval register (RT_interval) preferably records RT interval data by computing the equation RT_interval=RTIR+Calibration_rt.

(59) Similar to P.sub.onset-P.sub.peak segment measurement, the same wave onset detection algorithm is preferably applied to measure the timing from T wave onset (T.sub.onset) to T wave peak (T.sub.peak) and the value is preferably recorded in the T.sub.onset-T.sub.peak Register (TOTR). After Rp complete is detected, another pair of LnOR/LOR is preferably used to detect the offset point of the R-S segment. Then, the S offset counter (Sof counter) preferably starts to count until T_detect to acquire the timing value from S wave offset (S.sub.offset) to T.sub.peak, and records the value in the S.sub.offset-T.sub.peak register (S2TR). Finally, the ST segment recording register (ST_segment) preferably records the ST segment by computing the equation ST_segrnent=S2TRTOTR+Calibration_st. Here, Calibration_st is preferably used to compensate the timing error between the R-S segment offset and the S.sub.offset as illustrated in FIG. 14.

(60) Embodiments of algorithms LMaP and LMiP are different from some other QRS detection algorithms, such as pulse-triggered (PUT), time-assisted PUT (t-PUT), and input-feature-correlated (IFC). For example, an embodiment of the present invention can provide on-sensor signal processing to measure PR and RT intervals, and ST segments, without recovering the original signal. Moreover, embodiments of the present invention can introduce FEPM and PSPM to alleviate the challenge of signal baseline drifts, because low-frequency baseline drifts only generate very sparse ecgRs and ecgFs that would not change the processing state in LMaP and LMiP algorithms. A parallel algorithm for detecting a negative slope before a positive slope is also preferably provided, and in one embodiment it only operates when there is no normal QRS complex found.

(61) In one embodiment, the detection system of the present invention can detect and determine PR and RT intervals, as well as ST segment measurements while using an average of less than 10 W of power and more preferably an average of less than 1,000 nW of power.

INDUSTRIAL APPLICABILITY

(62) The invention is further illustrated by the following non-limiting examples.

Example 1

(63) A system according to an embodiment of the present invention was constructed and was used to analyze simulated datasets.

(64) D, P and T Wave Detection Algorithms Evaluations

(65) Embodiments of the present P and T wave detection algorithms were evaluated using 103 records from the QT Database. The QT Database contains a total of 105 records. Two records (sel35 and sel37) were not used in the evaluation due to the lack of professional manual annotations of the P and T waves. The most preferred data sources in the evaluation are MLII and V5, because the ECG signals from MLII and V5 have high amplitude and clear QRS complex, with more distinguishable P and T waves.

(66) The complete simulation results of the P and T wave detection are illustrated in the table of FIG. 17. The results of PR interval, RT interval and ST segment are illustrated in the table of FIG. 18. All records with notated P.sub.onset and Q.sub.onset (97 records), R.sub.peak and T.sub.peak (103 records), and S.sub.offset and T.sub.onset (45 records) from the QT Database were used to evaluate PR, RT interval and ST segment, respectively. Using an algorithm of the present invention, the performance in T wave detection is better than in the P wave detection.

(67) Measurement Results

(68) The three-state parallel Delta modulator chip was fabricated with a 0.13 m CMOS process. The core silicon area was 520560 m.sup.2. The power supply voltage was 0.6 V, The designed integration gains of Delta-qrs and Delta-pt were 0.04166 and 0.02083, respectively. The reference voltage of Delta-qrs and Delta-pt were set to 70 mV and 20 mV, respectively. With a 1 kHz sampling clock, the measured power consumption of the parallel Delta modulator was 720 nW. The digital processing circuits for QRS and PT wave detections were implemented in a Xilinx Spartan-6 FPGA board on an Opal Kelly XEM6001 module.

(69) In the test of the hardware prototype, the input ECG signal came from a Rigol DG4102 signal generator with 0.4 V.sub.PP and 60 beats-per-minute (bpm) heart rate. The output of the system included the detected QRS sign, P wave sign, T wave sign, and the wave polarities of the P waves and T waves. The system also measured PR intervals, RT intervals, and ST segments. The waveforms recorded from the oscilloscope in the experiment are illustrated in FIG. 19. The PR interval of the input ECG signal was around 140 ns, the RT interval was around 250 ns, and the ST segment was around 160 ins.

(70) In the test, the reading of the average PR interval was 152.7 ms, taking a 13 ms from the programmable patient-specific Calibration_pr, so the average recorded PR interval from the prototype was 139.3 ms. In the RT interval measurement, the average recording reading of the RT interval was 212.5 ms and the patient-specific Calibration_rt delay was set at 40 ms. So, the total average RT interval was 252.5 ms. In the ST segment measurement, the average recording value was 174.3 ms and the calibrated value is 160.3 ms with Calibration_st equaled 14 ms.

(71) A. Performance Evaluation

(72) Table IV of FIG. 20 compares these results with the state-of-the-art QRS and PT wave detection systems. For QRS detection, although asynchronous Delta modulators have very low power consumption, embodiments of the present invention achieve accuracy in QRS detection. In addition, embodiments of the present invention are able to detect P and T waves, and measure PR, RT intervals and ST segments. On the other hand, compared to other P and T wave detection algorithms, embodiments of the present invention achieve reasonably good accuracy, while providing low complexity algorithms that can be implemented on hardware.

(73) B. Discussions

(74) Embodiments of the present invention provide a Delta modulator-based ECG monitor with the capability of low power on-sensor measurement of PR/RT intervals and ST segments. These features can enable future studies of long-term ECG recording involving PR/RT/ST measurements while avoiding continuous raw data transmission, which is power consuming. Specifically, embodiments of the present invention can improve the sensitivity of automatic arrhythmia detection algorithms in intermittent monitoring systems. Biasing circuits, fully differential front-end circuits and buffers in analog front-end can be added to embodiments of the present invention to reduce the effects of power supply noise. Embodiments of the present invention can preferably be calibrated for patient-specific physical activities and other factors that can affect accuracy and problems of onset and offset detection of the first-derivative based algorithm. Embodiments of the present invention can include special detection models for certain arrhythmias, such as premature ventricular contractions (PVCs) and/or premature atrial contractions (PACs).

Example 2, QRS Complex Detection Algorithms Evaluations

(75) The QRS detection algorithm of an embodiment of the present invention was evaluated by all 48 records in the MIT-BIH Arrhythmia Database. The complete simulation results are illustrated in the table of FIG. 21. Three parameters (Se, +P, and Error) were simulated to evaluate the performance of the algorithm. The sensitivity (Se) represents the capability of the algorithm to correctly detect the QRS complex. The positive prediction (+P) illustrates the discriminability of the algorithm in detecting true and false QRS complexes. The detection error rate (Error) stands for the accuracy. Also, to measure the time error of R peak detection, m represents the mean errors in time domain between the cardiologist annotations and the detection by the proposed algorithm. Also, s stands for the average of the intra-recording standard deviations.

(76) $\begin{array}{cc}\mathrm{Se}\left(\%\right)=\frac{\mathrm{TP}}{\mathrm{TP}+\mathrm{FN}}& \left(2\right)\end{array}$$\begin{array}{cc}+P\left(\%\right)=\frac{\mathrm{TP}}{\mathrm{TP}+\mathrm{FP}}& \left(3\right)\end{array}$$\begin{array}{cc}\mathrm{Error}\left(\%\right)=\frac{\mathrm{FN}+\mathrm{FP}}{\mathrm{TP}+\mathrm{FN}}& \left(4\right)\end{array}$
where TP is the number of true positive detections, FN is the number of false negative detections, and FP stands for the number, of false positive detection errors. The overall sensitivity and positive prediction were both above 99%. Except for one record (203), all other 47 records were above 95% for Se. The overall mean error and average intra-recording standard deviation was 0.09 ms and 12.12 ms, respectively. All records had positive predictions greater than 97%.

(77) In one embodiment, the parallel Delta modulator chip was fabricated with 0.13 m CMOS technology with 0.6 V power supply voltage. The chip consumed 720 nW with a sampling rate of 1 kHz. The system was verified through simulations using data from the MIT-BIH Arrhythmia Database and the QT Database. The system was found to achieve 99.17%, 91.12%, and 98.36% sensitivity and 99.55%, 92.44% and 98.99% predictivity for QRS complex, P wave, and T wave detections, respectively, and the respective mean errors and intra-recording standard deviations were 0.1817.47 ms, 2.1317.02 ms and 0.51995 ms for PR interval, RT interval and ST segment detections respectively. The hardware prototype system was found to perform real-time PR and RT interval, and ST segment measurements.

(78) Note that throughout this application, the term about means within twenty percent (20%) of the numerical amount cited.

(79) Embodiments of the present invention can include every combination of features that are disclosed herein independently from each other. Although the invention has been described in detail with particular reference to the disclosed embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all references, applications, patents, and publications cited above are hereby incorporated by reference. Unless specifically stated as being essential above, none of the various components or the interrelationship thereof are essential to the operation of the invention. Rather, desirable results can be achieved by substituting various components and/or reconfiguration of their relationships with one another.