Method and measuring system for continuously determining the intra-arterial blood pressure

Abstract

The invention describes a measuring system for the continuous non-invasive determination of blood pressure at one or more fingers. The fingers chosen for measurement and the adjacent parts of the palm rest on a supporting surface of a housing, which has the shape of a computer mouse. Inside the housing of the “CNAP Mouse”, i.e. underneath the supporting surface for the hand, the pressure generating system is located. The finger sensors are mounted on the supporting surface for the hand. The forearm and the back of the hand are left free and may be used to place intra-venous or intra-arterial access elements. Since the hand will rest on the supporting surface motion artefacts are largely avoided. Tilting or turning of the sensors is hardly possible since the fit of the sensors and thus the coupling of light and pressure are optimized.

Claims

1. A measuring system for the continuous determination of intra-arterial blood pressure at two fingers of a hand having a palm, the measuring system comprising: a double-finger sensor having inflatable cuffs for the two fingers; a plethysmographic system comprising at least two light sources, with one or more wavelengths, and at least two light detectors; a pressure generating system comprising at least one valve, controlled in real time by the plethysmographic system, for generating pressure in the cuffs, which pressure is available over a frequency range with an upper limit frequency of at least 20 Hz and corresponds to the intra-arterial blood pressure in the finger; and a housing comprising: an upper supporting surface configured to support the two fingers and adjacent areas of the palm of the hand, the upper supporting surface having a curved outer profile; and a flat bottom surface configured to rest on a supporting plane; wherein the double-finger sensor projects from the upper supporting surface; wherein the upper supporting surface comprises a receiving slot that partitions the upper supporting surface mirror-symmetrically into two partial surfaces, the receiving slot containing elements for connecting the plethysmographic system to the double-finger sensor; wherein the elements for connecting the plethysmographic system to the double-finger sensor comprises two outer elements disposed in the receiving slot that are slideable in the direction of a central element; wherein a part of the double-finger sensor which accommodates the inflatable cuffs is fastened to the elements in a removable and exchangeable manner; wherein the two outer elements contain the two light sources while the central element contains the two light detectors; and wherein the pressure generating system with the at least one controlled valve is located in the housing underneath the upper supporting surface.

2. The measuring system according to claim 1, wherein a control system is disposed in the housing underneath the upper supporting surface for the hand.

3. The measuring system according to claim 1, wherein a pump and an air reservoir are disposed in the housing underneath the upper supporting surface for the hand.

4. The measuring system according to claim 1, wherein the housing of the measuring system is made air-tight underneath the upper supporting surface for the hand, and acts as an air reservoir.

5. The measuring system according to claim 1, wherein a battery or a power pack, and elements for wireless transmission of signals are disposed in the housing underneath the upper supporting surface for the hand.

6. A measuring system for the continuous determination of intra-arterial blood pressure at two fingers of a hand having a palm, the measuring system comprising: a double-finger sensor having inflatable cuffs for the two fingers; a plethysmographic system comprising at least two light sources, with one or more wavelengths, and at least two light detectors; and a pressure generating system comprising at least one valve, controlled in real time by the plethysmographic system, for generating pressure in the cuffs, which pressure corresponds to the intra-arterial blood pressure in the finger; and wherein the measuring system has a housing comprising: an upper supporting surface configured to support the two fingers and adjacent areas of the palm of the hand, the upper supporting surface having a curved outer profile, and a flat bottom surface configured to rest on a supporting plane, wherein the double-finger sensor projects from the upper supporting surface; wherein two outer elements are disposed in a receiving slot on the upper supporting surface of the housing so as to be slideable in the direction of a central element; wherein a part of the double-finger sensor which accommodates the inflatable cuffs is fastened to the elements in a removable and exchangeable manner; and wherein the two outer elements contain the two light sources while the central element contains the two light detectors.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows a state-of-the-art measuring system, where the pressure generating system is placed at the distal end of the forearm;

(2) FIG. 2 shows the measuring system according to the invention in the shape of a computer mouse (CNAP mouse) in a three-dimensional view;

(3) FIG. 3 shows the measuring system of FIG. 2 in an exploded view;

(4) FIG. 4 shows the measuring system of FIG. 2 with the cuff part removed and the PPG elements visible;

(5) FIG. 5 shows the symmetrical design of the measuring system of FIG. 2 in a top view;

(6) FIG. 6 shows all possible ways of applying the measuring system to the right or left hand of a patient; and

(7) FIG. 7 shows a variant of the measuring system according to the invention in an exploded view.

DETAILED DESCRIPTION

(8) The invention describes a measuring system for the continuous non-invasive determination of blood pressure. The combination of pressure generating system and finger sensors comprising cuff and light system—the photoplethysmographic system—is presented in detail.

(9) In principle the Vascular Unloading Technique mimics the intra-arterial pressure in the finger cuff in real time. This is done by means of a control system which requires as input signal at least the light signal v(t) of the PPG system. More recent control systems, such as described in publications WO 2000/059369, WO 2005/037097, WO 2011/0511819 or WO 2011/051822, also use the cuff pressure p.sub.c(t) as an input variable.

(10) The pressures obey the following equation, which arises when the air-filled cuff is applied:
p.sub.c(t)=p.sub.a(t)−p.sub.t(t)   (1)

(11) where p.sub.t(t) is the socalled “transmural pressure”, i.e. the pressure difference between cuff pressure p.sub.c(t) and intra-arterial blood pressure p.sub.a(t).

(12) The transmural pressure p.sub.t(t) acts on the diameter of the artery, which diameter can indirectly be determined from the PPG signal or volume signal v(t). As has been described above, the control system will adapt the cuff pressure p.sub.c(t) in real time such that v(t) will be kept constant, i.e. that Δv(t)=0. If this condition Δv(t)=0 is fulfilled, the transmural pressure p.sub.t(t)=0, and therefore one has
p.sub.c(t)=p.sub.a(t)−0   (2)

(13) In order to fulfil this real-time condition, the measuring system must be able to follow the changes in intra-arterial blood pressure p.sub.a(t). Interpreting the control condition in the frequency domain the measuring system must be able to mimic pressure changes which transcend the upper limit frequency of intra-arterial blood pressure. This upper limit freqency of intra-arterial blood pressure p.sub.a(t) lies approximately at 20 Hz.

(14) All the methods and devices described require that the cuff pressure p.sub.c(t) must be available over the total relevant frequency range with an upper limit frequency of at least 20 Hz, a requirement which presents a challenge to the pressure generating system and its valves or valve system. It is therefore important that the pressure generating system and its valve or valve system be situated in the immediate vicinity of the finger cuff. Depending on the rigidity of the connecting tube, the maximum distance will be 30 to 50 cm.

(15) FIG. 1 shows how the pressure system 101 is connected to the finger sensor 102 in a state-of-the-art measuring system. The finger sensor 102 shown is designed as a double finger sensor for index and middle finger 100 of the right hand, permitting longer duration of measurement. The finger sensor 102 is supplied with pressure and electric energy via a double tube system 103. The pressure system 101 is attached to the distal end of the forearm by means of a fastening element 104 and is supplied via a cable 105 with electric energy and with basic pressure.

(16) FIG. 2 shows the measuring system according to the invention, the assembled CNAP mouse. On the body or on the housing 1 of the CNAP mouse there is provided a relatively large supporting surface 11 upon which the hand may rest during the measuring operation. The finger sensors 2 are attached to the housing 1. In the example there is shown a double finger sensor, with which each finger may be measured alternatingly. The double sensor has two light sources (LEDs) 21a, 21b and at least two light sensors 22a, 22b (partly visible). The two inflatable cuffs 23a and 23b are disposed on the interior areas 23 of the finger sensors 2. The part of the finger sensor 2 which carries the inflatable cuffs 23a, 23b is removable and may be exchanged, and is attached to a locking element on the housing 1 by means of snap-on element 24.

(17) FIG. 3 shows an exploded view of the CNAP mouse. First it is shown how the pressure generating system 12 is integrated into the body or the housing 1 of the mouse underneath the supporting surface 11 for the hand. In this case the pressure generating system 12 consists of an electronic circuit board 12a, which carries one or more valves 13a, 13b as well as the elements of the valve control electronics. Preferably, a connector 14 is also provided on the board 12a. The pressure generating system 12 is placed in the housing 1 and secured by a cover 15 on the bottom of the housing 1. A cable 16 supplies the pressure generating system 12 with basic pressure, electric energy and the signals emitted by the control system for the Vascular Unloading Technique.

(18) On the top face of the housing 1 there are provided fittings for one or more finger sensors. On the one hand these must have electric connectors for the PPG signal, on the other hand there must be provided pressure fittings for supplying pressure to the one or more cuffs. In the variant shown, which presents the double finger system as described above, this is achieved as follows: a receiving slot 17 partitions the top of the body of the mouse 1, which is also the supporting surface 11, into two partial surfaces 11a and 11b. Into this slot two outer slideable elements 21 and a central element 22 are inserted. The slideable elements 21 contain the two LEDs 21a and 21b necessary for generating the PPG signal. The outer elements are movable in order to accommodate various sizes of the finger sensor. The element 22 sits in the middle of the slot and is not movable. It contains the two light detectors 22a and 22b.

(19) Onto this body or housing 1 of the mouse carrying the elements 21, 22 for the PPG signal as shown, the detachable part of the finger sensor 2 may be mounted. The finger sensors may have diverse sizes or diameters. The optimum fit of the sensors may thus be adapted to the actual finger size. The sensors have an air-inflatable cuff 23a, 23b whose pressure can be controlled by the pressure generating system 12 with the required accuracy. For this purpose fitting elements are provided, which are integrated in the element 22 (not explicitly shown). Furthermore the PPG system must be integrated into the sensors. In the case shown this is realized by the double finger sensor 2 already known. The double finger sensor 2 contains on its interior wall 23 a cuff 23a, 23b for each finger. The advantage of the design of the invention lies in the fact that no electrical leads for the PPG system are necessary for the detachable part of the double finger sensor 2 containing the cuffs 23a, 23b. The PPG elements sit on the housing 1 of the mouse and are supplied with electrical power there.

(20) FIG. 4 shows how the PPG elements 21a, 21b and 22a, 22b are attached to the housing 1 of the mouse. One can see that the LED elements 21a, 21b are movable in order to adapt to the detachable part of finger sensors of varying size. The element 22 carrying the light detectors 22a, 22b, however, is fixedly attached in the middle of the supporting surface 11. In this variant the connector 14 supplies the CNAP mouse with basic air pressure, electrical energy and control signals.

(21) FIG. 5 shows a mirror-symmetrical variant of the invention in a top view. On the housing 1 of the CNAP mouse the finger sensor 2 is attached, which partitions the supporting surface 11 for the hand into two partial surfaces 11a and 11b. The CNAP mouse is supplied via a cable 16 with basic air pressure, electrical energy and the control signals from below (as shown) or proximally as regards the hand. There is no problem in turning the mouse around and supplying it from above or distally as regards the hand.

(22) FIG. 6 demonstrates the advantage derived from the mirror-symmetrical design of the CNAP mouse. The mouse may be used with the fingers of the left hand as well as with those of the right hand. The pair index finger, middle finger (as shown) may be used, as well as the pair middle finger, third finger (not shown). The mouse may be served by the cable 16 either from behind (proximally as regards the hand) or from in front (distally as regards the hand). This will afford maximum flexibility of application of the device on the patient.

(23) The supporting surface for the hand should preferably be soft, permitting hand and fingers to relax in order to avoid motion artefacts. A cushion or a gel pack may therefore be placed on the supporting surface 11.

(24) It is furthermore of advantage if the control system 3 with the microcontroller 31 for fulfilling the real-time condition of the Vascular Unloading Technique is also included in the housing 1 of the CNAP mouse (see FIG. 7). The electronic circuit board of the control system 3 may for instance be disposed underneath the pressure generating system 12. As a result the measuring system need only be supplied with basic pressure and electrical energy.

(25) Carrying this one step farther, it is also possible to integrate a miniature pump 33 in the housing 1 of the mouse. Between the pump 33 and the pressure generating system 12 there is usually an air reservoir 34 disposed, in order to compensate high frequency pressure fluctuations resulting from irregularities in the pump motor. According to one variant this small air reservoir 34 may also be integrated in the body of the mouse (see FIG. 7).

(26) According to a variant of the invention space may be saved in the housing 1 of the mouse by creating an airtight cavity underneath the supporting surface for the hand which can serve as an air reservoir. The electronic circuit board will then be contained in the housing under increased air pressure, but this will not harm the electronic circuits. The air reservoir will thus not require extra volume. In this case the measuring system only has to be supplied with electrical energy.

(27) As a further step of development a power pack or battery 35 may be integrated into the housing 1. Using wireless signal transmission via a wireless chip (see element 32 “wireless” in FIG. 7) the measuring system may function completely independently.

(28) According to the invention it is also possible to attach only a single finger sensor in a corresponding slot on the CNAP mouse, but two individual finger sensors or more would be possible as well. The variant shown in the drawings seems to be of special practical use since the double finger system will permit long measuring times by alternatingly measuring one and the other finger.

(29) The variants cited should be taken as examples only; any variants resulting from permutations of these will also be considered protected.