Method of and a system for determining a cardiovascular quantity of a mammal
The present invention relates to a method and a system for carrying out the method of determining at least one cardiovascular quantity of a mammal. The method comprises (i) selecting a measuring site of a vessel; (ii) determining or estimating a mean diameter of the vessel at the measuring site; (iii) determining a pulse wave velocity and/or another elasticity related quantity of the vessel at the measuring site; (iv) determining a distension of the vessel at the measuring site; and (v) calculating the at least one cardiovascular quantity from the determined mean diameter, elasticity related quantity and distension of the vessel at the measuring site. The cardiovascular quantity system comprises (i) a plurality of sets of electrodes where each set of electrodes comprising at least two electrodes can be attached to a skin surface of the mammal such that capacitive coupling through the skin surface and between the electrodes of the set of electrodes is provided when an electrical signal is applied over the electrodes at a measuring site of a vessel; (ii) electrical devices for applying an electric oscillating signal over the respective sets of electrodes; (iii) at least one processor and memory unit arranged to receive signals from the respective sets of electrodes; wherein said at least one processor is designed and programmed to calculate the at least one cardiovascular quantity according to the method using signals from the respective sets of electrodes. By calculating the cardiovascular quantity from such data a more accurate determination can be obtained, which determination further is exempt from the need for any individual calibration or any further type of calibration procedure at all, neither before or after measurements. An accurate determination in this context means a determination with a very low measurement uncertainty, such as in the order of about 10% or less, preferably about 5% or less.
1. A method of determining at least one cardiovascular quantity of a mammal,
comprising:
determining or estimating a mean diameter of a vessel at a measuring site by:
applying a plurality of electrical oscillating signals selected from an oscillating current and an oscillating voltage to at least one set of electrodes, wherein the plurality of electrical oscillating signals comprises at least two different excitation frequencies and wherein electric field lines between the at least one set of electrodes penetrate the vessel at the measuring site; and
determining an impedance between the at least one set of electrodes for each excitation frequency;
determining an elasticity related quantity of the vessel at the measuring site;
determining a distension of the vessel at the measuring site; and
calculating the at least one cardiovascular quantity from the determined mean diameter, the elasticity related quantity and the distension of the vessel at the measuring site.
2. The method as claimed in claim 1 , wherein the vessel is an artery selected from a Brachial artery, a Radial artery, an Ulnar artery, a Femoral artery, a Digital artery and a Carotid artery.
3. The method as claimed in claim 1 , wherein the method comprises applying the at least one set of electrodes comprising at least two electrodes within a selected distance to the measuring site.
4. The method as claimed in claim 3 , wherein each of the electrodes of the at least one set of electrodes being attached to a skin surface of the mammal.
5. The method as claimed in claim 4 , further comprising determining at least one impedance parameter of the at least one set of electrodes as a function of time.
6. The method as claimed in claim 3 , wherein the method further comprises determining at least one impedance parameter by measuring over the at least one set of electrodes, where an excitation electrode set and a detection electrode set constitutes the same set.
7. The method as claimed in claim 1 , wherein the method comprises applying at least two sets of electrodes, a first and a second electrode set, within a selected distance to the measuring site, applying an electrical signal selected from an oscillating current and an oscillating voltage of at least one excitation frequency to the first set of electrodes, and determining at least one impedance parameter by measuring over the second set of electrodes, where an excitation electrode set and a detection electrode set constitute the first set and second set, respectively.
8. The method as claimed in claim 1 , wherein the determination of the mean diameter comprises determining the mean diameter using multi frequency excitation.
9. The method as claimed in claim 1 , wherein the determination of the mean diameter of the vessel at the measuring site comprises providing a priory estimation of a structure of a cross-section anatomy at the measuring site and adjacent region penetrated by the field lines of the at least one set of electrodes, setting up a set of mathematical formulas based on this priory estimation by an equivalent circuit for the impedance between the at least one set of electrodes, where the mathematical formulas divide electric field lines into at least one length part of field lines passing through skin, one length part of field lines passing through fat layer, one length part of field lines passing through muscles and one length part of field lines passing through the vessel and determining an actual length part of field lines passing through the vessel based on the determined impedance between the at least one set of electrodes at the least two different excitation frequencies and the set of mathematical formulas.
10. The method according to claim 1 wherein four electrodes are applied and an excitation current is applied to at least a first set of electrodes comprising at least two electrodes that are configured in such a way that when applied to skin they are displaced both in a direction of an artery and perpendicular to the artery, and a voltage is measured on at least a second set of electrodes comprising at least two electrodes that are configured in such a way that when applied to the skin they are displaced also in the direction of the artery and perpendicular to the artery, and configured in such a way that the diagonals of the first and second set of electrodes are crossing.
11. The method as claimed in claim 1 , wherein the determination of the elasticity related quantity of the vessel at the measuring site comprises determining a pulse wave velocity in the vessel at the measuring site, wherein the determination of the pulse wave velocity in the vessel at the measuring site comprises placing at least two sensors with a selected mutual distance along a length section L of the vessel comprising at least a part of the measuring site, and determining the pulse as a function of time by each sensor and thereby determining the pulse wave velocity.
12. The method as claimed in claim 11 , wherein there is provided the at least two sensors, a first sensor comprising a first set of electrodes and a second sensor comprising a second set of electrodes, the respective sets of electrodes being electrically connected in electrical circuits such that electric field lines between the respective set of electrodes penetrate the vessel at respectively a first pulse wave sensing site and a second pulse wave sensing site.
13. The method as claimed in claim 1 , wherein there is provided at least three electrode sets, a first electrode set being detection electrodes, a second electrode set being detection electrodes, and a third electrode set being excitation electrodes, the third electrode set being placed such that at least the electrical field lines excited from the third set of electrodes penetrate the vessel at the measuring site.
14. The method as claimed in claim 13 wherein the third set of electrodes is placed in between the first and second electrode sets.
15. The method as claimed in claim 1 , further comprising determining the impedance of two respective sets of electrodes as a function of time and determining a temporal displacement of one impedance signal with respect to the other impedance signal.
16. The method as claimed in claim 1 , where the determined cardiovascular quantity is a differential blood pressure, which is the difference between a systolic and a diastolic pressure, which differential blood pressure is determined from the velocity v of a pulse wave in the vessel at the measuring site by applying the following equation
v
≅
Δ
P
Δ
A
A
ρ
,
where ΔP is the differential blood pressure, ΔA is the distension of the vessel, ρ the blood density, and A is expressed by the mean vessel cross-section area.
17. The method as claimed in claim 1 , wherein the determination of the distension of the vessel at the measuring site comprises an electrical circuit comprising the at least one set of electrodes such that electric field lines between the at least one set of electrodes penetrate the vessel at the measuring site, and determining a temporal variation of impedance of the at least one set of electrodes.
18. The method as claimed in claim 1 wherein the method comprises applying the at least one set of electrodes within a selected distance to the measuring site, applying an electrical oscillating signal to the at least one set of electrodes and determining at least one impedance parameter selected from mean impedance, minimum impedance, maximum impedance, temporal variations of impedance, impedance as a function of time or any combination thereof over the at least one set of electrodes.
19. The method as claimed in claim 18 , wherein the method comprises determining or estimating temporal spacing between pulses of the mammal, determining the mean impedance using a bridge and automatic balancing the bridge through a feedback loop with a loop response time that is at least as large as the temporal spacing between pulses by adjusting at least two resistance components of the bridge.
20. The method as claimed in claim 18 , wherein the method comprises determining or estimating temporal spacing between pulses of the mammal, determining the temporal variation of impedance using a bridge and automatic balancing the bridge through a feedback loop with a loop response time that is at least as large as the temporal spacing between pulses, the method comprises determining temporal variations of the imbalance of the bridge.
21. The method as claimed in claim 1 , wherein
the determination of mean diameter,
the determination of the elasticity related quantity,
the determination of distension of the vessel, and
the determinations of two thereof, or the determination of all three thereof is/are based on determination of at least one impedance parameter selected from mean impedance, minimum impedance, maximum impedance, temporal variations of impedance, impedance as a function of time or any combination thereof over the at least one set of electrodes.
22. The method as claimed in claim 1 , wherein the calculation of the at least one cardiovascular quantity from the determined mean diameter, the elasticity related quantity, and the distension of the vessel at the measuring site comprises calculating a differential blood pressure, calculating a systolic blood pressure, calculating a diastolic pressure or calculating a vascular compliance.
23. The method as claimed in claim 1 , wherein the method further comprises determining at least one additional dimension of the vessel at the measuring site, selected from the thickness of a vessel wall, a maximum diameter of the vessel, a minimum diameter of the vessel, a temporal variation of the vessel diameter or vessel diameter as a function of time.
24. The method as claimed in claim 1 , wherein the method further comprises determining a pulse rate.
25. The method as claimed in claim 1 , wherein the method is non-invasive.
26. The method as claimed in claim 1 , not comprising application of pressure to the vessel.
27. The method as claimed in claim 1 , where any one of pulse amplitude, pulse pressure, or pulse rate selected, is determined by applying a counter pressure for calibration purposes.
28. A method of determining at least one cardiovascular quantity of a mammal, the method comprises:
determining or estimating a mean diameter of a vessel at a measuring site;
applying at least one set of electrodes comprising at least two electrodes within a selected distance to the measuring site;
applying an electrical oscillating signal to the electrodes such that electric field lines from the electrodes penetrate the vessel at the measuring site;
determining at least one impedance parameter of a detection set of electrodes as a function of time;
determining an elasticity related quantity of the vessel at the measuring site;
determining a distension of the vessel at the measuring site; and
calculating the at least one cardiovascular quantity from the determined mean diameter, the elasticity related quantity and the distension of the vessel at the measuring site,
wherein the determination of the at least one impedance parameter is performed using signal processing by at least one of a voltage follower and an instrumentation amplifier for sensing and amplifying the signal from the detection electrodes, and at least one mixer for demodulation of the signal by quadrature detection, and amplifying the demodulated signal, comprising an in-phase signal and the quadrature signal, by a known value.
29. A cardiovascular quantity system for determining at least one cardiovascular quantity in a vessel of a mammal, the cardiovascular quantity system comprising:
a plurality of sets of electrodes where each set of electrodes comprising at least two electrodes that can be attached to a skin surface of the mammal such that capacitive coupling through the skin surface and between the electrodes of the plurality of sets of electrodes is provided when an electrical signal is applied over the electrodes at a measuring site of the vessel;
electrical devices apply the electrical signal comprising an electric oscillating signal over the plurality of sets of electrodes; and
at least one processor and a memory unit arranged to receive electrical response signals from the plurality of sets of electrodes; wherein the at least one processor is coupled to the plurality of sets of electrodes and is configured with software instructions to calculate the at least one cardiovascular quantity by:
determining or estimating a mean diameter of the vessel at the measuring site by:
applying a plurality of electrical oscillating signals selected from an oscillating current and an oscillating voltage to at least one set of electrodes, wherein the plurality of electrical oscillating signals comprises at least two different excitation frequencies and wherein electric field lines between the at least one set of electrodes penetrate the vessel at the measuring site; and
determining an impedance between the at least one set of electrodes for each excitation frequency;
determining an elasticity related quantity of the vessel at the measuring site;
determining a distension of the vessel at the measuring site; and
calculating the at least one cardiovascular quantity from the determined mean diameter, the elasticity related quantity and the distension of the vessel at the measuring site.