CARDIAC MONITORING SYSTEM

• US 20090082679A1
• Filed: 06/21/2005
• Published: 03/26/2009
• Est. Priority Date: 06/21/2004
• Status: Abandoned Application

• Current Assignee: Aorora Technologies Pty., Ltd.
• Original Assignee: Impedance Cardiology Systems, Inc.
• Inventors: Chetham, Scott Matthew
• Application Number:
  US11/629,804
• Publication Number:
  US 20090082679A1
• Time in Patent Office: Days
• US Class Current: 600/508
• CPC Class Codes: A61B 5/029 Measuring or recording blood output from the heart, e.g. minute volume A61B8/065 takes precedence
• CPC Class Codes: A61B 5/0295 using plethysmography, i.e. measuring the variations in the volume of a body part as modified by the circulation of blood therethrough, e.g. impedance plethysmography
• CPC Class Codes: A61B 5/0535 Impedance plethysmography for measuring blood flow A61B5/0295

Abstract

A method of analyzing cardiac functions in a subject using a processing system. The method includes causing one or more electrical signals to be applied to the subject using a first set of electrodes, the one or more electrical signals having a plurality of frequencies. The method includes determining an indication of electrical signals measured across a second set of electrodes applied to the subject in response to the applied one or more signals. Following this, and for a number of sequential time instances, the method includes determining from the indicating data and the one or more applied signals, an instantaneous impedance value at each of the plurality of frequencies, and determining, using the using instantaneous impedance values, an intracellular impedance parameter. The intracellular impedance parameter over at least one cardiac cycle is the used to determine one or more parameters relating to cardiac function.

Claims

1. A method of analyzing cardiac functions in a subject, the method including, in a processing system:

a) causing one or more electrical signals to be applied to the subject using a first set of electrodes, the one or more electrical signals having a plurality of frequencies;

b) determining an indication of electrical signals measured across a second set of electrodes applied to the subject in response to the applied one or more signals;

c) for a number of sequential time instances;

i) determining from the indicating data and the one or more applied signals, an instantaneous impedance value at each of the plurality of frequencies;

ii) determining, using the instantaneous impedance values, an intracellular impedance parameter; and

,

d) determining, using the intracellular impedance parameter over at least one cardiac cycle, one or more parameters relating to cardiac function.

2. A method according to claim 1, wherein the impedance parameter is a variable intracellular resistance parameter.

3. A method according to claim 1, wherein the method includes, in the processing system:

e) determining, using the instantaneous impedance values, at least one impedance value; and

,

f) determining the intracellular impedance parameter using the at least one impedance value and a predetermined equation.

4. A method according to claim 3, wherein the predetermined equation is:

$R_1=\frac{R_{\mathrm{var}}}{{\left({\mathrm{\tau <br><br>}}_Y<br><br>{\mathrm{\omega <br><br>}}_{\mathrm{Ym}}\right)}^{-a}}$

5. A method according to claim 3, wherein the at least one impedance value includes at least one of:

g) the impedance at zero frequency;

h) the impedance at infinite frequency; and

,

i) the impedance at a characteristic frequency.

6. A method according to claim 1, wherein method includes, in the processing system, determining the intracellular impedance parameter is determined using a CPE model.

7. A method according to claim 1, wherein the method includes, in the processing system, and for impedances determined at a time instance:

j) fitting a function to the instantaneous impedance values; and

,

k) using the fitted function to determine the intracellular impedance parameter.

8. A method according to claim 7, wherein the method includes, in the processing system:

l) fitting a function to the instantaneous impedance values;

m) determining any outlier instantaneous impedance values;

n) for any outlier instantaneous impedance values;

i) removing the instantaneous impedance value;

ii) recalculating the function; and

,

iii) using the recalculated function if the recalculated function is a better fit for the instantaneous impedance values.

9. A method according to claim 7, wherein the method includes, in the processing system, using the fitted function to determine one or more impedance values.

10. A method according to claim 7, wherein the function includes at least one of:

o) a polynomial fitted using a curve fitting algorithm; and

,

p) a function based on a Wessel plot.

11. A method according to claim 1, wherein the method includes, in the processing system:

q) determining an indication of one or more subject parameters; and

,

r) using the one or more subject parameters to determine the one or more parameters relating to cardiac function.

12. A method according to claim 1, wherein the method includes, in the processing system, determining one or more parameters relating to cardiac function using the equation:

$\mathrm{CO}=k_1<br><br>{c_1\left(\frac{<br><br>{\left(\frac{<br><br>R_{\mathrm{var}}<br><br>\left(t\right)}{<br><br>t}\right)}_{\mathrm{MAX}}<br><br>}{Z_0}\right)}^n*{\left(\frac{1}{T_{\mathrm{RR}}}\right)}^m\times <br><br>T_{\mathrm{LVE}}$

where;

i) CO denotes cardiac output (litres/min),

ii) k₁ is an optional population specific correction factor based on one or more subject parameters, such as at least the height and weight, but can also include distance between the electrodes and age;

iii) c₁ is an optional calibration coefficient used to convert the units from Ohmic units to litres (which may be uniquely defined at manufacture for each monitoring device used to implement the method),

iv) Z₀ is an optional baseline Impedance measured at the characteristic frequency (between 10 Ohms and 150 Ohms),

v) TRR is the interval between two R waves obtained from the ECG (found from the ECG or impedance or conductance data),

vi) TLVE is left ventricular ejection time (measured from either the conductance or impedance curve or preferably a combination of other physiological measurement techniques) and

vii) n(range −

4>

n<

4) and m(range −

4>

m<

4) are optional constants.

13. A method according to claim 1, wherein the method includes, processing electrical signals measured across a second set of electrodes applied to the subject to perform at least one of:

b) removal of respiratory effects;

c) extraction of ECG signals; and

,

d) removing unwanted signals.

14. A method according to claim 1, wherein the method includes, in the processing system, displaying an indication of at least one of:

e) impedance values;

f) one or more intracellular impedance parameter values; and

,

g) one or more parameters relating to cardiac function.

15. A method according to claim 1, wherein the method includes, in the processing system, determining at least one of:

h) stroke volume;

i) cardiac output;

j) cardiac index;

k) stroke index;

l) systemic vascular resistance/index;

m) acceleration;

n) an acceleration index;

o) velocity;

p) velocity index;

q) thoracic fluid content;

r) left ventricular ejection time;

s) pre-ejection period;

t) systolic time ratio;

u) left cardiac work/index;

v) heart rate; and

,

w) mean arterial pressure.

16. A method according to claim 1, wherein the intracellular impedance parameter models at least resistance changes caused by the re-orientation of cellular components of the subject'"'"'s blood over the cardiac cycle.

17. Apparatus for analyzing cardiac functions in a subject, the apparatus including a processing system for:

x) causing one or more electrical signals to be applied to the subject using a first set of electrodes, the one or more electrical signals having a plurality of frequencies;

y) determining an indication of electrical signals measured across a second set of electrodes applied to the subject in response to the applied one or more signals;

z) for a number of sequential time instances;

i) determining from the indicating data and the one or more applied signals, an instantaneous impedance value at each of the plurality of frequencies;

ii) determining, using the instantaneous impedance values, an intracellular impedance parameter; and

,

aa) determining, using the intracellular impedance parameter over at least one cardiac cycle, one or more parameters relating to cardiac function.

18. Apparatus according to claim 17, wherein the impedance parameter is a variable intracellular resistance parameter.

19. Apparatus according to claim 17, the apparatus including:

bb) a signal generator coupled to the processing system for generating electrical signals to be applied to the subject; and

,

cc) a sensor for sensing electrical signals across the subject.

20. Apparatus according to claim 19, wherein the signal generator is a current generator.

21. Apparatus according to claim 19, wherein the sensor is a voltage sensor.

22. Apparatus according to claim 19, wherein the apparatus includes a number of electrodes for coupling the signal generator and the sensor to the subject.

23. Apparatus according to claim 19, wherein the processing system is coupled to at least one of the signal generator and the sensor via a wireless connection.

24. Apparatus according to claim 19, wherein the sensor includes an analogue to digital converter.

25. (canceled)