Method and apparatus for determining cardiac performance in a patient with a conductance catheter
An apparatus for determining cardiac performance in the patient. The apparatus includes a conductance catheter for measuring conductance and blood volume in a heart chamber of the patient. The apparatus includes a processor for determining instantaneous volume of the ventricle by applying a non-linear relationship between the measured conductance and the volume of blood in the heart chamber to identify mechanical strength of the chamber. The processor is in communication with the conductance catheter. Methods for determining cardiac performance in a patient. Apparatuses for determining cardiac performance in a patient.
1. A method for determining cardiac performance in a patient from a plurality of desired wave forms received from electrodes placed in the patient and in communication with a heart chamber of the patient comprising the steps of :
receiving and separating with a data acquisition mechanism the plurality of desired wave forms at desired frequencies from the electrodes; and
determining with a computer instantaneous volume of the heart chamber by applying a non-linear relationship between measured conductance and volume of blood in the heart chamber to identify mechanical strength of the chamber from the desired wave forms at desired frequencies, the processor in communication with the data acquisition mechanism.
2. The method of claim 1 including the step of a signal synthesizer producing the plurality of desired wave forms at desired frequencies for the electrodes.
3. The method of claim 2 including the steps of the signal synthesizer producing the plurality of desired wave forms at desired frequencies simultaneously, and the data acquisition mechanism separating the plurality of desired wave forms at desired frequencies the data acquisition mechanism receives from the electrodes.
4. The method of claim 3 wherein the non-linear relationship depends on a number of the electrodes, dimensions and spacing of the electrodes, and an electrical conductivity of a medium in which the electrodes of the catheter are disposed.
5. The method of claim 4 wherein the non-linear relationship is
β( G )(σ=0.928 S/m )=1+1.774(10 7.481×10 −4 (G−2057) )
where: G is the measured conductance (S), the calculations have been corrected to the conductivity of whole blood at body temperature (0.928 S/m), and 2057 is the asymptotic conductance in μS when the cuvette is filled with a large volume of whole blood.
6. The method of claim 5 wherein the nonlinear relationship further includes:
Vol
(
t
)
=
[
β
(
G
)
]
[
L
2
σ
b
]
[
Y
(
t
)
-
Y
p
]
where: β(G)=the field geometry calibration function (dimensionless), Y(t)=the measured combined admittance, σ b is blood conductivity, L is distance between measuring electrodes, and Y p =the parallel leakage admittance, dominated by cardiac muscle.
7. The method of claim 6 including the step of measuring instantaneous pressure of the heart chamber with a pressure sensor in communication with the computer.
8. The method of in claim 7 wherein the pressure sensor is in contact with the conductance catheter to measure ventricular pressure in the chamber.
9. The method of claim 8 including the step of the computer converting conductance into a volume.
10. The method of claim 9 including the step of the computer producing a drive signal having a plurality of desired wave forms at desired frequencies to drive the electrodes.
11. The method of claim 4 wherein
Vol( t )=ρ L 2 g b ( t )exp[γ·( g b ( t )) 2 ]
where Vol(t) is the instantaneous volume, ρis the blood resistivity, L is the distance between the sensing electrodes, g b (t) is the instantaneous blood conductance, and γis an empirical calibration factor.
12. A method for determining cardiac performance in a patient from a plurality of desired wave forms received from electrodes placed in the patient and in communication with a heart chamber of the patient comprising the steps of receiving and separating the plurality of desired wave forms at desired frequencies from the electrodes with a data acquisition mechanism; and determining instantaneous volume of the heart chamber with a computer by removing a muscle contribution from conductance that includes contributions from blood and muscle with respect to the heart chamber, the processor in communication with the data acquisition mechanism.
13. The method of claim 12 including the step of producing the plurality of desired wave forms at desired frequencies for the electrodes with a signal synthesizer.
14. The method of claim 13 including the step of the signal synthesizer producing the plurality of desired wave forms at desired frequencies simultaneously, and the data acquisition mechanism separates the plurality of desired wave forms at desired frequencies the data acquisition mechanism receives from the electrodes.
15. The method of claim 14 including the step of the computer measuring complex admittance with the electrodes to identify the muscle contribution.
16. The method of claim 15 wherein the complex admittance is defined as
Yp=Gm+jωCm ( Y subscript p )
where
Cm=capacitance component of muscle (F=Farads) (C subscript m)
ω=angular frequency (radians/second) (greek “omega” =2 pi f)
Gm=conductance of muscle (S=Siemens) (G subscript m).
17. The method of claim 16 wherein the conductance is defined as
Y ( t )= Gb+Gm+jωCm
where Gb=conductance of blood (S) (G subscript b).