Non-invasive monitoring of hemodynamic parameters using impedance cardiography
First Claim
1. A method for processing a bioimpedance signal and electrocardiogram for deriving heart rate, heart stroke volume, and cardiac output comprising:
- registering gain-phase-frequency (GPF) characteristics of input analog devices for measuring bioimpedance;
registering gain-phase-frequency (GPF) characteristics of input analog devices for measuring electrocardiogram;
measuring bioimpedance as a function of time over a given time period with said input analog devices for measuring bioimpedance and generating a bioimpedance signal;
measuring electrocardiogram as a function of time over said given time period with said input analog devices for measuring electrocardiogram and generating an electrocardiogram (ECG) signal;
correcting the bioimpedance signal for distortions based on the GPF characteristics previously registered;
correcting the electrocardiogram signal for distortions based on the GPF characteristics previously registered;
determining valid QRS complexes associated with each cardiocycle of said electrocardiogram signal in the given time period;
locating check-points on said valid QRS complexes;
processing one of said ECG signal and said corrected bioimpedance signal to estimate heart rate;
time-differentiating the corrected bioimpedance signal;
determining check-points for each said cardiocycle of the time-differentiated bioimpedance signal in the given time period;
determining effective left ventricular ejection time (ELVET) using said time differentiated bioimpedance check-points in relation to said corresponding QRS check-points;
determining a novel correction factor Zs-q using said time differentiated bioimpedance check-points in relation to said corresponding QRS check-points;
calculating stroke volume as a function of said ELVET, maximum time-differentiated bioimpedance (dZ/dt)max, specific blood resistivity (P), distance (L) between two bioimpedance voltage sensing electrodes of the bioimpedance analog input device, baseline bioimpedance (Z0), said correction factor Zs-q, and a novel scale factor (K); and
calculating cardiac output by multiplying said stroke volume by said heart rate.
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Abstract
A method and apparatus for determination of heart rate, heart stroke volume, heart stroke volume, and cardiac output from thoracic bioimpedance signals and electrocardiograms. A unique bioimpedance electrode arrangement is employed, and the bioimpedance signals are corrected for gain-phase-frequency distortion through the use of sinusoidal test signals through the measuring or detection electrodes to identify distortions and correct for same during actual measurements. Time-derivative bioimpedance signals are employed, the power spectrum calculated, and a novel autoconvolution procedure used to emphasize the heart rate harmonic. Breath waves and other signals not indicative of the patient'"'"'s cardiocycles are removed. Left ventricular ejection time is derived from the bioimpedance signals, and an improved version of Kubicek'"'"'s equation is employed to derive heart stroke volume and thus cardiac output.
138 Citations
67 Claims
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1. A method for processing a bioimpedance signal and electrocardiogram for deriving heart rate, heart stroke volume, and cardiac output comprising:
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registering gain-phase-frequency (GPF) characteristics of input analog devices for measuring bioimpedance; registering gain-phase-frequency (GPF) characteristics of input analog devices for measuring electrocardiogram; measuring bioimpedance as a function of time over a given time period with said input analog devices for measuring bioimpedance and generating a bioimpedance signal; measuring electrocardiogram as a function of time over said given time period with said input analog devices for measuring electrocardiogram and generating an electrocardiogram (ECG) signal; correcting the bioimpedance signal for distortions based on the GPF characteristics previously registered; correcting the electrocardiogram signal for distortions based on the GPF characteristics previously registered; determining valid QRS complexes associated with each cardiocycle of said electrocardiogram signal in the given time period; locating check-points on said valid QRS complexes; processing one of said ECG signal and said corrected bioimpedance signal to estimate heart rate; time-differentiating the corrected bioimpedance signal; determining check-points for each said cardiocycle of the time-differentiated bioimpedance signal in the given time period; determining effective left ventricular ejection time (ELVET) using said time differentiated bioimpedance check-points in relation to said corresponding QRS check-points; determining a novel correction factor Zs-q using said time differentiated bioimpedance check-points in relation to said corresponding QRS check-points; calculating stroke volume as a function of said ELVET, maximum time-differentiated bioimpedance (dZ/dt)max, specific blood resistivity (P), distance (L) between two bioimpedance voltage sensing electrodes of the bioimpedance analog input device, baseline bioimpedance (Z0), said correction factor Zs-q, and a novel scale factor (K); and calculating cardiac output by multiplying said stroke volume by said heart rate. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13)
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14. A method of heart rate estimation, comprising:
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calculation of a power spectrum of a bioimpedance signal; multiplication of said power spectrum by a selected amplitude-frequency function to differentiate the signal and suppress breath harmonics; autoconvoluting the resulting power spectrum according to the formula
space="preserve" listing-type="equation">AS1(f)=PSa(f)·
PSa(2f)·
PSa(3f) . . . ;and determining a maximum amplitude value of autoconvolution in a predefined frequency range as an estimation of heart rate.
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15. A method for determining cardiocycles, comprising:
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filtering a bioimpedance signal to emphasize fronts of cardiocycles; calculating a time-amplitude envelope of said cardiocycles by analyzing the first five harmonics of the power spectrum of said bioimpedance signal after said filtration; selecting said cardiocycle fronts by comparison with said calculated time-amplitude envelope; and rejecting erroneously-detected fronts.
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16. A method of selecting valid cardiocycles from corrected bioimpedance signals to eliminate cardiocycles having interference artifacts, comprising:
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detecting time and amplitude relations referencing check points within individuals of a plurality of cardiocycles; comparing said time and amplitude relations between individuals of a said plurality of cardiocycles; and further examining selected cardiocycles which exhibit the presence of artifacts according to a plurality of comparison criteria. - View Dependent Claims (17)
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18. A method of deriving effective left ventricular ejection time from measured bioimpedance signal and measured electrocardiogram signal, comprising:
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filtering said measured bioimpedance signal and suppressing breath waves therein; filtering said measured electrocardiogram signal; detecting a valid cardiocycle; calculating the time-derivative of said bioimpedance signal Y(x); determining the maximum value of the time-derivative (dZ/dt)max, determining effective ejection start time (S-point); determining effective ejection end time (T-point); and calculating effective left ventricular ejection time (ELVET) as change in time between effective ejection start time and end time. - View Dependent Claims (19, 20, 21, 22)
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23. A method of determining stroke volume for a patient, comprising:
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determining specific blood resistivity P; measuring a distance L between two bioimpedance electrodes applied to the patient; determining the base thoracic impedance Z0 ; determining ELVET; determining Δ
Z, impedance changes due to blood influx;and calculating stroke volume SV according to the equation
space="preserve" listing-type="equation">SV=K·
P·
(L/Z.sub.η
).sup.2 ·
Δ
Zwhere K is a novel scale factor related to body composition of the patient. - View Dependent Claims (24, 25, 26)
- 25. The method of claim 24, wherein K0, K1, K2, K3 are gender and age dependent and lie in ranges of
- space="preserve" listing-type="equation">K.sub.η
ε
[1-4];
K.sub.8 ε
[3-16];
K.sub.0 ε
[0-1];
K.sub.r ε
[0.1-2].
- space="preserve" listing-type="equation">K.sub.η
-
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26. A method of claim 23, wherein determining Δ
- Z comprises;
locating the start of QRS complex of an ECG signal and labelling it point Q; determining the impedance at point S, Zs ; determining the impedance at point Q, Zq ; calculating the impedance difference Zs-q between points S and Q; estimating Δ
Z according to the formula
space="preserve" listing-type="equation">Δ
Z=(dZ/dt).sub.τ
τ
·
ELVET+Z.
- Z comprises;
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27. A method of breath wave suppression for a bioimpedance signal, comprising:
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calculating the Fourier transform of the signal; locating the first and second frequency harmonics of cardiocycles in the calculated spectrum of the signal; estimating the width of each of the harmonics; suppressing frequency harmonics below the lower bound of the second harmonic except for harmonics within the bounds of the first frequency harmonic; and calculating the inverted Fourier transform of the signal.
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28. A system for non-invasive monitoring of hemodynamic parameters using detected thoracic bioimpedance and electrocardiogram indications, comprising:
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an electrode arrangement adapted for detecting thoracic bioimpedance and electrocardiogram indications of a patient to provide analog signals representative of said indications; means for correcting errors associated with said analog signals; means for converting said corrected analog signals to digital signals; means for processing said digital signals to effectuate at least one of; estimation of heart rate; suppression of breath artifact in said bioimpedance signal; cardiocycle recognition; arrangement of check points; and selection of cardiocycles devoid of artifact. - View Dependent Claims (29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67)
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58. A system of claim 55, wherein Δ
- Z is determined by;
locating the start of QRS complex of an ECG signal and labeling it point Q; determining the impedance at point S, Zs ; determining the impedance at point Q, Zq ; calculating the impedance difference Zs-q between points S and Q; estimating Δ
Z according to the formula
space="preserve" listing-type="equation">Δ
Z=(dZ/dt).sub.τ
τ
·
ELVET+Z.
- Z is determined by;
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59. The system of claim 28, wherein said processing means is adapted to suppress breath artifact by:
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calculating the Fourier transform of the signal; locating the first and second frequency harmonics of cardiocycles in the calculated spectrum of the signal; estimating the width of each of the harmonics; suppressing frequency harmonics below the lower bound of the second harmonic except for harmonics within the bounds of the first frequency harmonic; and calculating the inverted Fourier transform of the signal.
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60. The system of claim 59, wherein said electrode arrangement comprises:
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an upper influencing electrode placed on the subject'"'"'s head; a lower influencing electrode placed on the left lower extremity of the subject; an upper pair of detecting electrodes placed on the subject'"'"'s neck; and a lower pair of detecting electrodes placed on the trunk of the subject.
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61. The system of claim 60, wherein the electrode arrangement placement geometry further comprises:
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an upper influencing electrode placed on the subject'"'"'s forehead; a lower influencing electrode placed in the general area of the subject'"'"'s left knee; a pair of upper detecting electrodes placed on the subject'"'"'s neck; and a pair of lower detecting electrodes placed laterally on opposite sides of the subject'"'"'s chest.
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62. The system of claim 61, wherein said upper influencing electrode comprises a spot electrode for orientation on vertical and horizontal center lines of said subject'"'"'s forehead.
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63. The system of claim 60, wherein said lower influencing electrode comprises a spot electrode, the placement of which satisfies the relationship L<
- 5R, where L is the vertical distance between said upper and said lower influencing electrodes and R is the radius of said subject'"'"'s chest.
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64. The system of claim 60, wherein said upper detecting electrodes comprises a pair of spot electrodes oriented symmetrically on opposite sides of said subject'"'"'s neck along a horizontal line approximately 4 centimeters above the base of said subject'"'"'s neck.
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65. The system of claim 61, wherein said lower detecting electrodes further comprise a pair of electrode assemblies, each assembly providing contact surface area between about 12 square centimeters and about 30 square centimeters oriented laterally on opposite sides of said subject'"'"'s chest at approximately xiphoid process level.
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66. The system of claim 65, wherein each assembly further comprises four spot electrodes, wherein each spot electrode comprises approximately 4 square centimeters contact surface area with each said spot electrode centered at corners of a square with sides measuring 5 centimeters, and all 4 spot electrodes of said assembly electrically connected to each other.
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67. The system of claim 66, wherein the top spot electrodes of each assembly lie on the xiphoid process level of the subject.
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Specification