Impedance cardiograph and method of operation utilizing peak aligned ensemble averaging

Claim 2

An impedance cardiograph comprising: means for applying an excitation signal to the thorax of an animal; means, responsive to the excitation signal, for producing a first electrical signal which varies with thoracic impedance changes of the animal; means for producing a second electrical signal that corresponds to the first derivative of the first electrical signal; means for generating a plurality of periodic samples of the second electrical signal, the samples being divided into a number of groups with the samples in each group representing the second electrical signal during a different cardiac cycle of the animal; means for identifying a signal sample in each group which occurs during a cardiac ejection interval and which corresponds to the smallest rate of change of the thoracic impedance; means for ensemble averaging the groups of the samples of the second electrical signal to produce a set of ensemble averaged signal samples, said means for ensemble averaging aligning each group of samples of the second electrical signal about a sample in each group which occurs during a cardiac ejection interval and which corresponds to the smallest rate of change of the thoracic impedance; and means for computing a cardiac performance parameter value from the ensemble averaged signal samples.

Claim 3

The impedance cardiograph as recited in claim 1 further comprising means for selecting the groups of samples of the second electrical signal for ensemble averaging based upon whether each group of samples meets a predefined acceptance criteria.

Claim 4

The impedance cardiograph as recited in claim 2 wherein said means for selecting includes: means for measuring an interval between samples of a group which correspond to two predefined points on the second electrical signal; and means for determining whether the interval for each group is within a predefined range of values.

Claim 5

The impedance cardiograph as recited in claim 2 further comprising a means for producing an electrocardiogram signal having a QRS wave during each cardiac cycle.

Claim 6

The impedance cardiograph as recited in claim 4 wherein said means for selecting includes: means for measuring an interval between a reference point on the QRS wave and a predefined point on the second electrical signal during each of the cardiac cycles; and means for determining whether the measured interval is within a predefined range of values.

Claim 7

The impedance cardiograph as recited in claim 4 wherein said means for selecting includes: means for measuring a first interval point on the QRS wave and a predefined point on the second electrical signal during each of the cardiac cycles; and means for determining whether the measured interval is within one standard deviation of an arithmetic mean of a plurality of first intervals.

Claim 8

The impedance cardiograph as recited in claim 6 wherein said means for selecting further includes: means for measuring a second interval between a reference point on two consecutive QRS waves in the electrocardiogram signal; and means for determining whether the first interval is less than a given fraction of the second interval.

Claim 9

The impedance cardiograph as recited in claim 4 wherein said means for selecting includes: means for determining whether a QRS wave has a duration that is within an given range of values; and means for determining whether a QRS wave has a height that is within an given range of values.

Claim 10

The impedance cardiograph as recited in claim 4 wherein said means for selecting includes: means for selecting a QRS wave as a reference QRS wave; means for deriving a shape fit value representing the degree to which another QRS wave matches the reference QRS wave; and means for determining whether the shape fit value is within a given range of shape fit values.

Claim 11

The impedance cardiograph as recited in claim 4 wherein said means for selecting includes: means for measuring intervals between a reference point on each consecutive QRS wave in the electrocardiogram signal; and means for determining whether each interval is within a given range of standard deviations of an arithmetic mean of a plurality of the intervals.

Claim 12

The impedance cardiograph as recited in claim 2 further comprising a means for sensing blood pressure of the animal.

Claim 13

The impedance cardiograph as recited in claim 11 wherein said means for selecting includes: means for measuring a first interval between the occurrence of systolic blood pressure and a predefined point on the second electrical signal during each of the cardiac cycles; and means for determining whether each measured interval is within a defined range of reference values.

Claim 14

The impedance cardiograph as recited in claim 12 wherein the defined range of reference values is specified by a given number of standard deviations of an arithmetic mean of a plurality of the first intervals.

Claim 15

The impedance cardiograph as recited in claim 12 wherein said means for selecting further includes: means for measuring a second interval between consecutive occurrences of systolic blood pressure; and means for determining whether the first interval is less than a given fraction of the second interval.

Claim 16

An impedance cardiograph comprising: means for applying an excitation signal to the thorax of an animal; means, responsive to the excitation signal, for producing an electrical signal which varies with thoracic impedance changes of the animal; means for producing an electrocardiogram signal containing a cardiac QRS wave; means for determining a heart rate of the animal from the electrocardiogram signal; means for selecting segments of the electrical signal for which a predefined point of a segment occurs within a given interval from an occurrence of a predetermined point on a cardiac QRS wave, said means for selecting including means for determining the given interval; and means, responsive to said first means for selecting, for computing a cardiac performance parameter value from the selected segments of the electrical signal.

Claim 17

An impedance cardiograph comprising: means for applying an excitation signal to the thorax of an animal; means, responsive to the excitation signal, for producing an electrical signal which varies with thoracic impedance changes of the animal; means for producing an electrocardiogram signal containing a cardiac QRS wave; means for determining a heart rate of the animal from the electrocardiogram signal; means for selecting segments of the electrical signal for which a predefined point of a segment occurs within a given interval from an occurrence of a predetermined point on a cardiac QRS wave, said means for selecting including means for determining the given interval; and means, responsive to said first means for selecting, for computing a cardiac performance parameter value from the selected segments of the electrical signal.

Claim 18

The impedance cardiograph as recited in claim 15 wherein the predetermined point on a cardiac QRS wave is a peak of an R spike.

Claim 19

The impedance cardiograph as recited in claim 16 wherein the predefined point of a segment of the electrical signal corresponds to the smallest rate of change of the thoracic impedance.

Claim 20

An impedance cardiograph comprising: means for applying an excitation signal to the thorax of an animal; means, responsive to the excitation signal, for producing a first electrical signal which varies with thoracic impedance changes of the animal; means for producing a second signal corresponding to the first derivative of the first electrical signal; means for producing an electrocardiogram signal having a QRS wave during each cardiac cycle; means for generating a plurality of periodic samples of the first electrical signal, second electrical signal and the electrocardiogram signal, the samples of each signal being divided into a number of groups with the samples in each group representing the respective signal during a different cardiac cycle of the animal; means for averaging the samples of the first electrical signal to produce an average thoracic impedance value; means for ensemble averaging the groups of the samples of the second electrical signal to produce a first set of ensemble averaged signal samples, said means for ensemble averaging aligning each group of samples of the second electrical signal about a sample in each group which occurs during a cardiac ejection interval and which corresponds to the smallest rate of change of the thoracic impedance; means for ensemble averaging the groups of the samples of the electrocardiogram signal to produce a second set of ensemble averaged signal samples; and means for computing cardiac stroke volume from the first set of ensemble averaged signal samples and the average thoracic impedance value.

Claim 21

An impedance cardiograph comprising: means for applying an excitation signal to the thorax of an animal; means, responsive to the excitation signal, for producing a first electrical signal which varies with thoracic impedance changes of the animal; means for producing a second signal corresponding to the first derivative of the first electrical signal; means for producing an electrocardiogram signal having a QRS wave during each cardiac cycle; means for generating a plurality of periodic samples of the first electrical signal, second electrical signal and the electrocardiogram signal, the samples of each signal being divided into a number of groups with the samples in each group representing the respective signal during a different cardiac cycle of the animal; means for averaging the samples of the first electrical signal to produce an average thoracic impedance value; means for ensemble averaging the groups of the samples of the second electrical signal to produce a first set of ensemble averaged signal samples, said means for ensemble averaging aligning each group of samples of the second electrical signal about a sample in each group which occurs during a cardiac ejection interval and which corresponds to the smallest rate of change of the thoracic impedance; means for ensemble averaging the groups of the samples of the electrocardiogram signal to produce a second set of ensemble averaged signal samples; and means for computing cardiac stroke volume from the first set of ensemble averaged signal samples and the average thoracic impedance value.

Claim 22

The impedance cardiograph as recited in claim 18 further comprising means for displaying the first and second sets of ensemble averaged signal samples.

Claim 23

The impedance cardiograph as recited in claim 18 further comprising means for selecting the groups of samples of the second electrical signal for ensemble averaging based upon whether each group of samples meets a predefined acceptance criteria.

Claim 24

The impedance cardiograph as recited in claim 20 wherein said means for selecting includes means for determining whether each cardiac cycle has a duration within a first predefined range of values; and whether the QRS wave has a duration within a second predefined range of values.

Claim 25

The impedance cardiograph as recited in claim 20 wherein said means for selecting includes: means for selecting a QRS wave as a reference QRS wave; means for deriving a shape fit value representing the degree to which another QRS wave matches the reference QRS wave; and means for determining whether the shape fit value is within a given range of shape fit values.

Claim 26

The impedance cardiograph as recited in claim 20 wherein said means for selecting includes: means for measuring an interval between a reference point on the QRS wave and a predefined point on the second electrical signal during each of the cardiac cycles; and means for determining whether the measured interval is within a predefined range of values.

Claim 27

The impedance cardiograph as recited in claim 20 wherein said means for selecting includes: means for measuring an interval between samples of a group which correspond to two predefined points on the second electrical signal; and means for determining whether the interval for each group is within a predefined range of values.

Claim 30

The method of measuring cardiac activity as recited in claim 25 further comprising selecting the groups of samples of the second electrical signal for ensemble averaging based upon whether each group of samples meets a predefined acceptance criteria.

Claim 31

The method of measuring cardiac activity as recited in claim 26 wherein selecting step includes: measuring an interval between samples of a group which correspond to two predefined points on the second electrical signal; and determining whether the interval for the group is within a predefined range of values.

Claim 32

The method of measuring cardiac activity as recited in claim 26 further comprising producing an electrocardiogram signal, which signal has a QRS wave during each cardiac cycle.

Claim 33

The method of measuring cardiac activity as recited in claim 28 wherein said selecting step includes: determining whether each cardiac cycle has a duration within a first predefined range of values; and determining whether each QRS wave has a duration within a second predefined range of values.

Claim 34

The method of measuring cardiac activity as recited in claim 28 wherein said selecting step includes: measuring an interval between a reference point on the QRS wave and a predefined point on the second electrical signal during each cardiac cycle; and determining whether each measured interval is within a defined range of values.

Claim 35

The method of measuring cardiac activity as recited in claim 26 wherein said selecting step includes: producing an electrocardiogram signal having a QRS wave during each cardiac cycle; selecting a QRS wave as a reference QRS wave; deriving a shape fit value representing the degree to which another QRS wave matches the reference QRS wave; and determining whether the shape fit value is within a given range of shape fit values.

Claim 36

The method of measuring cardiac activity as recited in claim 25 wherein said step of computing a cardiac performance parameter value includes calculating the stroke volume.

Claim 37

The method of measuring cardiac activity as recited in claim 25 wherein said step of computing a cardiac performance parameter value includes calculating cardiac output.

Claim 38

The method of measuring cardiac activity as recited in claim 25 further comprising evaluating a plurality of cardiac performance parameter values, and determining whether the evaluation indicates an undesirable trend in the cardiac activity of the animal.

Claim 39

An impedance cardiograph comprising: means for applying an excitation signal to the thorax of an animal; means, responsive to the excitation signal, for producing an electrical signal which varies with thoracic impedance changes of the animal; means for producing an electrocardiogram signal containing a cardiac QRS wave; means for determining a heart rate of the animal from the electrocardiogram signal; means for measuring an interval between two predefined points within a segment of the electrical signal; means for selecting segments of the electrical signal, a given segment being selected in response to a measurement of the interval of the segment being within a given range of values; and means, responsive to said first means for selecting, for computing a cardiac performance parameter value from the selected segments of the electrical signal.

Claim 40

An impedance cardiograph comprising: means for applying an excitation signal to the thorax of an animal; means, responsive to the excitation signal, for producing an electrical signal which varies with thoracic impedance changes of the animal; means for producing an electrocardiogram signal containing a cardiac QRS wave; means for determining a heart rate of the animal from the electrocardiogram signal; means for measuring an interval between two predefined points within a segment of the electrical signal; means for selecting segments of the electrical signal, a given segment being selected in response to a measurement of the interval of the segment being within a given range of values; and means, responsive to said first means for selecting, for computing a cardiac performance parameter value from the selected segments of the electrical signal.

Claim 41

The impedance cardiograph recited in claim 35 wherein said means for selecting acquires measurements of intervals for segments from a plurality of cardiac cycles, and the given range of values is defined by a number of standard deviations from an arithmetic mean of the acquired measurements.

Claim 42

The impedance cardiograph as recited in claim 35 wherein the two predefined points correspond to half-height points on a peak in the electrical signal segment.