Apparatus and method for indicating cardiac output
An apparatus for indicating cardiac output comprises means for monitoring a patient's transthoracic impedance and generating a corresponding impedance signal, and signal processing means for (a) deriving a signal S 1 which is a measure of the average amplitude of the impedance signal, (b) filtering the impedance signal at a plurality of different wavelengths within a predetermined frequency band, (c) for each filter deriving a signal S 2 which is a measure of the average amplitude of the respective filter output, (d) calculating the ratio of the maximum one of the signals S 2 derived from step (c) to the signal S 1 derived from step (a), and (e) using the ratio from step (d) in a decision tree to provide a signal indicating cardiac output or not.
1. An external defibrillator, comprising:
a processor;
a frequency generator;
a pair of electrodes; and
a non-transitory computer-readable storage medium having instructions stored which, when executed by the processor, cause the processor to perform operations comprising:
signaling the frequency generator to generate an impedance test signal;
when the pair of electrodes are in contact with a patient, applying the impedance test signal to the patient via the pair of electrodes:
receiving an impedance measurement in response to the impedance test signal; repeating the impedance test signal with the patient to acquire a plurality of impedance measurements;
averaging the plurality of impedance measurements over a defined period of time, to yield an average impedance measurements;
filtering each of the plurality of impedance measurements at a plurality of different spaced frequencies within a predetermined band, to yield a plurality of filtered impedance measurements;
averaging each of the plurality of filtered impedance measurements over a defined period of time, to yield a plurality of average filtered impedance measurements;
identifying a maximum average filtered impedance measurement from the plurality of average filtered impedance measurements;
calculating a ratio of the maximum average filtered impedance measurement to the average impedance measurement;
comparing the calculated ratio with a predetermined threshold value to provide a signal indicating the presence or absence of a cardiac output; and providing the signal indicating the presence or absence of the cardiac output to a user.
2. The external defibrillator of claim 1 ,
wherein the threshold value is in a range inclusively comprising 0.25 to 0.75; and
when the calculated ratio exceeds the threshold value, a first signal is provided indicating the presence of a cardiac output; and
when the calculated ratio is below the threshold value, a second signal is provided indicating the absence of a cardiac output.
3. The external defibrillator of claim 2 , the computer-readable storage medium having additional instructions stored which, when executed by the processor, result in operations, comprising:
when the calculated ratio is below the first threshold value,
comparing the calculated ratio with a second threshold value that is lower than the first threshold value; and
when the calculated ratio is less than the second threshold value, providing a distinct signal indicating the absence of a cardiac output.
4. The external defibrillator of claim 3 , wherein the second threshold value is in a range inclusively comprising 0.25 to 0.03.
5. The external defibrillator of claim 1 , wherein the defined period of time is in a range inclusively comprising 1-12 seconds.
6. The external defibrillator of claim 1 , wherein the predetermined frequency band has a bandwidth in a range inclusively comprising 1 to 5 Hz.
7. The external defibrillator of claim 6 , wherein the plurality of different spaced frequencies number at least 6 distinct frequencies within the predetermined frequency band.
8. The external defibrillator of claim 6 , wherein the plurality of different spaced frequencies are consecutively grouped at intervals of a least 0.166 Hz.
9. A method for use with an external defibrillator, comprising:
signaling a frequency generator to generate an impedance test signal;
when a pair of electrodes are in contact with a patient, applying the impedance test signal to the patient via the pair of electrodes;
receiving an impedance measurements in response to the impedance test signal;
repeating the impedance test signal with the patient to acquire a plurality of impedance measurements;
averaging the plurality of impedance measurements over a defined period of time, to yield an average impedance measurements;
filtering each of the impedance measurements at a plurality of different spaced frequencies within a predetermined frequency band, to yield a plurality impedance measurements;
averaging each of the plurality of filtered impedance measurements over a defined period of time, to yield a plurality of average filtered impedance measurements;
indentifying a maximum average filtered impedance measurement from the plurality of average filtered impedance measurements;
calculating a ratio of the maximum average filtered impedance measurement to the average impedance measurement;
comparing the calculated ratio with a predetermined threshold value to provide a signal indicating the presence or absence of a cardiac output; and providing the signal indicating the presence or absence of the cardiac output to a user.
10. A method of claim 9 , wherein the threshold value is in a range inclusively comprising 0.25 to 0.75; and
when the calculated ratio exceeds the threshold value, a first signal is provided indicating the presence of a cardiac output; and
when the calculated ratio is below the first threshold value, a second signal is provided indicating the absence of a cardiac output.
11. The method of claim 10 , comprising:
when the calculated ratio is below the first threshold value,
comparing the calculated ratio with a second threshold value that is lower than the first threshold value; and
when the calculated ratio is less than the second threshold value, providing a signal indicating the absence of a cardiac output.
12. A method of claim 11 , wherein the second threshold value is in range inclusively comprising 0.25 to 0.30.
13. The method of claim 9 , wherein the defined period of time is in a range inclusively comprising 1-12 seconds.
14. The method of claim 9 , wherein the predetermined frequency band has a bandwidth in a range inclusively comprising 1 to 5 Hz.
15. The method of claim 9 , wherein the plurality of different spaced frequencies number at least six distinct frequencies in the predetermined frequency band.
16. The method of claim 9 , wherein the plurality of different spaced frequencies are consecutively grouped at intervals of at least 0.166 Hz.