Body-worn system for continuous, noninvasive measurement of cardiac output, stroke volume, cardiac power, and blood pressure
The invention provides a system for measuring stroke volume (SV), cardiac output (CO), and cardiac power (CP) from a patient that features: 1) impedance sensor connected to at least two body-worn electrodes and including an impedance circuit that processes analog signals from the electrodes to measure an impedance signal (e.g. a TBEV waveform); 2) an ECG sensor connected to at least two chest-worn electrodes and including an ECG circuit that processes analog signals from the electrodes to measure and ECG signal; 3) an optical sensor connected to a body-worn optical probe and including an optical circuit that processes signals from the probe to measure at least one optical signal (e.g. a PPG waveform) from the patient; 4) a processing system, typically worn on the patient's wrist and connected through a wired interface to the optical sensor, and through either a wired or wireless interface to the TBEV and ECG sensors.
1. A method for determining stroke volume from a patient using a body-worn monitor, comprising:
(a) measuring an impedance signal with an impedance sensor operably connected to the body-worn monitor, the impedance sensor comprising at least two electrodes attached to the patient's body and comprising an impedance circuit configured and arranged to measure trans-brachial electrovelocimetry (TBEV) waveform from the patient;
(b) measuring an optical signal with an optical sensor operably connected to the body-worn monitor, the optical sensor comprising an optical probe worn on the patient's hand and comprising an optical circuit;
(c) processing the TBEV waveform and the optical signal using a processing component of the body-worn monitor, wherein the processing component is configured to receive the TBEV waveform and the optical signal and to perform the following processing steps:
(i) processing the TBEV waveform to determine (d(TBEV)dZ/dt), a first zero value in d(TBEV)dZ/dt indicating onset of a pulse in d(TBEV)dZ/dt representing opening of the patient's aortic valve, a second zero value in d(TBEV)dZ/dt after onset of the pulse indicating closing of the patient's aortic valve, and a d(TBEV)dZ/dt max value;
(ii) processing the first zero value in d(TBEV)dZ/dt and the second zero value in d(TBEV)dZ/dt to determine a first systolic flow time (SFT) value;
(iii) collectively processing the (TBEV)dZ/dt max value and the first SFT value to determine a first stroke volume (SV) value;
(iv) processing the optical signal to determine a second SFT value;
(v) collectively processing the (TBEV)dZ/dt max value and the second SFT value to determine a second SV value;
(vi) collectively processing the first and second SV values to determine a third SV value; and
(vii) displaying the third SV value on a display device.
2. A method according to claim 1 , further comprising measuring an electrocardiogram (ECG) signal with an ECG sensor operably connected to the body-worn monitor, the ECG sensor connected to at least two electrodes attached to the patient's body and comprising an ECG circuit; and
processing the ECG signal using the processing component of the body-worn monitor, wherein the processing component of the body-worn monitor is further configured to receive the ECG signal and to perform the following processing steps:
processing the ECG signal to determine a value of heart rate (HR);
collectively processing HR and the first SV value to determine a first cardiac output (CO) value from the patient;
collectively processing HR and the second SV value to determine a second CO value from the patient;
collectively processing the first and second CO values to determine a third CO value; and
displaying the third CO value on the display device.
3. A method according to claim 1 , wherein the impedance sensor is worn on the patient's chest and the at least two electrodes are attached proximal to the patient's brachium; and the optical sensor is worn on one of the patient's fingers.
4. The method of claim 1 , wherein the impedance sensor comprises four electrodes attached proximal to the patient's brachial artery.
5. The method of claim 1 , wherein the optical probe is worn on one of the patient's fingers.
6. The method of claim 1 , wherein the optical probe is worn on the patient's thumb.
7. The method of claim 1 , wherein collectively processing the (TBEV)dZ/dt max value and the first SFT value comprises determining a square root transform of (dZ/dt) max /Z o to produce a value and processing this value and the first SFT value to determine the first SV value.
8. The method of claim 1 , wherein the optical signal is a photoplethysmogram (PPG).
9. The method of claim 8 , wherein processing the optical signal to determine the second SFT value comprises processing the PPG to determine a dichrotic notch, and analyzing a time-dependent value associated with the dichrotic notch to determine the second SFT value.
10. The method of claim 1 , wherein collectively processing the (TBEV)dZ/dt max value and the first SFT value to determine the first SV value comprises using an equation:
SV
=
C
×
(
dZ
dt
)
ma
x
Z
0
SFT
,
where C is a patient-specific constant, or a mathematical derivative of the equation.
11. The method of claim 1 , wherein the impedance sensor is operably connected to the body-worn monitor through a wired interface.
12. The method of claim 11 , wherein the impedance sensor is further configured to transmit a digital representation of the impedance signal through a cable to the body-worn monitor.
13. The method of claim 1 , wherein the impedance sensor is operably connected to the body-worn monitor through a wireless interface.
14. The method of claim 13 , wherein the impedance sensor is further configured to transmit a digital representation of the impedance signal through the wireless interface to the body-worn monitor.
15. The method of claim 1 , wherein processing the optical signal to determine the second SFT value comprises determining a pulse rate (PR) from the optical signal, and estimating the second SFT value from the PR.
16. The method of claim 15 , wherein estimating the second SFT value comprises processing PR with an equation:
SFT=−0.0017*PR+0.413, or a mathematical derivative of the equation.