Noninvasive Monitoring of Transient Cardiac Changes with Impedance Cardiography

Impedance cardiography (IC) has been used to monitor the pumping function of the heart in terms of stroke volume or cardiac output. We explored the extent to which IC can be used for monitoring transient effects of cardiac alterations on a beat-to-beat basis. A four-spot-electrode configuration was used in the present design. The spatial arrangement of the four electrodes was carefully selected to obtain an optimal result. A constant current source provided a 4 mA, 20 kHz sinusoidal excitation current. Analog signal processing was developed to demodulate and amplify the resultant impedance information, eliminate low frequency motion artifacts and incidental breathing artifacts. Experiments were performed on normal healthy volunteers. Impedance waveforms were recorded from each subject beginning from a supine position then moving to an upright position. Breath holding was maintained throughout the procedure. The impedance waveform, Z, its first derivative dZ/dt, the DC component, Z (0), and electrocardiogram were recorded simultaneously and continuously. The results showed a significant increase in the relative impedance levels from supine to transient state and then decreased during the upright position. Additionally, the supine position had a higher (dZ/dt)(max) than sitting resulting in a reduction in stroke volume. The increased impedance from supine to sitting verified the physiological findings obtained by others. The results of this investigation illustrate the ability of impedance cardiography to effectively monitor the dynamic cardiovascular function under transient conditions.