Measurement of cardiac output using Physio Flow with different positions of electrode placement

Physio Flow is a non-invasive impedance cardiograph device that measures cardiac output. Recommended electrode placements involve six electrodes, including two near the xiphisternum (Z3 and Z4/ ECG3/neutral). This study aims to evaluate if changing the positions of these two leads to the left fourth and fifth intercostal spaces along the mid-axillary line results in a change in the cardiac output measurement.

This was a prospective, controlled, crossover, paired study of 30 patients where electrodes were placed in the recommended positions and cardiac output (CO1) obtained after two minutes. The second cardiac output (CO2) was then obtained with the electrodes Z3 and Z4/ECG3/neutral repositioned at the left mid-axillary line at the fourth and fifth intercostal spaces. The final step involved switching the Z3 and Z4/ECG3/neutral leads back to the recommended position and the cardiac output (CO3) was measured.

The average of the initial and third readings (COave) was compared with the measured CO2 and analysed. The regression equation was: CO at the proposed site (CO2) = COave at the recommended site + 0.058. The paired samples correlation was 0.995. Within the 95 percent limits of agreement, the bias with CO measured at the proposed site of electrode placement was 0.046 L/min with the limits at -0.24 L/min and 0.34 L/min. The mean difference was 0.86% of the average CO.

A small positive bias was demonstrated when Physio Flow measurements were taken with the leads Z3 and Z4/ECG3/neutral placed in the mid-axillary line fourth and fifth intercostal spaces.