Determination of an electrode configuration for measuring right ventricular stroke volume using impedance changes

Right ventricular stroke volume was estimated using impedance changes by equating the ventricle to a conductivity cell of variable dimensions. The lower resistive blood is surrounded by the higher resistive myocardium; therefore a constant current delivered between electrodes in the chamber will be preferentially constrained to the blood. When the dimensions of the ventricle change during the cardiac cycle, the impedance changes. The objective of the research was to find an optimal electrode configuration (electrode length, separation and location within the right ventricle) for measuring right ventricular impedance (RVZ) amplitude changes that correspond to changes in stroke volume. In separate studies, bipolar catheter-based electrodes (5 dogs), bipolar needle electrodes (6 dogs) and monopolar catheter-based electrodes (10 dogs) were evaluated. Different electrode geometries, separations and locations were evaluated for all three configurations. To assess the RVZ vs. stroke volume relationship, stroke volume was altered either by ventricular pacing or by inferior vena cava occlusion. Electrode configurations were evaluated by comparing RVZ to stroke volume calculated by integrating the pulmonary artery blood flow (PABF). RVZ was measured three ways: (1) ejection RVZ defined as the impedance change occurring during right ventricular ejection, (2) ejection-to-peak RVZ defined as the impedance change occurring from the beginning of ejection to the maximum impedance amplitude and (3) peak-to-peak RVZ was identified as the amplitude of the impedance change. Linear regression analysis was used to determine the correlation between RVZ and stroke volume. The results showed that bipolar catheter and needle electrodes in the right ventricle are unreliable for tracking changes of stroke volume by RVZ. However, a monopolar electrode configuration provided a definitive improvement. A 0.4 cm electrode positioned 1 cm from the right ventricular apex proved optimal for following changes of stroke volume by RVZ