In vivo measurement of real-time aortic segmental volume using the conductance catheter.

The goal of this investigation was to determine if the conductance catheter technique for chamber volume measurement could be applied in vivo to determine real-time phasic aortic segmental volume. A four-electrode conductance catheter was used to measure time-varying resistance of the descending thoracic aorta in open-chest, anesthetized dogs. Resistance was converted to segmental volume and the slope correction factor (alpha) and parallel conductance volume (Vp) were determined. The results showed excellent linear correlation between conductance and sonomicrometric segmental volume. The correction factors alpha and Vp were found to be empirically related to average vessel diameter. The relatively high values for the slope correction factor (alpha=4.59+/-0.17 SEM) were found to be primarily related to low-resistivity shunt paths probably originating in the periadventitial aortic wall and to a lesser extent to changes in flow-induced increases in blood resistivity, hematocrit, catheter position, and other adjacent tissue resistivity. The results demonstrate that correction factors empirically derived from measurements of mean aortic diameter could be used to determine absolute real-time phasic segmental volume, cross-sectional area, or diameter. The conductance technique may possess the same potential for determining aortic mechanical properties which has already been demonstrated for determining ventricular mechanical properties.