Geometry and temperature dependence of conductance ventriculography

The conductance catheter allows continuous monitoring of ventricular volume in vivo, but shape and temperature dependence have not been defined. Accordingly, an in vitro experiment was performed to assess the potential utility of conductance for volume measurement during hypothermia and in asymmetric chambers resembling the right ventricle. A 5 Fr conductance catheter was placed inside an ellipsoidal latex balloon, and known volumes of normal saline were infused, while conductance volumes were measured. The geometry of the right ventricle was then approximated by compressing the balloon in a mandrel constructed of two hemi-cylinders. Measurements were repeated in this geometry. Temperature was varied at volumes of 90 ml and 120 ml in an ellipsoidal balloon, and conductance was measured. Conductance volumes were plotted versus known volumes and regression analysis was applied. Compared with the line of identity, there was no statistical difference between slopes of left ventricular geometry and RV geometry, but conductance was highly dependent upon temperature. This was reflected in rho, the resistivity of saline. Rho varied from 80 to 51 (omega-cm) while temperature varied from 8 to 38 degrees C (p < 0.01). These observations suggest that conductance may be useful for measuring right as well as left ventricular volumes. During changes in blood temperature, multiple measurements of rho, in appropriately drawn blood samples, are necessary to maintain accuracy.