The trans-cardiac conductance method for on-line measurement of left ventricular volume: assessment of parallel conductance offset volume

The trans-cardiac conductance (TCC) method provides on-line left ventricular (LV) volume signals by determining the electrical conductance of blood in the LV by means of central venous and epithoracic electrodes. Conductive structures outside the LV blood pool cause a "parallel conductance" offset term (V^p) that can be determined by bolus injections of hypertonic saline in the pulmonary artery (V^p_saline), which cause a transient increase in blood conductivity. This study in anesthetized sheep evaluates the accuracy of the saline calibration method and the variabilities of V^p between animals, between hemodynamic conditions and during the cardiac cycle. The conventional intra-cardiac conductance catheter method was used to obtain independent estimates of V^p by the zero-volume method (V^p_zerovolume). Mean baseline V^p_saline and V^p_zerovolume were 104 ± 6 ml and 106 ± 6 ml, respectively. Bland-Altman analysis showed a small nonsignificant bias (-2.5 ml) and narrow limits of agreement (4.6 ml). V^p was not significantly different between hemodynamic conditions (baseline, dobutamine, volume load, propranolol), but had a substantial interanimal variability (IAV) (38%). Average variations during the cardiac cycle were <10% of mean V^p. We conclude that the saline method can be applied to determine V^p for TCC. IAV is substantial, so that V^p must be determined in each animal, but within-animal variability is relatively small.