Assessment of parallel conductance for the trans-cardiac conductance method: can we use the hypertonic saline method with pulmonary artery injections?

Abstract

The trans-cardiac conductance (TCC) method provides on-line left ventricular (LV) volume signals by determining the electrical conductance of blood in the LV using central venous and epithoracic electrodes. Conductive structures outside the LV cause a ‘parallel conductance’ offset term (Vp) that is determined by bolus injections of hypertonic saline in the pulmonary artery (Vpsaline). Analysis of the increased conductance signal during passage of the bolus through the LV yields Vpsaline. Since TCC signals are picked up by epithoracic electrodes, concern has been raised that hypertonic saline remaining in the lungs might lead to overestimation. The decrease in blood conductivity induced by injection of non-ionic contrast medium during a LV angiogram may also be used to determine Vp (Vpcontrast). Since the contrast is injected directly into the LV, lung conductance should be unaltered. Thus, we compared Vpsaline with Vpcontrast in six anaesthetized sheep during different hemodynamic conditions. Linear regression showed that Vpsaline = 0.99 Vpcontrast + 2.45 ml (r2 = 0.99). Bland–Altman analysis yielded a small non-significant bias (±2SD) of 1.8 (±6.8) ml. We conclude that parallel conductance for TCC can be accurately determined with the conventional hypertonic saline method.