Comparison of intravenous and pulmonary artery injections of hypertonic saline for the assessment of conductance catheter parallel conductance.

OBJECTIVE: The conductance catheter provides a continuous measure of left ventricular volume. Conversion of raw data to calibrated absolute volume requires assessment of parallel conductance. Conventionally, parallel conductance is determined by injecting a small bolus hypertonic saline into the pulmonary artery and analyzing the signal obtained during passage of the bolus through the left ventricle. However, in some cases, a pulmonary artery catheter is not practicable. Therefore, we investigated whether intravenous hypertonic saline injections yield reliable parallel conductance estimates.
METHODS: In 13 anesthetized sheep (33+/-5 kg) parallel conductance was obtained by pulmonary artery and by intravenous injections. Measurements (triplicate) were done at baseline, during dobutamine and pacing, and repeated after embolization of the right coronary artery in order to assess the effects of enlarged right ventricular volumes. We used a multiple linear regression model to determine the relation between parallel conductance obtained by the two methods and to quantify the effects of dobutamine, pacing, and embolization.
RESULTS: The two methods show an excellent correlation with a systematic overestimation for intravenous injection. The mean parallel conductance obtained by pulmonary artery injection was 0.690+/-0.009 ohm(-1) whereas intravenous injection yielded 0.739+/-0.015 ohm(-1). Interanimal variability was 0.138 ohm(-1). The difference between the two methods was relatively small, but highly significant (+0.049+/-0.012 ohm(-1), P<0.001). Embolization resulted in significantly higher values (+0.141+/-0.017 ohm(-1), P<0.001), but dobutamine and pacing did not significantly affect parallel conductance (+0.021+/-0.016 ohm(-1), NS). There was no interaction between these interventions and the injection method, indicating that the relation between parallel conductances obtained by the two methods was maintained in all conditions.
CONCLUSION: Parallel conductance obtained by intravenous injection was significantly higher (+7%) than by pulmonary artery injection. However, the relation between the two methods is highly linear with an excellent correlation and is not affected by large hemodynamic changes. The systematic difference between the two methods is likely due to increased conductivity of blood in the right ventricle which is present with intravenous injection but not with pulmonary artery injection. Determination of parallel conductance by intravenous injection is a good alternative for conventional pulmonary artery injection and may be applied in studies where pulmonary artery injection is problematic. This may include studies in very small animals or studies in patients prone to arrhythmias or with cardiac anomalies such as pulmonary artery stenosis. In addition, intravenous injection could be used in biventricular studies to obtain right and left ventricular parallel conductances from a single saline injection.