Parallel conductance estimation by hypertonic dilution method with conductance catheter: effects of the bolus concentration and temperature

The conductance catheter has gained momentum since its introduction in cardiovascular dynamics back in 1980. However, measuring errors are still blurring its clinical acceptance. The main objective here was to study the effects of the injected saline concentration and-temperature on the evaluation of the parallel conductance, G _p, and thus, on the correction volume V _p. That conductance, G _p, and its associated volume, V _p, were computed using 167 saline dilution curves obtained with boluses at different concentrations and temperatures, injected in seven anesthetized closed-chest dogs. The excursion of the total conductance relative to the steady-state value during a saline manoeuvre showed good correlation with the injected concentration at both studied temperatures. The reference parallel volume (one reference per dog) was defined as the average value obtained with three successive manoeuvres, at 6-M concentration and at body temperature; therefore, the method acted as its own reference. The variation of V _p relative to the reference value was clearly dependent on the injected concentration and on its temperature; dispersion was greater at 22°C than at 40°C. The variability would recognize also other causes, such as uncertainty of the extrapolation procedure and the thoracic redistribution of electrical field lines. As a conclusion, it is recommended to characterize each manoeuvre by its concentration and temperature. Body temperature and 6-M concentration appear as the most recommendable combination for the injectate in most animals. Finally, these results intend to characterize the V _p estimation procedure in order to minimize errors. The variability of V _p, in different experimental conditions, demonstrated that both concentration and temperature are additional parameters that may modify the G _p estimate.