Parallel conductance determination in cardiac volumetry using dilution manoeuvres: theoretical analysis and practical implications

Left ventricular volume calibration based on the conductance catheter depen ds on the correct determination of the parallel conductance (G(p)). Baan's saline manoeuvre procedure leads to G(p) by finding the end-systolic (G(es) ) and end-diastolic (G(ed)) conductances, for each beat of the dilution cur ve rising limb. After plotting such values in an xy-system, their linear re gression is back-projected to intersect the identity line, so yielding an e stimated G(p). The objective is to theoretically analyse all possible lines , G(es) = aG(ed) + b (Baan's line) and, based on experimental results, to e stablish their limitations. This was attained by calculating the regression lines using, first G(ed) = f(1)(G(es)) and thereafter, G(es) = f(2)(G(ed)) , which led to two values, G(p2) and G(p1), for the parallel conductance. T he morphology of the saline curve was also modified to assess its effect on the extrapolation. Multiple dilutions were recorded in eight experimental dogs injecting different concentrations. Each curve was classified accordin g to the maximum change (VAR) reached by the total average conductance. Ove r 138 manoeuvres, 276 regressions were processed yielding correlations high er than 0.65. Of this total, 92.4% gave positive parallel conductances. The rest produced negative values and, thus, were neglected. If the two (G(ed) , G(es)) statistical relationships were ideal, they should yield G(p) = G(p 1) = G(p2); however, there were differences which, when G(p1) was studied a gainst G(p2), led to: G(p1) = 0.97 G(p2) + 0.055, with r = 0.9476, and n = 85. The remaining 53 were discarded because either some G(p) values were ne gative, or the correlation of G(es) which G(ed) (or vice versa) was < 0.85, and/or VAR < 15%; the two latter conditions were found necessary for relia ble calibration. Baan's line high correlation is not a unique condition to ensure the accuracy and precision of G(p) determination because the slope a depends on VAR and, thus, different intersections with the identity line m ay be obtained. Its recommended that manoeuvres be used with at least eight data points, with VAR > 15% and, finally, with (G(es), G(ed)) correlation better than 0.85. Theoretical analysis of Baan's line offers a reference fr ame, which contains only a limited number of practical possibilities.