CONDUCTANCE CATHETER MEASUREMENT OF LEFT-VENTRICULAR VOLUME - EVIDENCE FOR NONLINEARITY WITHIN CARDIAC CYCLE

The conductance catheter gain factor, alpha, is usually determined by an independent measure of stroke volume and, as such, is assumed to be constant. However, nonlinearity of the conductance-volume relation has been proposed on theoretical grounds. The present study was designed to establish the extent of nonlinearity, or variability of alpha, within the cardiac cycle using magnetic resonance imaging (MRI) as the reference method. Pentobarbital-anesthetized minipigs (n = 10, 10-13 kg) were instrumented with left ventricular (LV) conductance and Millar catheters. Conductance catheter signals were recorded, and volumes were corrected for parallel conductance using a saline-dilution technique. Animals were then placed in a 4.7-T magnet, and first time derivative of LV pressure-gated transverse MRI images (5-mm slices) acquired during isovolumic contraction (end diastole) and relaxation (end systole). LV cavity volumes were then determined using a third-order polynomial model. The gain cc was computed three ways: by dividing conductance stroke volume by MRI stroke volume (alpha(SV)), by dividing conductance end-diastolic volume by MRI end-diastolic volume (alpha(ED)), and by dividing conductance end-systolic volume by MRI end-systolic volume (alpha(ES)). alpha(SV) was 0.62 +/- 0.15, with alpha(ED) (0.71 +/- 0.17) significantly lower than alpha(ES) (0.81 +/- 0.21; P < 0.001). Using alpha(SV) to adjust conductance gain (i.e., assuming constant gain) resulted in a significantly larger end-diastolic volume (25.8 +/- 4.6 mi) and smaller ejection fraction (46.8 +/- 7.2%) than those obtained with MRI (23.0 +/- 4.1 mi and 53.1 +/- 7.3%, respectively; P < 0.001). These results indicate that cc is inversely related to LV volume within the cardiac cycle. Although alpha(SV) corrects conductance stroke volume, such assumption of constant gain within the cardiac cycle can result in a slight overestimation of end-diastolic volume and therefore underestimation of ejection fraction.