Effects of respiration and vasodilation on venous volume in animals and man, as measured with an impedance catheter

Effects of physical parameters on the cylindrical model for volume measurement by conductance

Despite its undisputed utility for determining changes in ventricular pressure-volume relationships, the conductance catheter technique has not been proven reliable for measuring absolute volume. This limitation is due to violations of the assumptions inherent in the cylindrical model on which the method is based (i.e., homogeneous electric field and no leakage current). The purpose of this investigation was to relate cylindrical model correction factors to the physical environment of the catheter and to the cylindrical equation. Physical measurements of saline-filled, nonconductive cylinders using a four-electrode conductance catheter were compared with a three-dimensional finite element model of the physical apparatus. These measurements were incorporated into a parallel conductance model to relate physical parameters to corrections in the cylindrical equation for volume measurement. Excellent agreement between measured and modeled data was found. Results demonstrated a nonlinear relationship between the field nonhomogeneity correction factor (alpha) and cylinder diameter. The relationship between alpha and diameter was consistent with a theoretical extrapolation of cylinder diameter toward infinity. An inverse relationship between alpha and the parallel conductance volume (Vp) was also clarified. The parallel conductance model was able to demonstrate opposite effects of the physical presence of the catheter body and electrodes, which tended to cancel out any net effect on measured conductance. Results of this investigation and the developed finite element model clarify the nature of the correction terms in the cylindrical model and may lead to greater application of the conductance technique.

Does inferior vena cava size predict right atrial pressures in patients receiving mechanical ventilation?

The inferior vena cava diameter and its respiratory response are used to estimate right atrial pressures in spontaneously breathing patients but its value in patients receiving mechanical ventilation is unvalidated. Forty-nine patients undergoing mechanical ventilation were prospectively evaluated in the intensive or coronary care units with two-dimensional echocardiography of the inferior vena cava and simultaneous measurements of mean right atrial pressures by central venous or pulmonary artery catheter. Correlation between inferior vena cava diameter at expiration and mean right atrial pressure was only 0.58. The correlation between inspiratory change in inferior vena cava diameter and mean right atrial pressure was poor (r = 0.13). Despite these correlations, an inferior vena cava diameter of < or = 12 mm predicted a right atrial pressure of 10 mm Hg or less 100% of the time, but sensitivity was only 25%. An inferior vena cava diameter > 12 mm had no predictive value for right atrial pressure.

Simultaneous conductance catheter and dimension assessment of left ventricle volume in the intact animal

We compared left ventricle (LV) volume (V) simultaneously measured using the conductance catheter (VM) with volume calculated from three LV dimensions (VD) determined ultrasonically from endocardial crystals. Seven adult mongrel dogs (20-30 kg) were anesthetized and instrumented to measure micromanometer LV pressure and V. Three pairs of crystals were placed orthogonally in subendocardial positions and a conductance catheter was placed in the LV retrograde across the aortic valve. Under steady-state conditions, over the range of a single cardiac cycle, the relation between VM and VD was well described by a straight line. There was an excellent correlation of conductance and dimension volumes with r equal to 0.97 +/- 0.04 and SEE 0.8 +/- 0.5 ml. The gain (1/alpha) and parallel conductance volume (alpha VC) were constant. At lower volumes obtained during bicaval occlusion, however, the relation between VM and VD was curvilinear. 1/alpha and alpha VC both decreased as LVV fell. Thus, determination of absolute volume using the conductance catheter depended on the conditions under which the data were obtained. Under steady-state conditions, alpha VC calculated by both the saline method (mean +/- SD, 50 +/- 15 ml) and by regression of VM and VD, (45 +/- 21 ml) were similar. Consequently, absolute LV end-diastolic volumes and end-systolic volumes by the conductance and dimension methods were similar (53 +/- 14 ml and 38 +/- 14 ml vs. 56 +/- 17 ml and 44 +/- 16 ml, respectively, p = NS). When volume decreased during bicaval occlusion, there was a progressively greater decrease in VM as compared with VD. The absolute slope (EES) of the end-systolic pressure-volume relation (ESPVR) was consistently higher by the dimension method, group average, 16.3 +/- 7.6, than by the catheter, 8.5 +/- 5.9, p less than 0.05. The direction and magnitude of the change in EES at different inotropic states (autonomic blockade; dobutamine), however, was similarly measured by both the conductance catheter and dimension method. We conclude that the gain and offset of the conductance catheter are relatively constant at steady state but vary when volume is reduced by caval occlusion. Thus, the conductance catheter accurately measures absolute volumes at steady state but can underestimate the slope and position of the ESPVR when it is determined by caval occlusion. The conductance catheter does, however, accurately measure the directions and magnitude of change in contractile state.