Methods for volume assessment of an air filled compliance chamber

The air-filled (120 ml) compliance chamber employs a Dacron covered Hexsyn-butyl bilaminar diaphragm to minimize gas diffusion. Finite permeability dictates the need for a means to detect the remaining air volume and a refill port for gas addition. X-ray studies were used to detect radiopaque chamber markers in cadavers. Tangential X-ray cinematography enabled detection of chamber volumes less than 100 ml and diaphragm center deflection predicted volume to within +/- 6 ml of the actual volume. An electrically driven left ventricular assist pump (90 cc) was used to evaluate motor current (MC) and intracompliance pressure (IP) as indicators of low compliance volume. Peak MC was a function of both hemodynamic output power and compliance volume. Thus, if the peak current for a given hemodynamic output is known, variances may be indicative of low compliance volumes. The cyclic minimum IP vs. volume was +1, -3, and -13 mmHg for compliance volumes of 120, 90, and 80 cc, respectively. Ejection velocity did not significantly affect IP. In conclusion, X-ray studies and MC are useful as noninvasive means of assessing the need for compliance refill. IP, though measured invasively, is the most sensitive volume-dependent parameter.