Use of Impedance Ratio for the Continuous Measurement of Stroke Volume of a Valveless Pouch Used as a Cardiac-Assist Device

A technique is described for measuring the volume of a valveless compressible plastic pouch and its volume change when used as a cardiac-assist device. The method employs measuring the pouch impedance at high frequency with sleeve electrodes at both ends of the pouch. The use of an adequately high frequency eliminates the electrode impedance and the impedance measured is that of the resistance of the electrolyte in the pouch. By equating the compressible pouch to two truncated cones with their bases adjacent, an equation is derived that relates pouch impedance to volume. It is shown that by plotting the stroke volume ejected (ΔV) versus the ratio of systolic (Rs) to diastolic (Rd) impedance, the resulting relationship is independent of the resistivity of the fluid in the pouch. Validation tests were made with a 100 mL pouch filled with solutions having resistivities of 60, 102, 145, and 192 Ω-cm. The method described herein permits calibration of the volume change of a valveless pouch used as a circulatory-assist device and in use the calibration will not be affected by a change in packed-cell volume which changes resistivity.