Use of electrical impedance for continuous measurement of stroke volume of a skeletal muscle-powered cardiac assist device

Abstract

This study describes the use of electrical impedance Z to continuously measure the stroke volume SV of a skeletal muscle-powered ventricle (SMV). An SMV was constructed surgically in four anaesthetised dogs. The rectus abdominis (two dogs) or latissimus dorsi (two dogs) muscle was wrapped around a compressible pouch, the ends of which were connected to a saline-filled (0·9 per cent) mock circulation. The motor nerves to the muscle were stimulated to produce tetanic contractions at a rate of 10 min⁻¹. Z was measured between brass sleeve electrodes within the end conduits of the pouch. To derive a simple expression relating pouch volume V to Z, the pouch was represented as two truncated cones with their bases joined. For V ranging from 53 to 103 ml, the relationship between Z and$1/\sqrt V$ was nearly linear; i.e.$Z = m(1/\sqrt V ) + b$. Impedance-derived stroke volume SV (ΔZ) was calculated using this linear approximation and the impedance measured just before and after muscle contraction. The stroke volume SV (EM) ejected by the pouch during muscle contraction was measured with an electromagnetic flowmeter. The linear regression coefficients ranged from 0·99 to 2·55; the correlation coefficients ranged from 0·90 to 0·98. In general, SV(ΔZ) tracked SV(EM) very well, although SV(ΔZ) tended to overestimate SV(EM).