Relation between muscle contraction speed and hydraulic performance in skeletal muscle ventricles

The fatigue resistance and power-to-weight ratio of skeletal muscle that has been conditioned by electrical stimulation makes cardiac assistance from a graft of such muscle a realistic prospect. A skeletal muscle must be surgically reconfigured to act on the circulating blood, but little is known about the power losses that accompany such interventions. We investigated in acute experiments the hydraulic performance of approximately cylindrical pumps made from sheep latissimus dorsi (LD) muscles, having first characterized the performance of each muscle in situ.

Force-length and force-velocity relations were measured in situ for LD that had received either 8 weeks of stimulation at 2 Hz or no chronic stimulation. Two sizes of skeletal muscle ventricle (SMV) were formed from the same muscles, and their hydraulic performance was measured. The hydraulic performance was also calculated from the linear data, models of the force-length and force-velocity curves, and a description of the stress distribution within the SMV wall. The model predicted well the isovolumetric function of the ventricles and the optimum afterload but overestimated the flow and therefore the power. In conditioned ventricles the performance was particularly poor because of the slow contractile properties of the muscles.

If SMVs are to pump effectively against the arterial impedance, the pressure drop caused by flow (or the internal resistance) should be lower than that of the ventricles we constructed. Progress can be made through refinement of surgical technique and stimulation protocols that generate faster fatigue-resistant muscles.