In vivo measurements of skeletal muscle in a linear configuration powering a hydraulically actuated VAD.

Linear contracting skeletal muscle can provide more power and physiologic efficiency for cardiac assistance than muscle wrapping configurations. In this study, the insertion of the porcine latissimus dorsi muscle was removed from the humerus and reattached to a muscle powered ventricular assist device (MVAD), consisting of a mechanical to hydraulic piston energy convertor coupled to a Thoratec VAD. Effects of muscle preload stretch and thoracodorsal nerve stimulation parameters on in vivo unconditioned muscle work and MVAD stroke volume were studied. Stroke work increased linearly with muscle preload, and the slope of this relationship (Mprsw) provided an index of muscle "contractility," similar to the preload-recruitable stroke work relationship for the heart. With 5 V, 220 microseconds stimulation pulses over a 200 msec contraction period at 60 bpm, the Mprsw increased with stimulation frequencies from 0.055 J/cm at 30 Hz to 0.149 J/cm at 60 Hz, and to 0.212 J/cm at 90 Hz. Stroke work up to 1 J was achieved during muscle shortening of 2.5 cm with forces up to 6 kgf and energy convertor pressure of 112 psi (approximately 760 kPa). This produced an ejected MVAD stroke volume of 40 ml into a systolic pressure of 92 mmHg on a mock loop, at a filling pressure of 10 mmHg. The MVAD is designed as an alternative to cardiac transplantation, to provide completely implantable circulatory support free from batteries and other power conditioning hardware required with electromechanical systems.