Biomechanical circulatory assistance. Importance of aerobic capacity of normal and conditioned skeletal muscles.

To define more efficient use of biomechanical circulatory assistance, the contracting and fatigue characteristics of normal and conditioned latissimus dorsi of seven adult mongrel dogs were analyzed. The conditioned muscles underwent vascular adaptation and electrical stimulation with burst frequency of 50 Hz in rates of 60 and 120 contractions per minute for 6 to 8 weeks. They were compared with unconditioned contralateral controls by isometric force measurements. Normal and transformed muscles exhibited the same optimum pacing parameters (burst frequencies from 50 Hz up and train durations from 150 msec) with supramaximal amplitude stimulation. Conditioned muscles showed initial smaller forces (-27%) and longer contraction times (+32%). Fatigue curves of unconditioned muscles for different duty cycles showed marked initial decreases and proportional stable values of force after some minutes, with equal final tension time indexes under these conditions (20 +/- 3 KgF seg/min 100 g). Conditioned muscles maintained stable force for same duty cycles and a greater tension time index during prolonged conditions (68 +/- 6 KgF seg/min 100 g). This study revealed that normal and conditioned skeletal muscles are capable of maintaining steady state work depending upon their aerobic capacity and suggests that these muscles may undergo adaptive transformation while contracting in a synchronous fashion with the heart during variable duty cycles.