Comparison of three methods of electrical stimulation for converting skeletal muscle to a fatigue resistant power source suitable for cardiac assistance

Twelve dogs were sorted into 3 equal groups, and the in-situ right latissimus dorsi muscle of each dog was stimulated via its motor nerve for a period of 6 weeks. The resulting isotonic contractions were used to pump fluid in an implanted, 2-chambered, compressible pouch system. Three methods of electrical stimulation were used: (a) continuous 2 sec^-1 single pulses that caused muscle twitching, (b) a 250 msec train of pulses (36 sec^-1) that caused tetanic muscle contractions and was repeated every 2 sec for 15 min followed by a 15 min period of rest, and (c) alternating 15 min periods of the above 2 stimulation methods to cause alternating twitch and tetanic contractions. The 2 sec^-1 twitch stimulation and the combined twitch/tetanic stimulation methods resulted in a 100% conversion to fatigue-resistant fibers within 6 weeks. Standardized muscle function tests were performed weekly. With the twitch stimulation (Method 1), the time to fatigue increased from 9 to 116 min (p<0.001), but fluid pumping ability of the muscle decreased substantially from 0.25 to 0.14 liters min^-1 (p<0.05). With the intermittent tetanic stimulation (Method 2), the fatigue resistance increased only slightly from 7 to 11 minutes (p=NS), and pumping ability was unchanged. With the combined (twitch-tetanic) stimulation (Method 3), the time to fatigue increased from 9 to 107 min (p<0.001), and the pumping ability did not significantly change from 0.20 to 0.22 liters min^-1 (p=NS). These results suggest that a combined electrical stimulation method which produces both twitches and tetanic contractions can achieve rapid fiber conversion and increased fatigue resistance without loss of muscle strength.