In-vitro studies of myocardial asynchrony and regional

Ten pairs of cat papillary muscles were arranged in tandem to study their individual and combined length-tension and force-velocity relationships. Physi-ologic asynchronous contraction (up to 60 msec) of the pair of muscles produced a change in the individual force-velocity relations of each muscle which was characteristic of a change in muscle length; i.e., there was an alteration of tension developed without a change in the maximum measured velocity of shortening. On the ascending limb of the length-active tension curve, asynchronous stimulation (average time interval 32 msec) increased the force developed (5.3 ± 1.4%) by the tandem pair, as compared to synchronous stimu-lation. When one muscle was made hypoxic for 20 minutes, its force development was reduced by 64 ± 3%, and paradoxical motion occurred. After 2 minutes of reoxygenation, the time to peak tension was slightly prolonged, while the time for isometric tension to relax to one-half its peak value was increased t