In‐Vitro Studies of Myocardial Asynchrony and Regional Hypoxia

Ten pairs of cat papillary muscles were arranged in tandem to study their individual and combined length-tension and force-velocity relationships. Physiologic 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 stimulation. 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 to 234±27% of control. During this period the contraction pattern of the tandem pair was out of phase (asynergic), even though they were stimulated synchronously. Thus transient asynergy associated with regional myocardial ischemia may be explained in part by these observed alterations in the time course of contraction and relaxation.