Time dependence of regional systolic left ventricular relationships in intact hearts

In intact hearts the beat-to-beat left ventricular systolic pressure-length relationship is not uniquely defined, but rather is history dependent and appears hysteretic. The purpose of these studies was to assess the time course of this phenomenon. In an open chest swine heart preparation, left ventricular pressure, regional lengths, and wall thickness were measured. Loading conditions were altered by the infusion of volume into the left ventricle to increase peak systolic pressure approximately 30 mmHg. The resultant interbeat systolic pressure-dimension relationships were hysteretic. The first part of the study (n = 13) was to test whether the myocardium exhibited a long-term memory of prior mechanical events. Measurements obtained 30 and 120 s after volume loading revealed no residual differences in pressure or dimension compared with preinjection values. Thus the phenomenon was transient and complete (reversible) by 30 s. To better characterize this event, the temporal effects of myocardial loading were studied (n = 8). A randomized comparison of two different rates of volume infusions (27 +/- 2 vs. 56 +/- 2 ml/s, P less than 0.001) was performed. Varying the volume loading resulted in similar increases in peak left ventricular pressure (35 +/- 3 vs. 31 +/- 3 mmHg) but at different increments of development, i.e., greater than 5-7 cardiac cycles at the slower rate of volume infusion vs. greater than 2-3 cardiac cycles (P less than 0.001) at the faster rate. The isochronal systolic pressure-dimension relationships were quantitated by measuring the area of the hysteretic relationship and its slope.(ABSTRACT TRUNCATED AT 250 WORDS)