Regional end-systolic pressure-length relationships using a volume-loading technique in the intact pig heart.

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Abstract
Left ventricular systolic elastances as derived from pressure-segment length and pressure-diameter relationships were characterized, and the resulting regional and global elastances were compared to known changes in inotropy. Left ventricular pressure-segment length and pressure-diameter were vaired in a series of consecutive beats by means of a volume-loading technique, and both regional and global relationships at 20-ms intervals throughout systole were nonlinear and exhibited hysteresis. In 8 animals, regional hysteresis was present after vagotomy, propranolol (1.0 mg/kg) and atropine (0.1 mg/kg), and was present whether hearts were loaded by volume (45-60 ml/s) or by pressure (partial aortic occlusion) over a similar range of left ventricular systolic pressures. Elastance was linearly approximated by the slope of the major axis of the hysteresis loops. In each instance, elastance increased to a maximum and then decreased, thus defining end-systole. In 7 animals, maximum elastance-length and -diameter were compared before and after treatments with dobutamine (5-13 .mu.g/kg per min) and propranolol (6-51 .mu.g/kg per min), or after induction of global ischemia. Dobutamine increased maximum elastance-diameter by 37% (P < 0.01) and maximum elastance-length by 159% (P < 0.05). Propranolol decreased maximum elastance-diameter by 27% (P < 0.05) and maximum elastance-length by 6% (P = NS). Global ischemia (50% reduction in coronary flow) did not significantly change either maximum elastance-diameter or -length. However, with ischemia, the diameter intercept of maximum elastance-diameter increased by 24% (P < 0.025), and the time to maximum elastance-length decreased by 33% (P < 0.01). Comparing all interventions, the percent changes in maximum elastance-length and -diameter related directly but with a low correlation coefficient (r = 0.49). These differences in regional and global elastance suggest a complex relationship between regional and global myocardial mechanics, and may not necessarily reflect specific changes in contractility.