RELATIVE MAGNITUDE AND ASYNCHRONY OF DIRECTIONAL COMPONENTS OF CONTRACTION IN SEDATED DOG LEFT-VENTRICLE

The purpose of this study was to quantitate the temporal relationships and the extent and speed of shortening in segments of myocardium resp onsive to contraction in circumferential, longitudinal, and oblique fi ber groups. Measurements were made in five sedated dogs (morphine, dia zepam) with and without alterations in preload and afterload (nitropru sside, phenylephrine). The measurement interval was the phase of rapid contraction, determined by differentiation of the segment length vs. time. In the control state, percentage segment shortening was greater in circumferential than in longitudinal [15.2 +/- 0.24 (SE) vs. 10.5 /- 0.80%; P = 0.0020] and in the subepicardial oblique than in the sub endocardial oblique fiber directions (16.6 +/- 0.65 vs. 9.7 +/- 0.36%; P = 0.0010). Shortening was proportional to both maximum speed and du ration of shortening (r = 0.735 +/- 0.015 and 0.757 +/- 0.017, respect ively). Duration of shortening was significantly longer in circumferen tial than in longitudinal (mean difference 39.3 +/- 6.6 ms; P = 0.0039 ) and in subepicardial oblique than in subendocardial oblique directio ns (mean difference 27.7 +/- 5.5 ms; P = 0.0072). Velocities of up to 3.0 segment lengths/s were attained in response to nitroprusside. Thes e data reveal the local anisotropy and asynchrony of contraction in th e myocardium; however, they also support the concept of the myocardium as a functional continuum. The dominance of circumferential over long itudinal and subepicardial over subendocardial oblique contractile com ponents indicates their relative contributions to the constriction of the midmyocardial shell.