NONUNIFORMITY OF 2-DIMENSIONAL MYOCARDIAL DEFORMATION IN RESPONSE TO CHRONOTROPIC AND INOTROPIC STIMULATION IN CATS

The degree of uniformity of myocardial deformation for cross-oriented segments in the anterior wall of the left ventricle is influenced both by loading conditions and by infusion of isoprenaline. The aim of thi s study was to clarify the chronotropic influence (atrial pacing) comp ared to the combined inotropic and chronotropic effects of isoprenalin e on uniformity of contraction. In eight open-chest pentobarbitone-ana esthetized cats segment performance was measured by orthogonal sonomic rometry. Heart rate (HR) increased from control state, 199 +/- 5 (mean +/- SEM) beats min-1, to 224 +/- 6 and 227 +/- 7 beats min-1 during a trial pacing and isoprenaline infusion, respectively (P < 0.0005). Cir cumferential segment shortening remained unchanged during pacing but i ncreased with isoprenaline (P < 0.0005). Longitudinal segments showed reduced shortening during pacing (P < 0.05), whereas shortening during isoprenaline infusion did not differ from control shortening. The rat io between shortening of longitudinal and circumferential segments, lo ng/circ ratio, changed from 0.52 +/- 0.13 in the control state to 0.36 +/- 0.10 during pacing and 0.39 +/- 0.11 with isoprenaline (P < 0.05) . End-systolic pressure-length relations were unchanged by atrial paci ng but showed leftward shifts during isoprenaline infusion. Myocardial tissue blood flow (microspheres) was unchanged during interventions. We conclude that both atrial pacing and isoprenaline infusion lead to a more non-uniform deformation in the anterior wall. However, non-unif ormity during atrial pacing was primarily related to reduced longitudi nal shortening whereas the non-uniformity during isoprenaline infusion was due to increased circumferential shortening.