Non-uniformity of two-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 this study was to clarify the chronotropic influence (atrial pacing) compared to the combined inotropic and chronotropic effects of isoprenaline on uniformity of contraction. In eight open-chest pentobarbitone-anaesthetized cats segment performance was measured by orthogonal sonomicrometry. Heart rate (HR) increased from control state, 199 +/- 5 (mean +/- SEM) beats min-1, to 224 +/- 6 and 227 +/- 7 beats min-1 during atrial pacing and isoprenaline infusion, respectively (P < 0.0005). Circumferential segment shortening remained unchanged during pacing but increased 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 ratio between shortening of longitudinal and circumferential segments, long/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 pacing 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-uniformity during atrial pacing was primarily related to reduced longitudinal shortening whereas the non-uniformity during isoprenaline infusion was due to increased circumferential shortening.