Regional differences in the contractile function of the subendocardium and subepicardium of the left ventricle in the dog

: With the objective to study the regional differences in myocardial function of different left ventricular (LV) segments, twelve pairs of ultrasound crystals were implanted in six open-chest dogs. Six pairs in the subendocardium and six in the subepicardium in three zones of the left ventricle (4 in the apex, 4 in the left anterior descending region and 4 in the circumflex region). In each zone of subendocardium one pair was placed parallel to the epicardial fibers (long axis) and the other following the circumferential plane of the left ventricle (short axis); the same orientation was followed in the subepicardium. Left ventricular pressure, coronary flow (electromagnetic flow-meters), peripheral ECG leads, dp/dt, and segmental shortening were measured. The extent of shortening and the fraction of shortening were considered. The extent of shortening is always higher in the long axis than in the short axis crystals. Similarly, the shortening in the subendocardium is higher than in the subepicardium. The greater extent of shortening and fraction of shortening appeared in the apex, above all in the subendocardium. There are differences in the time-course of fibrillar shortening between subepicardium and subendocardium; the subendocardial contraction was simultaneous with a lengthening of the subepicardial segments. In conclusion, there are differences in myocardial contraction depending on the myocardial region and the layer. Also the shortening of the subendocardium is not synchronous with the subepicardium.