ABSTRACT
Clinical observations of pulsus alternans have led to conflicting theories of the mechanism of its occurrence. In particular, these are the Frank-Starling mechanism and the alternating contractile state theory. In addition, observations were mostly at either the global level or at the excised cardiac muscle level. Regional ventricular alternans in the intact heart has not been clearly examined. To investigate this, experiments were performed in open-chest anesthetized dogs with either induced or spontaneous pulsus alternans. The electrocardiogram, left ventricular and aortic pressures, and aortic flow were measured simultaneously, together with segmental muscle lengths measured with ultrasonic dimension gauges placed in the endocardial regions of the myocardium. Results show that there were no significant differences in cardiac periods, in end-diastolic pressures, or in end-diastolic lengths. There was clear alternation, however, in end-systolic pressure and end-systolic length. Segmental work of strong and weak beats differs. The strength of alternation intensifies with the imposition of myocardial ischemia. It is therefore concluded that pulsus alternans reflects a depressed ventricular regional phenomenon.
