Anatomy and Physiology of Left Ventricular Suction Induced by Rotary Blood Pumps

This study in five large greyhound dogs implanted with a VentrAssist left ventricular assist device focused on identification of the precise site and physiological changes induced by or underlying the complication of left ventricular suction. Pressure sensors were placed in left and right atria, proximal and distal left ventricle, and proximal aorta while dual perivascular and tubing ultrasonic flow meters measured blood flow in the aortic root and pump outlet cannula. When suction occurred, end-systolic pressure gradients between proximal and distal regions of the left ventricle on the order of 40-160mmHg indicated an occlusive process of variable intensity in the distal ventricle. A variable negative flow difference between end systole and end diastole (0.5-3.4L/min) was observed. This was presumably mediated by variable apposition of the free and septal walls of the ventricle at the pump inlet cannula orifice which lasted approximately 100ms. This apposition, by inducing an end-systolic flow deficit, terminated the suction process by relieving the imbalance between pump requirement and delivery from the right ventricle. Immediately preceding this event, however, unnaturally low end-systolic pressures occurred in the left atrium and proximal left ventricle which in four dogs lasted for 80-120ms. In one dog, however, this collapse progressed to a new level and remained at approximately -5mmHg across four heart beats at which point suction was relieved by manual reduction in pump speed. Because these pressures were associated with a pulmonary capillary wedge pressure of -5mmHg as well, they indicate total collapse of the entire pulmonary venous system, left atrium, and left ventricle which persisted until pump flow requirement was relieved by reducing pump speed. We suggest that this collapse caused the whole vascular region from pulmonary capillaries to distal left ventricle to behave as a Starling resistance which further reduced right ventricular output thus contributing to a major reduction in pump flow. We contend that similar complications of manual speed control also occur in the human subject and remain a major unsolved problem in the clinical management of patients implanted with rotary blood pumps.