Vena caval interruption in pulmonary hypertensive canines.

Massive pulmonary embolism regularly causes a reduction in systemic arterial pressure. This is accompanied by an increase in pulmonary arterial pressure. If right heart strain is sufficient, peripheral venous pressures also rise. Initial cardiac output and heart rate responses are variable, but with the progression of time, cardiac output almost uniformly falls. When either vena caval clipping or vena caval ligation are performed in the face ot these altered hemodynamics, there is a further reduction of systemic arterial pressure and cardiac output. After a period of 90 to 120 minutes, the effects of the two procedures appear to be equivalent. This is thought to be on the basis of opening of venous collateral pathways in the case of vena caval ligation. Therefore, when sufficient venous collaterals are present, the major determinant of the ultimate outcome is the impact of the embolus itself rather than the type of procedure used to interrupt the inferior vena cava.