Regional myocardial blood flow and myocardial function during acute right ventricular pressure overload in calves

Hemodynamics, myocardial function, and regional myocardial blood flow (MBF) were measured during acute right ventricular (RV) pressure overload created by inflation of a previously implanted cuff to constrict the pulmonary artery trunk (PAC) in seven closed-chest, anesthetized calves during normoxia (PaO₂: 90-110 mm Hg) and hypoxia (PaO₂ ~ 40 mm Hg). MBF was determined by the microsphere method. With PAC, mean RV systolic pressure approached 90 mm Hg or higher and tricuspid regurgitation occurred frequently. There was no indication of increased RV contractility when alterations in preload and afterload were taken into account, nor was there evidence to suggest deterioration of left ventricular contractility during PAC with normoxia or hypoxia. During normoxia + PAC, there was an insignificant increase in blood flow to the RV free wall and right side of the interventricular septum over their respective control values. However, with hypoxia + PAC, myocardial perfusion to these regions registered a significant increase over the control hypoxic values despite similar coronary driving pressure and RV tension time index. This suggests that coronary vascular reserve had not been exhausted during normoxia + PAC in these areas of the ventricular myocardium. Selective underperfusion of RV endocardium did not occur during either PAC period. The tendency of blood flow to increase in the right side of the septum with PAC during normoxia and hypoxia supports the view that the right side of the septum aids the RV contraction.