Arteriovenous-shunt-mediated increase in venous return causes apparent right coronary arterial autoregulation

Reports of autoregulation in the right coronary vasculature have varied from non-existent to almost perfect. At least some of this discrepancy may be due to failure to account for changes in myocardial metabolism secondary to the method used to vary perfusion pressure. The aim of this study was to determine if the potent autoregulation reported when right coronary perfusion pressure was lowered by opening a large arteriovenous shunt was due to increased right ventricular myocardial oxygen consumption (MVO2) induced by augmented preload and afterload.

Two protocols were used to produce right coronary perfusion pressures of 100, 80, and 60 mm Hg in anaesthetised dogs. In both protocols the right coronary artery was cannulated and supplied with blood from a pressurised chamber. In protocol 1, right coronary perfusion pressure was decreased independently of aortic pressure, and in protocol 2, aortic pressure was decreased in parallel with right coronary perfusion pressure by opening a large arteriovenous shunt. Right coronary blood flow, central venous pressure, and pulmonary arterial pressure were measured, and right ventricular oxygen extraction and MVO2. Central venous pressure (right ventricular preload) and pulmonary arterial pressure (right ventricular afterload) did not change. In protocol 2, opening the arteriovenous shunt increased venous return, as shown by increased central venous pressure and pulmonary arterial pressure. This increased right ventricular MVO2 at the lower right coronary perfusion pressures and maintained right coronary blood flow at the level recorded when right coronary perfusion pressure was 100 mm Hg.

This apparently potent autoregulation resulted from the shunt induced increase in oxygen consumption at low right coronary perfusion pressures, in contrast to the decreased right ventricular oxygen consumption and right coronary blood flow observed when right coronary perfusion pressure is selectively decreased.