In Response

Abstract

It was not our intention to discuss the details of coronary microvascular control, because the focus of our reviews was on the effects of receptors throughout the circulation.1,2 Dr. Crystal3 and others4 have shown that to variable degrees, the myocardium is capable of regulating oxygen supply in response to changes in demand, an ability not unique to the myocardium.5 Like all organs capable of autoregulation, when demand is excessive or supply too scarce, autoregulation fails. This phenomenon is the primary interest of our manuscripts, and this inability to autoregulate ad infinitum has been shown in multiple organ systems. Our allusion to Dr. Antonopoulos's manuscript6 was for the sake of completeness; we acknowledged the difficulty of interpreting this interesting manuscript and therefore supported its conclusions with 3 additional studies. Alpha-adrenergic agonists have differential end-organ effects. Few organs can autoregulate as efficiently as the heart, which likely explains its relative resistance to changes in myocardial flow following α-adrenergic stimulation. As Dr. Crystal notes, when myocardial work is increased by systemic arteriolar constriction and a marked increase in blood pressure, the local autoregulation in the coronary microvasculature of healthy hearts can counterbalance increased α-adrenergic stimulation in these vessels, as demonstrated by his studies7 and others. We agree that the work of Dr. Crystal and others suggests potential mechanisms for this observation; whether this remains clinically significant in the face of significantly increased demand is unknown. However, various clinical studies suggest that in patients8 in whom coronary circulation is diseased at the level of the large or small vessels (e.g., mechanical or biochemical disruption of endothelial function; inflammatory activation within the vascular smooth muscle), the microcirculatory control may become imbalanced. Consequently, the statement “the myocardium is not at risk when phenylephrine is used to treat hypotension in patients with adequate cardiac function and coronary vasodilator reserve” must be viewed in the context of experimental work or the more ambiguous clinical setting. Edward C. Nemergut, MD Carl Lynch III, MD, PhD H. Thiele, MD Departments of Anesthesiology and Neurological Surgery University of Virginia Health System Charlottesville, VA Department of Anesthesiology Duke University Hospital Durham, NC [email protected]