Coronary physiology

The major areas of normal coronary physiological research since Berne's 1964 review have been the measurement of ventricular transmural blood flow distribution with microspheres, the adenosine hypothesis of local metabolic control of coronary blood flow, and the autonomic control of coronary blood flow. There is an improved understanding of intramyocardial tissue pressure and extravascular compressive forces on coronary vessels. However, the unexpected finding of zero flow during a prolonged diastole with a coronary artery pressure of 40 mmHg (PZF) is a reminder that the physical forces, including vascular smooth muscle contraction, that determine coronary vascular resistance are incompletely understood. During normal circumstances, the left ventricular subendocardium probably receives more blood flow than the subepicardium does, but the difference is small. When the coronary circulation is compromised by stenosis or aortic valve lesions, the subendocardium is much more vulnerable to underperfusion than is the subepicardium. The coronary vasodilating effect of arterial hypoxia has been confirmed in many studies, but the role of tissue oxygen tension in local metabolic control of coronary blood flow during normoxia is unknown. The coronary vasodilating action of carbon dioxide has received renewed attention, but its role in local control is also unknown. The adenosine hypothesis has passed several critical tests, but despite much research the importance of adenosine in normal coronary regulation is not established. Local metabolic control of coronary blood flow probably involves more than just one factor, but a unified hypothesis has not been put forward. Sympathetic alpha-receptor-mediated coronary vasoconstriction has been demonstrated by nerve stimulation and during a carotid sinus baroreceptor reflex. Sympathetic coronary vasoconstriction is capable of competing with local metabolic control to lower coronary venous oxygen tension under experimental circumstances, but its importance during normal resting conditions is not established. Parasympathetic muscarinic coronary vasodilation has been shown by vagal nerve stimulation, but a role for it during normal blood flow regulation has yet to be demonstrated. There have been elegant descriptive studies of the coronary blood flow response during excitement and exercise, where coronary blood flow increases pari passu with myocardial metabolism; however, there are also data that indicate a concomitant sympathetic vasoconstrictor effect during strenuous exercise. Overall there has been encouraging progress in coronary physiology. Inevitably new knowledge has focused old questions and presented new ones.