CORONARY VASCULAR RESERVE IN HUMANS - A CRITICAL-REVIEW OF METHODS OF EVALUATION AND OF INTERPRETATION OF THE RESULTS

The concept of coronary vascular reserve stems from the description of coronary autoregulation by Mosher in 1964. Schematically, the coronary blood flow at rest depends on the dterminants of myocardial oxygen demand (heart rate, contractility and ventricular load). If maximal coronary vasodilation is produced, it results in a linear and steep pressure-flow relationship. The increment of coronary flow above its basal value is called the coronary flow reserve, which indicates the flow added to the basal flow for a given coronary perfusion pressure when the coronary vascular bed is maximally dilated. Thus, in the absence of epicardial coronary artery stenosis, the maximal coronary flow is function of the coronary perfusion pressure and of the maximal surface area of the coronary circulation at the resistance vessel level. Evaluation of the coronary reserve in humans raises three major concerns: (1) methods for measuring or evaluating coronary flow are imprecise and inconvenient (diffusible indicators), invasive (coronary sinus thermodilution, intracoronary Doppler), expensive and require sophisticated technology (ultrafast computed tomography, positron emision tomography), or in the process of being validated (contrast echocardiography, transoesophageal doppler echocardiography, myocardial scintigraphy); (2) the compounds used to produce maximal coronary vasodilation are administered either by the intracoronary route (papaverine) or intravenously (dipyridamole, adenosine); (3) how to interpret the date: is the alteration of the coronary reserve due to a modification of the basal coronary flow or/and of the maximal coronary flow? Have the haemodynamic conditions changed between the measurement of the basal and maximal flow? Thus, each method has its own limitations and constraints that must be taken into account in order to avoid crude misinterpretations. In conclusion, the measurement of coronary reserve may be an important means of understanding coronary physiology and pathophysiloogy. recently developed non-invasive methods (ultrafast computed tomography, position emission tomography, echography) and the ability to produce maximal coronary vasoditlation by the intravenous route are promising.