Myocardial perfusion in humans: what can we measure?

Though angina suggests myocardial ischaemia, this symptom is poorly related to the extent, severity, or site of impaired myocardial perfusion. Electrocardiography during graded exercise is commonly used as a clinical tool to assess the extent and site of ischaemia but it is a poor guide. Myocardial oxygen extraction is almost maximal in the resting mammalian heart, an obligatory aerobic organ. As a result increases in oxygen requirement are met predominantly by increases in myocardial blood flow and many of the commonest cardiovascular diseases are manifest as a relative or absolute reduction in myocardial perfusion. The measurement of myocardial perfusion is the cornerstone to understanding the pathophysiological mechanisms involved. All currently available methods for the measurement of myocardial perfusion in humans are limited. A technique should ideally provide measurements of absolute blood flow with a high degree of spatial and temporal resolution and without perturbing the circulation under investigation. In animals the use of radiolabelled microspheres allows up to nine measurements of absolute perfusion with high spatial resolution and is considered the "gold standard" of available techniques.' Because of the need for tissue samples the method is inapplicable to human studies. All the methods available in humans have limitations. Some do not assess absolute flow, and underestimation of high perfusion rates is a feature of many of the methods based on tracers: All have limited spatial resolution so that none can be used to assess differential perfusion in the layers of the myocardium. Improvement in spatial resolution may come with the development of techniques such as contrast echocardiography, magnetic resonance, and ultrafast x ray computed tomography scanning. All methods assess arterial inflow, venous outflow, or tissue perfusion itself. Methods based on the measurement of arterial inflow or venous drainage have the advantage of high temporal resolution, but limited spatial resolution, and are unable to quantify absolute blood flow per unit myocardial mass.