Measurement of coronary flow reserve by noninvasive cardiac imaging

Excerpt

Myocardial blood flow (MBF) must respond to changes in metabolic conditions and oxygen requests to meet the needs of myocytes, and autoregulation plays a mayor role in the control of the coronary circulation [1‐3]. It has been demonstrated that as the coronary artery progressively narrows, resting flow does not change at first, but maximal flow resulting from injection of a vasodilator decreases progressively [4, 5]. Coronary flow reserve (CFR) indicates the amount of additional blood flow that can be supplied to the heart over baseline blood flow. The absence of CFR implies maximal vasodilatation of the resistance vessels at rest and an inability to further increase MBF. Different terms are used to describe CFR. Absolute flow reserve is the ratio of blood flow during maximal hyperaemia in a stenotic artery to blood flow in the same artery at rest [6, 7]. The invasive, Doppler-based technique that measures coronary blood velocity at rest and during hyperaemia is a good example of absolute CFR measurement. Relative flow reserve is the ratio of hyperaemic flow in a stenotic artery to hyperaemic flow in a normal artery [8, 9]. Fractional flow reserve is the ratio of the maximum achievable flow in the presence of a stenosis to the theoretical maximum flow in the same artery if the artery were normal [10, 11]. This is the basis of the pressure-derived method that is the invasive method of choice to determine the significance of a coronary stenosis [12]. Several factors influence CFR measurement, such as the ability to achieve maximal coronary vasodilatation, heart rate and myocardial contractility, right atrial pressure, serial coronary stenosis, coronary resistance, and coronary collateral circulation [13]. Each of these factors has a different impact according to the method used for CFR evaluation, and therefore the various techniques are not interchangeable. While problems with each of these techniques may explain the difference, the choice of technique used is often governed by the availability of local resources, and the important practical issue may not be which method is best, but that the techniques do not provide comparable results. …