Maximal vasodilation of the epicardial artery (intracoronary nitrates) and of the microvasculature is mandatory to obtain meaningful CFR and IMR values. Due to the need to achieve a proper Doppler signal or to inject multiple 3 ml boluses of saline (at least 3 boluses at rest and 3 boluses during hyperaemia), intravenous administration of adenosine (at 140 mcg/kg/min) is mandatory. Maximal hyperaemia is achieved within two minutes after the start of the infusion. Using this dose, a significant number of patients experience dyspnoea, hypotension and, in a few cases, AV block.
General considerations when assessing CFR
CFR represents the function of the entire coronary system (including the epicardial artery and the microcirculation), as shown in Figure 2. In case of epicardial coronary stenosis (even in cases with apparently mild stenosis), CFR should be read with caution because hyperaemic flow can be reduced to the epicardial stenosis. Therefore, proper vasodilation of the epicardial coronary arteries is mandatory (i.e., with nitroglycerine) before CFR assessment; the ruling-out of epicardial stenosis with hyperaemic or non-hyperaemic indices is also advisable.
CFR is the ratio between the resting coronary flow and the hyperaemic coronary flow. For this reason, it is extremely important to achieve optimal resting...