Coronary flow reserve is a measure of the ability of the coronary artery to increase coronary blood flow in response to a hyperemic stimulus. A normal coronary flow reserve is a value above 2.0. . In other words, ordinarily the coronary artery can double its blood flow in response to the administration of adenosine. Coronary flow reserve interrogates both the epicardial blockage(s) as well as the resistance in the downstream capillary bed. This is in contrast to fractional flow reserve or FFR which reflects only abnormalities of the epicardial stenosis, and does not reflect abnormalities of the downstream microvascular bed.

When compared to reversible defects on myocardial perfusion imaging, a coronary flow reserve < 2.0 has:

• Sensitivity: 86% to 92%
• Specificity: 89% to 100%
• Predictive accuracy: 89% to 96%
• Positive predictive value: 84% to 100%
• Negative predictive value: 77% to 95%

In some studies, a coronary flow reserve greater than 1.3 immediately after reperfusion in the setting of STEMI is predictive of preservation of the microvasculature.

The risk of intracoronary Doppler velocity wire measurements are as follows (n=906):

• Bradycardia: 1.7%
• Coronary spasm: 2%
• Ventricular fibrillation: 0.2%

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