PET-measured longitudinal flow gradient correlates with invasive fractional flow reserve in CAD patients

Aims: We aimed to evaluate whether a PET-determined longitudinal decrease in myocardial blood flow (MBF) or gradient, assumed as a more specific flow parameter for epicardial resistance, correlates with invasively measured fractional flow reserve (FFR) in coronary artery disease (CAD) patients. Methods and Results: In 29 patients with suspected or known CAD, myocardial perfusion and MBF in mL/g/min was determined with ¹³N-ammonia PET/CT during regadenoson stimulation and at rest, and corresponding myocardial flow reserve (MFR = MBF stress/MBF rest) was calculated. MBF parameters were assessed in the myocardial region with stress-related perfusion defect and with stenosis ≥50% (Region 1), without defect but with stenosis ≥50% (Region 2), or without stenosis ≥50% (Region 3). Hyperaemic MBFs were significantly lower in the mid-distal than in the mid-left ventricular myocardium in Regions 1-3 [median and IQ range: 1.57 (1.24, 1.84) vs. 1.87 (1.61, 2.00), and 1.23 (1.11, 1.86) vs. 1.89 (1.80, 1.97), and 1.78 (1.48, 2.00) vs. 1.94 (1.84, 2.05) mL/g/min, P < 0.0001]. Resulting longitudinal MBF gradient during hyperaemic flows was more pronounced in Region 2 than in Regions 1 and 3, respectively [-0.46 (-0.70, -0.10) vs. -0.17 (-0.29, -0.11) and -0.15 (-0.25, -0.09) mL/g/min, respectively, P < 0.01]. There was a significant correlation between the hyperaemic longitudinal MBF gradient and FFR (r = 0.95; P < 0.0001), while this association was less pronounced for corresponding MFR (r = 0.50; P = 0.006). Conclusion: The observed close correlation between a longitudinal MBF gradient during hyperaemic flows and invasively measured FFR suggests the longitudinal flow gradient as an emerging non-invasive index of flow-limiting CAD.