Normal values of thermodilution-derived absolute coronary blood flow and microvascular resistance in humans

Background: Absolute hyperaemic coronary blood flow (Q, in mL/min) and resistance (R, in Wood units [WU]) can be measured invasively by continuous thermodilution. Aims: The aim of this study was to assess normal reference values of Q and R. Methods: In 177 arteries (69 patients: 25 controls, i.e., without identifiable coronary atherosclerosis; 44 patients with mild, non-obstructive atherosclerosis), thermodilution-derived hyperaemic Q and total, epicardial, and microvascular absolute resistances (R-tot, R-epi, and R-micro) were measured. In 20 controls and 29 patients, measurements were obtained in all three major coronary arteries, thus allowing calculations of Q and R for the whole heart. In 15 controls (41 vessels) and 25 patients (71 vessels), vessel-specific myocardial mass was derived from coronary computed tomography angiography. Results: Whole heart hyperaemic Q tended to be higher in controls compared to patients (668 +/- 185 vs 582 +/- 138 mL/min, p=0.068). In the left anterior descending coronary artery (LAD), hyperaemic Q was significantly higher (293 +/- 102 mL/min versus 228 +/- 71 mL/min, p=0.004) in controls than in patients. This was driven mainly by a difference in R-epi (43 +/- 23 vs 83 +/- 41 WU, p=0.048), without significant differences in R-micro. After adjustment for vessel-specific myocardial mass, hyperaemic Q was similar in the three vascular territories (5.9 +/- 1.9, 4.9 +/- 1.7, and 5.3 +/- 2.1 mL/min/g, p=0.44, in the LAD, left circumflex and right coronary artery, respectively). Conclusions: The present report provides reference values of absolute coronary hyperaemic Q and R. Q was homogeneously distributed in the three major myocardial territories but the large ranges of observed hyperaemic values of flow and of microvascular resistance preclude their clinical use for inter-patient comparison.