Optimization of Absolute Coronary Blood Flow Measurements to Assess Microvascular Function: In Vivo Validation of Hyperemia and Higher Infusion Speeds

Background: Reliable assessment of coronary microvascular function is essential. Techniques to measure absolute coronary blood flow are promising but need validation. The objectives of this study were: first, to validate the potential of saline infusion to generate maximum hyperemia in vivo. Second, to validate absolute coronary blood flow measured with continuous coronary thermodilution at high (40-50 mL/min) infusion speeds and asses its safety.

Methods: Fourteen closed-chest sheep underwent absolute coronary blood flow measurements with increasing saline infusion speeds at different dosages under general anesthesia. An additional 7 open-chest sheep underwent these measurements with epicardial Doppler flow probes. Coronary flows were compared with reactive hyperemia after 45 s of coronary occlusion.

Results: Twenty milliliters per minute of saline infusion induced a significantly lower hyperemic coronary flow (140 versus 191 mL/min; P=0.0165), lower coronary flow reserve (1.82 versus 3.21; P≤0.0001), and higher coronary resistance (655 versus 422 woods units; P=0.0053) than coronary occlusion. On the other hand, 30 mL/min of saline infusion resulted in hyperemic coronary flow (196 versus 192 mL/min; P=0.8292), coronary flow reserve (2.77 versus 3.21; P=0.1107), and coronary resistance (415 versus 422 woods units; P=0.9181) that were not different from coronary occlusion. Hyperemic coronary flow was 40.7% with 5 mL/min, 40.8% with 10 mL/min, 73.1% with 20 mL/min, 102.3% with 30 mL/min, 99.0% with 40 mL/min, and 98.0% with 50 mL/min of saline infusion when compared with postocclusive hyperemic flow. There was a significant bias toward flow overestimation (Bland-Altman: bias±SD, -73.09±30.52; 95% limits of agreement, -132.9 to -13.27) with 40 to 50 mL/min of saline. Occasionally, ischemic changes resulted in ventricular fibrillation (9.5% with 50 mL/min) at higher infusion rates.

Conclusions: Continuous saline infusion of 30 mL/min but not 20 mL/min induced maximal hyperemia. Absolute coronary blood flow measured with saline infusion speeds of 40 to 50 mL/min was not accurate and not safe.