Impact of vessel volume on thermodilution measurements in patients with coronary microvascular dysfunction

Abstract Background Two invasive methods are available to estimate microvascular resistance: bolus and continuous thermodilution. Comparative studies have revealed a lack of concordance between measurements of microvascular resistance obtained through these techniques. Aims This study aimed to examine the influence of vessel volume on bolus thermodilution measurements. Methods We prospectively included patients with angina with non‐obstructive coronary arteries (ANOCA) undergoing bolus and continuous thermodilution assessments. All patients underwent coronary CT angiography to extract vessel volume. Coronary microvascular dysfunction was defined as coronary flow reserve (CFR) < 2.0. Measurements of absolute microvascular resistance (in Woods units) and index of microvascular resistance (IMR) were compared before and after volumetric adjustment. Results Overall, 94 patients with ANOCA were included in this study. The mean age was 64.7 ± 10.8 years, 48% were female, and 19% had diabetes. The prevalence of CMD was 16% based on bolus thermodilution, while continuous thermodilution yielded a prevalence of 27% (Cohen's Kappa 0.44, 95% CI 0.23–0.65). There was no correlation in microvascular resistance between techniques ( r = 0.17, 95% CI −0.04 to 0.36, p = 0.104). The adjustment of IMR by vessel volume significantly increased the agreement with absolute microvascular resistance derived from continuous thermodilution ( r = 0.48, 95% CI 0.31–0.63, p < 0.001). Conclusions In patients with ANOCA, invasive methods based on coronary thermodilution yielded conflicting results for the assessment of CMD. Adjusting IMR with vessel volume improved the agreement with continuous thermodilution for the assessment of microvascular resistance. These findings strongly suggest the importance of considering vessel volume when interpreting bolus thermodilution assessment.