Predictors of Variability in Coronary Flow Reserve and Microvascular Resistance Measurement Using the Bolus Thermodilution Technique

BACKGROUND: Bolus thermodilution is the most commonly used invasive assessment for coronary microvascular dysfunction (CMD). However, variability in thermodilution transit time measurements (TTM) affects calculated Coronary Flow Reserve (CFR) and Index of Microvascular Resistance (IMR) values. This study determined variables associated with transit time variability and quantified confidence intervals for the range of measured CFR and IMR values. METHODS: The CoroFlow Cardiovascular System (Coroventis, Sweden) and PressureWire™ X Guidewire (Abbott) were used to collect 2541 TTM from 259 vessels at 14 US sites included in the FlowLab Study. Correlations with baseline patient and procedural variables were determined by both linear and mixed linear models. Mixed linear models with the physician or vessel as the random effect were computed. Monte Carlo simulations of CFR and IMR were performed to determine 95% confidence intervals for thresholds of 2.5 for CFR and 25 for IMR. RESULTS: = 0.948). In this model, RCA vessel location (p < 0.001) and pressure sensor distance < 5 cm (p < 0.005) were significantly associated with transit time. Monte Carlo simulations identified a "gray zone" range of 2.1-3.1 for CFR and 19-31 for IMR. CONCLUSIONS: Minimizing variability in invasive CMD assessment is essential for improving diagnostic accuracy and guiding clinical decision-making as testing becomes more widespread. In this study, target vessel choice (RCA) and pressure sensor placement were identified as key modifiable sources of transit time variability. Clinically applicable confidence intervals around CFR and IMR values were defined.