Validation of a New Non-Hyperemic Physiological Index: The Constant-Resistance Ratio (cRR)

Objectives. The instantaneous wave-free ratio (iwFR) has limited availability. A new resting index called the constant-resistance ratio (cRR), which dynamically identifies cardiac intervals with constant and minimum resistance, has been developed; however, its diagnostic performance is unknown. The aim of this study was to validate the cRR by retrospectively calculating the cRR values from raw pressure waveforms of 2 publicly available datasets and compare them with those of the iwFR. Methods. Waveform data from the CONTRAST and VERIFY 2 studies were used. The primary endpoint was Bland–Altman bias between cRR and iwFR. Secondary endpoints included diagnostic agreement, correlation, receiver operating characteristic (ROC) analysis, and success rates of cRR and iwFR. Results. Among the 1036 waveforms, 871 were successful in determining paired cRR and iwFR values, while cRR was 6% more successful than iwFR (P < .0001). The mean bias between cRR and iwFR was 0.003, with 95% limits of agreement [-0.021,0.028]. These 2 indices were highly correlated (r = 0.991; P < .0001). Using an iwFR of 0.89 or less as the reference standard, the optimal cRR cutoff was 0.89, with an area under the ROC curve of 0.991 (P < .001) and a diagnostic accuracy of 96.9% (95% CI [96%, 98%]). Conclusions. The cRR, a new resting index for identifying dynamic cardiac intervals with constant and minimum resistance, demonstrated high numerical agreement, diagnostic consistency, and a higher success rate than the iwFR based on the 2 publicly available datasets.