To overcome these barriers, the instantaneous wave-free ratio (iFR), the ratio of distal coronary pressure and aortic pressure at the wave-free period during resting state, was introduced and has demonstrated non-inferiority to FFR in large randomized trials, leading to Class I recommendations in current guidelines.3)4) After the success of iFR, the field continues to explore other non-hyperemic pressure ratios (NHPRs) that might offer practical or physiologic advantages. One such index is the diastolic hyperemia-free ratio (DFR)—an innovative metric that restricts its analysis to the diastolic phase, theoretically reducing variability and improving measurement stability.7) However, clinical validation for this index has been limited—making the current study an important step in bridging the gap between theoretical rationale and real-world evidence.

In this issue of the Korean Circulation Journal, Roh et al.8) report the findings of the ICE-HEAT study—a prospective, observational trial that assessed the performance of DFR in real-world clinical practice. A total of 324 intermediate coronary lesions in 300 patients were evaluated using both DFR and FFR. The study found a strong correlation between DFR and FFR (r=0.80; 95% confidence interval, 0.76–0.84), with a diagnostic accuracy of 92.0% for identifying functionally significant stenosis (FFR ≤0.80). These results were consistent across major coronary vessels—including the left anterior descending artery, left circumflex artery, and right coronary artery—as well as in both proximal and non-proximal lesions, offering insight into the potential applicability of DFR across various lesion locations, including areas where iFR has previously shown limitations.

The higher concordance between DFR and FFR across various target vessels and lesion locations, compared to what has been previously reported for FFR and iFR, may provide supportive evidence for the clinical use of DFR. However, the absence of a direct comparison between FFR and iFR within the same dataset represents a limitation of this study. Given that previous studies have demonstrated a very high concordance and practical equivalence between iFR and DFR, the present findings do not support the conclusion that DFR correlates better with FFR than iFR, regardless of target vessels or lesion locations.7) In addition to the current limitations discussed above, further considerations are warranted regarding the inherent limitations of resting indices. Resting indices, including DFR, may be susceptible to hemodynamic variability across different patient populations. Factors such as tachycardia, hypertension, left ventricular dysfunction, or microvascular dysfunction can affect diastolic flow dynamics and compromise the reproducibility of resting pressure ratios.9) Furthermore, because resting indices typically exhibit smaller pressure gradients compared to FFR, they may be more susceptible to signal noise and less reliable in borderline cases.

Even though current guidelines endorse the iFR-guided PCI with Class 1A recommendation based on the large randomized trials, a recent pooled analysis of long-term outcomes from these trials has raised concerns regarding whether iFR-guided strategies are truly comparable to those guided by FFR.3)4)10) These differences in long-term clinical outcomes are thought to stem from areas of discordance between iFR and FFR. While the present study showed higher concordance between DFR and FFR across various target vessels and lesion locations, these findings are not sufficient to suggest that DFR-guided PCI would more closely replicate the clinical outcomes of FFR-guided PCI compared to iFR-guided PCI. Rather, given the very high concordance among NHPRs, it is more plausible that DFR-guided PCI would yield clinical outcomes more closely aligned with those of iFR-guided PCI than with FFR-guided PCI based on the concurrent evidence. Therefore, DFR-guided PCI may not resolve the concerns raised by iFR-guided PCI.

Accordingly, DFR may provide a practical and less burdensome alternative for assessing the physiological significance of coronary artery stenosis, particularly in clinical settings where hyperemia agents are limited. Given the high concordance demonstrated between DFR and FFR in this study, its use may facilitate quicker decision-making, improve patient comfort, and potentially reduce procedural costs. However, whether DFR-guided PCI can match the clinical efficacy and prognostic value of FFR-guided PCI remains controversial. While DFR shows promising diagnostic performance, its role in guiding revascularization decisions should be interpreted cautiously until robust data on long-term outcomes become available. At present, DFR should be considered a complementary tool rather than a replacement for FFR in routine clinical decision-making.