The reasons why fractional flow reserve and instantaneous wave-free ratio are similar using wave separation analysis

BACKGROUND AND OBJECTIVES: Fractional flow reserve (FFR) and instantaneous wave-free ratio (iFR) are the two most commonly used coronary indices of physiological stenosis severity based on pressure. To minimize the effect of wedge pressure (P(wedge)), FFR is measured during hyperemia conditions, and iFR is calculated as the ratio of distal and aortic pressures (P(d)/P(a)) in the wave-free period. The goal of this study was to predict P(wedge) using the backward wave (P(back)) through wave separation analysis (WSA) and to reflect the effect of P(wedge) on FFR and iFR to identify the relationship between the two indices. METHODS: An in vitro circulation system was constructed to calculate P(wedge). The measurements were performed in cases with stenosis percentages of 48, 71, and 88% and with hydrostatic pressures of 10 and 30 mmHg. Then, the correlation between P(back) by WSA and P(wedge) was calculated. In vivo coronary flow and pressure were simultaneously measured for 11 vessels in all patients. The FFR and iFR values were reconstructed as the ratios of forward wave at distal and proximal sites during hyperemia and at rest, respectively. RESULTS: Based on the in vitro results, the correlation between P(back) and P(wedge) was high (r = 0.990, p < 0.0001). In vivo results showed high correlations between FFR and reconstructed FFR (r = 0.992, p < 0.001) and between iFR and reconstructed iFR (r = 0.930, p < 0.001). CONCLUSIONS: Reconstructed FFR and iFR were in good agreement with conventional FFR and iFR. FFR and iFR can be expressed as the variation of trans-stenotic forward pressure, indicating that the two values are inferred from the same formula under different conditions.