220 Sensitivity of computed coronary physiology to murray’s law of vascular scaling

Introduction Murray's law relates blood flow (Q) to vessel diameter (D) (Q ∝ D3) in coronary trees. As a fundamental physiological law, it is used to inform various computational techniques for deriving coronary physiology, including virtual fractional flow reserve (vFFR), absolute distal flow (Qd) and microvascular resistance (CMVR). However, the cubed exponent of 3.0 is disputed; the Huo Kassab law suggests that it is 2.33. The aim of this study was to assess the sensitivity of the flow-diameter exponent in deriving computed coronary physiology. Methods Using an established 1D model of coronary flow, we evaluated vFFR, Qd and CMVR in 403 idealised coronary arteries, derived from angiography and invasive physiology data. The flow-diameter exponent was used to determine the magnitude of side-branch flow, which was sequestered along the length of the vessel according to local taper and pressure. Results Median vFFR was 0.87 [0.75 – 0.94] using an exponent of 2.33 and 0.90 [0.79 – 0.95] using an exponent of 3.0. Correlation was strong (r = 0.985, p < 0.0001). Relative to an exponent of 2.33, Murray's original exponent of 3.0 overestimated vFFR by +0.02 (95% CI 0.00 to +0.10) and resulted in a change in management (vFFR ≤ 0.80) in 28 (6.9%) cases. Correlation was also significant for both Qd and CMVR (r = 0.992, p < 0.0001 and r = 0.997, p < 0.0001 respectively). However, compared with vFFR, agreement was relatively poorer for Qd (-15 mL/min, 95% CI -5 to -28 mL/min) and CMVR (+361 WU, 95% CI +40 to +2176 WU). Discussion The flow-diameter exponent influences the diagnostic accuracy of vFFR, with even more pronounced implications for advanced measures of coronary physiology. Determination of the optimum flow-diameter exponent for human coronary arteries is, therefore, an important question in clinical cardiology. Conflict of Interest None