Pressure-Derived Indices in the Left Main Coronary Artery: Insights From Comprehensive In Vivo Hemodynamic Studies of Diseased and Unobstructed Vessels.

Background: Pressure-based physiological evaluation of coronary artery disease is well established, but its application is limited in left main coronary artery (LMCA) disease. Our aim was to investigate whether pressure-based indices are comparable in the left anterior descending (LAD) and left circumflex artery (LCx) branches of the LMCA, and if discordance is due to differences in microvascular function in these territories.

Methods: Simultaneous measurements of coronary pressure and flow were made in patients with (1) isolated LMCA disease and (2) unobstructed coronary arteries. Fractional flow reserve, instantaneous wave-free ratio, and microvascular resistance reserve values in the LAD were compared with those of the LCx.

Results: A total of 80 patients were enrolled (mean age 65±10 years, 56% male). In those with isolated LMCA disease, fractional flow reserve in the LAD was lower than in the LCx (0.74±0.11 versus 0.81±0.11; P<0.0001). Instantaneous wave-free ratio was also lower in the LAD (0.89 [0.76-0.92] versus 0.94 [0.88-0.97]; P<0.0001). The misclassification rates of functionally significant coronary disease, when these indices were measured in the LCx, were 21% for fractional flow reserve and 28% for instantaneous wave-free ratio. Microvascular resistance reserve was higher in the LAD than the LCx, in cohorts with diseased (3.57±1.40 versus 2.50±0.81; P<0.0001) or unobstructed LMCA (3.40±0.78 versus 2.47±0.68; P<0.0001). Microvascular resistance reserve in the LAD territory was similar regardless of whether the LMCA was obstructed or not (P=0.56). Similarly, microvascular resistance reserve in the LCx territory was comparable between cohorts (P=0.88).

Conclusions: Microvascular resistance in the LAD is lower than in the LCx territory. Consequently, fractional flow reserve and nonhyperemic pressure-derived indices are lower in the LAD than the LCx. These findings have important implications for how LMCA atheroma should be assessed in clinical practice and also suggest the need for territory-specific thresholds for defining abnormal microvascular function or epicardial conductance.