Background— The effect of epicardial artery stenosis on myocardial microvascular resistance remains controversial. Recruitable collateral flow, which may affect resistance, was not incorporated into previous measurements.

Methods and Results— In an open-chest pig model, distal coronary pressure was measured with a pressure wire, and the apparent minimal microvascular resistance was calculated during peak hyperemia as pressure divided by flow, measured either with a flow probe around the coronary artery (R_{micro app}) or with a novel thermodilution technique (apparent index of microcirculatory resistance [IMR_app]). These apparent resistances were compared with the actual R_micro and IMR after the coronary wedge pressure and collateral flow were incorporated into the calculation. Measurements were made at baseline (no stenosis) and after creation of moderate and severe epicardial artery stenoses. In 6 pigs, 189 measurements of R_micro and IMR were made under the various epicardial artery conditions. Without consideration of collateral flow, R_{micro app} (0.43±0.12 to 0.46±0.10 to 0.51±0.11 mm Hg/mL per minute) and IMR_app (14±4 to 17±7 to 20±10 U) increased progressively and significantly with increasing epicardial artery stenosis (P<0.001 for both). With the incorporation of collateral flow, neither R_micro nor IMR increased as a result of increasing epicardial artery stenosis.

Conclusions— After collateral flow is taken into account, the minimum achievable microvascular resistance is not affected by increasing epicardial artery stenosis.