Objectives: We assess microvascular integrity as a marker of myocardial viability after coronary stenting, using only a pressure guidewire. Background: Microvascular integrity generally is not assessed using pressure-only guidewires because the transducer lies upstream of microvasculature. We partially inflate a balloon inside a coronary stent to achieve a specific normalized pressure drop at rest (distal coronary/aortic pressure = 0.8) and then infuse a vasodilator, to render the wire sensitive to microvascular function. We hypothesize that the further decline in pressure (ΔFFR0.8) predicts MRI myocardial viability. Methods: We studied 29 subjects with acute coronary syndrome including myocardial infarction. After successful culprit stenting, the resting coronary/aortic pressure was set to 0.8 using temporary balloon obstruction. ΔFFR0.8 was defined as 0.8-(distal coronary/aortic pressures) during adenosine-induced hyperemia. The average transmural extent of infarction was defined as the average area of MRI late gadolinium enhancement (after 2.8 ± 1.5 days) divided by the corresponding full thickness of the gadolinium enhanced sector in short axis slices, and was compared with ΔFFR0.8. Results: ΔFFR0.8 corresponded inversely and linearly with the average transmural extent of infarction (r2 = 0.65, P < 0.001). We found that a transmural extent of infarction of 0.50 corresponded to a ΔFFR0.8 threshold of 0.1, and had high sensitivity and specificity (100% and 94.4%, respectively). Conclusions: Using only an upstream pressure-sensitive guidewire and a partially obstructing balloon during pharmacologic hyperemia, we were able to predict MRI myocardial viability with high accuracy after relief of epicardial stenosis. With further validation, this may prove a useful clinical prognostic tool after percutaneous intervention. © 2011 Wiley Periodicals, Inc.