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Figure 3 Angiographic findings, electrocardiographic and pressure measurements during recanalization of a chronic right coronary artery occlusion (black asterisk), with collateral circulation (white arrows) supported from the left coronary system at baseline. Measurements of mean aortic pressure (Pa) and coronary wedge pressure (Pw) documented a collateral pressure index (CPI) (collateral pressure index = Pw/Pa) of 0.61 during the first balloon inflation (Balloon 1). After the first balloon inflation (Post 1), the percentage diameter stenosis was 65% (white arrowhead). During the second balloon inflation (Balloon 2), CPI decreased to 0.46, and a slight ST-segment modification (black arrow) was evident on the electrocardiogram. A residual diameter stenosis of 42% (double white arrowheads) was documented by angiography (Post 2). During the drug-eluting stent (DES) deployment, collateral pressure index dropped to 0.33, the patient experienced chest pain (grade 4/10), and a marked ST-segment elevation (double black arrows) was evident on the electrocardiogram. Angiography showed an 8% residual diameter stenosis (Final).