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Figure 1 Schematic illustration of the coronary collateral circulation with an inflated angioplasty balloon and a Doppler and pressure guidewire located distal to the stenosis. A coronary flow velocity spectrum obtained distal to the stenosis is shown on the right side (horizontal axis: time, second; vertical axis: average peak velocity [APV], cm/sc, i.e., time average value of instantaneous peak velocity samples over two cardiac cycles). The flow velocity trend depicts a graphic illustration of the concept of the velocity-derived collateral flow index (CFI_v, no unit) showing that APV during occlusion amounts to more than half of APV during vessel patency. For the actual calculation of CFI_v, ratios of distal velocity time integrals during (VI_occl or PV_i, cm) to that after occlusion and following cessation of reactive hyperemia (Vi_-occl, cm) are used (see equations above). Abbreviations: CFI_p (no unit): pressure-derived collateral flow index; CVP: central venous pressure (mm Hg); P_ao: mean aortic pressure (mm Hg); P_occl: intracoronary distal occlusive pressure (mm Hg).