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Figure 1 Schematic diagram of the heart showing the left anterior descending (LAD) and the left circumflex coronary arteries. The Doppler guide wire is positioned in the left circumflex coronary artery distal to the stenosis (occluder symbol) to be dilated. The IC (i.c.) ECG lead is obtained from the Doppler guide wire. During balloon occlusion of the stenosis, an instantaneous flow velocity signal (cm/s) and a flow velocity trend over 90 s (both shown on the black panel to the right) can be recorded. Compared with the status shortly before occlusion (white arrow), there are almost no flow velocity signals. The velocity-derived collateral flow index (no unit) is calculated as distal flow velocity time integral during (Vi_occl or peak velocity integral [PVi] [cm]; in this case, 2.3 cm) divided by that after occlusion and after cessation of reactive hyperemia (Vi_ø-occl [cm]; in this case, 23 cm). During stenosis occlusion, marked ST segment elevation on the IC ECG lead is present (arrowheads) (i.e., this patient has insufficient collateral channels. The difference between distal IC pressure and mean aortic pressure further illustrates the period of stenosis occlusion and also the low collateral flow index present in this case. APV = average peak velocity (cm/s; BAPV = basal average peak velocity (cm/s); DPVi = diastolic peak velocity time integral (cm); PAPV = peak average peak velocity; SPVi = systolic peak velocity time integral (cm).