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Figure 2 Three pulmonary vein flow patterns identified in fetuses with left heart obstruction. (A) Continuous forward flow with a small a-wave reversal (VTIR/VTIF ratio <0.18). (B) Continuous forward flow with an increased a-wave reversal (VTIR/VTIF ratio 0.18). (C) To-and-fro flow pattern with absent early diastolic forward flow. VTIF = velocity-time integral for forward flow; VTIR = velocity-time integral for reverse flow.