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Background: Studies in isolated tissues and myocytes show different repolarisation properties in subepicardium, midmyocardium and subendocardium. Whether these differences are present in vivo and are relevant to humans has been the subject of controversy, Our objectives were (1) to ascertain whether transmural repolarisation gradients are present in humans, (2) to determine whether the greater sensitivity of subepicardial cells to ischaemia in vitro is manifest during early ischaemia in humans in vivo, Methods and results: We studied 21 patients during routine coronary artery surgery. Unipolar activation recovery intervals (ARI) were recorded from five transmural locations between subepicardium and subendocardium in the left ventricular wall. A pacing protocol spanned a range of cycle lengths from a cycle length of 300 ms to the maximum permitted by the intrinsic atrial activity. Following the onset of cardiopulmonary bypass recordings were obtained before (control) and during a 3-min period of global ischaemia. During control transmural ARIs were homogeneous between 300 and 1500 ms (ventricular pacing) and 750 and 1500 ms (atrial spontaneous brats), During ischaemia, ARIs shortened similarly at all transmural electrode sites and transmural homogeneity was maintained, Conclusions: Transmural repolarisation differences within the ventricular wall of the human heart were absent at cycle lengths within the physiological range but also during prolonged cycles. During early (global) ischaemia repolarisation changed equally in subepicardial and subendocardial regions and transmural homogeneity of repolarisation was preserved. (C) 2001 Elsevier Science B.V. All rights reserved.