The incidence of ECG changes related to different doses of intravenous bolus doses of oxytocin (5 and 10 units) during caesarean section is unknown. In a pilot study preceding the present study, 5 units oxytocin were administrated and the incidence of ECG changes was 15%. With the assumption that ECG changes could occur in 50% of subjects receiving 10 units oxytocin and based on 0.8 power to detect a significant difference (P = 0.05, two-sided), 25 patients were required for each study group. An interim analysis performed after inclusion of 50 patients revealed that ECG-changes occured in 8 and 24 % respectively in the two groups of 5 and 10 units of oxytocin. An additional 50 patients were allocated and a second interim analysis planned.