Background: When monitoring postoperative urine output there is no guidance specific to obstetrics. Factors such as peri-operative oxytocin infusions add further complexity. Our aim was to determine a normal range for urine output after elective caesarean section under neuraxial anaesthesia.
Methods: Sixty women were recruited and for 24h from the time of urethral catheterisation, we recorded urine output and fluid input. We also measured intra-operative blood loss, use of prophylactic oxytocin infusion and markers of renal function. Data were compared with Mann-Whitney U-tests or paired t tests.
Results: Oxytocin infusions were used in 45 women (75%). Median (95% CI) urine output in the first 6h was 0.8 (0.4-1.9) mL kg(-1)h(-1) in women receiving oxytocin compared to 1.4 (0.7-2.2)mL kg(-1)h(-1) in those who did not (P=0.02). Urine output for all women at 12 and 18 h was 2.0 (0.7-5.7) and 1.9 (0.5-4.5)mL kg(-1)h(-1). Blood loss was 0.4 (0.2-0.8)L in women with oxytocin infusions and 0.3 (0.1-0.4)L in those without (P=0.003). Mean (SD) pre- and postoperative urine osmolality was 622.5 (185.7) and 293.0 (135.1) mosm/kg, respectively (P<0.0001).
Conclusions: Urine output varied widely between subjects, especially after the first 6h and was further reduced by the use of oxytocin infusion. This may have been a direct effect or related to increased blood loss in this group. Oxytocin use should be accounted for when setting a minimum postoperative urine output. We also found high pre-operative urine osmolalities suggesting significant dehydration.
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