Normal urine output after elective caesarean section: an observational study.
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.
Mackenzie
M J
MJ
Magill Department of Anaesthesia, Imperial College, Chelsea and Westminster Hospital, London, UK. drmmackenzie@mac.com
Woolnough
M J
MJ
Barrett
N
N
Johnson
M R
MR
Yentis
S M
SM
D000328:Adult; D000773:Anesthesia, Obstetrical; D016063:Blood Loss, Surgical; D015992:Body Mass Index; D002585:Cesarean Section; D003404:Creatinine / Q000652:urine; D005260:Female; D005440:Fluid Therapy; D005919:Glomerular Filtration Rate; D006801:Humans; D009994:Osmolar Concentration; D010120:Oxytocics; D010121:Oxytocin; D010298:Parity; D019990:Perioperative Care; D011184:Postoperative Period; D011247:Pregnancy; D018401:Sample Size; D014508:Urea / Q000652:urine; D014563:Urodynamics / Q000502:physiology*; D014882:Water-Electrolyte Balance / Q000502:physiology
D010120:Oxytocics; D010121:Oxytocin; D014508:Urea; D003404:Creatinine