The effect of epidural analgesia on postoperative sodium balance

The effect of a high epidural blockade on postoperative sodium (Na) retention has been studied in 8 patients undergoing cholecystectomy, with a further 8 patients, who received conventional anaesthesia and analgesia, acting as a control group. Preoperatively, all the patients received 90 mmol of Na per day and were in Na balance at the start of operation; this intake was continued for 48 h after the operation by intravenous infusion, and the epidural blockade was maintained with regular supplements of bupivacaine throughout. In addition to urine Na excretion and blood pressure changes, plasma levels of aldosterone, cortisol, renin activity and glucose were measured at appropriate intervals. The Na retention in both the epidural and control groups was the same (139 mmol and 135 mmol respectively at 48 h), but there were significant differences in the measurements of all the other factors. It is concluded that the epidural was providing effective blockade, but that the factors which cause Na retention were not affected by the epidural blockade. These results are at variance with another study of the effect of epidural blockade on postoperative Na retention (Bevan, 1971), and the possible reasons for this are discussed.