The hourly pattern of urine solute and electrolyte excretion following standard surgical trauma

Although urine volume (and, less frequently, concentration) is often measured in the perioperative period, little attempt has been made to separate temporal phases of the intra- and postoperative response to surgery. In 7 patients undergoing standard severe single trauma and managed by a conventional regimen which included intraoperative Hartmann's solution, we have investigated the hourly pattern of urine solute and electrolyte excretion over the first 48 h. Contrary to expectation, in the first 5 h Na+ excretion increases in association with overall solute excretion, and thereafter progressively diminishes. K+ excretion increases 4 h postoperatively and remains elevated for 24 h, after which it returns to normal, even though Na+ excretion remains low. Free water excretion is negative for the first 24 h, though urine osmolality does not suggest a maximal antidiuretic response by the kidney--the highest concentration achieved being just below 800 mosmol/kg. In order to distinguish between the physiological adaptive changes due to starvation and those due to injury, the hourly pattern of urine solute and electrolyte excretion was further investigated in 12 healthy volunteers mimicking postoperative conditions. Apart from the early postoperative period, the hourly pattern of Na+, K+ and osmolar excretion shows no discernible difference from the operated group. These results show that, particularly in relation to Na+, the changes seen in the post-injury patient, even after major uncomplicated surgery, are largely adaptive, and this is especially striking at 24 h after surgery.