Effect of the head-down tilt position during lower abdominal surgery on endocrine and renal function response.

A head-down tilt position in awake subjects induces natriuresis, accompanied by reduced plasma levels of catecholamines, renin, angiotensin II, aldosterone, and antidiuretic hormone. We hypothesized that the head-down tilt position would counteract the surgical stress response which induces sodium and water retention and increases plasma levels of these hormones. We studied endocrine and renal function responses in patients during lower abdominal surgery, performed under sevoflurane anesthesia, at a 6 degree head-down tilt position (n = 10) versus a horizontal position (n = 10). The mean arterial pressure was maintained constant by adjusting the inspired concentration of sevoflurane. Heart rate, and the dose of sevoflurane, decreased significantly in the head-down tilt position. Increases in plasma norepinephrine and epinephrine levels during surgery were significantly less in the head-down tilt position; in contrast, plasma aldosterone and cortisol levels were increased significantly in this position compared to the horizontal position. Both fractional and total urinary excretion of sodium increased significantly in the head-down tilt position compared to the horizontal position. Plasma renin activity, and antidiuretic hormone and atrial natriuretic peptide levels, as well as urine volume, creatinine clearance, and water clearance showed no positional variation. We conclude that the 6 degree head-down tilt position reduces increased sympathetic activity and lessens renal tubular sodium reabsorption during lower abdominal surgery under sevoflurane anesthesia. Other stress hormones, however, increase more in the head-down tilt position than in the horizontal position.