Whole body clearance of norepinephrine. The significance of arterial sampling and of surgical stress.

The whole body clearance of norepinephrine (NE) was measured in seven patients pre- and postoperatively. L[(3)H]NE was infused intravenously for 90 min and steady-state concentrations of L[(3)H]NE were measured at 75 and 90 min in both arterial and peripheral venous blood. Preoperatively, in the resting supine position, the clearance values based on arterial and venous sampling averaged 1.4 and 2.5 liter/min, respectively (P < 0.02). The difference in clearance values was due to a peripheral uptake of NE averaging 45%. The mean plasma NE increased from 1.70 nmol/liter preoperatively to 5.20 nmol/liter postoperatively (P < 0.02). The plasma appearance rate of NE averaged 2.4 nmol/min before surgery and it increased to 9.5 nmol/min postoperatively (P < 0.02). The plasma clearance of NE averaged 1.4 and 1.6 liter/min pre- and postoperatively, respectively (not significantly different). Our study demonstrates that the calculation of plasma NE clearance based on venous sampling results in values that are too high. Furthermore, such values may be influenced by individual variations in the peripheral uptake of NE, since we found no correlation between clearance values based on venous and arterial sampling. The increase in plasma NE postoperatively was due to an increase in the plasma appearance rate of NE because the clearance rate did not change.