Antidiuretic hormone response to osmotic stimulus under fentanyl anaesthesia

The effect of fentanyl anaesthesia on the plasma antidiuretic hormone (ADH) response to osmotic stimulus was studied in eight patients. Eight g (137 mmol) NaCl were rapidly injected intravenously the day before anaesthesia and blood samples were collected 5, 10, 20 and 30 min after the injection. This protocol was repeated in the same subjects, under anaesthesia with thiopental, nitrous oxide and fentanyl before surgical incision. Plasma ADH markedly increased after NaCl administration and was significantly correlated with plasma sodium (r = 0.67, P less than 0.005) when the patients were awake, whereas it did not change over 30 min and was not correlated with plasma sodium (r = 0.18, P greater than 0.05) under fentanyl anaesthesia. This inhibitory effect of anaesthesia occurred in spite of a significant fall in the mean arterial pressure during the study. In order to eliminate the role of overnight fasting, premedication and fluid load, the same protocol was performed in six control patients who were fasted overnight, premedicated and fluid loaded. These results demonstrate that fentanyl anaesthesia abolishes the plasma ADH response to both osmotic and low arterial pressure stimuli.