METHOHEXITAL IMPAIRS OSMOREGULATION - STUDIES IN CONSCIOUS AND ANESTHETIZED VOLUME-EXPANDED DOGS

Background: Anesthetic agents influence central regulations. This stud y investigated the effects of methohexital anesthesia on renal and hor monal responses to acute sodium and water loading in dogs in the absen ce of surgical stress. Methods: Fourteen experiments (two in each dog) were performed in seven well-trained, chronically tracheotomized beag le dogs kept in highly standardized environmental and dietary conditio ns (2.5 mmol sodium and 91 ml water/kg body weight daily). Experiments lasted 3 h, while the dogs were conscious (7 experiments) or, after 1 h control, while they were anesthetized (7 experiments) with methohex ital (initial dose 6.6 mg/kg body weight and maintenance infusion 0.34 mg . min(-1). kg(-1) body weight) over a period of 2 h. In both exper iments, extracellular volume expansion was performed by intravenous in fusion of a balanced isoosmolar electrolyte solution (0.5 ml . min(-1) kg(-1) body weight). Normal arterial blood gases were maintained by c ontrolled mechanical ventilation. In another five dogs the same protoc ol was used, and vasopressin (0.05 mU . min(-1) kg(-1) body weight) wa s infused intravenously during methohexital anesthesia. Results: Value s are given as means. During methohexital anesthesia, mean arterial pr essure decreased from 108 to 101 mmHg, and heart rate increased from 9 5 to 146 beats/min. Renal sodium excretion decreased; urine volume inc reased; and urine osmolarity decreased from 233 to 155 mosm/l, whereas plasma osmolarity increased from 301 to 312 mosm/l because of an incr ease in plasma sodium concentration from 148 to 154 mmol/l. Plasma ren in activity, plasma aldosterone concentration, plasma atrial natriuret ic peptide, and plasma antidiuretic hormone concentrations (range 1.8- 2.8 pg/ml) did not change in either protocol. In the presence of exoge nous vasopressin (antidiuretic hormone 3.3 pg/ml), water diuresis did not occur, and neither plasma osmolarity nor the plasma concentration of sodium changed. Conclusions: Methohexital may impair osmoregulation by inhibiting adequate pituitary antidiuretic hormone release in resp onse to an osmotic challenge.