EXTRACELLULAR VOLUME EXPANSION INHIBITS ANTIDIURETIC-HORMONE INCREASEDURING POSITIVE END-EXPIRATORY PRESSURE IN CONSCIOUS DOGS

1. This study in conscious dogs examined the effects of extracellular volume expansion on plasma antidiuretic hormone, atrial natriuretic pe ptide and aldosterone concentrations, plasma renin activity, and haemo dynamic and renal responses during controlled mechanical ventilation w ith 20 cmH2O positive end-expiratory pressure. 2. Twenty experiments ( 10 controls, 10 expansion experiments with 0.5 ml min-1 kg-1 body weig ht of a balanced electrolyte solution given intravenously throughout) were performed in five trained, conscious, tracheotomized dogs over 4h : first and fourth hour, spontaneous breathing; second and third hour, 20 cmH2O positive end-expiratory pressure. 3. In the control experime nts positive end-expiratory pressure increased plasma antidiuretic hor mone concentration from 1.4+/-0.2 to 10.0+/-3.3 pg/ml, plasma aldoster one concentration from 113+/-19 to 258+/-58 pg/ml and heart rate from 77+/-5 to 94+/-5 beats/min. Positive end-expiratory pressure did not c hange plasma atrial natriuretic peptide concentration (55+/-5 pg/ml), plasma renin activity (2.6+/-0.4 pmol of angiotensin I h-1 ml-1) and m ean arterial pressure 103+/-3 mmHg). 4. In the expansion experiments, positive end-expiratory pressure did not change plasma antidiuretic ho rmone concentration (1.1+/-0.1 pg/ml), plasma aldosterone concentratio n (25+/-2 pg/ml), plasma atrial natriuretic peptide concentration (82/-8 pg/ml), plasma renin activity (0.8+/-0.15 pmol of angiotensin I h- 1 ml-1), heart rate (92+/-6 beats/min) and mean arterial pressure (111 +/-4 mmHg). 5. In the control experiments, urine volume, sodium excret ion and fractional sodium excretion remained in a low range during pos itive end-expiratory pressure, whereas potassium excretion increased. In the expansion experiments, urine volume, sodium excretion and fract ional sodium excretion increased continuously. Glomerular filtration r ate was decreased during positive end-expiratory pressure in the contr ol experiments when compared with the expansion experiments (3.4+/-0.3 versus 3.9+/-0.2 ml min-1 kg-1 body weight). 6. Arterial blood gases and plasma osmolality did not change in either protocol. 7. It is sugg ested that the striking increase in antidiuretic hormone may play a pa rt in the circulatory control mechanisms that maintain mean arterial p ressure during positive end-expiratory pressure when the extracellular volume is not expanded.