RECTAL NERVES ARE NOT INVOLVED IN SODIUM AND WATER-RETENTION DURING MECHANICAL VENTILATION IN AWAKE DOGS

Background: The role of renal nerves during positive end-expiratory pr essure ventilation (PEEP) has only been investigated in surgically str essed, anesthetized, unilaterally denervated dogs. Anesthesia, sedatio n, and surgical stress, however, decrease urine volume and sodium excr etion and increase renal sympathetic nerve activity independent of PEE P. This study investigated in awake dogs the participation of renal ne rves in mediating volume and water retention during PEEP. Methods: Eig ht tracheotomized, trained, awake dogs were used. The protocol consist ed of 60 min of spontaneous breathing at a continuous positive airway pressure of 4 cm H2O, followed by 120 min of controlled mechanical ven tilation with a mean PEEP of 15-17 cm H2O (PEEP), and 60 min of contin uous positive airway pressure. Two protocols were performed on intact dogs, in which volume expansion had (hypervolemic; electrolyte solutio n, 0.5 ml. kg(-1).min-1) and had not (normovolemic) been instituted. T his was repeated on the same dogs 2 or 3 weeks after bilateral renal d enervation. Results: Hypervolemic dogs excreted more sodium and water than did normovolemic dogs. There was no difference between intact and renal-denervated dogs. Arterial pressure did not decrease when contin uous positive airway pressure was switched to PEEP. Plasma renin activ ity, aldosterone, and antidiuretic hormone concentrations were greater in normovolemic dogs. The PEEP increased aldosterone and antidiuretic hormone concentrations only in normovolemic dogs. Conclusions In cons cious dogs, renal nerves have no appreciable contribution to sodium an d water retention during PEEP. Retention in normovolemic dogs seems to be primarily caused by an activation of the renin-angiotensin system and an increase in the antidiuretic hormone. Excretion rates depended on the volume status of the dogs.