Respiration after phrenicotomy and hydrocortisone treatment in anaesthetized rats

The study was designed to determine the extent to which respiratory muscle wasting, resulting from corticosteroid-induced atrophy, may affect respiration in normal rats and in rats with denervated diaphragm. Twenty four male Wistar rats were divided into four groups: 1) controls with sham operation (SX) and vehicle injections: 2) SX with eight hydrocortisone (HC) injections (60 mg.kg-1.day-1 i.m.); 3) phrenicotomized (PX), injected with vehicle; 4) PX and HC-treated. HC treatment was started on the thirteenth day after surgery. Under urethane anaesthesia, tidal volume, respiratory rate, arterial carbon dioxide tension (PaCO2) and occlusion pressure were measured at rest and after 5 min of stimulated-breathing induced by added dead space 22nd day after surgery. All HC-treated animals decreased body weight by 32% compared to untreated rats. The diaphragm weight was reduced in PX rats by 29%, and after HC by 44%, while in PX rats with HC treatment diaphragm weight decreased by only 21%. PX rats (HC-untreated) had the lowest minute ventilation and occlusion pressure. There was no difference in ventilation between control and both HC-treated groups at rest. However, ventilation in PX and HC-treated rats did not increase upon stimulation, and the occlusion pressure increased significantly only in the HC-untreated animals. We conclude that in the rat, HC treatment did not affect resting ventilation, but it impaired ventilation performance, during increased demand, in animals handicapped by diaphragm denervation.