Pulmonary function in dogs after intercostal thoracotomy: comparison of morphine, oxymorphone, and selective intercostal nerve block.

A thoracotomy was performed at the left 5th intercostal space in 24 dogs. Dogs were assigned to 4 groups of 6 dogs each. Postoperative analgesia was administered as follows: group 1--control, no analgesia; group II--morphine (0.5 mg/kg of body weight); group III--oxymorphone (0.1 mg/kg); group IV--selective intercostal nerve block with bupivacaine HC1. Respiratory rate, minute volume (VE), and arterial blood gases were measured during the recovery period. Ventilation-perfusion mismatch was estimated by calculation of the alveolar-arterial oxygen tension difference. Arterial carbon dioxide tension (Paco2) in the control and selective intercostal nerve block groups remained within the normal range and did not differ significantly (P less than 0.05) between groups. During the first 60 to 90 minutes after surgery, Paco2 tension was increased significantly (P less than 0.05) in the groups given morphine and oxymorphone. Hypoventilation in the groups given narcotics resulted from significant reductions (P less than 0.05) in the respiratory rate and VE and produced significant (P less than 0.05) respiratory acidosis and hypoxemia. Three dogs in the groups given narcotics had a panting response that resulted in increased respiratory rates and VE. This response did not improve alveolar ventilation in these dogs, which was evidenced by increased Paco2 values. Hypoventilation, respiratory acidosis, and hypoxemia in the groups given narcotics improved significantly with time, presumably because of drug clearance. Values for alveolar-arterial oxygen tension difference indicated moderate ventilation-perfusion mismatch secondary to anesthesia in all groups; however, significant differences (P less than 0.05) between the groups were not observed.