The Effect of Epidural Anesthesia on Respiratory Distress Induced by Airway Occlusion in Isoflurane-Anesthetized Cats

The role of afferent information from the chest wall in the genesis of dyspnea is not fully elucidated. We have developed an animal model for the study of airway occlusion (AO) and proposed new concepts of minimum alveolar anesthetic concentration for AO (MACaor) and the duration from the start of AO to the onset of the positive motor response (Doccl) to evaluate respiratory distress quantitatively. We examined the effects of thoracic epidural anesthesia on respiratory distress by using our animal model. Adult cats (n = 24) were anesthetized with isoflurane, and an epidural catheter was placed after T9 laminectomy. After determination of MACaor, Doccl was measured. Animals were then randomly assigned into three groupsthe EPD Group (n = 12) received epidural 1% lidocaine (0.4 mL/kg), IM saline (0.4 mL/kg), and saline infusion. The IM Group (n = 6) received epidural saline (0.4 mL/kg), IM 1% lidocaine (1 mL/kg), and saline infusion. The PHE Group (n = 6) received epidural 1% lidocaine (0.4 mL/kg) and IV phenylephrine (0.5–1 μg · kg−1 · min−1) to maintain a stable arterial blood pressure. Doccl and MACaor were measured in each animal at 15 min after the administration of drugs. Plasma lidocaine concentrations were measured before and after epidural or IM injection. Doccl was significantly longer after epidural injection in all groups than before the injection. Although there was no significant difference in the values of MACaor between before and after the epidural injection in the EPD Group, the IM administration of lidocaine in the IM Group significantly reduced MACaor. Plasma concentrations of lidocaine were similar in all groups at all measurement points. Our data indicate that thoracic epidural anesthesia using 1% lidocaine significantly reduced respiratory distress induced by AO. This effect is most likely caused by a systemic effect of lidocaine rather than by reduced afferent information from the chest wall.