Thoracic epidural anesthesia causes rib cage distortion in anesthetized, spontaneously breathing dogs

The effects of thoracic epidural anesthesia on the performance of the parasternal intercostal muscles were investigated by measuring electromyographic activity and length changes of the parasternals (EMG activities and length, respectively, of the parasternals) in seven pentobarbital anesthetized, spontaneously breathing dogs. Epidural injection of 0.1 mL/kg of 2% lidocaine decreased tidal volume and minute ventilation during unstimulated breathing. These changes were accompanied by complete abolishment of EMG activities of the parasternals and passive elongation of the parasternals during inspiration. At equivalent end-tidal PCO2 values (70 and 80 mm Hg) during CO2 rebreathing, tidal volume and minute ventilation were lower after epidural block compared to the corresponding values before the block. Thoracic epidural anesthesia impaired contraction of the parasternals and conceivably other respiratory muscles in the rib cage as well and could induce a distortion of the rib cage. The authors conclude that respiratory muscles in the rib cage contribute considerably to the maintenance of ventilation in anesthetized dogs.