Epidural anesthesia and intrathoracic blood volume

To investigate how epidural anesthesia changes intrathoracic gas volume, high epidural anesthesia was administered to five pentobarbital-anesthetized dogs lying supine, and the total intrathoracic volume at end-expiration was measured with a high-speed three-dimensional x-ray scanner. The amount of gas in the lungs at end-expiration [the functional residual capacity (FRC)] was measured with a nitrogen washout technique, and the intrathoracic tissue volume, including the intrathoracic blood volume, was calculated as the difference between intrathoracic volume at end-expiration and FRC. High epidural anesthesia with the local anesthetic etidocaine to a T-1 myotomal level significantly (P < 0.05) increased intrathoracic volume at end-expiration [76 +/- 35 mL (M +/- SD)] by significantly increasing both intrathoracic tissue volume (33 +/- 15 mL) and FRC (43 +/- 26 mL). Increases in intrathoracic tissue volume were probably caused by increases in intrathoracic blood volume. We conclude that increases in FRC caused by epidural anesthesia in anesthetized dogs lying supine may be minimized by a concurrent increase in intrathoracic blood volume. These results suggest that measurements of thoracic gas volume alone may be insufficient to describe chest wall responses to epidural anesthesia.