THORACIC EPIDURAL-ANESTHESIA INCREASES DIAPHRAGMATIC SHORTENING AFTER THORACOTOMY IN THE AWAKE LAMB

Background. Prolonged inhibition of diaphragmatic function occurs after thoracic and upper abdominal surgery. It was hypothesized that thoracic epidural anesthesia on the day after a thoracotomy could block inhibitory neural pathways and increase the shortening of costal and crural diaphragmatic segments. Methods. Pairs of sonomicrometer crystals were implanted into the costal and crural regions of the diaphragm through a right lateral thoracotomy in 14 30-kg, 4-5-month-old lambs. One day after surgery, a thoracic epidural catheter was placed at the T8-T9 level. Regional diaphragmatic shortening normalized to end-expiratory length (%L(FRC)), was measured by sonomicrometry in these awake lambs. Changes in gastric (DELTAP(gas)), esophageal (DELTAP(es)), and transdiaphragmatic (DELTAP(dl)) pressures were measured with transnasal balloon catheters. End-tidal carbon dioxide (FET(CO2)), costal and crural electromyogram (E(dl)), and tidal volume (V(T)) were measured. Inductance plethysmography was used in four lambs to assess relative contributions of the rib cage and abdomen to V(T). Control values were obtained during quiet breathing and while rebreathing at up to 10% FET(CO2). To block thoracic dermatomes, 1% or 2% lidocaine was injected through the epidural catheter. Measurements were repeated after each lidocaine injection. Results: There was no change of resting length with 1% lidocaine; costal resting length increased by 22% with 2% lidocaine. After 2% lidocaine, costal %L(FRC) increased from control both during quiet breathing (8.7 +/- 0.7 to 18.1 +/- 1, xBAR +/- SEM%) and at FET(CO2) 10% (22.1 +/- 2 to 33.7 +/- 3%). V(T) during quiet breathing was unchanged after 1% lidocaine but increased from 235 +/- 16 to 283 +/- 28 ml after 2% lidocaine. At 10% FET(CO2) DELTAP(dl) was unchanged after 1% lidocaine and decreased from 36.5 +/- 4.3 to 26.3 +/- 4.9 cmH2O after 2% lidocaine. Regional DELTAE(dl) was unchanged with both 1% and 2% lidocaine at rest and during carbon dioxide rebreathing. Plethysmography in three lambs showed a reduction in rib cage contribution to tidal volume with 2% lidocaine during quiet breathing. Conclusions: Improved postoperative tidal volume and diaphragmatic shortening after thoracic epidural blockade may be due to changes of chest wall conformation and resting length and a shift of the workload of breathing from the rib cage to the diaphragm caused by intercostal muscle paralysis.