EFFECTS OF THORACIC EXTRADURAL BLOCK ON DIAPHRAGMATIC ELECTRICAL-ACTIVITY AND CONTRACTILITY AFTER UPPER ABDOMINAL-SURGERY

Background. Upper abdominal surgery (UAS) induces diaphragmatic dysfunction. Thoracic extradural block (TEB) using 0.5% bupivacaine improves some pressure and motion indices of diaphragmatic function. However, no direct information on diaphragmatic activity is available after UAS. The aim of this study was to assess diaphragmatic electrical activity (Edi) after UAS before and after TEB. Methods. A postoperative electromyogram was obtained, using intramuscular electrodes inserted by the surgeon in the costal and crural parts of the diaphragm, in 14 patients undergoing abdominal aortic surgery. Tidal changes in abdominal (V(AB)) and rib-cage (V(RC)) volumes, and gastric (DELTAPgas), esophageal (DELTAPes), and transdiaphragmatic (DELTAPdi) pressures were used to measure tidal volume (V(T)) and respiratory rate and to provide indirect indices of diaphragmatic activity from the two ratios V(AB)/V(T) and DELTAPgas/DELTAPdi. These respiratory variables were obtained preoperatively. Postoperatively, measurements including Edi were obtained before and after a segmental epidural block, reaching a T4 level was achieved with 0.5% plain bupivacaine. Results: Upper abdominal surgery induced an increase in respiratory rate (+28 +/- 15%; P < .01), associated with a decrease in V(AB)/V(T) (from 0.75 +/- 0.11 to 0.07 +/- 0.08; P < .01), DELTAPgas/DELTAPdi (from 0.3 +/- 0.08 to 0.01 +/- 0.19; P < .05), and V(T) (- 30 +/- 14%; P < .01). After surgery, all patients exhibited electrical diaphragmatic activity that increased with TEB by 48 +/- 28% (P < .01) and 60 +/- 22% (P < .001) for the cural and costal segments, respectively. The ratio DELTAPdi/Edi, used to evaluate diaphragmatic contractility, was not modified by TEB. Tidal volume, respiratory rate, and DELTAPgas/DELTAPdi returned to preoperative levels, whereas V(AB)/V(T) increased but remained different from preoperative values. Conclusions: This study demonstrates that TEB produces an increase in diaphragmatic activity, identical for the two segments of the muscle. Interruption of afferents that produce an inhibitory effect on diaphragmatic activity appears the most attractive hypothesis to explain the consequences of TEB after UAS.