Modifications of diaphragmatic activity induced by midline laparotomy and changes in abdominal wall compliance

a b s t r a c t Introduction and Objective. Diaphragmatic activity varies with the initial length of the muscle. Our objective was to evaluate the influence of surgery and changes in abdominal wall compliance on diaphragmatic activity. Methods. Both phrenic nerves in 7 mongrel dogs were stimulated electrically with single supramaximal pulses (twitch). The gastric (Pga) and transdiaphragmatic (Pdi) pressures generated and muscle shortening (sonomicrometry) were used to evaluate diaphragmatic activity, which was determined at baseline, after midline laparotomy, with an elastic abdominal bandage, and with a rigid circular cast. Abdominal pressure was then gradually increased in order to induce progressive lengthening of the diaphragm. Results. After laparotomy, the pressures were somewhat lower (by 12%) than at baseline. The elastic bandage produced a slight increase in the pressure generated by the diaphragm (mean (SE) values: Pga, from 4.2 (0.3) cm H2O to 6.3 (0.9) cm H2O, P<.01; Pdi, from 12.1 (2.0) cm H2O to 15.4 (1.8) cm H2O, P<.05)), and these values increased even further with the rigid cast (Pga, to 12.6 (1.5) cm H2O; Pdi, to 20.2 (2.3) cm H2O; P<.01 for both comparisons); this occurred despite smaller degrees of muscle shortening: by 57% (5%) of the initial length at functional residual capacity at baseline, by 49% (5%) with the bandage (P<.05), and by 39% (6%) with the cast (P<.01). With progressive lengthening of the muscle, its contractile efficacy increased up to a certain point (105% of the length at functional residual capacity), after which it began to decline. Conclusions. Abdominal wall compliance plays an important role in the diaphragmatic response to stimulation. This appears to be due mainly to changes in its length at rest.