The effect of anesthesia on abdominal muscle resting length and shortening in awake dogs.

The objectives of this study were to examine the effects of anesthesia and implantation of sonomicrometer transducers on tonic and phasic expiratory activity of the abdominal muscles. Eight tracheotomized dogs were chronically instrumented with sonomicrometer transducers placed in each of the four abdominal muscles. The dogs were studied in the lateral decubitus position immediately after transducer implantation, while under halothane anesthesia, and then in the awake dogs 2 to 3 days postimplantation, and repeatedly over a 2- to 8-week period. The resting length (LRL) of the rectus abdominis (RA) was reduced in the first awake study compared to during anesthesia, but there was no change in LRL of the other abdominal muscles. The abdominal muscles phasically, actively, shortened during expiration more in the awake state than in the anesthetized state. The internal abdominal muscle layer (transversus abdominis and internal oblique) shortened phasically more than the external layer (RA and external oblique). Neither the LRL nor the amount of phasic shortening of the abdominal muscles changed significantly over the period in which transducers were in situ (2-8 weeks). Muscle sections excised at the end of each study exhibited small capsules of fibrosis immediately surrounding the transducers with normal muscle tissue between pairs of transducers. In conclusion, both tonic and phasic respiratory activity of the abdominal muscles, reflected by changes in resting length and the amount of active shortening, respectively, were absent during halothane anaesthesia and chronic implantation of sonomicrometer transducers per se had no effect.