Vagal control of end-expiratory lung volume in anaesthetized dogs

We have studied the mechanism underlying the increase in end-expiratory lung volume (VEEL) after vagotomy in dogs anaesthetized with thiopentone and gluco-chloralose. Dogs (n = 10) were studied during three phases: a) baseline, b) after bilateral vagotomy, and c) during paralysis with suxamethonium after vagotomy. To examine the influence of posture, dogs were randomly studied either in the upright (n = 5) or in the supine (n = 5) position. After vagotomy, VEEL, as determined by spirometry, increased by 27.8 +/- 13.6% (SD) and 15.3 +/- 9.6% in upright and supine dogs, respectively. After paralysis, further small increases in VEEL were observed in all upright and three supine dogs. Static lung compliance did not change after vagotomy or paralysis. Chest wall compliance decreased after vagotomy, but returned towards baseline values during paralysis. The rectus abdominis electromyogram recorded expiratory muscle activity during the expiratory pause; vagotomy markedly reduced it in upright dogs and it was undetectable in supine dogs. We conclude that VEEL is actively maintained by expiratory muscles, predominantly under reflex vagal control. This reflex may serve to minimize the increase in VEEL that occurs on assuming the upright posture. We suggest that reflex vagal mechanisms are also involved in the inhibition of expiratory muscle activity during lung inflation.