Effect of tidal volume and anesthetic agent on airway responsiveness to histamine

Dose-response relationships for bronchoconstriction in response to aerosal histamine were assessed before and after vagotomy in 11 dogs anesthetized with barbiturates and in 9 dogs anesthetized with alpha-chloralose-urethan. The dose-response relationships following vagotomy were assessed during spontaneous ventilation and during muscular paralysis and mechanical ventilation with tidal volume (VT) similar to each animal's VT prior to vagotomy. After vagotomy the spontaneous VT of both groups increased but the VT of the alpha-chloralose-urethan group was significantly less than that of the barbiturate group. The histamine responsiveness of the animals anesthetized with barbiturates was significantly greater during mechanical ventilation when VT was reduced to prevagotomy levels compared with during spontaneous ventilation. In contrast, the histamine responsiveness of the alpha-chloralose-urethan group was not significantly changed by reducing VT to prevagotomy levels. In six other dogs anesthetized with pentobarbital sodium and studied after vagotomy, responsiveness to histamine aerosol during controlled ventilation with breaths of prevagotomy VT was greater than responsiveness during mechanical ventilation with large volume breaths given immediately afterward. Thus the magnitude of VT of dogs after vagotomy may influence airway responsiveness, and the influence of anesthetic agents on airway responsiveness after vagotomy may in part be due to their effects on VT. Furthermore, bronchodilation accompanying large volume ventilation persists after vagotomy, suggesting that it is not exclusively mediated by changes in parasympathetic activity.