3-methylindole-induced pulmonary toxicosis in ponies

In unanesthetized ponies, arterial blood gas tensions, pulmonary mechanics, and lung volumes were determined before and 24 to 48 hours after oral administration of 500 ml of corn oil or 100 mg of 3-methylindole (3MI)/kg of body weight in 500 ml of corn oil. In the latter group, variables were also measured after bilateral cervical vagotomy. Respiratory rate and minute ventilation were increased by 3MI treatment and decreased after vagotomy, suggesting that the tachypnea induced by 3MI was vagally mediated. The arterial O2 tension (PaO2) was unaffected but arterial CO2 tension (PaCO2) decreased below base line after 3 MI and vagotomy. Specific respiratory conductance and dynamic compliance wee decreased by 3MI. Specific conductance increased again after vagotomy. Functional residual capacity, which increased after 3MI, was unaffected by vagotomy. Total lung capacity and quasistatic compliance were unaffected by either treatment. Minimal volume was larger in 3MI-treated ponies than in untreated ponies. Decreased dynamic compliance and specific respiratory conductance and increased functional residual capacity and minimal volume are all compatible with small airway obstruction produced by the necrotizing bronchiolitis and bronchiolar obstruction observed histologically in 3MI-treated ponies.