Mechanisms of bronchoprotection by anesthetic induction agents: propofol versus ketamine.

BACKGROUND: Propofol and ketamine have been purported to decrease bronchoconstriction during induction of anesthesia and intubation. Whether they act on airway smooth muscle or through neural reflexes has not been determined. We compared propofol and ketamine to attenuate the direct activation of airway smooth muscle by methacholine and limit neurally mediated bronchoconstriction (vagal nerve stimulation).
METHODS: After approval from the institutional review board, eight sheep were anesthetized with pentobarbital, paralyzed, and ventilated. After left thoracotomy, the bronchial artery was cannulated and perfused. In random order, 5 mg/ml concentrations of propofol, ketamine, and thiopental were infused into the bronchial artery at rates of 0.06, 0.20, and 0.60 ml/min. After 10 min, airway resistance was measured before and after vagal nerve stimulation and methacholine given via the bronchial artery. Data were expressed as a percent of baseline response before infusion of drug and analyzed by analysis of variance with significance set at P< or =0.05.
RESULTS: Systemic blood pressure was not affected by any of the drugs (P>0.46). Baseline airway resistance was not different among the three agents (P = 0.56) or by dose (P = 0.96). Infusion of propofol and ketamine into the bronchial artery caused a dose-dependent attenuation of the vagal nerve stimulation-induced bronchoconstriction to 26+/-11% and 8+/-2% of maximum, respectively (P<0.0001). In addition, propofol caused a significant decrease in the methacholine-induced bronchoconstriction to 43+/-27% of maximum at the highest concentration (P = 0.05)
CONCLUSIONS: The local bronchoprotective effects of ketamine and propofol on airways is through neurally mediated mechanisms. Although the direct effects on airway smooth muscle occur at high concentrations, these are unlikely to be of primary clinical relevance.