Inhibitory effects of CO2 on airway defensive reflexes in enflurane-anesthetized humans

We investigated responses of respiration, blood pressure, and heart rate to tracheal mucosa irritation induced by injection of distilled water at three different levels of CO2 ventilatory drive in 11 spontaneously breathing female patients under a constant depth of enflurane anesthesia [1.1 minimum alveolar concentration (MAC)]. The airway irritation at the resting level of spontaneous breathing caused a variety of respiratory responses such as coughing, expiration reflex, apnea, and spasmodic panting, with considerable increases in blood pressure and heart rate. Although the latency of respiratory responses after water injection was much shorter than those of blood pressure and heart rate responses, blood pressure and heart rate responses, once elicited, were prolonged much longer than was the respiratory response. An increase in CO2 ventilatory drive decreased the degree and duration of respiratory, blood pressure, and heart rate responses to the airway irritation, whereas a decrease in CO2 ventilatory drive had the opposite effect on these responses. Our results indicate that changes in CO2 ventilatory drive can modify reflex responses of respiration, blood pressure, and heart rate to airway irritation.