Central contribution to hypoventilation during severe inspiratory resistive loads

Recent observations suggest that central hypoventilation with slowing of respiratory frequency contributes to hypoventilation during severe inspiratory resistive loads. We carried out a study to further characterize this bradypneic response. DESIGN:Prospective, controlled laboratory study. SETTING:Basic science laboratory of a university hospital. SUBJECTS:Anesthetized adult cats (loading experiment n = 7, nonloaded hypercapnic controls n = 11). INTERVENTIONS:Experimental inspiratory loads increased transdiaphragmatic pressure to 75% of the maximum for each animal. Respiratory responses were observed at midrun or moderate conditions of respiratory insufficiency (defined as Paco2 ≤60 torr [≤8.0 kPa]) and failure (Paco2 ≤80 torr [≤10.6 kPa]). Nonloaded hypercapnic controls were studied with similar durations of exposure to CO2 in the same CO2 range. MEASUREMENTS:Inspiratory airflow, tidal volume, respiratory frequency, airway pressure, transdiaphragmatic pressure, transdiaphragmatic pressure response to phrenic nerve electrical stimulation, blood gas analysis. Severe inspiratory loads were applied to anesthetized adult cats to determine whether bradypnea could be observed in an anesthetized model that eliminated conscious responses. Experiments were performed in hyperoxic conditions to determine whether bradypnea develops in the absence of hypoxia. An additional control group was studied under hypercapnic conditions without loading to determine whether comparable hypercapnia is a sufficient stimulus to elicit bradypnea. RESULTS:From midrun until failure, minute ventilation decreased by 16% in loaded animals. Hypoventilation was associated with a decrease in respiratory frequency from 40.1 to 29.9 breaths/ min, whereas tidal volume, spontaneous transdiaphragmatic pressure, and transdiaphragmatic pressure response to phrenic nerve electrical stimulation remained unchanged. Control animals had no significant reduction in ventilation or respiratory frequency over similar levels and durations of hypercapnia. CONCLUSIONS:Centrally mediated bradypnea contributed to hypoventilation in respiratory failure associated with inspiratory loading. Bradypnea preceded evidence of muscle fatigue. This change in respiratory cycle timing occurred under anesthesia, and thus, did not depend on conscious perception of dyspnea. Bradypnea does not depend on either hypercapnia or hypoxia. (Crit Care Med 1993; 21:1915–1922)