Responses of respiratory drive and breathing pattern to inspiratory loading during nitrous oxide and isoflurane sedation

Background: Increased inspiratory resistance in combination with mild gas narcosis is common during recovery after a general anesthesia, but there are only few previous studies on inspiratory loading during subanesthetic gas narcosis. Methods: Responses of respiratory drive (central inspiratory activity P 0.1 ) and ventilatory pattern to an inspiratory threshold load of –6 cm H 2 O were studied in 16 healthy subjects during mild subanesthetic gas narcosis. One group (n=9) was exposed to 13, 26 and 39% nitrous oxide (N 2 O) and air control (Group N). Another group (n=7) was exposed to 0.1, 0.2 and 0.3% isoflurane and air control (Group I). Measurements were done after 1 min adaptation to the load. Results: Nitrous oxide and isoflurane had no effect on respiratory drive and V T either during unloaded breathing or during inspiratory threshold loading. Across all gas concentrations (including 0% control), inspiratory threshold loading resulted in significant P 0.1 increases, amounting to 62% in group N and 38% in group I. At the same time V T decreased by 11 and 12%, respectively. A significantly increased end‐expired CO 2 and decreased minute volume compared to air control was found during isoflurane inhalation but could be ascribed to normalization of the hyperventilation in the control situation. Conclusions: It is concluded that the steady‐state ventilatory responses to loading, consisting of increased P 0.1 and decreased V T , are maintained during inhalation of subanesthetic doses of N 2 O (0.13–0.38 MAC) and isoflurane (0.09–0.26 MAC).