RESPIRATORY EFFECTS OF SEVOFLURANE USED IN COMBINATION WITH NITROUS-OXIDE AND SURGICAL STIMULATION

Study Objective: To evaluate the respiratory effects of sevoflurane anesthesia with and without nitrous oxide (N2O) during surgical stimulation. Design: Randomized study. Setting: Operating theater at a university hospital. Patients: 10 patients scheduled for minor head or neck surgery Interventions: Sevoflurane anesthesia was administered alone or in combination with N2O. After basal measurements were recorded, the following end-tidal anesthetic concentrations were administered: Group I = 1.3 minimum alveolar concentration (MAC) sevoflurane alone; Group 2 = 0.9 MAC sevoflurane with 0.4 MAC N,O; Group 3 = 1.5 MAC sevoflurane alone; Group 4 = 1.1 MAC sevoflurane with 0.4 MAC N2O. Measurements and Main Results: PaCO2, minute volume (V-E), respiratory rate, tidal volume (V-T), percentage of rib cage contribution to tidal volume (%RC), rate of inspiratory time in a breath cycle (T-I/T-tot, where T-I = inspiratory time and T-tot = tidal respiratory time), and mean inspired flow (V-T/T-I) were measured. The substitution of 0.4 MAC N2O for sevoflurane decreased PaCO2 and increased V-E with a consequent increase in V-T. At 1.3 MAC sevoflurane-N2O anesthesia, spontaneous respiration maintained PaCO2 at appropriate levels (42.7 +/- 3.6 mmHg). At 1.3 MAC sevoflurane alone and 1.5 MAC sevoflurane-N2O anesthesia, spontaneous respiration was moderately depressed. Sevoflurane and N2O combined did not change %RC or T-I/T-tot. Conclusion: Sevoflurane administered at an appropriate anesthetic depth maintained spontaneous respiration at acceptable levels during surgical stimulation, especially when combined with N2O.