To discover if reducing respiratory system impedance would increase tidal volume and improve ventilation during inhalation anesthesia.

Prospective, randomized cross-over study.

Nine ASA physical status I and II adult female oncology patients undergoing breast operations with or without lymph node dissection and general anesthesia while breathing spontaneously.

Patients underwent alternating trials of constant positive airway pressure, with or without pressure support. Constant positive airway pressure and pressure support were titrated to maximize respiratory system compliance and equal inspiratory pressure gradient across tracheal tube, respectively. Variables reflecting cardiovascular function, pulmonary mechanics and lung gas exchange, and respired gases and isoflurane concentrations were measured.

End-tidal concentration of isoflurane (1.3 ± 0.2%), Fio₂ (0.43 ± 0.09 ), and CO₂ elimination (209 ± 42 mL min⁻¹) was unchanged throughout study in patients aged 63 ± 12 years, weighing 72 ± 12 kg. Constant positive airway pressure (12 ± 2 cm H₂O) increased respiratory system compliance from 52 ± 8 to 80 ± 9 mL cm H₂O⁻¹ (P < .001), tidal volume from 156 ± 32 to 325 ± 52 mL (P < .001), and minute ventilation from 4.37 ± 0.86 to 6.18 ± 0.92 L min⁻¹ (P < .001). Respiratory rate decreased from 29 ± 7 to 19 ± 2 min⁻¹ (P < .001), Paco₂ decreased from 54 ± 8 to 44 ± 6 mm Hg (P < .001), and Pao₂ increased from 137 ± 37 to 160 ± 64 mm Hg (P < .001). Pressure support (3.1 ± 0.3 cm H₂O) did not alter ventilation or gas exchange.

We conclude that constant positive airway pressure titrated to optimal respiratory system compliance will increase efficiency of inspiratory muscles and improve ventilation. Constant positive airway pressure facilitates a pattern of breathing that minimizes some of the adverse pulmonary effects of inhalation anesthesia.