S177 INTRODUCTION: During general anesthesia, most patients have their ventilation controlled mechanically, in spite of touted advantages of spontaneous breathing. Further, modes of partial ventilatory support ubiquitous in the ICU, have gained little popularity in the operating room. We developed methodology to augment gas exchange of spontaneously breathing, anesthetized patients. METHODS: Twenty-seven adult patients (52 +/- 16 yrs; 75 +/- 16 kg) scheduled to undergo inhalation anesthesia signed a consent form approved by the Institutional Review Board. Anesthesia was induced with IV propofol, a laryngeal mask airway was positioned, and patients were allowed to breathe spontaneously. Anesthesia was maintained with isoflurane, nitrous oxide, oxygen and air. A flow transducer and gas sampling tube were connected to a monitor for determination of respiratory rate (RR), tidal volume (VT), minute ventilation (VE), inspired oxygen concentration, end-tidal carbon dioxide tension (PETCO2) and concentration of anesthetic agents. Heart rate (HR), mean arterial blood pressure (MAP), and SpO2 (%) were recorded. Baseline data were collected after surgical incision and patients breathed spontaneously for 15-min. They then randomly were assigned to undergo alternating 15-min trials of CPAP, or Apneustic Anesthesia Ventilation (AAV). CPAP was titrated to a level that, when released to atmospheric pressure, would produce a VT of 6 mL/kg body weight. The rate of AAV was titrated to reduce spontaneous breathing to [approximate]30% of the baseline value. Data are summarized as mean +/- SD. Data obtained during AAV and CPAP were compared using Student's t test for paired observations and were compared to baseline with Dunnett's test. RESULTS: End-tidal concentration of isoflurane (1.1 +/- 0.4%) and N2 O (62 +/- 11%), FIO2 (0.33 +/- 0.07), HR (73 +/- 14 /min), MAP (78 +/- 16 mmHg) and SpO2 (97 +/- 2%) statistically were comparable throughout the study. Total RR and E during AAV were less than baseline values (P<.001), but VT was significantly greater when generated by decrease in airway pressure than by spontaneous breathing during AAV. Movement of the surgical field was barely detectable. Variables reflecting respiratory function during baseline (Base), CPAP (9 +/- 2 cmH2 O) and AAV (Spont=spontaneous breath, Mech=mechanical breath) are summarized in Table 1 (*P<.001 v. Base; [dagger]P<.001 v. CPAP).Table 1DISCUSSION: Application of AAV in anesthetized, spontaneous breathing patients reduced respiratory rate and minute ventilation, but not alveolar ventilation. Breathing was facilitated with a peak airway pressure much lower than observed during traditional intermittent positive pressure ventilation. We conclude that AAV provides an efficient means of supplementing spontaneous breathing with minimal disruption of the surgical field. Therefore, AAV may be useful in patients who require general anesthesia, who might benefit from lack of paralysis and/or hyperventilation, and who are deemed to benefit from partial ventilatory support.