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Airway pressure-release ventilation provides ventilation comparable to controlled mechanical ventilation (CMV), but with lower peak airway pressures and less deadspace ventilation. To obtain these advantages for patients administered general anesthesia, the authors (1) designed a mode similar to airway pressure-release ventilation, intermittent continuous positive airway pressure (CPAPI), and compared its efficiency with that of CMV; and (2) assessed the accuracy of end-tidal carbon dioxide tension (PET (CO)2) as a monitor of the partial pressure of carbon dioxide in arterial blood (PaCO2) during CPAPI compared with during CMV.Twenty anesthetized, tracheally intubated patients received baseline CMV that produced a PETCO2 of approximately 35 mmHg and a pulse oximetry value > 90%. Patients were assigned to undergo alternating trials of CMV and CPAPI. During CPAPI, CPAP was applied to the airway, removed for 1 s, and reapplied at a rate equal to the ventilator rate during CMV. The difference between the carbon dioxide tension in arterial blood and end-tidal gas [P(a - ET)CO2] and the calculation of PaCO2/minute ventilation quantified the efficiency of ventilation. Data were summarized as mean +/- SD and compared using the Student's test.Peak airway pressure (13 +/- 2 vs. 23 +/- 5 cm H2 O; P < 0.001) and minute ventilation (3.5 +/- vs. 4.6 +/- 1.2 l/min; P < 0.0001) were lower during CPAPI than during CMV. The value for PaCO2/minute ventilation (11.1 +/- 2.9 vs. 7.9 +/- 2.6 mmHg [middle dot] 1-1 [middle dot] min-1; P < 0.0001) was greater during CPAPI. P(a - ET)CO2 was always greater during CMV (6.3 +/- 1.6 vs. 1.7 +/- 0.9 mmHg; P < 0.0001) and was never > 3.5 mmHg during CPAPI.During CPAPI, less ventilation was necessary to produce a PaCO2 comparable to that during CMV. This represents a significant reduction in dead-space ventilation, improved efficiency of ventilation, and a lower value for P(a - ET)CO. Compared with CMV, CPAPI also improves the accuracy of PETCO2 as a monitor PaCO2.