Intermittent CPAP: a new mode of ventilation during general anesthesia

Airway pressure-release ventilation provides ventilation comparable to controlled mechanical ventilation (CMV), but with lower peak airway pressures and less dead-space 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 (PETCO2) 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 t-test.

Peak airway pressure (13+/-2 vs. 23+/-5 cm H2O; P < 0.001) and minute ventilation (3.5+/-1 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 x l(-1) x 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)CO2. Compared with CMV, CPAPI also improves the accuracy of PETCO2 as a monitor of PaCO2.