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* Apnea * Deteriorating Alveolar Ventilation 
- pH < 7.25 
- Rising PaCO2: in appropriate clinical setting, especially with falling pH; PaCO2 > 60 mm Hg; * Inadequate oxygenation: (Pul. Edema, Consilidation, Atelectasis); 
- Falling PaO2 
- PaO2 < 70 mm Hg on 50% O2 by mask; 
- PaO2 < 55 mm on room air in the appropriate clinical setting