Maximum PaO2 with 100% Oxygen
When breathing 100% oxygen, PaO2 can reach extremely high levels—typically 350-500 mm Hg or higher—but an oxygen saturation of 100% may correspond to a PaO2 anywhere between 80 and 500 mm Hg, making saturation alone an unreliable indicator of actual arterial oxygen tension. 1
Physiological Range on 100% Oxygen
In clinical studies of post-cardiac arrest patients ventilated with 100% oxygen, mean PaO2 values reached 345±174 mm Hg (range approximately 343-500 mm Hg) at 60 minutes after return of spontaneous circulation 1
Animal studies demonstrate that 100% oxygen can generate PaO2 values of 250-450 mm Hg during the first 10-60 minutes of ventilation 1
The specific PaO2 achieved depends on multiple factors including lung function, cardiac output, and ventilation-perfusion matching 2
Critical Clinical Implications
Why High PaO2 Matters
Hyperoxia (PaO2 >350 mm Hg) is associated with increased brain lipid peroxidation, metabolic dysfunction, neurodegeneration, and worse neurological outcomes in post-cardiac arrest patients 1
In cardiac arrest models, PaO2 of 250-350 mm Hg during reperfusion resulted in worse cardiac function compared to PaO2 of 40-90 mm Hg 1
The Saturation Paradox
A pulse oximetry reading of 100% saturation is dangerously misleading because it cannot distinguish between a safe PaO2 of 80 mm Hg and a potentially harmful PaO2 of 500 mm Hg 1
This is why guidelines recommend weaning FiO2 when saturation reaches 100%, provided saturation can be maintained ≥94% 1
Current Guideline Recommendations
Target Oxygen Levels
For most critically ill patients, target oxygen saturation should be 94-98%, which typically corresponds to PaO2 of approximately 60-100 mm Hg 1
In patients at risk of hypercapnic respiratory failure (COPD, obesity hypoventilation), target 88-92% saturation 1
Post-Resuscitation Care
After return of spontaneous circulation, titrate FiO2 to the minimum concentration needed to achieve arterial oxyhemoglobin saturation ≥94%, avoiding routine use of 100% oxygen beyond initial resuscitation 1
The 2010 American Heart Association guidelines specifically state this as Class I, Level of Evidence C recommendation 1
Special Circumstances
Carbon Monoxide Poisoning Exception
Carbon monoxide poisoning is the only common scenario where targeting 100% saturation and using 100% oxygen (via reservoir mask at 15 L/min) is appropriate, regardless of oximetry readings 1
This is because carboxyhaemoglobin produces falsely "normal" oximetry readings 1
Initial Resuscitation
- During active CPR and immediate post-arrest period, 100% oxygen via reservoir mask at 15 L/min is appropriate until reliable oximetry monitoring is established 1
Practical Monitoring Approach
Once a patient is stabilized, arterial blood gas measurement is essential when saturation is 100% to determine actual PaO2 and guide appropriate FiO2 weaning 1
Conservative oxygen therapy targeting PaO2 of 70-100 mm Hg (saturation 94-98%) reduced ICU mortality compared to conventional therapy allowing PaO2 up to 150 mm Hg 3
In the conservative oxygen group, median PaO2 was 87 mm Hg versus 102 mm Hg in the conventional group, with significantly lower mortality (11.6% vs 20.2%) 3