Is a PaO2 (partial pressure of arterial oxygen) goal of 55-80 mmHg adequate for patients with Acute Respiratory Distress Syndrome (ARDS)?

PaO2 Goal of 55-80 mmHg in ARDS Management

A PaO2 goal of 55-80 mmHg is appropriate for most ARDS patients, but targeting the higher end of this range (70-90 mmHg) may be safer based on the most recent evidence. The most recent guidelines from the American Thoracic Society recommend maintaining PaO2 between 70-90 mmHg or SaO2 between 92-97% to balance the risks of hypoxemia and hyperoxia 1.

Evidence for Oxygenation Targets in ARDS

Current Recommendations

• The National Heart, Lung, and Blood Institute ARDS Clinical Trials Network has historically recommended a target PaO2 between 55-80 mmHg 2
• More recent guidelines from the American Thoracic Society recommend slightly higher targets of 70-90 mmHg or SaO2 92-97% 1
• These targets aim to balance the risks of hypoxemia against the potential harms of hyperoxia

Safety Considerations

The LOCO2 trial (2020) compared conservative oxygen therapy (PaO2 55-70 mmHg) with liberal oxygen therapy (PaO2 90-105 mmHg) and was stopped early due to safety concerns 2:

• At 90 days, mortality was higher in the conservative-oxygen group (44.4% vs 30.4%)
• Five mesenteric ischemic events occurred in the conservative-oxygen group
• This suggests that targeting the very low end of the 55-80 mmHg range may increase risk

ARDS Severity-Based Approach

Oxygen targets should be considered in the context of ARDS severity:

Severity	PaO2/FiO2	Recommended Management
Mild	201-300 mmHg	• Target PaO2 70-90 mmHg • Lower PEEP (5-10 cmH2O)
Moderate	101-200 mmHg	• Target PaO2 70-90 mmHg • Higher titrated PEEP
Severe	≤100 mmHg	• Target PaO2 70-90 mmHg • Higher titrated PEEP • Consider prone positioning • Consider neuromuscular blockade

Implementation of Oxygenation Strategy

1. Ventilation Strategy:

   • Use low tidal volume ventilation (6 mL/kg predicted body weight) 3
   • Maintain plateau pressures ≤30 cmH2O 3
   • Adjust PEEP based on ARDS severity 3

2. For Severe ARDS (PaO2/FiO2 ≤100 mmHg):

   • Implement prone positioning for >12 hours/day 3, 1
   • Consider neuromuscular blockade for ≤48 hours 3, 1
   • Use higher PEEP strategy with careful hemodynamic monitoring 3

3. For Moderate ARDS (PaO2/FiO2 101-200 mmHg):

   • Consider higher PEEP strategy 3, 1
   • Monitor for need to escalate therapy if oxygenation worsens

4. For Mild ARDS (PaO2/FiO2 201-300 mmHg):

   • Use low PEEP strategy (defined as <10 cmH2O) 3
   • Consider conservative fluid management 3, 1

Monitoring and Adjustment

• Regularly assess arterial blood gases to monitor PaO2 levels and calculate PaO2/FiO2 ratio 1
• Consider standardized ventilatory settings when assessing PaO2/FiO2 ratio for accurate classification 4
• Monitor for signs of tissue hypoxia (lactate elevation, organ dysfunction) when targeting lower PaO2 values
• Adjust FiO2 and PEEP to maintain target PaO2, prioritizing FiO2 reduction when possible

Potential Pitfalls

1. Targeting excessively low PaO2 values:

   • The LOCO2 trial suggests increased mortality with PaO2 55-70 mmHg 2
   • Mesenteric ischemia risk increases with lower oxygenation targets

2. Excessive PEEP to achieve oxygenation targets:

   • High PEEP can impede venous return and cardiac preload 3
   • Particularly problematic in patients with vasodilated states

3. Ignoring patient-specific factors:

   • Patients with chronic hypoxemia may tolerate lower PaO2 values
   • Patients with cardiac or cerebrovascular disease may require higher targets

In conclusion, while the historical range of 55-80 mmHg is reasonable, current evidence suggests targeting the higher portion of this range (70-80 mmHg) or slightly above (up to 90 mmHg) may provide a better balance of risks and benefits in ARDS management.