What is the role of FiO2 (fraction of inspired oxygen), PEEP (positive end-expiratory pressure), and flow rate in maintaining adequate oxygenation in a patient with respiratory distress?

Optimizing Oxygenation in Respiratory Distress: The Role of FiO2, PEEP, and Flow Rate

The optimal approach to maintaining adequate oxygenation in respiratory distress requires careful titration of FiO2, PEEP, and flow rate, with PEEP selection based on gas exchange, hemodynamic status, lung recruitability, and driving pressure, while targeting oxygen saturation of 92-97% or PaO2 of 70-90 mmHg. 1

FiO2 (Fraction of Inspired Oxygen)

• FiO2 is the primary method to increase oxygen delivery to the lungs and should be titrated to maintain arterial oxygen saturation between 92-97% or PaO2 between 70-90 mmHg 1, 2
• High FiO2 (>0.60) should be avoided when possible due to potential oxygen toxicity and oxidative lung injury 1, 2
• FiO2 should be reduced as soon as oxygenation improves, with PEEP adjustments made simultaneously to maintain adequate oxygenation 1, 3
• In severe hypoxemia, initial high FiO2 may be necessary but should be reduced as soon as possible with appropriate PEEP optimization 1

PEEP (Positive End-Expiratory Pressure)

• PEEP improves oxygenation by recruiting collapsed alveoli, improving lung homogeneity, and reducing atelectrauma from repeated opening and closing of alveoli 1

• Higher PEEP levels (10-15 cmH2O) are recommended for moderate to severe ARDS (PaO2/FiO2 ≤ 200 mmHg) 1

• PEEP selection should be individualized based on:

  • Gas exchange response (improvement in oxygenation) 1
  • Hemodynamic stability (maintaining MAP ≥ 65 mmHg) 3
  • Lung recruitability (potential for alveolar recruitment) 1
  • End-expiratory transpulmonary pressure 1
  • Driving pressure (keeping ≤15 cmH2O when possible) 1

• Recruitment maneuvers (transient increase in inspiratory pressure to 40-45 cmH2O) before PEEP selection may improve oxygenation in selected patients, though routine use is not associated with reduced mortality 1

• For most patients, the therapeutic range of PEEP is relatively narrow, making the ARDS Network PEEP/FiO2 tables a reasonable approach 4

• Consider esophageal pressure measurement to guide PEEP settings in patients with abnormal chest wall compliance 1

Flow Rate

• High-flow nasal cannula (HFNC) oxygen therapy with flow rates of 50-60 L/min can be beneficial in acute hypoxemic respiratory failure 1

• HFNC provides several physiological benefits:

  • Matches high inspiratory demands of dyspneic patients 1
  • Delivers reliable FiO2 up to 100% 1
  • Provides low levels of PEEP in upper airways 1
  • Decreases work of breathing 1
  • Reduces dead space through carbon dioxide washout 1
  • Provides reliable humidification 1

• In non-intubated patients with acute hypoxemic respiratory failure, HFNC may reduce intubation rates compared to conventional oxygen therapy or NIV 1

• For mechanically ventilated patients, inspiratory flow should be set to match patient demand and minimize work of breathing 2

Integrated Approach to Oxygenation Management

Initial Management

1. For mild respiratory distress:

   • Start with conventional oxygen therapy or HFNC 1
   • Target SpO2 92-97% or PaO2 70-90 mmHg 1, 2
   • Monitor closely for deterioration 1

2. For moderate to severe respiratory distress requiring mechanical ventilation:

   • Implement lung-protective ventilation with low tidal volumes (4-8 ml/kg predicted body weight) 1, 2
   • Set initial PEEP based on severity of hypoxemia:
     • Mild ARDS (PaO2/FiO2 200-300): PEEP 5-10 cmH2O 1, 5
     • Moderate ARDS (PaO2/FiO2 100-200): PEEP 10-15 cmH2O 1, 5
     • Severe ARDS (PaO2/FiO2 <100): PEEP >15 cmH2O 1, 5

Titration Strategy

1. Start with appropriate PEEP and FiO2 combination based on severity of hypoxemia 1, 3
2. Perform recruitment maneuver if indicated (moderate to severe ARDS with evidence of recruitability) 1
3. Adjust PEEP and FiO2 to maintain target oxygenation while monitoring:
   • Plateau pressure (keep ≤30 cmH2O) 1, 3
   • Driving pressure (keep ≤15 cmH2O) 1
   • Hemodynamic stability (MAP ≥65 mmHg) 3
4. Consider PEEP/FiO2 titration tables as used in the ARDS Network trials for systematic adjustment 4, 6

Special Considerations

• PEEP and inhaled nitric oxide have synergistic effects on oxygenation; optimizing PEEP may convert non-responders to responders for inhaled nitric oxide therapy 7

• Standardizing ventilatory settings (PEEP ≥10 cmH2O and FiO2 ≥0.5) when measuring PaO2/FiO2 ratio improves risk stratification and clinical decision-making 5

• For patients with high PEEP requirements, monitor for potential adverse effects including:

  • Hemodynamic compromise from decreased venous return 3
  • Barotrauma (pneumothorax, pneumomediastinum) 1
  • Overdistension of non-dependent lung regions 1

• In patients with severe ARDS not responding to conventional management, consider:

  • Prone positioning for >12 hours per day 1, 2
  • Neuromuscular blockade 1
  • Extracorporeal membrane oxygenation (ECMO) 1