What FiO₂ should be targeted to achieve the desired oxygen saturation in an adult patient, taking into account normal lungs, chronic hypercapnic disease (e.g., COPD), and severe hypoxemic respiratory failure (e.g., ARDS, pneumonia, COVID‑19)?

Target FiO₂ for Oxygen Therapy in Adults

The target oxygen saturation—and therefore FiO₂—depends critically on whether the patient is at risk for hypercapnic respiratory failure: aim for SpO₂ 94–98% (FiO₂ as needed) in patients with normal lungs or severe hypoxemic respiratory failure, but target SpO₂ 88–92% (lower FiO₂) in patients with chronic hypercapnic disease such as COPD. 1

Patients with Normal Lungs or Acute Hypoxemic Respiratory Failure

Target Oxygen Saturation

• SpO₂ target: 94–98% in patients without risk of hypercapnia 1, 2
• This applies to pneumonia, ARDS, COVID-19, pulmonary embolism, pleural effusion, acute heart failure, myocardial infarction, and stroke 1
• Avoid hyperoxia (SpO₂ >96%) as multiple guidelines emphasize not exceeding 96% saturation 1

Initial FiO₂ Settings

• Start with FiO₂ 0.35–1.0 (35–100%) depending on severity of respiratory distress 3
• For severe hypoxemic respiratory failure on high-flow nasal cannula (HFNC): initiate at 40–60 L/min flow with FiO₂ titrated to achieve SpO₂ ≥92% 4
• For CPAP in COVID-19: start at 10 cmH₂O with FiO₂ 0.6, escalating to 12–15 cmH₂O with FiO₂ 0.6–1.0 if needed 1

Mechanical Ventilation Guidance

• Titrate PEEP and FiO₂ together using established tables (ARDSnet protocol) to achieve SpO₂ >90% while minimizing oxygen toxicity 1
• Target PaO₂/FiO₂ ratio >150; consider prone positioning if ratio remains <150 after 12 hours of optimization 1

Patients with Chronic Hypercapnic Disease (COPD, Obesity Hypoventilation, Neuromuscular Disease)

Target Oxygen Saturation

• SpO₂ target: 88–92% in all patients at risk of hypercapnic respiratory failure 1, 5
• This lower target prevents worsening hypercapnia and CO₂ narcosis 1, 5
• Risk factors include moderate-to-severe COPD, obesity, neuromuscular disease, cystic fibrosis, and chest wall disorders 2, 5

Initial FiO₂ Settings

• Start with 24% Venturi mask at 2–3 L/min or 28% Venturi mask at 4 L/min 5, 3
• For patients with known severe chronic hypercapnia (on home oxygen): begin at FiO₂ 0.24 (24%) even if renal compensation has occurred 3
• For patients in extremis with severe hypoxemia and acidosis: still start at 24% FiO₂ unless mechanically ventilated 3

Titration Strategy

• Obtain arterial blood gas within 30–60 minutes of starting oxygen to assess for respiratory acidosis 5
• If SpO₂ remains <88%, increase FiO₂ incrementally (e.g., 24% → 28% → 35%) but do not exceed SpO₂ 92% without blood gas confirmation 5, 3
• If pH <7.35 with PaCO₂ >6.0 kPa (45 mmHg), initiate non-invasive ventilation immediately while maintaining SpO₂ 88–92% through the NIV circuit 1, 5

Non-Invasive Ventilation with Oxygen

• When using NIV for acute hypercapnic respiratory failure, maintain SpO₂ 88–92% regardless of underlying cause 1
• Entrain oxygen as close to the mask as possible (not at the ventilator end) 1
• Optimize NIV settings (pressure support, PEEP) before increasing FiO₂ 1
• If oxygen flow >4 L/min is needed, use a ventilator with integral oxygen blender to avoid mask leak and trigger delays 1

Practical Oxygen Delivery Considerations

Nasal Cannula Limitations

• Each liter of oxygen adds approximately 4% to baseline FiO₂ of 21% 2
• At 7 L/min, estimated FiO₂ is 49% (range 44–50%) 2
• Do not exceed 6 L/min via standard nasal cannula due to nasal discomfort, unpredictable FiO₂, and potential harm 2
• For FiO₂ requirements near 50%, switch to Venturi mask at 40% or 60% for more precise delivery, or consider HFNC 2

Oxygen Toxicity Prevention

• 100% oxygen is safe for <6 hours; 70% oxygen is probably safe for 24 hours 3
• After 24 hours, 45% should be the approximate upper limit to FiO₂ 3
• Balance oxygen toxicity risk against hypoxemia severity 3

Critical Monitoring Requirements

Continuous Monitoring

• Pulse oximetry must be continuous to adjust FiO₂ to target range 2
• Arterial blood gas analysis is necessary if risk of hypercapnia exists or if clinical deterioration occurs 2, 5
• For patients on NIV, monitor continuously for the first 24 hours 5

Special Population Considerations

• In Black patients, SpO₂ 92% may correspond to PaO₂ as low as 49 mmHg; consider targeting SpO₂ 95% to ensure adequate oxygenation 6
• In pregnant patients with COVID-19: aim for SpO₂ 92–95% 1
• In sickle cell crisis: target SpO₂ 94–98% or the patient's usual baseline saturation 1

Common Pitfalls

• The PaO₂/FiO₂ ratio varies with FiO₂ level; always specify the FiO₂ at which it was measured when documenting acute lung injury severity 7
• Avoid rebreathing from paper bags for hyperventilation—this is dangerous and not advised 1
• In paraquat or bleomycin poisoning: give oxygen only if SpO₂ <85%, and reduce if SpO₂ rises >88% to avoid toxicity 1