When should a non‑rebreather (reservoir) mask be used instead of a simple face mask for oxygen therapy?

Non-Rebreather (Reservoir) Mask vs Simple Face Mask

Use a non-rebreather (reservoir) mask at 15 L/min when initial SpO₂ is below 85% in patients not at risk of hypercapnic respiratory failure; otherwise, start with nasal cannulae (2-6 L/min) or a simple face mask (5-10 L/min) for less severe hypoxemia. 1

Primary Indication for Non-Rebreather Mask

The 2017 BTS guideline provides clear, algorithmic guidance on oxygen device selection based on severity of hypoxemia:

Severe Hypoxemia (SpO₂ <85%)

• Start immediately with reservoir (non-rebreather) mask at 15 L/min 1
• Target saturation: 94-98%
• This applies to:
  • Acute hypoxemia (cause not yet diagnosed)
  • Deterioration of lung fibrosis or interstitial lung disease
  • Pneumothorax requiring observation (aim for 100% saturation to accelerate clearance)

Moderate Hypoxemia (SpO₂ 85-94%)

• Start with nasal cannulae at 2-6 L/min (preferred) or simple face mask at 5-10 L/min
• Titrate to achieve target saturation of 94-98%
• Escalate to reservoir mask if target saturation cannot be maintained with these devices 1

Critical Illness Exception

In critical illness (sepsis, shock, anaphylaxis, major trauma), initiate treatment with reservoir mask at 15 L/min regardless of initial SpO₂ until reliable pulse oximetry is established 1. This is the preferred method for delivering high-concentration oxygen to critically ill patients 1.

Important Caveats for Non-Rebreather Masks

Flow Rate Requirements

Non-rebreather masks MUST be run at 10-15 L/min to function properly 2. Running them at lower flows (e.g., 4 L/min) creates serious risks:

• Inadequate oxygen delivery
• Increased risk of CO₂ rebreathing
• Can precipitate CO₂ narcosis, particularly in COPD patients 2

Patients at Risk of Hypercapnia

For patients with COPD or other risk factors for hypercapnic respiratory failure:

• Even with SpO₂ <85%, use caution with reservoir masks
• Target lower saturation range: 88-92% pending blood gas results
• Consider starting with 24-28% Venturi mask or nasal cannulae at 1-2 L/min instead 1
• Check arterial blood gases within 30-60 minutes 1

Duration of Use

Prolonged use (>2 hours) of non-rebreather masks in patients with respiratory failure due to pulmonary disease may increase mortality (HR 1.3) 3. Once stabilized, titrate down to lower-flow devices (nasal cannulae or simple mask) to maintain target saturation 1.

Practical Algorithm

1. Assess initial SpO₂ and clinical status
2. Determine hypercapnia risk (COPD, obesity hypoventilation, neuromuscular disease)
3. Select device:
   • SpO₂ <85% + no hypercapnia risk → Reservoir mask 15 L/min
   • SpO₂ <85% + hypercapnia risk → Venturi 24-28% or nasal cannulae 1-2 L/min (target 88-92%)
   • SpO₂ 85-94% → Nasal cannulae 2-6 L/min or simple mask 5-10 L/min
   • Critical illness → Reservoir mask 15 L/min regardless
4. Reassess within 5 minutes and adjust 1
5. Obtain arterial blood gas within 1 hour if requiring increased oxygen 1
6. Escalate to reservoir mask if target not achieved with nasal cannulae/simple mask
7. De-escalate once stable to avoid prolonged high-flow oxygen exposure

Key Pitfall to Avoid

Never run a non-rebreather mask at low flows (<10 L/min) thinking you're being cautious about oxygen delivery—this creates a dangerous situation where the patient receives inadequate oxygen AND risks CO₂ rebreathing 2. If concerned about hypercapnia, use a different device (Venturi mask or nasal cannulae at low flow) rather than a reservoir mask at inadequate flow.