Why is bag-mask ventilation performed after pre-oxygenation and before intubation?

Bag-Mask Ventilation After Pre-Oxygenation: Rationale and Evidence

Bag-mask ventilation with CPAP should be performed after pre-oxygenation and before intubation attempts to prevent alveolar de-recruitment, extend safe apnea time, improve oxygenation, and assess the ease of mask ventilation—particularly in patients with respiratory failure, obesity, or when hypercarbia is problematic. 1

Primary Physiological Rationale

Alveolar De-recruitment Prevention

• With the onset of apnea and neuromuscular blockade, alveolar de-recruitment occurs immediately and will lead to hypoxemia if untreated 1
• Even after optimal pre-oxygenation, critically ill patients experience rapid oxygen desaturation due to reduced functional residual capacity and increased oxygen consumption 1
• Bag-mask ventilation with CPAP (5-10 cm H₂O) counteracts this de-recruitment and maintains alveolar patency 1

Extension of Safe Apnea Time

• Facemask ventilation with CPAP may improve oxygenation and extend the safe apnea time beyond what pre-oxygenation alone provides 1
• In a comparative analysis, bag-mask ventilation was associated with a 4.2% higher lowest oxygen saturation compared to apneic oxygenation alone (95% CI: 0.7%-7.8%; P=0.02) 2
• The incidence of severe hypoxemia (SpO₂ <80%) was 6.6% with bag-mask ventilation versus 15.6% with apneic oxygenation alone 2

Specific Clinical Indications

High-Risk Populations Requiring Bag-Mask Ventilation

The British Journal of Anaesthesia guidelines explicitly recommend facemask ventilation with CPAP before attempting intubation in the following scenarios: 1

• Respiratory failure patients where hypoxia occurs or is likely to occur
• Obese patients who have reduced functional residual capacity and desaturate rapidly (as quickly as 2.5 minutes in supine position) 1, 3
• When hypercarbia is problematic, including patients with:
  • Metabolic acidosis
  • Raised intracranial pressure
  • Pulmonary hypertension 1

Assessment of Airway Management Difficulty

• Bag-mask ventilation before the first intubation attempt serves as a functional test to indicate the ease of facemask ventilation 1
• This assessment helps predict whether rescue oxygenation will be feasible if intubation attempts fail 1
• If facemask ventilation proves difficult, cricoid force should be reduced or removed, and a two-person technique with oral airway adjuncts should be employed 1

Technical Execution

Optimal Technique

• Use a tight-fitting facemask with a circuit capable of delivering CPAP (e.g., Waters circuit) 1
• Apply 5-10 cm H₂O CPAP if oxygenation is impaired 1
• Employ a two-person technique where the mask is held with two hands by one operator while a second compresses the bag 1
• High respiratory rates and volumes are rarely necessary and may cause hypotension or breath-stacking in patients with expiratory airflow limitation 1

Concurrent Oxygen Delivery

• Continue nasal oxygen at 15 L/min during bag-mask ventilation for additional apneic oxygenation 1
• Exercise caution with concomitant high-flow nasal oxygen during facemask ventilation, as this can result in dangerously high airway pressures with a tight-fitting mask 1

Common Pitfalls and How to Avoid Them

Cricoid Force Interference

• Inexpert cricoid force may obstruct the laryngeal inlet or upper airway and render both bag-mask ventilation and nasal oxygen ineffective 1
• Cricoid force should be reduced or removed if there is difficulty with facemask ventilation 1
• Gastric insufflation during mask ventilation is reduced by proper application of cricoid force (1 kg awake, increasing to 3 kg after loss of consciousness) 1

Equipment-Related Issues

• Some bag-valve-mask devices fail to deliver adequate FiO₂ in spontaneously breathing patients due to design variability 4
• Devices with a duckbill non-rebreather valve and without a dedicated expiratory valve perform worst 4
• Eight of 40 tested devices failed to deliver FiO₂ above 0.85, with three delivering FiO₂ below 0.55 4
• Clinicians must be aware of their specific BVM device characteristics and limitations 4

Mask Leak Complications

• Even with flush rate oxygen (40-60 L/min), bag-valve-mask performance is severely compromised by mask leaks, with mean FeO₂ dropping to 30% (95% CI: 26%-35%) 5
• A tight mask seal is essential—use of oral airway adjuncts and two-handed technique improves seal, particularly in obese patients 1

Modified Rapid Sequence Induction Context

Integration with RSI Protocol

• The British Journal of Anaesthesia guidelines emphasize a "modified" rapid sequence induction approach for critically ill patients that includes facemask ventilation with CPAP as a core component 1
• This differs from traditional RSI teaching that avoided any ventilation between induction and intubation 1
• The risk of pulmonary aspiration is reduced by cricoid force application, which also reduces gastric insufflation during mask ventilation 1

COVID-19 and Aerosol-Generating Procedures

• During the COVID-19 pandemic, guidelines recommended that gentle continuous positive airway pressure may be applied after reliable loss of consciousness to minimize the need for mask ventilation 1
• Bag-mask ventilation should be used to assist ventilation and prevent hypoxia if indicated, using minimal oxygen flows and airway pressures consistent with achieving this goal 1
• A second-generation supraglottic airway may be inserted after loss of consciousness to replace bag-mask ventilation if this is difficult 1

Between Intubation Attempts

Facemask ventilation with CPAP is specifically recommended between intubation attempts where hypoxia occurs or is likely to occur 1

• If facemask ventilation between intubation attempts is unsuccessful, rescue oxygenation using a second-generation supraglottic airway device may be required 1
• This represents Plan B/C in failed intubation algorithms 1