Purpose of Pre-oxygenation in Anesthesia
Pre-oxygenation is performed to increase oxygen reserves in the lungs and prevent life-threatening arterial oxygen desaturation during the apneic period of anesthetic induction and intubation attempts. 1
Primary Mechanism and Goal
Pre-oxygenation works by replacing alveolar nitrogen with oxygen (denitrogenation), creating an intrapulmonary oxygen reservoir that extends the safe apnea time—defined as the time until arterial saturation drops to 88-90%. 1, 2
- In healthy adults breathing room air, desaturation to SpO2 90% occurs within only 1-2 minutes of apnea 1
- With effective pre-oxygenation, this safe apnea time extends to 6-8 minutes, providing critical additional time to secure the airway 1, 3
- Without pre-oxygenation, even ASA I patients experience arterial oxygen desaturation (SpO2 <90%) in 30-60% of cases 1
The target is achieving an end-tidal oxygen fraction (FeO2) ≥90%, which represents adequate lung denitrogenation and optimal oxygen reserves. 1
Critical Importance for Patient Safety
Inability to adequately control airways is frequently associated with arterial oxygen desaturation and represents a major cause of anesthetic morbidity and mortality. 1
- The UK's NAP4 audit revealed that difficult or failed intubation represented 39% of incidents related to airway control 1
- Hypoxemia during induction remains a major cause of preventable anesthetic mortality 1
- By increasing oxygen reserves and prolonging apnea tolerance, pre-oxygenation prevents hypoxemia during intubation attempts 1
Physiologic Rationale
The rate of arterial desaturation during apnea depends on three main factors: 3
- Volume of oxygen stored in the lungs (functional residual capacity)
- Mixed venous oxygen saturation
- Presence of intrapulmonary shunt
These factors explain why certain populations desaturate more rapidly and require optimized pre-oxygenation techniques. 3
High-Risk Populations Requiring Enhanced Pre-oxygenation
Obese patients have reduced functional residual capacity and increased oxygen consumption, leading to desaturation in as little as 2.5 minutes when supine. 4, 2 They benefit from 25-30° head-up positioning, which increases functional residual capacity and extends safe apnea time by approximately 30%. 1, 4
Pregnant women have decreased functional residual capacity and increased metabolic demands. 1 A 2-minute pre-oxygenation period is adequate for term pregnant women, though most anesthetists use ≥3 minutes even during category-1 cesarean sections. 1
Critically ill patients often have reduced lung compliance, atelectasis, and shunt, making them particularly vulnerable to rapid desaturation. 2 These patients may require non-invasive positive pressure ventilation (NIPPV) with CPAP 5-10 cmH2O during pre-oxygenation to prevent critical desaturation. 4
Technical Requirements for Effective Pre-oxygenation
A tight mask-to-face seal is essential to reduce air entrainment and achieve adequate denitrogenation. 1
- Fresh gas flow rate of ≥10 L/min is required for effective denitrogenation 1
- Breath-by-breath oxygen monitoring should be corroborated with capnography, as erroneous FeO2 values may occur due to apparatus deadspace 1
- Absence of a capnograph trace indicates significant mask leak and inadequate pre-oxygenation 4
Common Pitfalls to Avoid
Failure to position the patient head-up is the most common error, particularly in obese patients where supine positioning dramatically reduces apnea tolerance. 4 A 20-30° head-up position should be standard practice as it increases functional residual capacity, improves laryngoscopy view, and may reduce gastro-esophageal reflux. 1, 4
Inadequate mask seal negates pre-oxygenation efforts, which is particularly problematic in edentulous or bearded patients. 5 Always verify adequate seal by checking for a capnograph waveform. 4
Using vital capacity maneuvers instead of tidal volume breathing is inferior for most patients, as 3 minutes of tidal volume breathing or 8 deep breaths in 60 seconds are more effective techniques. 4, 5