Preoxygenation & Apneic Oxygenation 

1. Introduction to Preoxygenation

Preoxygenation is a critical step in airway management aimed at increasing oxygen reserves before intubation. It involves replacing nitrogen in the lungs with oxygen, thereby creating an oxygen reservoir that extends the time to critical desaturation during apnea.

🔹 Why is Preoxygenation Important?

• Prevents hypoxemia-related cardiac arrest
• Increases the “safe apnea time” (time before SpO₂ drops below 90%)
• Essential for difficult airway, critically ill, obese, and pediatric patients

2. Physiology of Preoxygenation

• The lungs normally contain a fraction of nitrogen (~79%), which does not contribute to oxygenation.
• Preoxygenation with 100% O₂ replaces nitrogen in the alveoli, increasing functional residual capacity (FRC)as an oxygen reservoir.
• Oxygen continues to diffuse into the bloodstream even in the absence of active breathing (apneic oxygenation).

🔹 Safe Apnea Time in Different Populations

3. Preoxygenation Techniques

1. 8 deep breaths of full vital capacity at an oxygen flow rate of 10 L/min within  60 seconds 
2. 3 minutes of tidal volume breathing at 5 L/min of oxygen flow
3. 4 Vital Capacities Method in 30 sec(not preferred)
4. Transnasal Humidified Rapid Insufflation Ventilator Exchange (THRIVE)

endpoint is to  achieve ETO2 > 90% (100% is not achievable due to the presence of CO2 and water vapour). 

1. Concept of Apneic Oxygenation

preoxygenated patient the safe apnea time = 8 or 9 minutes, safe apnea time on room air ~1 min

Apneic oxygenation (AO) refers to the passive movement of oxygen from the upper airway to the alveoli due to the pressure gradient created by oxygen consumption.

🔹 Even in apnea, alveoli continue to take up oxygen (due to continuous diffusion), allowing oxygenation to persist despite the absence of active ventilation.

🔹 The process relies on Fick’s principle, which states that as long as oxygen is available in the alveoli, it will diffuse into pulmonary capillaries.

📌 Example: Even in complete apnea, oxygenation can continue for several minutes if high-flow nasal oxygen (HFNO) is provided.

🔹 Key Concept: The negative pressure generated by alveolar gas uptake allows fresh oxygen to flow down from the pharynx to the alveoli even without active respiration.

📌 Key Point: CO₂ does not diffuse out as efficiently as O₂ → leading to gradual respiratory acidosis if prolonged.

2. Clinical Applications of Apneic Oxygenation

🔹 A. Difficult Airway Management

• Used in rapid sequence intubation (RSI) and anticipated difficult airway cases.

• Prevents rapid desaturation in patients with low functional residual capacity (FRC) (e.g., obesity, pregnancy, pediatric, critical illness).

🔹 B. Trauma and Emergency Airway Management

• Reduces hypoxia-related cardiac arrest during crash intubations.

• Especially useful in TBI, hypovolemic shock, and facial trauma.

🔹 C. Procedural Sedation and Anesthesia

• Used in sedated patients (e.g., bronchoscopy, endoscopic procedures, awake craniotomies).

• Extends the safe apnea period in induction of anesthesia.

🔹 D. ICU and Critical Care Transport

• Prevents desaturation in ventilator disconnections (e.g., ECMO, ARDS patients).

4. Methods of Apneic Oxygenation

📌 Key Point: High-Flow Nasal Oxygen (HFNO) is the most effective method to prolong safe apnea time in trauma and critical patients.

THRIVE (Transnasal Humidified Rapid-Insufflation Ventilatory Exchange)

1. Introduction

THRIVE is an advanced apneic oxygenation technique using high-flow nasal oxygen (HFNO) to maintain oxygenation and extend safe apnea time during airway management, particularly in difficult intubation scenarios and physiologically compromised patients.

✅ Key Concept:

• High-flow humidified oxygen (up to 70 L/min) is delivered via a nasal cannula.
• Generates positive airway pressure (PEEP) and flushes CO₂, prolonging apnea tolerance.
• Allows continuous oxygenation even during laryngoscopy and apnea.
• THRIVE extends apnea time up to 17 minutes in optimized conditions.
• Useful for critically ill, obese, pediatric, and difficult airway patients.

• High-flow oxygen flushes out CO₂ from the airway, reducing hypercapnia buildup.
• Generates a mild continuous positive airway pressure (CPAP/PEEP effect) (~3–7 cmH₂O).
• Prevents airway dryness and bronchoconstriction.
• Enhances mucociliary function and reduces airway resistance.
• THRIVE provides continuous oxygenation throughout apnea(during induction and intubation)

3. Indications for THRIVE

THRIVE is beneficial in situations where prolonged apnea time is required and hypoxia must be prevented.

✅ Ideal for:

• Difficult airway scenarios (anticipated or unanticipated)
• Obese patients (BMI > 30 kg/m²) – High desaturation risk
• Pediatric patients – Smaller oxygen reserves
• Critically ill patients (ARDS, sepsis, trauma)
• Difficult preoxygenation cases (airway obstruction, agitation)
• Patients with high metabolic demands (pregnancy, sepsis, burns, DKA)

🚫 Not Suitable for:

• Complete airway obstruction – No oxygen flow to alveoli
• Massive upper airway bleeding or secretions – Reduces effectiveness
• Severe respiratory acidosis requiring immediate CO₂ clearance

6. THRIVE vs. Conventional Preoxygenation

MCQs on Apneic Oxygenation

Question 1:What is the physiological principle behind apneic oxygenation?

A. Fick’s Principle

B. Bohr’s Effect

C. Haldane Effect

D. West’s Zones of Perfusion

✅ Answer: A. Fick’s Principle

Question 2:Which of the following is the most effective method for apneic oxygenation in a critically ill trauma patient?

A. Simple nasal cannula at 4 L/min

B. Bag-Valve-Mask at 5 L/min

C. High-Flow Nasal Oxygen (HFNO) at 60 L/min

D. Non-Rebreather Mask at 10 L/min

✅ Answer: C. High-Flow Nasal Oxygen (HFNO) at 60 L/min

Question 3:What is the main limitation of apneic oxygenation?

A. Oxygen desaturation occurs rapidly

B. Carbon dioxide retention leads to respiratory acidosis

C. Requires active ventilation

D. Only useful in awake patients

✅ Answer: B. Carbon dioxide retention leads to respiratory acidosis