Preoxygenation

Definition and Physiologic Goal

• Roberts and Hedges' Clinical Procedures in Emergency Medicine
• Miller's Anesthesia, 10e

Physiology: Why It Works

• At baseline, FRC (functional residual capacity) is ~2.5 L in a healthy adult and is mostly nitrogen.
• Induction of anesthesia causes hypoventilation/apnea, supine positioning, muscle paralysis, and reduced FRC from anesthetic effects - all accelerating hypoxemia.
• Replacing N2 with O2 converts FRC into a usable oxygen store. During apnea, O2 diffuses from this reservoir into the blood far faster than CO2 accumulates.
• Barash's Clinical Anesthesia, 9e

Methods of Preoxygenation

1. Tidal Volume Breathing - Standard Method

• Breathe 100% O2 for 3 minutes via tight-fitting face mask.
• Exchanges ~95% of lung gas with oxygen.
• Flow rate must be ≥10-12 L/min on an anesthesia circuit, or ≥30-40 L/min ("flush rate") on an emergency wall flowmeter to prevent rebreathing and room air entrainment.
• Preferred method when time allows.
• Miller's Anesthesia, 10e; Roberts and Hedges'

2. Vital Capacity Breaths - Rapid Method

• 4 maximal breaths over 30 seconds achieves high PaO2 (~339 mmHg) but shorter time to desaturation than the 3-minute method.
• 8 deep breaths over 60 seconds is more effective and shows promise as a time-saving alternative.
• Reserved for situations where 3 minutes is not available.
• Barash's Clinical Anesthesia, 9e; Miller's Anesthesia, 10e

3. Non-Rebreather (NRB) Mask for Emergency

• Administer 100% O2 for ≥3 minutes at a minimum of 15 L/min, or turn flowmeter to flush rate (40-60 L/min) to deliver 90-97% FiO2.
• A standard NRB at low flow delivers only 60-70% O2 - insufficient.
• Tintinalli's Emergency Medicine; Roberts and Hedges'

Positioning

• Minimizes atelectasis
• Reduces pressure of abdominal contents against the diaphragm
• Enables continued deep breathing

• Roberts and Hedges'; Miller's Anesthesia, 10e

High-Risk Populations - Shorter Safe Apnea Time

• Barash's Clinical Anesthesia, 9e

• BiPAP or CPAP during preoxygenation
• Head-up (reverse Trendelenburg) positioning
• If SpO2 cannot be raised > 93-95%, add NIPPV or BVM with PEEP valve.

Adjunct: Apneic Oxygenation

• Nasal cannula at 15 L/min (NO DESAT) - simple and effective adjunct
• High-Flow Nasal Cannula (HFNC) at up to 60-70 L/min delivers warmed, humidified O2 and provides low-level PEEP
• THRIVE (Transnasal Humidified Rapid-Insufflation Ventilatory Exchange) - HFNC at 60 L/min for 3 minutes is as effective as face mask tidal volume preoxygenation; apneic durations up to 55 minutes reported in select non-obese patients

• Miller's Anesthesia, 10e; Barash's Clinical Anesthesia, 9e; Roberts and Hedges'

Delayed-Sequence Intubation (DSI)

• Give ketamine 1 mg/kg IV (dissociative dose) to allow the patient to cooperate
• Apply HFNC, NRB mask, or BVM with PEEP valve for 2-3 minutes
• Once adequately preoxygenated, proceed to intubation

• Roberts and Hedges'; Roberts and Hedges' (DSI section)

Pitfalls and Common Failures

• Inadequate flow rate - <30 L/min on an emergency flowmeter allows room air entrainment; always use flush rate
• Poor mask seal - even a 4 mm leak significantly reduces FiO2; BVM requires a two-hand technique to seal properly
• Self-inflating bags - do NOT deliver 100% O2 during spontaneous breathing; not suitable for preoxygenation without O2 reservoir
• SpO2 100% ≠ complete denitrogenation - use EtO2 monitoring when available (target > 90%)
• Flat (supine) positioning - reduces FRC further; always elevate head
• Barash's Clinical Anesthesia, 9e; Roberts and Hedges'

Summary Table

Apneic Oxygenation

Definition

• Miller's Anesthesia, 10e; Roberts and Hedges' Clinical Procedures in Emergency Medicine

Physiologic Mechanism

• Oxygen continues diffusing from alveoli into the bloodstream at roughly 250 mL/min (whole body oxygen consumption)
• CO2 enters the alveoli much more slowly, only about 8-20 mL/min initially (most CO2 remains dissolved in blood/tissues due to high blood solubility and carbonate buffering)
• This creates a net negative alveolar pressure - the alveoli are being emptied faster than they are being filled
• This pressure gradient passively draws gas from the nasopharynx/oropharynx down into the lungs

• Roberts and Hedges'; Miller's Anesthesia, 10e

1. Oxygen diffuses across the alveolar membrane far more readily than CO2 (per unit pressure gradient)
2. CO2 has much higher solubility in blood, so it is buffered in tissues and does not rapidly accumulate in alveoli
3. Hemoglobin has high O2 affinity, driving continued O2 extraction from alveoli

• Roberts and Hedges'

• Miller's Anesthesia, 10e; Barash's Clinical Anesthesia, 9e

Clinical Applications

1. Adjunct During Intubation (RSI / Emergency Airway)

• Apply nasal cannula beneath the preoxygenation mask before induction
• If awake: 5-15 L/min (higher flows uncomfortable)
• Once unconscious / at laryngoscopy: ≥15 L/min, ideally as high as possible
• Keep nasal cannula in place throughout laryngoscopy and intubation
• If nasal obstruction: insert a nasopharyngeal airway to bypass the obstruction and deliver O2 to the posterior pharynx
• Ensure upper airway patency (jaw thrust, head tilt) to maximize flow to the glottis
• Roberts and Hedges'

• Roberts and Hedges'

2. High-Flow Nasal Cannula (HFNC) for Apneic Oxygenation

• Higher flows create a larger nasopharyngeal O2 reservoir
• Provides low-level CPAP (~1-3 cm H2O), reducing atelectasis
• Humidification makes high flows tolerable
• Shown superior to standard nasal cannula in an ICU study, though both groups had low hypoxemia rates
• Roberts and Hedges'; Barash's Clinical Anesthesia, 9e

3. THRIVE (Transnasal Humidified Rapid-Insufflation Ventilatory Exchange)

• In 25 patients with difficult airways, median safe apnea time was 14 minutes (range 5-65 minutes)
• CO2 rose at only 1.1 mmHg/min on average
• CO2 clearance attributed to: turbulent flow at the glottis, gas mixing in anatomic dead space, cardiac oscillations (cardiogenic mixing / "cardiabalism"), and other factors
• Used for awake intubation and prolonged procedures (e.g., suspension laryngoscopy), where apneic durations up to 55 minutes have been reported in select non-obese, healthy patients
• Conflicting data on CO2 clearance in pediatric and adult studies - hypercapnia still accumulates, just more slowly
• Miller's Anesthesia, 10e; Barash's Clinical Anesthesia, 9e

4. Apnea Test for Brain Death Confirmation

• Goal: Allow CO2 to rise to ≥60 mmHg (which is the threshold to trigger respiratory drive) while preventing hypoxia
• Simply disconnecting the ventilator would cause dangerous hypoxia - apneic oxygenation prevents this
• Standard technique:
  1. Pre-oxygenate and normalize PaCO2
  2. Disconnect the ventilator
  3. Insert a catheter into the trachea and deliver 10-15 L/min O2
  4. Observe for any respiratory effort for 8 minutes
  5. Obtain ABG at end - a PaCO2 ≥60 mmHg (or rise of ≥20 mmHg from baseline) with no respiratory effort = positive apnea test (supports brain death)
• Alternative: set ventilator rate to zero while maintaining continuous O2 flow and CPAP

• Roberts and Hedges'; Bradley and Daroff's Neurology in Clinical Practice

Delivery Methods - Comparison

Recent Evidence

• Apneic oxygenation increased first-pass intubation success (RR 1.27, 95% CI 1.03-1.57)
• Higher SpO2 during intubation (mean difference +3.6%)
• Reduced incidence of hypoxemia (RR 0.24, 95% CI 0.17-0.33)

• NIV + HFNC combination was superior to HFNC alone or conventional oxygen therapy
• HFNC alone provided longer apnea time than conventional O2 (mean difference -50 seconds)
• NIV alone ranked highest for maintaining SpO2 >80% and >90%

Limitations and Pitfalls

• CO2 accumulation is inevitable - apneic oxygenation maintains SpO2 but not pH; respiratory acidosis progresses
• Requires patent upper airway - if the pharynx is obstructed, the pressure gradient cannot be maintained; jaw thrust is mandatory
• Standard nasal cannula causes desiccation of the nasopharyngeal mucosa at high flows, though this is clinically minor given the short duration of use
• Not a substitute for ventilation - should not be used as a long-term strategy outside of very specific contexts (THRIVE in controlled settings)
• Barash's Clinical Anesthesia, 9e; Roberts and Hedges'