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Preoxygenation At High flow nasal 02

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preoxygenation "high flow nasal oxygen"

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Preoxygenation with High-Flow Nasal Oxygen (HFNO/HFNC)

Physiological Basis of Preoxygenation

Preoxygenation replaces nitrogen in the lungs with oxygen, converting the lungs into an oxygen reservoir during the apneic period of rapid-sequence intubation (RSI). The goal is denitrogenation — not simply achieving SpO₂ 100%, since an SpO₂ of 100% can correspond to a PaO₂ anywhere from ~100 to 600 mmHg.
With maximal preoxygenation, safe apnea time before oxyhemoglobin desaturation to <80% ranges:
  • Healthy non-obese adult: ~9 minutes
  • Obese adults / children: ≤3 minutes
Those at greatest risk for rapid desaturation: obese, pregnant, critically ill, and pediatric patients.
— Roberts and Hedges' Clinical Procedures in Emergency Medicine; Miller's Anesthesia, 10e

Standard Preoxygenation Techniques

MethodDetails
Tidal volume breathing3–5 min via face mask, achieves exchange of ~95% lung gas; preferred
Vital capacity breaths8 breaths over 60 s (4 breaths/30 s less effective); for time-limited scenarios
End-tidal O₂ targetContinue until EtO₂ ≥90%
Flow rate is critical: FiO₂ delivered by any mask device is inadequate at <30 L/min. The flowmeter should be turned to flush rate (typically >40 L/min; up to 70–90 L/min on some flowmeters). High-flow oxygen washes out CO₂ from dead space, fills the nasopharynx with O₂, compensates for mask leaks, and generates 1–2 cmH₂O of passive CPAP. — Roberts and Hedges'

HFNC for Preoxygenation

High-flow nasal cannula (HFNC) delivers warmed, humidified oxygen at 30–70 L/min (sometimes called THRIVE — Transnasal Humidified Rapid-Insufflation Ventilatory Exchange at 60 L/min).

Mechanisms of benefit

  1. High FiO₂ delivery — eliminates nasopharyngeal dead space nitrogen, delivers near-100% FiO₂
  2. Low-level PEEP (~1–3 cmH₂O) — reduces atelectasis, preserves FRC
  3. CO₂ washout from upper airway — delays hypercapnia during apnea
  4. Continuous flow during apnea — enables simultaneous apneic oxygenation without interruption

THRIVE evidence (Miller's Anesthesia, 10e)

THRIVE at 60 L/min for 3 minutes is as effective as tidal-volume preoxygenation by face mask. In 25 patients with anticipated difficult airways, THRIVE extended safe apnea time significantly beyond standard techniques, preventing the development of severe respiratory acidosis. — Miller's Anesthesia, 10e

When to use HFNC for preoxygenation

Clinical ContextRecommendation
Mild hypoxemia (P/F >200–300)HFNC is a reasonable alternative to face mask
Moderate–severe hypoxemia (P/F <200)NIV preferred over HFNC — fewer severe hypoxemic events (FLORALI-2 trial)
Cooperative patient, elective intubationHFNC ± face mask acceptable
Uncooperative/agitated patientHFNC may be preferred due to tolerability
Obese patientHFNC + head-up position (25° or ramped); delays desaturation
"Current evidence does not support the use of HFNC for preoxygenation before intubation as the sole modality. HFNC should be considered for mild hypoxemia; NIV used for more severe cases." — Murray & Nadel's Textbook of Respiratory Medicine

HFNC as Apneic Oxygenation (During Intubation)

Even after preoxygenation ends and laryngoscopy begins, HFNC left in situ provides apneic oxygenation via mass-flow oxygen delivery to the alveoli — exploiting the fact that O₂ diffuses across alveoli far faster than CO₂ is produced, creating a net pressure gradient that pulls O₂ distally.
  • HFNC shown superior to simple nasal cannula for apneic oxygenation in ICU studies, though both had low hypoxemia rates
  • For standard apneic oxygenation: nasal cannula at 15 L/min placed under the face mask during preoxygenation, then maintained throughout intubation
  • HFNC provides higher and more consistent FiO₂ than simple cannula at equivalent flows, and the humidification prevents mucosal desiccation
— Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 113–114

Key Clinical Evidence

Trial/StudyFinding
PREOXYFLOW (RCT, n=124; P/F <300)No difference HFNC vs. face mask in SpO₂, adverse events, or ventilator-free days
FLORALI-2 (RCT, n=313; P/F ≤300)No overall outcome difference HFNC vs. NIV; NIV fewer severe hypoxemic events in P/F <200 subgroup
PROTRACH (RCT, n=192; RSI)HFNC vs. bag-valve mask: fewer desaturations <95% (12% vs. 23%, P=0.045) and fewer peri-intubation complications (6% vs. 19%, P=0.007)
THRIVE (observational)60 L/min HFNC = tidal volume face mask preoxygenation; extended apnea time
Recent systematic review (2024): Crístian de Carvalho et al., Br J Anaesth (PMID: 38599916) — network meta-analysis of preoxygenation strategies.

Positioning

Head-up 20–25° (reverse Trendelenburg if spinal precautions needed) improves preoxygenation quality in both obese and non-obese adults by:
  • Reducing abdominal pressure on the diaphragm
  • Minimizing atelectasis formation
  • Increasing FRC

Practical Protocol Summary

  1. Position: Head-up 20–25°
  2. Device: Non-rebreather mask at flush rate (≥40 L/min) or HFNC at 40–60 L/min
  3. Duration: 3–5 min tidal breathing (or 8 vital capacity breaths if time-critical)
  4. Augment: Place nasal cannula (15 L/min) or leave HFNC in place underneath the mask
  5. Target: EtO₂ ≥90% if measurable; SpO₂ + adequate time if not
  6. Severe hypoxemia (P/F <200): Add NIV/BiPAP for preoxygenation; HFNC alone insufficient
  7. Apneic phase: Keep HFNC or nasal cannula at 15 L/min throughout laryngoscopy
Sources: Roberts and Hedges' Clinical Procedures in Emergency Medicine | Miller's Anesthesia 10e | Murray & Nadel's Textbook of Respiratory Medicine | Barash Clinical Anesthesia 9e | PMID: 38599916 (network meta-analysis, 2024)
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