Preoxygenation long note including calculation
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preoxygenation rapid sequence intubation techniques
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Preoxygenation - Long Note (Including Calculations)
1. Definition
Preoxygenation (also called denitrogenation) is the process of replacing the nitrogen present in the lungs (primarily within the functional residual capacity, FRC) with 100% oxygen before the induction of anesthesia or performance of airway management. This creates an oxygen reservoir in the lungs that maintains arterial oxygenation during the apneic period of rapid sequence intubation (RSI) or difficult airway management.
- Miller's Anesthesia, 10e - "Preoxygenation, the process of replacing nitrogen in the lungs with oxygen, provides an increased length of time before oxyhemoglobin desaturation occurs in an apneic patient."
2. Physiological Basis
Functional Residual Capacity (FRC) and Oxygen Stores
The key concept underpinning preoxygenation is the FRC as an oxygen reservoir:
| Compartment | Volume (average adult) |
|---|---|
| FRC (normal adult, upright) | ~2,500 mL (2-3 L) |
| FRC (supine position) | Reduced by ~300-500 mL |
| FRC (obese, supine) | Reduced by up to 50% |
| FRC (pregnant at term, supine) | Reduced ~20% |
Before preoxygenation (room air):
- FRC gas composition: ~79% N2, ~16% O2, ~4% CO2, ~1% other
- Available oxygen in FRC = 0.16 × 2,500 mL = 400 mL
After complete preoxygenation (FiO2 = 1.0):
- FRC gas composition: ~95% O2, ~5% CO2/other (near-complete denitrogenation)
- Available oxygen in FRC = 0.90 to 0.95 × 2,500 mL = ~2,250-2,375 mL of O2
Why SpO2 = 100% Alone is NOT Enough
An SpO2 of 100% is achievable from PaO2 of ~100 mmHg (room air, normal FRC composition) to PaO2 of ~600 mmHg (full denitrogenation, 100% O2). The difference represents vastly different oxygen reserves. Achieving SpO2 of 100% with simple face mask on low-flow O2 does NOT confirm adequate denitrogenation.
"It is not enough to achieve a peripheral oxygen saturation (SpO2) value of 100% prior to intubation, because an SpO2 of 100% does not necessarily correspond with denitrogenation of the lungs; furthermore, PaO2 at 100% SpO2 can range from approximately 100 mmHg to 600 mmHg." - Roberts & Hedges' Clinical Procedures in Emergency Medicine
3. Calculation of Safe Apnea Time
Step-by-Step Oxygen Reserve Calculation
Formula:
Safe Apnea Time (min) = Oxygen Store after preoxygenation (mL) ÷ Oxygen Consumption (mL/min)
Values:
| Parameter | Normal Adult | Notes |
|---|---|---|
| FRC | 2,500 mL | Varies 2-3 L |
| % O2 after full preoxygenation | 90-95% | EtO2 ≥90% is target |
| O2 available in FRC | 90% × 2,500 = 2,250 mL | |
| Basal O2 consumption | 200-250 mL/min | ~3.5 mL/kg/min |
| Also: O2 in blood, tissues | ~300-400 mL additional | |
| Total available O2 reserve | ~2,500-2,800 mL |
Calculation for healthy adult:
2,250 mL ÷ 250 mL/min = ~9 minutes of safe apnea time
This aligns with textbook data: "With maximal preoxygenation, time to oxyhemoglobin desaturation below 80% can vary from 9 minutes in a healthy, non-obese adult." (Miller's Anesthesia, 10e)
Without preoxygenation (room air):
400 mL ÷ 250 mL/min = ~1.5-2 minutes to desaturation
Safe Apnea Times by Patient Category
| Patient Group | Safe Apnea Time (to SpO2 <90%) | Reason for Reduction |
|---|---|---|
| Healthy adult, non-obese | 8-10 minutes | Baseline |
| Obese adult (BMI >40) | 2-3 minutes | Reduced FRC, increased O2 consumption |
| Pregnant (term) | 2-3 minutes | Reduced FRC, increased O2 demand |
| Pediatric (infant) | <1-2 minutes | Very small FRC/weight ratio, high metabolic rate |
| Critically ill / septic | 2-4 minutes | Increased O2 consumption, V/Q mismatch |
| Healthy child (1-5 yrs) | ~4 minutes | Intermediate |
| Healthy adult breathing room air (no preoxygenation) | 1-2 minutes | Low FRC O2 content |
These numbers represent time to SpO2 drop to <90% (some sources use <80%).
Numerical Example: Obese Patient
- FRC (obese, supine) = ~1,200 mL (reduced ~50%)
- O2 available after preoxygenation = 90% × 1,200 = 1,080 mL
- O2 consumption (obese) = ~300 mL/min (increased metabolic rate)
- Safe apnea time = 1,080 ÷ 300 = ~3.6 minutes
This matches Barash's data: "Obese patients had 2.7 ± 0.25 minutes vs. 6 ± 0.5 minutes in healthy nonobese patients."
Numerical Example: Pediatric (10 kg infant)
- FRC ≈ 30 mL/kg × 10 kg = 300 mL
- O2 available = 90% × 300 = 270 mL
- O2 consumption = 6-8 mL/kg/min × 10 kg = 60-80 mL/min
- Safe apnea time = 270 ÷ 70 = ~3.9 minutes (in ideal conditions - less in sick infants)
Note: Neonates can desaturate in <1 minute if not preoxygenated.
4. Goal of Preoxygenation
The clinical target for adequate preoxygenation:
- End-tidal O2 (EtO2) ≥ 90% - the gold-standard endpoint
- SpO2 = 100% (necessary but not sufficient alone)
- PaO2 > 300-400 mmHg (confirms full denitrogenation; usually assessed with ABG in ICU)
EtO2 is measured by the anesthesia machine's gas analyzer and directly reflects the FRC O2 content.
5. Methods of Preoxygenation
A. Tidal Volume Breathing (Standard Method - Gold Standard)
- Technique: Breathing normally (tidal volume) via tight-fitting face mask delivering 100% O2 for 3 minutes
- Flow rate required: 10-12 L/min (anesthesia circuit) or ≥30-40 L/min (emergency setting)
- Efficacy: Achieves EtO2 >90% in most patients; exchanges ~95% of FRC gas
- Advantage: Most reliable, suitable for all patients including those who cannot cooperate with deep breathing
B. Vital Capacity Breaths (Rapid Method)
| Variant | Duration | Efficacy |
|---|---|---|
| 4 maximal vital capacity breaths | 30 seconds | Achieves high PaO2 (~339 mmHg) but shorter desaturation time than tidal method |
| 8 deep breaths | 60 seconds | More effective than 4-breath method; approaches tidal volume technique |
- When to use: Time-critical emergencies when 3-minute preoxygenation is not possible
- Limitation: Requires cooperative patient; does NOT achieve same O2 reserve as 3-minute method
C. Non-Rebreather Mask (NRM) - Emergency Setting
- Flow rate: ≥15 L/min minimum; ideally turned to flush rate (≥40-60 L/min)
- At standard 15 L/min: FiO2 ~60-70%
- At flush/max rates (40-90 L/min): FiO2 ~90-97%
- Duration: 3-5 minutes of tidal breathing
"At lower flow rates (<30 L/min) the FiO2 will not be high enough for adequate preoxygenation... the oxygen flow rate should be at least 30 L/min." - Roberts & Hedges'
D. Bag-Valve-Mask (BVM)
- Requires perfect mask seal AND one-way valves
- Without proper seal, FiO2 approaches room air
- Self-inflating bags without reservoir do NOT deliver FiO2 of 100% during spontaneous breathing
- Less preferred than face mask for preoxygenation in spontaneously breathing patients
E. High-Flow Nasal Cannula (HFNC) / THRIVE
THRIVE (Transnasal Humidified Rapid-Insufflation Ventilatory Exchange):
- Delivers warmed, humidified O2 at 30-70 L/min
- 60 L/min for 3 minutes = as effective as tidal volume face mask preoxygenation
- Additional benefits: CO2 washout from dead space, small amount of PEEP (~2-3 cmH2O)
- Extends safe apnea time beyond standard face mask
- Can be used simultaneously with face mask for maximal effect
Standard HFNC (15 L/min via nasal cannula):
- Evidence does NOT strongly support HFNC alone as primary preoxygenation in most patients
- Useful as adjunct for apneic oxygenation during laryngoscopy
- Murray & Nadel: "Current evidence does not support the use of HFNC for preoxygenation before intubation. At this time, HFNC for preoxygenation should only be considered for those cases of mild hypoxemia."
A 2025 systematic review and meta-analysis (PMID: 39854507) examined HFNC for pre- and apneic oxygenation during RSI in emergency surgery, confirming evolving evidence in this area.
F. Non-Invasive Ventilation (NIV) - BiPAP/CPAP
- Best for: Patients who cannot achieve SpO2 >95% with spontaneous breathing
- BiPAP during preoxygenation prolongs apnea time, especially in:
- Obese patients (recruits collapsed alveoli)
- Hypoxemic ICU patients (improves V/Q matching)
- Patients with pulmonary edema, pneumonia
- CPAP/PEEP prevents atelectasis formation during preoxygenation
- Tintinalli: "If unable to achieve oxygen saturation >95% with spontaneous breathing, consider the use of BVM + PEEP valve, or CPAP/BiPAP."
6. Positioning During Preoxygenation
Head-Up / Reverse Trendelenburg Position
| Position | Effect on FRC | Recommendation |
|---|---|---|
| Supine | Reduced FRC (vs. upright) | Baseline for most |
| Head-up 20-30° | Increases FRC vs. supine | Recommended for all |
| Head-up 25-45° (obese) | Maximizes FRC | Strongly recommended |
| Ramped position (obese) | Head/shoulders elevated | Improves FRC + laryngoscopy view |
- Head-up positioning improves preoxygenation quality in both obese and non-obese patients
- In pregnancy: left lateral tilt + head-up reduces aortocaval compression and increases FRC
- Tintinalli: "Elevating the head of the patient 20 to 30 degrees may improve preoxygenation."
7. Apneic Oxygenation (Extension of Preoxygenation)
After preoxygenation is completed, apneic oxygenation extends safe apnea time during laryngoscopy by exploiting the physiologic phenomenon of mass flow:
Physiologic Basis
- During apnea, O2 is absorbed from alveoli into blood at ~250 mL/min
- CO2 enters alveoli far more slowly (~8-10 mL/min; most buffered in blood)
- This creates a net negative pressure in the alveolus
- If the airway is patent and O2 is flowing at the nose/mouth, oxygen is entrained down into the lungs passively
Technique
| Device | Flow Rate | Notes |
|---|---|---|
| Standard nasal cannula | 15 L/min | NO DESAT technique; can go under face mask |
| THRIVE / HFNC | 30-70 L/min | Most effective; CO2 washout also occurs |
| Pharyngeal catheter | 10-15 L/min | Catheter tip in pharynx |
- Place nasal cannula at 15 L/min concurrently with face mask preoxygenation
- Keep nasal cannula running during entire laryngoscopy attempt
- Pediatric apneic oxygenation: 1-2 L/min per year of age, maximum 15 L/min
8. Special Populations
Obese Patients
- Reduced FRC (up to 50% reduction in morbid obesity)
- Increased O2 consumption
- Prone to rapid atelectasis during preoxygenation
- Strategies:
- Ramped/head-elevated position (25-45°)
- BiPAP preoxygenation preferred
- HFNC + face mask combination
- Minimize apnea time; use apneic oxygenation
Pregnant Patients (at term)
- FRC reduced ~20% due to elevated diaphragm
- O2 consumption increased ~20-30%
- Aspiration risk (full stomach - RSI mandatory)
- Safe apnea time: ~2-3 minutes
- Left lateral tilt position
- ALWAYS preoxygenate with full 3-minute tidal volume technique
Pediatric Patients
- FRC/body weight ratio: adults ~30-35 mL/kg; neonates ~30 mL/kg (similar ratio)
- O2 consumption: 6-8 mL/kg/min (2x adult per kg)
- Therefore desaturate MUCH faster
- Preoxygenation time: 2-3 minutes with NRM
- Vital capacity breaths difficult in uncooperative children
- Always use apneic oxygenation: 1-2 L/min/year of age (max 15 L/min) during RSI
Critically Ill / ICU Patients
- High V/Q mismatch, shunt
- Increased O2 consumption
- Already hypoxemic despite supplemental O2
- NIV/BiPAP preoxygenation for 3-5 minutes superior to standard face mask
- Higher risk of desaturation during intubation - requires careful preparation
9. Factors Limiting Preoxygenation
| Factor | Mechanism | Countermeasure |
|---|---|---|
| Poor mask seal (even 4 mm gap) | Room air entrainment; FiO2 approaches room air | Proper mask fit; two-handed technique |
| Low O2 flow rate (<15-30 L/min) | Rebreathing of expired gas; CO2 rebreathing | Use flush rate (≥40 L/min) |
| Self-inflating bag (no reservoir) | Does not deliver 100% FiO2 during spontaneous breathing | Use anesthesia circuit or proper BVM with reservoir |
| Patient non-cooperation/claustrophobia | Removes mask, mouth breathing | Allow patient to hold mask; sedation; alternative device |
| Pre-existing pulmonary pathology | Shunt, V/Q mismatch; O2 does not replace N2 in flooded/collapsed alveoli | NIV; PEEP; accept limitation |
| Obesity / pregnancy | Reduced FRC; faster N2 displacement but less reservoir | Position + BiPAP |
| Nasal-only mask | Does not protect against mouth breathing room air | Use full face mask |
10. Endpoints of Adequate Preoxygenation
| Endpoint | Value | Comments |
|---|---|---|
| EtO2 (end-tidal O2) | ≥ 90% | Gold standard; requires gas analyzer |
| SpO2 | 100% | Necessary but NOT sufficient alone |
| EtN2 (end-tidal N2) | < 5% | Confirms denitrogenation |
| Duration | ≥ 3 minutes tidal breathing | Practical bedside standard |
| PaO2 (ABG) | > 300-400 mmHg | ICU/research setting |
11. Summary of Preoxygenation Techniques - Comparison Table
| Method | Time Required | Efficacy (EtO2/SpO2) | Best Setting |
|---|---|---|---|
| 3-min tidal volume (face mask, high flow) | 3 min | EtO2 >90% - Gold standard | Elective / emergency |
| 8 vital capacity breaths | 60 sec | ~EtO2 85-88% | Time-critical emergency |
| 4 vital capacity breaths | 30 sec | PaO2 ~339 mmHg; less O2 reserve | Last resort only |
| THRIVE (60 L/min) × 3 min | 3 min | = tidal volume method | Theatre/ICU |
| BiPAP/NIV × 3-5 min | 3-5 min | Superior in hypoxemic/obese | ICU/obese/hypoxemic |
| NRM flush rate (40-90 L/min) × 3-5 min | 3-5 min | FiO2 ~90-97% | Emergency department |
12. Practical Protocol (RSI in Emergency Setting)
- Start preoxygenation immediately on patient arrival - do not wait
- Position: Head-up 20-30° (or ramped if obese)
- Device: NRM at flush rate (≥40 L/min) or anesthesia face mask at 10-12 L/min
- Place nasal cannula at 15 L/min simultaneously (for apneic oxygenation during intubation)
- Duration: ≥3 minutes tidal breathing; if time allows, wait for EtO2 ≥90%
- If SpO2 cannot reach >95%: Switch to BVM + PEEP valve or BiPAP
- If <3 min available: Ask for 8 vital capacity breaths
- During laryngoscopy: Keep nasal cannula running at 15 L/min (apneic oxygenation - NO DESAT)
- Target SpO2 threshold for abort: >93-95% preferred; intervene with BVM if SpO2 drops <90-93%
13. Key Numbers to Remember
| Fact | Value |
|---|---|
| Normal FRC (adult) | ~2,500 mL |
| O2 in FRC (room air) | ~400 mL (FiO2 ~0.16 × 2,500) |
| O2 in FRC after full preoxygenation | ~2,250 mL (90% × 2,500) |
| Basal O2 consumption | 200-250 mL/min |
| Safe apnea time - healthy adult | ~8-10 minutes |
| Safe apnea time - obese adult | ~2-3 minutes |
| Safe apnea time - pregnant at term | ~2-3 minutes |
| Safe apnea time - infant | <2 minutes (often <1 min if sick) |
| Safe apnea time - no preoxygenation | ~1-2 minutes |
| Target EtO2 | ≥90% |
| Minimum O2 flow (emergency) | ≥30-40 L/min |
| Standard tidal volume method duration | 3 minutes |
| Rapid vital capacity breaths | 8 breaths/60 sec |
| Apneic oxygenation flow (adults) | 15 L/min nasal cannula |
| Apneic oxygenation flow (children) | 1-2 L/min/year of age (max 15) |
| THRIVE flow rate | 30-70 L/min |
Sources:
- Miller's Anesthesia, 10e (Elsevier)
- Barash, Cullen & Stoelting's Clinical Anesthesia, 9e
- Morgan & Mikhail's Clinical Anesthesiology, 7e
- Roberts & Hedges' Clinical Procedures in Emergency Medicine
- Tintinalli's Emergency Medicine, A Comprehensive Study
- Rosen's Emergency Medicine, Concepts and Clinical Practice
- Tang H, et al. "High-flow nasal cannula for pre- and apneic oxygenation during RSI in emergency surgery." PLoS One. 2025. PMID: 39854507
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