Acute Airway Management & RSI — Part 1: Airway Assessment & Preoxygenation

1. Principles of Airway Assessment

1.1 Overview

Every patient who may require airway management should undergo a rapid, structured assessment to identify features that predict difficulty with laryngoscopy, bag-valve-mask (BVM) ventilation, supraglottic airway (SGA) placement, and front-of-neck access (FONA).^{1 2} No single predictor reliably identifies all difficult airways; therefore, a multi-dimensional assessment incorporating several validated tools is recommended. In the emergency setting, time constraints often limit assessment to 30–60 seconds, making familiarity with structured mnemonics essential.

Key principles of emergency airway assessment include:

• Assume difficulty until proven otherwise — emergency patients have not been fasted, are often hemodynamically unstable, and present without prior airway records
• Assess multiple dimensions — difficulty with intubation, BVM ventilation, SGA placement, and surgical airway may coexist or exist independently
• Identify the “physiologically difficult airway” — hypoxemia, hypotension, metabolic acidosis, and right ventricular failure independently increase peri-intubation risk regardless of anatomy³
• Plan before you paralyze — every intubation requires a primary plan and at least two backup plans

2. The LEMON Assessment for Difficult Laryngoscopy

The LEMON mnemonic provides a rapid, systematic framework for predicting difficult direct laryngoscopy and intubation.⁴ Each component assesses a different anatomic or functional predictor.

2.1 L — Look Externally

Perform a rapid visual assessment for features associated with difficult intubation:

2.2 E — Evaluate 3-3-2

The 3-3-2 rule uses three simple bedside measurements to assess the geometry of the oral cavity, mandibular space, and laryngeal position relative to the tongue base.⁴

Clinical interpretation: Failure of any component increases the probability of a Cormack-Lehane grade III or IV view. Failure of multiple components substantially increases the likelihood of difficult or failed direct laryngoscopy.

2.3 M — Mallampati Score

The Mallampati classification (modified by Samsoon and Young) evaluates the relative size of the tongue to the oropharynx by examining the visible oropharyngeal structures with the mouth open and tongue protruded without phonation.⁵

Limitations in the ED:

• Mallampati was validated in elective surgical patients sitting upright and cooperative
• Supine patients, patients in cervical collar, obtunded patients, and those with oropharyngeal blood/secretions cannot be reliably assessed
• Sensitivity as a standalone predictor is only 55–65%; positive predictive value is approximately 15–30%
• Should always be used in conjunction with other predictors, never in isolation

2.4 O — Obstruction

Assess for conditions that obstruct the upper airway or laryngeal inlet:

Key principle: Any condition causing stridor, muffled voice, or progressive swelling indicates partial upper airway obstruction and should prompt an aggressive, early approach to airway management with a low threshold for intervention before complete obstruction occurs.

2.5 N — Neck Mobility

Adequate atlanto-occipital extension is essential for aligning the oral, pharyngeal, and laryngeal axes during direct laryngoscopy. Reduced neck mobility substantially impairs the laryngoscopic view.

Assessment: Ask the patient to touch their chin to their chest (flexion) and extend the neck to look at the ceiling. Normal atlanto-occipital extension is approximately 35 degrees. Extension of less than 20–25 degrees is associated with difficult laryngoscopy.

3. Cormack-Lehane Grading System

The Cormack-Lehane classification describes the laryngoscopic view obtained during direct laryngoscopy and is the standard system for communicating the degree of glottic visualization.⁶

Clinical application:

• Cormack-Lehane grade is assessed during laryngoscopy, not before — it confirms (or refutes) pre-procedure predictions
• External laryngeal manipulation (ELM), also termed bimanual laryngoscopy or BURP (backward-upward-rightward pressure), can improve the view by one to two grades
• A grade IIb or higher should prompt immediate use of a bougie (gum elastic bougie/tracheal tube introducer)
• Grade III with bougie achieves successful intubation in approximately 90% of cases
• Grade IV requires an alternative technique (video laryngoscopy, SGA, or FONA)

4. Predicting Difficult Bag-Valve-Mask Ventilation: The MOANS Mnemonic

When intubation fails, the immediate rescue maneuver is BVM ventilation. Predicting which patients will be difficult to ventilate by BVM is therefore essential for identifying those at highest risk (i.e., “cannot intubate, cannot oxygenate” scenarios).^{4 7}

4.1 MOANS

Management of difficult BVM ventilation:

• Two-person technique (two-handed mask hold + separate ventilation by second provider)
• Oropharyngeal airway (OPA) and/or nasopharyngeal airway (NPA)
• Head-tilt chin-lift and jaw thrust
• Leave dentures in place for edentulous patients during BVM attempts
• PEEP valve on BVM (5–15 cm H₂O) for obese/atelectatic patients
• If BVM ventilation fails → immediate SGA placement

5. Predicting Difficult Supraglottic Airway Placement: The RODS Mnemonic

If both intubation and BVM ventilation fail, an SGA (e.g., laryngeal mask airway, i-gel, King LT) becomes the next rescue device. The RODS mnemonic identifies patients in whom SGA placement may be difficult or ineffective.⁴

6. Predicting Difficult Cricothyrotomy: The SHORT Mnemonic

When intubation, BVM ventilation, and SGA all fail (the “cannot intubate, cannot oxygenate” or CICO scenario), front-of-neck access via cricothyrotomy is the final rescue. The SHORT mnemonic identifies patients in whom this may be technically difficult.^{4 8}

When SHORT risk factors are identified:

• Double down on Plans A and B — be prepared for longer attempts with VL and SGA
• Pre-identify the cricothyroid membrane (palpation; consider point-of-care ultrasound marking) before induction
• Have surgical cricothyrotomy equipment immediately at bedside
• Consider awake intubation or awake tracheostomy if time permits

7. The Physiologically Difficult Airway

Beyond anatomic difficulty, physiologic derangements independently increase peri-intubation morbidity and cardiac arrest risk. These derangements must be identified and optimized before induction whenever possible.^{3 9}

7.1 Key Physiologic Threats

7.2 The “Hemodynamic Optimization Bundle”

For patients with physiologic compromise, the following pre-intubation preparation is recommended:

1. Fluid bolus — 250–500 mL crystalloid (unless right heart failure or volume overload)
2. Push-dose vasopressor — phenylephrine 100 mcg (100 mcg/mL concentration, 1 mL boluses) or push-dose epinephrine 10–20 mcg (10 mcg/mL concentration, 1–2 mL boluses) drawn up and available at bedside
3. Vasopressor infusion — norepinephrine or vasopressin infusion prepared and ready to initiate at induction
4. Choose hemodynamically favorable induction agent — ketamine or etomidate
5. Dose-reduce induction agents — use lower end of dosing range (e.g., ketamine 1.0–1.5 mg/kg, etomidate 0.2–0.3 mg/kg)

8. Preoxygenation

8.1 Physiologic Rationale

Preoxygenation replaces the nitrogen content of the functional residual capacity (FRC) with oxygen, creating an intrapulmonary oxygen reservoir that extends the duration of safe apnea.¹⁰ In a healthy 70 kg adult with adequate preoxygenation:

• FRC ≈ 2.5 L; when filled with 100% oxygen, provides approximately 300 mL/min of oxygen reserve
• Safe apnea time (time to SpO₂ 90%): approximately 8 minutes in a healthy, preoxygenated adult
• Without preoxygenation, safe apnea time drops to approximately 1–2 minutes
• In obese patients (BMI > 40): safe apnea time may be as low as 2–3 minutes even with preoxygenation
• In critically ill patients with shunt physiology (ARDS, pneumonia): safe apnea time may be < 1 minute

The goal of preoxygenation is to achieve an end-tidal oxygen (EtO₂) of ≥ 85%, which correlates with > 90% nitrogen washout. SpO₂ of 100% does not guarantee adequate preoxygenation (the oxygen-hemoglobin dissociation curve is flat at the top — SpO₂ can be 100% with an EtO₂ of only 50%).

8.2 Standard Preoxygenation Techniques

Key technical points:

• Oxygen flow must be ≥ 15 L/min through a non-rebreather mask to approach FiO₂ of 0.8–0.9
• Standard nasal cannula at 2–6 L/min is inadequate for preoxygenation
• A BVM with oxygen at flush rate and a tight two-handed seal provides the highest FiO₂ (approaching 1.0)
• The reservoir bag on the NRB or BVM must remain inflated throughout — if it collapses, room air is being entrained

8.3 High-Flow Nasal Cannula (HFNC) for Apneic Oxygenation

HFNC delivers heated, humidified oxygen at flow rates of 15–70 L/min via specialized large-bore nasal prongs. When used during the apneic period of RSI (from induction through intubation), it provides continuous oxygen delivery to the pharynx, generating a positive end-expiratory pressure effect (approximately 1 cm H₂O per 10 L/min of flow) and maintaining oxygen delivery during apnea.^{11 12}

Apneic oxygenation protocol:

1. Apply HFNC nasal cannula at 60 L/min, FiO₂ 1.0 prior to induction
2. The HFNC nasal prongs remain in place under the BVM during preoxygenation and throughout the apneic period
3. HFNC continues during laryngoscopy — the nasal prongs do not interfere with oral intubation
4. After intubation is confirmed, the HFNC can be removed

If dedicated HFNC equipment is unavailable, a standard nasal cannula at 15 L/min (maximum flow for standard wall oxygen) placed under the BVM/NRB provides a simpler form of apneic oxygenation. This approach, sometimes called “nasal prong oxygenation during efforts to secure a tube” (NO DESAT), has been shown to extend safe apnea time, although evidence for clinical outcomes is mixed.¹²

Evidence:

• The FELLOW trial (2019) demonstrated that standard nasal cannula at 15 L/min during intubation of critically ill patients in the ICU did not significantly reduce the incidence of desaturation below 90% compared to no apneic oxygenation (primary outcome); however, it reduced the incidence of SpO₂ below 80%¹³
• The ENDAO trial and PREOXI trial provide additional data supporting apneic oxygenation in the emergency department setting
• Apneic oxygenation is most beneficial in patients with normal or near-normal lung function; in patients with significant shunt physiology, it may be less effective
• Despite mixed trial data, apneic oxygenation is low-cost, low-risk, and widely recommended as a routine adjunct

8.4 Non-Invasive Ventilation (NIV) for Preoxygenation

For patients who cannot achieve adequate preoxygenation with standard techniques — particularly those with significant shunt physiology, obesity, or respiratory failure — NIV (BiPAP or CPAP) provides pressure support to recruit atelectatic lung and improve oxygenation before induction.^{14 15}

NIV preoxygenation protocol:

Indications for NIV preoxygenation:

• SpO₂ < 93% despite standard preoxygenation with NRB or BVM
• Morbid obesity (BMI > 40)
• Known significant shunt (ARDS, bilateral pneumonia, pulmonary edema)
• Patients already receiving NIV for respiratory failure

Technique:

1. Apply NIV via face mask with settings above
2. After 3–5 minutes (or when SpO₂ optimized), prepare for rapid transition
3. Remove NIV mask → immediate induction and paralysis
4. If delayed sequence intubation (DSI) is used, ketamine can be administered while the patient remains on NIV
5. After paralysis onset, switch to BVM or proceed directly to laryngoscopy (with apneic oxygenation via nasal cannula in place)

Evidence: The PreVent trial demonstrated that BVM ventilation between induction and laryngoscopy (with a PEEP valve) reduced desaturation events compared to no ventilation during RSI, challenging the traditional teaching of strict avoidance of BVM during RSI.¹⁶

8.5 Preoxygenation in Specific Populations

9. Patient Positioning

9.1 Sniffing Position

The “sniffing the morning air” position — moderate neck flexion (approximately 35 degrees at the lower cervical spine) combined with extension at the atlanto-occipital joint — aligns the oral, pharyngeal, and laryngeal axes to create the optimal line of sight for direct laryngoscopy.¹⁷

How to achieve:

• Place a folded towel or blanket (approximately 7–10 cm height) under the occiput to flex the lower neck
• Extend the head at the atlanto-occipital joint (tilt chin up)
• The external auditory meatus should be at the level of or above the sternal notch (“ear-to-sternal-notch” position)
• This position is contraindicated in suspected cervical spine injury — use manual in-line stabilization (MILS) instead

9.2 Ramped Position for Obese Patients

In obese and morbidly obese patients, standard supine positioning results in posterior displacement of the pharyngeal structures and rapid functional residual capacity loss. The ramped (head-elevated) position dramatically improves both preoxygenation efficacy and laryngoscopic view.¹⁸

How to achieve:

• Elevate the head, shoulders, and upper body using folded blankets, a commercially available ramp device, or by inclining the head of the bed
• Build the “ramp” from the scapulae to the head, achieving a 25–30 degree head-up position
• The goal: align the external auditory meatus with the sternal notch horizontally (same “ear-to-sternal-notch” alignment as sniffing position, but achieved at a higher elevation)
• The face should be parallel to the ceiling

9.3 Head-Elevated Laryngoscopy Position

Emerging evidence supports performing laryngoscopy with the patient in a 25–30 degree head-up (back-up) position for all patients, not just obese patients.¹⁹

Benefits:

• Improved functional residual capacity and prolonged safe apnea time
• Reduced passive regurgitation risk (gravity assists lower esophageal sphincter)
• Improved laryngoscopic view (gravity assists tongue displacement)
• Improved hemodynamic stability compared to flat supine position

The HELP position (Head-Elevated Laryngoscopy Position):

• Raise the entire bed to bring the patient’s face to the laryngoscopist’s xiphoid level
• Back-up to 25–30 degrees
• Achieve ear-to-sternal-notch alignment
• Ensure the bed can be rapidly flattened (Trendelenburg) if regurgitation occurs

10. Systematic Pre-Intubation Assessment Checklist

Before proceeding to intubation, the clinician should rapidly complete the following structured assessment:

Decision framework after assessment:

• No predicted difficulty + no physiologic compromise → Proceed with standard RSI
• Predicted anatomic difficulty but can ventilate by BVM/SGA → RSI with optimized technique (VL first line, bougie ready, SGA immediately available)
• Cannot intubate AND cannot ventilate predicted → Awake intubation (fiberoptic or VL) or awake surgical airway
• Physiologic compromise → Optimize before induction (fluids, vasopressors, preoxygenation); consider DSI; prepare for hemodynamic collapse at induction