The Physiologically Difficult Airway: Blogpost Based on an Interview with Dr. Jarrod Mosier.

The concept of the “difficult airway” is often framed from the physician’s perspective, focusing on the technical challenges of securing the airway. However, a patient-centered approach reframes this to consider the physiological difficulties that place the patient at risk for complications. This blogpost explores the difficult airway, emphasizing a physiological approach.

The term “difficult airway” encompasses scenarios where securing the airway poses significant risk of harm to the patient. These challenges arise from:

• Procedural and Anatomical Factors: Risk or difficulty in airway “capture” or provision of ventilation and oxygenation during airway management. This includes process or team factors.
• Physiological Challenges: Underlying pathophysiology, such as right ventricular failure or refractory hypoxemia, which increases the risk of complications, in response to airway management.

Generally speaking, anatomical difficulties arise when the provider has difficulty with one or more of the following tasks:

• Bag-mask ventilation (BMV): Difficulty in achieving adequate ventilation with a mask.
• Endotracheal intubation (ETI): Difficulty in visualizing the vocal cords, passing the endotracheal tube, or securing the airway.
• Supraglottic airway device (SAD) placement: Difficulty in inserting or achieving adequate ventilation with devices like a laryngeal mask airway.
• Cricothyrotomy or surgical airway: Challenges in establishing an airway via invasive methods when non-invasive methods fail

Complications such as desaturation, esophageal intubation, hemodynamic compromise, trauma and cardiac arrest, become more likely after failed intubation attempts. Even with first-attempt success, physiologically difficult airways can result in significant morbidity.

Rather than focusing solely on technical (procedural or anatomical) risks, a nuanced understanding of physiological  factors is crucial, to manage potential risk.

Physiologically Difficult Airways

A physiologically difficult airway refers to a clinical scenario in which the patient’s underlying physiological derangements pose a significant risk of harm during airway management, even if the technical aspects of securing the airway are straightforward. These challenges stem from the patient’s compromised physiology, which can lead to severe hypoxemia, hypotension, or cardiac arrest during or after intubation. Key features include:

1. Apnea tolerance and pre oxygenation: Patients with conditions like acute lung injury or ARDS often have reduced functional residual capacity (FRC). This is a direct result of decreased alveolar aeration, making them vulnerable to rapid desaturation during apnea.
2. Hemodynamic stability: Patients with hemodynamic instability (e.g., shock states or significant volume depletion) are at high risk for worsening hypotension or cardiac arrest due to the effects of sedative or paralytic drugs (e.g., propofol, etomidate, succinylcholine). The application of positive pressure changes hemodynamics by reducing pre load and increasing pulmonary vascular resistance, putting the right ventricle at risk This is particularly important in patients with pre-existing right ventricular failure or pulmonary arterial hypertension.
3. Severe Acidosis or Hypercapnia: Inability to compensate for respiratory acidosis due to chronic obstructive pulmonary disease (COPD) or severe asthma. A prolonged apnea period can lead to critical pH derangements.

Mitigating Physiological Risks:

Preoxygenation and mitigating the risk of desaturation.

Effective preoxygenation is critical for extending apnea time and minimizing hypoxemia during intubation. The premise is to de-nitrogenize the patients’ functional residual capacity, replacing it with oxygen. Key strategies include:

1. Positioning the patient upright. This increased the functional residual capacity area, available for preoxygenation.
2. Use a face mask at flush flow (essentially turning the oxygen on the wall up all the way). This eliminates entrainment of room air, by creating an oxygen-rich reservoir in the form of a cloud around the patients face, delivering 100% oxygen.
3. High-flow nasal oxygen at 40–60 L/min to match the patient’s inspiratory flow rate and prevent entrainment of room air is another useful tool used in pre oxygenation. This can, and should be left in place during laryngoscopy for apneic oxygenation.
4. Non-invasive respiratory support such as CPAP or BiPAP to recruit alveoli and optimize lung volume is another very useful option.

In conditions like ARDS, heterogeneous lung compliance results in Pendeluft (redistribution of gas within a diseased lung, without being replaced by fresh gas). This is as a result of differences in compliance and resistance within the lung. As soon as the respiratory muscles are paralyzed by neuromuscular blockade, air distribution changes, potentially making V/Q mismatch worse, causing desaturation.

The Role of Ventilator Management

The transition from spontaneous breathing to mechanical ventilation requires careful consideration of:

1. Pressure Distribution: Mechanical ventilation can worsen heterogeneity in lung compliance, leading to uneven pressure distribution and increased RV afterload.
2. Pendelluft and V/Q Mismatch: Strategies to optimize alveolar recruitment and oxygenation include using PEEP or high-flow nasal oxygen to stabilize lung volumes.

Understanding Right Ventricular Physiology

Patients with conditions such as massive pulmonary embolism (PE) or chronic pulmonary arterial hypertension (PAH) present unique challenges. Positive pressure ventilation can worsen right ventricular function by increasing afterload and reducing preload. The interaction between the right and left ventricles can either compensate for or exacerbate dysfunction, depending on the underlying physiology. Positive pressure ventilation improves left ventricular performance by reducing preload and increasing pulmonary vascular resistance.

1. Diagnosis: Evaluate RV function using echocardiographic measures such as TAPSE (tricuspid annular plane systolic excursion) and tissue Doppler imaging, for instance, low S’ values indicate poor contractile reserve.
2. Management: Focus on reducing RV afterload with pulmonary vasodilators (e.g., nitric oxide or inhaled milrinone). Avoid systemic hypotension by carefully managing MAP to exceed PA pressure.

Induction Strategies

Pharmacological Considerations:

1. Hemodynamic Stability: Tailor induction agents to the patient’s physiology. For example:
   1. Avoid high doses of propofol in patients with vasoplegia or RV dysfunction.
   2. Use ketamine cautiously in patients with severely depressed LV function, as it can exacerbate myocardial depression.
2. Adjuncts for RV Dysfunction: Consider diuresis, vasodilators, or pulmonary vasodilators to optimize RV function before induction.
3. Use of vasopressors prior to induction: Although the evidence is not overwhelming, the early use of vasopressors may help to stay ahead of the hypotension curve.

Practical Recommendations

1. Prepare for the Worst: Anticipate complications by having vasopressors, advanced airway devices, and pulmonary vasodilators readily available.
2. Optimize Preoxygenation: Focus on denitrogenation and maximizing FRC with non-invasive respiratory support or high-flow oxygen.
3. Tailor the Approach: Customize induction and ventilation strategies based on the patient’s specific pathophysiology.
4. Monitor Hemodynamics: Use echocardiography to assess RV and LV function, ensuring interventions are aligned with the patient’s cardiovascular status.
5. Collaborate Across Specialties: Difficult airways often require input from intensivists, anesthesiologists, and cardiologists, particularly in cases involving complex cardiovascular dynamics.

Conclusion

Managing a difficult airway is not just about technical expertise but also about understanding and mitigating the physiological challenges unique to each patient. By adopting a patient-centered approach and leveraging advanced tools and techniques, clinicians can navigate these high-risk scenarios with greater confidence and improve outcomes for their patients

More Reading:

1. The Physiologically Difficult Airway
2. Evaluation and Management of the Physiologically Difficult Airway: Consensus Recommendations From Society for Airway Management
3. The Physiologically Difficult Airway and Management Considerations
4. Chapter 6: The Physiologically Difficult Airway – J Mosier, F Rischard