What is negative pressure pulmonary edema (NPPE) in anesthesia?

Negative Pressure Pulmonary Edema in Anesthesia

Negative pressure pulmonary edema (NPPE) is a potentially life-threatening non-cardiogenic pulmonary edema caused by forceful inspiratory efforts against an obstructed airway, most commonly due to laryngospasm (>50%) during emergence from anesthesia, occurring in approximately 0.1% of all general anesthetics. 1

Pathophysiology

The development of NPPE involves several mechanisms:

1. Primary mechanism: Forceful inspiratory efforts against an obstructed airway generate extreme negative intrathoracic pressures

   • These negative pressures increase the hydrostatic pressure gradient across pulmonary capillary walls
   • This causes fluid leakage into the interstitial space and alveoli 1

2. Hemodynamic effects:

   • Increased venous return (preload) to the right ventricle
   • Increased pulmonary capillary blood volume
   • Increased right ventricular afterload due to hypoxia, acidosis, and negative intrathoracic pressure
   • Interventricular septal shift affecting left ventricular function 1

3. Contributing factors:

   • Hypoxic pulmonary vasoconstriction
   • Reactive catecholamine release
   • Alveolar capillary membrane disruption (stress failure) 1

Clinical Presentation

NPPE typically presents immediately or within 2-3 hours post-extubation with:

• Dyspnea and respiratory distress
• Agitation
• Cough with pink, frothy sputum
• Decreased oxygen saturation
• Diffuse bilateral alveolar opacities on chest imaging 1, 2

Risk Factors

• Young, muscular adults (male:female ratio 4:1)
• Upper airway surgery (particularly ENT procedures)
• Difficult airway
• Obesity
• Short neck
• Obstructive sleep apnea 1, 3

Diagnosis

Diagnosis is primarily clinical, based on:

• Temporal relationship to an episode of airway obstruction
• Characteristic clinical presentation
• Radiographic findings:
  • Chest X-ray: Bilateral alveolar opacities
  • CT scan: Centrally accentuated consolidations with surrounding ground-glass opacities 2, 4

Important differential diagnosis considerations:

• Cardiogenic pulmonary edema
• Aspiration pneumonitis
• COVID-19 (during pandemic) - Note that COVID-19 typically shows peripheral ground-glass opacities with vascular dilatations, while NPPE shows central opacities with decreased vascular clarity 4

Management

Immediate Actions:

1. Relieve airway obstruction - highest priority

   • If due to laryngospasm:
     • Apply continuous positive airway pressure with 100% oxygen
     • Consider Larson's maneuver (pressure at the "laryngospasm notch")
     • Administer propofol (1-2 mg/kg IV) if persistent
     • Use succinylcholine (1 mg/kg IV) for severe cases 1

2. Respiratory support:

   • Supplemental oxygen
   • Continuous positive airway pressure (CPAP) or non-invasive ventilation
   • Re-intubation with mechanical ventilation for severe cases 3

3. Pharmacological interventions:

   • Nebulized epinephrine (0.5 mg/kg) for post-extubation laryngeal edema 5
   • Consider dexamethasone (5-8 mg IV every 6 hours for adults) to reduce inflammation 5

Prevention:

• Use bite blocks during emergence to prevent biting on endotracheal tubes 1
• If biting occludes the tracheal tube, deflate the cuff to allow some gas flow 1
• Careful extubation planning for high-risk patients

Prognosis

With prompt recognition and appropriate management:

• Most cases resolve within hours
• Death is rare and usually attributable to hypoxic brain injury at the time of airway obstruction 1
• Delayed presentation can occur up to 2.5 hours post-obstruction 1

Pitfalls to Avoid

1. Misdiagnosis - Confusing NPPE with cardiogenic pulmonary edema or aspiration
2. Delayed recognition - Failing to consider NPPE in patients with post-anesthetic respiratory distress
3. Inadequate monitoring - Not monitoring patients for at least 2 hours after initial treatment 5
4. Delayed reintubation - Hesitating to reintubate when indicated

Remember that NPPE is a rare but potentially serious complication that requires immediate recognition and intervention to prevent morbidity and mortality.