Hypoxemic Respiratory Failure

• Impaired oxygenation (SaO2 <95%, PaO2 <80 on RA). 
• Alveolar to arterial gradient (A-a gradient) (PAO2 – PaO2): 
  • Estimated normal gradient = (Age/4) + 4. 
  • Alveolar gas equation: PAO2 = (FiO2 x [Patm - PH2O]) - PaCO2/RQ. 
    • FiO2 = fraction of inspired oxygen. 
    • Patm = atmospheric pressure: 760mmHg at sea level. 
    • PH2O = saturated vapor pressure of water: 47mmHg at body temperature of 37° Celsius. 
    • PaCO2 = the arterial partial pressure of CO2: normally 40mmHg. 
    • RQ = the respiratory quotient: normally 0.8. 
    • Common FiO2 estimates: 

The five causes of hypoxemia: 

• Elevated A-a gradient:
  1. Right-to-left shunt:
  • Anatomic: intracardiac (R to L), pulmonary AVM, hepatopulmonary syndrome. 
  • Physiologic: pneumonia, atelectasis, any alveolar filling process (e.g., pus, blood, or water). Prevents alveoli from being properly ventilated, yet perfusion of these non-ventilated alveoli continues. 
  2. Ventilation-perfusion (V:Q) mismatch: PE, obstructive lung disease, ARDS, pulmonary artery vasospasm. 
  3. Diffusion impairment: ILD, emphysema. 
  • Normal A-a gradient: 
  4. Alveolar hypoventilation: drug overdose (oversedation), obesity hypoventilation syndrome, muscular weakness, ALS, Guillain-Barré syndrome 
  5. Low inspired FiO2: e.g. high altitude 

  Hypercarbic Respiratory Failure 

  • PaCO2 ≥45mmHg. 
  • Decreased minute ventilation (respiratory rate and tidal volume): 
    • Decreased respiratory drive. 
    • Decreased function: nerves/muscles of respiration, thoracic cage function. 
  • Increased dead space (e.g. PE, COPD). 
  • Hypoventilation may cause hypoxemia but A-a gradient will be normal. 

  Upper Airway Compromise 

  Physical obstruction due to infection, trauma, tumor, tissue swelling or foreign body. If a difficult intubation is anticipated, consider fiberoptic scope for intubation and early notification of OHNS or General Surgery if a cricothyrotomy or tracheostomy may be required.