Definition
Respiratory dysfunction resulting in failure of oxygenation or ventilation (alveolar ventilation inadequate for CO2 production resulting in elevated PaCO2).
Evaluation
Hypoxemic respiratory failure: check ABG, calculate Alveolar-arterial O2 gradient (see section Appendix A: Formulas) and assess for response to oxygen therapy.
|
Mechanism |
A-a gradient |
Oxygen therapy? |
Causes/Comments |
|---|---|---|---|
|
Low inspired FiO2 |
Normal |
Improves |
High altitude, inhalation of gas with FiO2 <0.21 |
|
Hypoventilation |
Normal |
Improves |
↑PaCO2 on ABG. See causes below |
|
Diffusion defect |
↑ |
Improves |
Alveolar and/or interstitial inflammation and fibrosis. Rarely the cause of acute hypoxemia |
|
V/Q mismatch |
↑ |
Improves |
Airway (asthma/COPD), interstitial (interstitial lung disease), alveolar (edema, pneumonia), vascular (pulmonary embolism) |
|
Shunt |
↑ |
Does not improve |
Intracardiac shunt, intrapulmonary shunt (AVM), hepatopulmonary syndrome, alveolar collapse (atelectasis), alveolar filling (edema, pneumonia) |
Hypercapnic/hypercarbic respiratory failure.
|
Mechanism |
Causes/Comments |
|
Increased CO2 production |
Fever, infection, thyrotoxicosis, seizures |
|
Impaired central respiratory drive |
Sedatives, brainstem stroke/mass, CNS infection or trauma |
|
Impaired neuromuscular system |
Spinal cord injury, phrenic nerve injury, poliomyelitis, ALS, Guillain-Barré, myasthenia gravis, muscular dystrophy, meds (neuromuscular blocking agents, organophosphates), hypophosphatemia |
|
Impaired ventilatory apparatus |
Chest wall (kyphoscoliosis, obesity), pleura (pneumothorax), pleural effusion, airway (COPD, OSA, obesity hypoventilation syndrome, acute obstruction), lung parenchyma (fibrosis, edema, pneumonia) |
Management
Treatment involves specific therapy directed toward the underlying disease and respiratory supportive care which may include oxygen therapy, noninvasive positive pressure ventilation, or endotracheal intubation with mechanical ventilation. See Critical Care: Mechanical Ventilation and noninvasive positive pressure ventilation.
Chen H, Eisner MD. Acute Respiratory Failure. In: Williams MV, Flanders SA. (Eds.) Comprehensive Hospital Medicine: An evidence based and systems approach. Philadelphia: Saunders, an imprint of Elsevier, Inc. 2007