Respiratory failure is failure of gas-exchange and can either be hypoxemic or hypercapnic. Acute respiratory failure may be type I or type II. In some patients, both types may co-exist.
| Type | Features |
| Type I or hypoxemic | PaO2 <60 mmHg; PaCO2 normal or low; most common type; presence of V/Q mismatch and shunt; increased A-a gradient; hypoxemia can be corrected by 100% oxygen in V/Q mismatch but not in shunts; seen in primarily lung diseases like pulmonary edema, pneumonia, pulmonary hemorrhage, COPD. |
| Type II or hypercapnic | PaCO2 >50 mmHg; PaO2 low; Normal A-a gradient; seen in chest wall and muscle diseases like myasthenia gravis, poliomyelitis, Guillain-Barre syndrome, asthma, kyphoscoliosis, flail chest, COPD, drug overdose, CNS depression. |
Acute deterioration in a patient with chronic respiratory failure is termed acute-on-chronic respiratory failure. It is seen in COPD in the presence of respiratory infections, bronchoconstriction, non-compliance to therapy etc.
ABG and history will help to differentiate between acute and chronic causes of respiratory failure.
*PCWP or pulmonary capillary wedge pressure of > 18mmHg suggests cardiac origin of pulmonary edema and excludes ARDS (in some cases both conditions may co-exist).
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