Respiratory failure is defined as inadequate gas exchange due to malfunction of one or more components of the respiratory system. There are two main types of acute respiratory failure: hypoxemic and hypercarbic. Hypoxemic respiratory failure is defined by arterial O₂ saturation <90% while receiving an increased inspired O₂ fraction. Acute hypoxemic respiratory failure can result from pneumonia, pulmonary edema (cardiogenic or noncardiogenic), and alveolar hemorrhage. Hypoxemia results from ventilation-perfusion mismatch and intrapulmonary shunting.

Hypercarbic respiratory failure is characterized by alveolar hypoventilation and respiratory acidosis. Hypercarbic respiratory failure results from decreased minute ventilation and/or increased physiologic dead space. Conditions associated with hypercarbic respiratory failure include neuromuscular diseases (e.g., myasthenia gravis), disease processes causing diminished respiratory drive (e.g., drug overdose, brainstem injury), and respiratory diseases associated with respiratory muscle fatigue (e.g., exacerbations of asthma and chronic obstructive pulmonary disease [COPD]). In acute hypercarbic respiratory failure, PaCO₂ is typically >50 mmHg. With acute-on-chronic respiratory failure, as is often seen with COPD exacerbations, considerably higher PaCO₂ values may be observed. The degree of respiratory acidosis, the pt's mental status, and the pt's degree of respiratory distress are better indicators of the need for mechanical ventilation than a specific PaCO₂ level in acute-on-chronic respiratory failure. Two other types of respiratory failure are commonly considered: (1) perioperative respiratory failure related to atelectasis, and (2) hypoperfusion of respiratory muscles related to shock.