Definition of Acute Respiratory Failure
Acute respiratory failure is defined as a failure to maintain adequate gas exchange, characterized by abnormalities in arterial blood gas tensions: Type 1 (hypoxemic) failure is PaO₂ <60 mmHg (8.0 kPa) with normal or low PaCO₂, while Type 2 (hypercapnic) failure is PaO₂ <60 mmHg (8.0 kPa) with PaCO₂ >50 mmHg (6.65 kPa). 1
Core Diagnostic Criteria
The fundamental distinction between the two types of acute respiratory failure is critical for management:
Type 1 (Hypoxemic) Respiratory Failure
- PaO₂ <60 mmHg (8.0 kPa)
- PaCO₂ normal or decreased
- Results from lung parenchymal disease, impaired gas exchange, or ventilation-perfusion mismatch
- Common causes include pneumonia, pulmonary edema, ARDS, and pulmonary embolism 1
Type 2 (Hypercapnic) Respiratory Failure
- PaO₂ <60 mmHg (8.0 kPa)
- PaCO₂ >50 mmHg (6.65 kPa)
- Often accompanied by respiratory acidosis (pH <7.35)
- Results from inadequate alveolar ventilation due to respiratory pump failure
- Common causes include COPD exacerbations, neuromuscular diseases, chest wall deformities, and severe airway obstruction 2, 1
ARDS as a Specific Subtype
When acute respiratory failure manifests as ARDS, the Berlin Definition provides additional specificity:
- Timing: Within 1 week of known clinical insult
- Oxygenation: PaO₂/FiO₂ ≤300 mmHg with minimum 5 cmH₂O PEEP
- Imaging: Bilateral radiographic opacities not fully explained by cardiac failure or fluid overload
- Severity classification:
- Mild: 200 < PaO₂/FiO₂ ≤300 mmHg
- Moderate: 100 < PaO₂/FiO₂ ≤200 mmHg
- Severe: PaO₂/FiO₂ ≤100 mmHg 3
Clinical Recognition
Arterial blood gas analysis is essential for diagnosis - transcutaneous oxygen saturation (SpO₂) monitoring alone is insufficient to diagnose or classify respiratory failure 2. The presence of respiratory acidosis (pH <7.35) in Type 2 failure indicates acute or acute-on-chronic decompensation and signals need for urgent intervention 1, 4.
Key Clinical Indicators Requiring ABG Assessment:
- SpO₂ <90% despite supplemental oxygen
- Respiratory rate >25 breaths/min
- Significant dyspnea or respiratory distress
- Altered mental status (may indicate hypercapnia)
- Patients with neuromuscular disease, chest wall deformity, or obesity (who may have respiratory failure without obvious dyspnea) 1
Important Caveats
The definition emphasizes acute onset - this distinguishes it from chronic respiratory insufficiency, though acute-on-chronic presentations are common, particularly in COPD patients who may be chronically hypercapnic at baseline 4. In these patients, the development of acidosis (pH ≤7.35) indicates acute decompensation requiring intervention.
Measurement timing matters: arterial blood gases can improve rapidly with initial medical treatment and supplemental oxygen, so repeat sampling after a short interval may be necessary to determine if respiratory failure persists and whether advanced respiratory support is indicated 1.