Diagnosis of Respiratory Failure
Respiratory failure is diagnosed by arterial blood gas (ABG) analysis showing PaO₂ <60 mmHg (8 kPa) and/or PaCO₂ >50 mmHg with pH <7.35, combined with clinical assessment of respiratory distress. 1, 2, 3
Diagnostic Criteria
Arterial Blood Gas Analysis (Essential)
- Obtain ABG immediately in any patient with suspected respiratory failure to measure PaO₂, PaCO₂, and pH—this is the definitive diagnostic test 2, 3
- Type 1 (Hypoxemic) Respiratory Failure: PaO₂ <60 mmHg (<8 kPa) with normal or low PaCO₂ 2
- Type 2 (Hypercapnic) Respiratory Failure: PaCO₂ >50 mmHg with pH <7.35, often accompanied by hypoxemia 1, 4
- Classify ARDS severity by PaO₂/FiO₂ ratio: mild (200-300 mmHg), moderate (100-200 mmHg), severe (≤100 mmHg) 2
Clinical Assessment Parameters
Assess work of breathing by observing:
- Respiratory rate >30 breaths/min indicates severe distress 2
- Use of accessory muscles (sternocleidomastoid, intercostals) 1
- Inability to speak in full sentences 2
- Paradoxical chest wall motion or abdominal breathing 1
Monitor mental status closely:
- Drowsiness, confusion, or agitation indicates impending respiratory failure requiring immediate escalation 2
- Deteriorating conscious level is a critical sign of treatment failure 1
Objective Respiratory Monitoring
Pulse oximetry should be monitored continuously for at least 24 hours, though SpO₂ alone cannot diagnose respiratory failure 1
Spirometry measurements can predict need for mechanical ventilation:
- Forced vital capacity (FVC) <20 mL/kg 1
- Maximum inspiratory pressure <30 cm H₂O 1
- Maximum expiratory pressure <40 cm H₂O 1
Alternative monitoring techniques when spirometry unavailable:
- Single breath count test: inability to count to ≥25 suggests respiratory muscle dysfunction 1
- Rising end-tidal CO₂ (EtCO₂) strongly predicts need for mechanical ventilation 1
Timing of Reassessment
- Initial ABG should be obtained at presentation 2, 3
- Repeat ABG after 1-2 hours of initial treatment to assess response 1, 2
- If no improvement after 4-6 hours, consider escalation to invasive ventilation 1
- Recheck ABG within 30-60 minutes after any change in FiO₂ or ventilator settings 1, 5
- Serial measurements every 4-6 hours once patient stabilizes 2
Supporting Diagnostic Studies
Obtain chest X-ray, electrocardiogram, and complete blood count immediately to identify underlying causes (pneumonia, pulmonary edema, pulmonary embolism, pneumothorax) 3
Critical Diagnostic Pitfalls
Do not rely on SpO₂ alone—patients with chronic CO₂ retention may have acceptable oxygen saturations despite severe hypercapnia 1
Avoid targeting SpO₂ >92% in COPD patients as excessive oxygen worsens V/Q mismatch and hypercapnia; target 88-92% instead 5, 3
Recognize that absence of respiratory distress does not exclude respiratory failure—central hypoventilation from drugs or CNS depression may present without dyspnea 4, 6
Failure to improve PaCO₂ and pH after 4-6 hours of non-invasive ventilation indicates treatment failure and need for intubation 1