Diagnostic Criteria for Acute Respiratory Failure
Acute respiratory failure is diagnosed by arterial blood gas analysis showing PaO₂ <60 mmHg (or SpO₂ <88%) and/or PaCO₂ ≥45 mmHg with pH <7.35, classified as Type 1 (hypoxemic) or Type 2 (hypercapnic) based on PaCO₂ levels. 1, 2
Arterial Blood Gas Thresholds
Type 1 Respiratory Failure (Hypoxemic)
- PaO₂ <60 mmHg (<8 kPa) with normal or low PaCO₂ 1, 3
- SpO₂ <88% indicates potentially lethal hypoxemia requiring immediate oxygen supplementation 3, 2
- pH typically normal or elevated (>7.40) due to compensatory hyperventilation 3
- Results from V/Q mismatch, shunt, diffusion limitation, or low inspired oxygen 2
Type 2 Respiratory Failure (Hypercapnic)
- PaO₂ <60 mmHg (<8 kPa)** AND **PaCO₂ >45 mmHg (>6 kPa) 1, 2
- pH <7.35 defines respiratory acidosis requiring intervention 1, 3
- PaCO₂ >49 mmHg (>6.5 kPa) with pH <7.35 after 60 minutes of optimal therapy indicates need for non-invasive ventilation 3, 4
- Respiratory rate typically >23 breaths/min in acute decompensation 4
Critical pH Thresholds
- pH <7.35 with PaCO₂ >49 mmHg: Acute respiratory acidosis requiring NIV consideration 1, 3
- pH <7.15: Severe mixed acidosis requiring immediate intervention with controlled oxygen and urgent NIV 4
- pH <7.1 with base excess <-10: Consider intravenous bicarbonate therapy 3
Clinical Assessment Requirements
Arterial blood gas measurement is mandatory in most patients with acute breathlessness to differentiate respiratory failure types and guide management. 1
High-Risk Populations Requiring ABG
- Neuromuscular diseases, chest wall deformity, or obesity (may have respiratory failure without significant dyspnea) 1
- Acute confusional states 1
- COPD exacerbations with suspected hypercapnia 1
- Severe spinal conditions causing chest wall restriction 4
Timing of ABG Measurement
- Repeat ABG after 30-60 minutes of initiating or changing oxygen therapy to verify response without precipitating respiratory acidosis 3
- Repeat ABG after 1-2 hours of NIV initiation to assess treatment response 4
- Immediate repeat if clinical deterioration occurs at any time 3
Oxygenation Targets by Patient Type
- Patients without CO₂ retention risk: Target SpO₂ 94-98% 3
- Patients with COPD or hypercapnic risk: Target SpO₂ 88-92% using controlled oxygen delivery (24-28% Venturi mask or 1-2 L/min nasal cannula) 3, 4
Associated Clinical Signs
Hypoxemic Failure
- Tachypnea and increased work of breathing 5
- Cyanosis (when SpO₂ <85%) 5
- Altered mental status from cerebral hypoxia 5
- Tachycardia and arrhythmias 5
Hypercapnic Failure
- Altered consciousness or confusion (CO₂ narcosis) 5
- Flapping tremor (asterixis) 5
- Bounding pulse and warm peripheries (CO₂ vasodilation) 5
- Morning headaches from nocturnal hypoventilation 5
Common Pitfalls
Pulse oximetry alone does not assess ventilation or acid-base status; ABG remains essential for critically ill patients. 3 SpO₂ may appear adequate while severe hypercapnia and respiratory acidosis develop undetected.
Do not delay ABG measurement in patients with neuromuscular disease or chest wall deformity, as they may have profound respiratory failure without subjective dyspnea due to chronic adaptation. 1
Venous blood gas cannot replace arterial sampling for diagnosis, though pH and HCO₃⁻ correlate reasonably well; PaCO₂ prediction from venous samples is unreliable in acute respiratory failure. 6