Type 1 and Type 2 Respiratory Failure: ABG Values
Type 1 respiratory failure is defined by a PaO2 <8 kPa (60 mmHg) with a normal or low PaCO2, while Type 2 respiratory failure is defined by a PaO2 <8 kPa (60 mmHg) and a PaCO2 >6 kPa (45 mmHg). 1
Type 1 Respiratory Failure
Type 1 respiratory failure is characterized by:
- PaO2 <8 kPa (60 mmHg)
- Normal or low PaCO2
- Normal or low bicarbonate (HCO3-)
- Usually normal or alkalotic pH (depending on compensation)
This pattern represents hypoxemic respiratory failure without carbon dioxide retention. It occurs when there is a problem with oxygen transfer from the alveoli to the blood, but ventilation remains adequate to clear CO2.
Common Causes of Type 1 Respiratory Failure:
- Pneumonia
- Pulmonary edema
- Pulmonary embolism
- Acute respiratory distress syndrome (ARDS)
- Interstitial lung disease
Type 2 Respiratory Failure
Type 2 respiratory failure is characterized by:
- PaO2 <8 kPa (60 mmHg)
- PaCO2 >6 kPa (45 mmHg)
- Elevated bicarbonate (in chronic cases due to renal compensation)
- Acidotic pH (unless compensated in chronic cases)
This pattern represents hypercapnic respiratory failure with carbon dioxide retention. It occurs when there is inadequate alveolar ventilation to clear CO2.
Common Causes of Type 2 Respiratory Failure:
- COPD exacerbation
- Severe asthma
- Neuromuscular diseases
- Chest wall deformities
- Obesity hypoventilation syndrome
- Central nervous system depression (drug overdose, stroke)
Clinical Implications
The British Thoracic Society (BTS) guidelines recognize the importance of arterial blood gas analysis in diagnosing respiratory failure and guiding treatment decisions:
- For patients with Type 1 respiratory failure, supplemental oxygen therapy should target SpO2 of 94-98% 1
- For patients with Type 2 respiratory failure or at risk of hypercapnia (e.g., COPD), oxygen therapy should target SpO2 of 88-92% to avoid worsening hypercapnia 1
Important Considerations
Acute vs. Chronic Respiratory Failure: In chronic Type 2 respiratory failure, renal compensation occurs with increased bicarbonate levels, which may normalize pH despite elevated PaCO2.
Mixed Respiratory Failure: Both types can coexist, particularly in conditions like COPD exacerbation with pneumonia 2.
Alveolar-arterial (A-a) Gradient: This measurement can help distinguish between different causes of respiratory failure and should be considered when classifying respiratory failure 2.
NIV Indications: For Type 2 respiratory failure, NIV should be started when pH <7.35 and PaCO2 ≥6.5 kPa persists after optimal medical therapy 1.
Sleep Effects: Normal oxygen saturation can decrease during sleep, with mean nadir SaO2 of 90.4% (±3.1%) in healthy individuals. This should be considered when interpreting ABGs in sleeping patients 1.
Pitfalls to Avoid
Pre-measurement Oxygen Therapy: Administering oxygen before ABG measurement can confound the diagnosis of respiratory failure when using PaO2 alone. Consider using PaO2/FiO2 ratio for more accurate assessment 2.
Isolated PaCO2 Assessment: Using PaCO2 alone to classify respiratory failure may be inaccurate as both types can coexist. The alveolar-arterial gradient provides additional diagnostic information 2.
Misinterpreting Compensated States: In chronic respiratory failure, compensatory mechanisms may normalize pH despite abnormal gas tensions, potentially masking the severity of the condition.
Overlooking Mixed Disorders: Patients may have both respiratory and metabolic acid-base disorders simultaneously, requiring careful interpretation of all ABG parameters including pH and bicarbonate.
By understanding these characteristic ABG patterns, clinicians can more accurately diagnose respiratory failure, determine its type, and implement appropriate management strategies.