What are the differences between Type 1 (hypoxemic) and Type 2 (hypercapnic) respiratory failure based on arterial blood gas (ABG) analysis?

Type 1 and Type 2 Respiratory Failure Based on ABG

Type 1 respiratory failure is defined by PaO₂ <8 kPa (60 mmHg) with normal or low PaCO₂, while Type 2 respiratory failure is defined by PaO₂ <8 kPa AND PaCO₂ >6.0 kPa (45 mmHg). 1

ABG Criteria for Classification

Type 1 (Hypoxemic) Respiratory Failure

• PaO₂ <8 kPa (60 mmHg) - equivalent to SpO₂ approximately 90% 1
• PaCO₂ normal (4.6-6.1 kPa) or low 1
• Results from failure of oxygenation despite adequate ventilation 1

Type 2 (Hypercapnic) Respiratory Failure

• PaO₂ <8 kPa (60 mmHg) 1
• PaCO₂ >6.0 kPa (45 mmHg) - this is the critical distinguishing feature 1
• Often accompanied by respiratory acidosis (pH <7.35) when acute 1

Important threshold clarification: The BTS/ICS guidelines specify that NIV should be considered when PaCO₂ is between 6.0-6.5 kPa, but NIV should be started when PaCO₂ ≥6.5 kPa with pH <7.35 after optimal medical therapy. 1

Key Distinguishing Features on ABG

Acute vs. Chronic Type 2 Failure

• Acute hypercapnic respiratory failure: pH <7.35 with elevated PaCO₂, indicating insufficient renal compensation 1
• Chronic compensated: Normal or near-normal pH despite elevated PaCO₂, due to renal bicarbonate retention over hours to days 1
• Acute-on-chronic: Elevated PaCO₂ with pH <7.35 despite baseline bicarbonate elevation 1

Critical pH Thresholds

• pH <7.35 (H⁺ >45 nmol/L) defines respiratory acidosis and triggers consideration for NIV 1
• pH <7.25 indicates severe acidosis with higher mortality risk 2
• Arterial hydrogen ion concentration is a more important prognostic factor than the degree of hypoxemia or hypercapnia alone 2

Common Pitfalls in ABG Interpretation

Oxygen Therapy Confounding

• Measuring PaO₂ after oxygen administration can mask the diagnosis of respiratory failure - use PaO₂/FiO₂ ratio instead when patients are already on supplemental oxygen 3
• In one study, using PaO₂/FiO₂ ratio identified an additional 16 cases of respiratory failure that would have been missed using PaO₂ alone 3

Mixed Type 1 and Type 2 Failure

• Both types can coexist - using PaCO₂ alone to classify can be misleading 3
• Calculate the alveolar-arterial (A-a) gradient to determine if hypercapnia is due to hypoventilation (Type 2) or V/Q mismatch (Type 1 with secondary hypercapnia) 3
• In one study, 22 of 57 cases initially classified as Type 2 using PaCO₂ alone were reclassified as Type 1 when A-a gradient was calculated, changing management in 18 cases 3

Practical ABG Interpretation Algorithm

1. First, assess oxygenation:

   • If on room air: PaO₂ <8 kPa indicates respiratory failure 1
   • If on supplemental oxygen: Calculate PaO₂/FiO₂ ratio (normal >400 mmHg or >53 kPa) 3

2. Second, assess ventilation:

   • PaCO₂ >6.0 kPa suggests Type 2 component 1
   • PaCO₂ ≥6.5 kPa with pH <7.35 mandates urgent intervention 1

3. Third, assess acuity:

   • pH <7.35 indicates acute or acute-on-chronic failure requiring immediate treatment 1
   • Normal pH with elevated PaCO₂ suggests chronic compensated failure 1

4. Fourth, calculate A-a gradient if both hypoxemia and hypercapnia present:

   • Normal A-a gradient with hypercapnia = pure hypoventilation (Type 2) 3
   • Elevated A-a gradient with hypercapnia = V/Q mismatch (Type 1) with secondary hypoventilation 3

Management Implications Based on ABG

Type 1 Failure

• Target SpO₂ 94-98% in most patients 1
• High-flow nasal oxygen may reduce intubation rates 4
• Does not typically respond to NIV unless specific indications present 4

Type 2 Failure

• Target SpO₂ 88-92% to avoid worsening hypercapnia 1, 4
• Repeat ABG after 1 hour of optimal medical therapy to determine if NIV is needed 1
• Start NIV when pH <7.35 and PaCO₂ ≥6.5 kPa persist after initial treatment 1
• Repeat ABG 1-2 hours after starting NIV, then again at 4-6 hours to assess response 1, 4

Critical Warning

Never administer high-flow oxygen to patients with suspected Type 2 failure without ABG confirmation and close monitoring - this can precipitate CO₂ narcosis and respiratory arrest. 4