What is Type 1 respiratory failure, characterized by low oxygen (O2) or carbon dioxide (CO2) levels?

Type 1 Respiratory Failure: Definition

Type 1 respiratory failure is characterized by LOW OXYGEN (PaO₂ <8 kPa or <60 mmHg) with NORMAL or LOW carbon dioxide levels (PaCO₂ ≤6 kPa or ≤45 mmHg). 1, 2

Key Distinguishing Features

Type 1 (Hypoxemic) Respiratory Failure:

• Low O₂: PaO₂ <8 kPa (<60 mmHg) 1, 2
• Normal or low CO₂: PaCO₂ normal or decreased (typically ≤6 kPa or ≤45 mmHg) 1, 2, 3
• Primary problem is oxygenation failure despite adequate ventilation 4, 3

Type 2 (Hypercapnic) Respiratory Failure:

• Low O₂: PaO₂ <8 kPa (<60 mmHg) 1, 2
• High CO₂: PaCO₂ >6 kPa (>45 mmHg) 1, 2
• Primary problem is ventilatory pump failure with inadequate CO₂ elimination 4, 2

Pathophysiological Mechanisms of Type 1 Failure

The underlying causes of Type 1 respiratory failure involve problems with oxygen transfer rather than ventilation:

• Ventilation-perfusion (V/Q) mismatch: Most common mechanism where blood flows past poorly ventilated alveoli 4, 3, 5
• Right-to-left shunting: Blood bypasses ventilated alveoli entirely 4, 3
• Diffusion impairment: Oxygen cannot cross the alveolar-capillary membrane adequately 4, 3
• Alveolar hypoventilation: Can contribute when severe 4, 3

Common Clinical Scenarios for Type 1 Failure

• Acute respiratory distress syndrome (ARDS) 4
• Pneumonia 4
• Pulmonary edema 4
• Pulmonary embolism 5

Clinical Significance

The critical distinction is that Type 1 failure patients maintain adequate ventilation (normal/low CO₂) but cannot oxygenate properly, while Type 2 failure patients cannot ventilate adequately (high CO₂). 4, 2 This distinction is essential because:

• Type 1 failure typically responds well to supplemental oxygen alone 4
• Type 2 failure requires ventilatory support (non-invasive or invasive ventilation) to address CO₂ retention 4, 2
• Giving high-flow oxygen to Type 2 patients can worsen hypercapnia and precipitate CO₂ narcosis 4, 2

Diagnostic Approach

Arterial blood gas (ABG) analysis is mandatory to properly classify respiratory failure and guide treatment 1, 2:

• Measure ABG in all patients with acute breathlessness 1
• PaO₂ <8 kPa (60 mmHg) confirms respiratory failure 1, 2
• PaCO₂ level determines Type 1 (normal/low) versus Type 2 (>6 kPa or 45 mmHg) 1, 2
• pH assessment identifies respiratory acidosis in Type 2 failure 1

Management Implications

For Type 1 respiratory failure:

• Target oxygen saturation 94-98% in most patients 6
• Oxygen therapy, CPAP, or high-flow nasal oxygen as first-line interventions 4, 3
• Consider invasive ventilation with lung-protective strategies if refractory hypoxemia develops 4

Critical pitfall to avoid: Do not assume all hypoxemic patients have Type 1 failure—always check the CO₂ level, as 22-27% of respiratory failure cases have hypercapnia (Type 2) and require different oxygen targets and management strategies. 1