What are the types of respiratory failure?

Types of Respiratory Failure

Respiratory failure is classified into two main types: Type 1 (hypoxemic) with PaO₂ <8 kPa (60 mmHg) and normal or low PaCO₂, and Type 2 (hypercapnic) with PaO₂ <8 kPa (60 mmHg) AND PaCO₂ >6 kPa (45 mmHg). 1

Primary Classification System

Type 1 Respiratory Failure (Hypoxemic)

• Defined by PaO₂ <8 kPa (60 mmHg) with normal or low PaCO₂ (typically ≤6 kPa or 45 mmHg), representing failure of oxygenation despite adequate ventilatory effort 1
• The primary pathophysiological mechanism is ventilation-perfusion (V/Q) mismatch, though right-to-left shunts, diffusion impairment, and alveolar hypoventilation also contribute 2, 1
• Common clinical scenarios include ARDS, pneumonia, and pulmonary edema 2
• Typically responds to oxygen therapy as the mainstay of treatment 2

Type 2 Respiratory Failure (Hypercapnic)

• Defined by PaO₂ <8 kPa (60 mmHg) AND PaCO₂ >6 kPa (45 mmHg), representing failure of the ventilatory pump to eliminate CO₂ 1
• The primary mechanism is alveolar hypoventilation, where minute ventilation is insufficient relative to CO₂ production 2, 1
• Contributing factors include increased airway resistance, dynamic hyperinflation with intrinsic PEEP (PEEPi), and inspiratory muscle dysfunction 2
• Common causes include COPD exacerbations, neuromuscular disorders, and chest wall deformities (scoliosis, thoracoplasty) 2

Additional Classification Dimensions

Temporal Classification

• Acute respiratory failure develops suddenly with rapid deterioration of arterial blood gases 2
• Chronic respiratory failure develops gradually over time, allowing compensatory mechanisms such as renal bicarbonate retention to develop 2
• Acute-on-chronic respiratory failure presents unique challenges due to altered baseline physiology and requires recognition of the patient's baseline status 2

ARDS Severity Classification (Subset of Type 1)

• Mild ARDS: PaO₂/FiO₂ 200-300 mmHg with minimum PEEP 5 cmH₂O 2
• Moderate ARDS: PaO₂/FiO₂ 100-200 mmHg 2
• Severe ARDS: PaO₂/FiO₂ ≤100 mmHg 2

Critical Diagnostic Approach

Essential Diagnostic Criteria

• Arterial blood gas analysis is mandatory for proper classification, as recommended by the British Thoracic Society 1
• The European Respiratory Society uses PaO₂ <8 kPa (60 mmHg) as the diagnostic threshold for respiratory failure 1
• Normal PaCO₂ range is 4.6-6.1 kPa (34-46 mmHg), and values >6 kPa indicate hypercapnia 2

Key Distinguishing Features

• Type 1 requires improvement of oxygenation through oxygen therapy, CPAP, or other interventions 1
• Type 2 requires ventilatory support to address both hypoxemia and hypercapnia, not just oxygen supplementation 1
• In Type 2 failure, administering high-flow oxygen without monitoring CO₂ can precipitate CO₂ narcosis and respiratory arrest 2

Management Implications by Type

Type 1 Management Principles

• High-flow nasal oxygen (HFNO) may reduce intubation rates compared to conventional oxygen therapy, with significant mortality reduction (ARD -15.8%) 2
• Target SpO₂ >94% in most cases of Type 1 failure 2
• Escalate to mechanical ventilation with lung-protective strategies if refractory hypoxemia persists 2

Type 2 Management Principles

• Target oxygen saturation of 88-92% to avoid worsening hypercapnia 2, 1
• NIV should be initiated when pH <7.35 and PaCO₂ >6 kPa (45 mmHg), particularly in COPD exacerbations with respiratory acidosis 2, 1
• Delaying NIV initiation when pH <7.35 and PaCO₂ >6.0 kPa misses the therapeutic window 2
• Consider invasive ventilation when pH <7.25 with persistent hypercapnia despite optimal therapy 1

Common Pitfalls to Avoid

• Do not administer high-flow oxygen to Type 2 patients without CO₂ monitoring, as this can worsen hypercapnia and precipitate respiratory arrest 2
• Do not assume all respiratory failure is Type 1 – always obtain arterial blood gas to distinguish between types, as management differs fundamentally 1
• Do not delay NIV in Type 2 failure when pH drops below 7.35 with elevated PaCO₂, as this represents the critical therapeutic window 2, 1
• NIV is contraindicated in patients with impaired consciousness, severe hypoxemia, or copious respiratory secretions 2