What are the types of respiratory failure?

Types of Respiratory Failure

Respiratory failure is classified into two main types: Type 1 (hypoxemic) characterized by low oxygen with normal or low carbon dioxide, and Type 2 (hypercapnic) characterized by elevated carbon dioxide with or without hypoxemia. 1

Type 1 Respiratory Failure (Hypoxemic)

Diagnostic Criteria:

• PaO₂ <60 mmHg (or <8 kPa) or SpO₂ <88% with normal or low PaCO₂ 1, 2
• Represents failure of oxygenation despite adequate ventilatory effort 1

Pathophysiological Mechanisms:

• Ventilation-perfusion (V/Q) mismatch - blood flows through poorly ventilated lung regions 1
• Intrapulmonary shunting - blood bypasses ventilated alveoli entirely, flowing through completely unventilated or fluid-filled lung units 1
• Diffusion impairment - impaired gas transfer across alveolar-capillary membrane 1
• Alveolar hypoventilation - reduced minute ventilation 1

Common Clinical Causes:

• Acute Respiratory Distress Syndrome (ARDS) - bilateral infiltrates with severe hypoxemia, classified as mild (PaO₂/FiO₂ 200-300 mmHg), moderate (100-200 mmHg), or severe (≤100 mmHg) with mortality of 30-40% 1
• Pneumonia - alveolar consolidation creating shunt physiology 1
• Pulmonary edema - fluid-filled alveoli causing severe V/Q mismatch 1
• Pulmonary embolism - increased dead space ventilation 1

Type 2 Respiratory Failure (Hypercapnic)

Diagnostic Criteria:

• PaCO₂ ≥45 mmHg (>6.0 kPa) with pH <7.35 1, 2
• Often accompanied by hypoxemia 1
• Represents failure of the ventilatory pump 1

Pathophysiological Mechanisms:

• Alveolar hypoventilation - minute ventilation insufficient relative to CO₂ production 1, 3
• Increased work of breathing - flow-limited expiration during tidal breathing, initially with exercise then at rest 4, 1
• Dynamic hyperinflation with intrinsic PEEP (PEEPi) - slowed lung emptying prevents expiration to relaxation volume, creating an inspiratory threshold load 4, 1
• Inspiratory muscle dysfunction - chronic hypercapnia related to impaired muscle function with increased mechanical workload 4, 1
• Increased dead space ventilation - ventilation of non-perfused alveoli 3

Common Clinical Causes:

• COPD exacerbations - account for the majority of Type 2 failures, with worsening V/Q abnormalities and increased airway resistance 4, 1
• Obesity hypoventilation syndrome - combines restrictive mechanics with central drive abnormalities 1, 5
• Neuromuscular disorders (ALS, muscular dystrophy, myasthenia gravis) - progressive ventilatory pump failure 1
• Chest wall deformities (scoliosis, thoracoplasty) - restrictive mechanics 1
• Central nervous system depression - reduced respiratory drive 3

Temporal Classification

Acute Respiratory Failure:

• Sudden onset with rapid deterioration of arterial blood gases 4, 1
• No time for compensatory mechanisms to develop 1
• In COPD exacerbations, V/Q abnormalities increase substantially with mild to moderate intrapulmonary shunt suggesting complete airway occlusion by secretions 4

Chronic Respiratory Failure:

• Gradual development over time with compensatory mechanisms (e.g., renal bicarbonate retention) 1
• Patients adapt breathing patterns to avoid dyspnea and exhaustion at the expense of reduced alveolar ventilation 3

Acute-on-Chronic Respiratory Failure:

• Presents unique challenges due to altered baseline physiology 1
• Common in COPD patients with baseline hypercapnia who develop acute exacerbations 4

Critical Clinical Pitfalls

For Type 2 Respiratory Failure:

• Administering high-flow oxygen without monitoring CO₂ can precipitate CO₂ narcosis and respiratory arrest - oxygen worsens V/Q balance and contributes to PaCO₂ increase 1
• Target oxygen saturation should be 88-92% in Type 2 failure to avoid worsening hypercapnia 1
• Delaying NIV initiation when pH <7.35 and PaCO₂ >6.0 kPa misses the therapeutic window 1

For Type 1 Respiratory Failure:

• Standard chest radiographs are poor predictors of oxygenation defect severity - classic ARDS findings may be asymmetric, patchy, or focal 1
• NIV failure is an independent risk factor for mortality in Type 1 failure - delayed intubation in patients with ARDS or pneumonia who fail to improve within 1 hour should be avoided 1