What is hypercapnic respiratory failure?

Hypercapnic Respiratory Failure

Hypercapnic respiratory failure is defined as an inability of the respiratory pump, in concert with the lungs, to provide sufficient alveolar ventilation to maintain a normal arterial PCO2, conventionally characterized by a pH <7.35 and a PCO2 >6.5 kPa (48.75 mmHg). 1

Definition and Pathophysiology

• Hypercapnic respiratory failure results from inadequate alveolar ventilation leading to carbon dioxide retention 1
• It is typically accompanied by mild hypoxemia that is usually easily corrected 1
• When pH falls below 7.25, this represents severe acidosis that may require consideration for mechanical ventilation 1
• The condition may present as acute, chronic, or acute-on-chronic respiratory failure 1

Common Causes

• Chronic Obstructive Pulmonary Disease (COPD) - most common cause 2, 3
• Obesity Hypoventilation Syndrome (OHS) 2, 3
• Neuromuscular diseases (NMD) - including motor neurone disease, muscular dystrophies 1
• Chest wall deformities (CWD) - such as severe kyphoscoliosis 1
• Sleep-disordered breathing 1
• Respiratory muscle weakness 1
• Drug-induced respiratory depression (opioids, benzodiazepines) 3
• Lower respiratory tract infections 3
• Congestive heart failure 3

Clinical Presentation

• Breathlessness is a common presenting symptom 1
• Varying levels of consciousness - from alert to severely obtunded 1
• Tolerance of hypercapnia varies considerably between patients 1
• Some patients may be excessively sleepy with minimal elevation of pCO2, while others remain alert despite severe hypercapnia 1
• In neuromuscular disease, any elevation of pCO2 may herald an impending crisis 1
• In COPD, the degree of acidosis is more important than the absolute level of hypercapnia 1

Diagnosis

• Arterial blood gas (ABG) measurement is essential for diagnosis 1
• Chest radiography to identify underlying causes or complications 1
• Assessment for reversible factors contributing to respiratory failure 1
• In some cases, echocardiography may be needed to exclude acute pulmonary edema 1

Management

Initial Management

• Controlled oxygen therapy targeting SpO2 88-92% in COPD and other at-risk patients 1
• This reduces mortality and the frequency/severity of hypercapnic respiratory failure 1
• Treat underlying causes and reversible factors 1

Non-invasive Ventilation (NIV)

• NIV should be started when pH <7.35 and pCO2 >6.5 kPa persist despite optimal medical therapy 1
• NIV is the treatment of choice for ventilatory support in most cases of hypercapnic respiratory failure 2, 4
• Severe acidosis alone does not preclude a trial of NIV in an appropriate setting with access to staff who can perform endotracheal intubation if needed 1
• NIV settings must be tailored to the underlying condition:
  • For COPD: longer expiration and shorter inspiration times to avoid hyperinflation 2
  • For NMD: lower pressure support (8-12 cm) unless significant skeletal deformity exists 1
  • For severe kyphoscoliosis: higher inspiratory positive airway pressure (>20, sometimes up to 30 cm) may be required 1
  • For OHS: higher expiratory positive airway pressure (10-15 range) often needed 1

Invasive Mechanical Ventilation (IMV)

• NIV should not delay escalation to IMV when appropriate 1
• IMV should be considered when NIV fails or is contraindicated 2
• Continued use of NIV when the patient is deteriorating, rather than escalating to IMV, increases mortality 1

Advanced Therapies

• Extracorporeal CO2 removal (ECCO2R) may be considered in cases where severe respiratory acidosis cannot be managed by conventional ventilation alone 2, 4
• This approach is still being evaluated and requires further research 4

Prognosis and Outcomes

• In-hospital mortality for hypercapnic respiratory failure is approximately 12.8% 3
• Prognostic factors affecting survival include:
  • Age (older patients have higher mortality) 1
  • Severity of acidosis (pH <7.25 associated with worse outcomes) 5
  • Presence of hypotension 5
  • Elevated urea levels 1
  • Comorbidities (respiratory tract infections and neuromuscular disease associated with increased risk of death) 3
• AHRF signals advanced disease with high risk for future episodes and limited long-term prognosis in COPD 1

Special Considerations

• In neuromuscular disease, diaphragm involvement may precede locomotor disability 1
• Minor infections can trigger progressive hypercapnia over 24-72 hours in NMD/CWD patients 1
• Bulbar dysfunction in NMD renders voluntary cough less effective and makes NIV more difficult to implement 1
• Regular auditing of NIV practice is recommended to maintain standards 1
• Multidisciplinary involvement is essential for both inpatient care and follow-up after discharge 6