What is acute hypercapnic respiratory failure?

Acute Hypercapnic Respiratory Failure

Acute hypercapnic respiratory failure (AHRF) 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, characterized by a pH <7.35 and a PCO2 >6.5 kPa, which persists despite initial medical therapy. 1

Definition and Pathophysiology

AHRF results from an imbalance between respiratory load and the capacity of the respiratory muscles to cope with that load, leading to:

• Elevated arterial carbon dioxide levels (PaCO2 >6.5 kPa or >45 mmHg)
• Respiratory acidosis (pH <7.35)
• Usually mild hypoxemia that is easily corrected 1

The severity of AHRF is classified by pH levels:

• Mild to moderate: pH 7.25-7.35
• Severe: pH <7.25 (indicates consideration for invasive mechanical ventilation) 1

Common Causes

AHRF complicates approximately 20% of acute exacerbations of COPD and signals advanced disease with limited long-term prognosis 1. Other common causes include:

1. Obstructive lung diseases:

   • COPD exacerbations
   • Severe asthma
   • Bronchiectasis
   • Cystic fibrosis

2. Restrictive disorders:

   • Neuromuscular diseases (e.g., motor neurone disease, muscular dystrophies)
   • Chest wall deformities (e.g., severe kyphoscoliosis, thoracoplasty)
   • Obesity hypoventilation syndrome

3. Central respiratory drive depression:

   • Drug overdose (opioids, benzodiazepines)
   • Central nervous system disorders

Clinical Manifestations

Clinical features of AHRF are often non-specific and may include:

• Dyspnea
• Tachypnea
• Use of accessory respiratory muscles
• Altered mental status (confusion, drowsiness)
• Asterixis (flapping tremor)
• Cyanosis (in severe cases)
• Cor pulmonale signs in chronic cases

Management Approach

1. Initial Assessment and Oxygen Therapy

• Controlled oxygen therapy is the first step to maintain SpO2 94-98% in non-chronic CO2 retainers 2
• Use nasal cannulae (2-6 L/min) or simple face mask (5-10 L/min) 2
• For patients with known COPD or other risk of chronic CO2 retention, target SpO2 88-92% 1
• Avoid high-flow oxygen in patients at risk of hypercapnia as this may worsen respiratory acidosis

2. Medical Management

• Treat the underlying cause (e.g., bronchodilators and steroids for COPD exacerbation)
• Optimize fluid balance
• Consider respiratory stimulants like doxapram only as a temporary measure in specific situations 3
• Doxapram may be used as a short-term aid in hospitalized patients with acute respiratory insufficiency superimposed on COPD to prevent elevation of arterial CO2 during oxygen administration 3

3. Non-Invasive Ventilation (NIV)

NIV should be initiated when respiratory acidosis persists despite maximal medical treatment and appropriate oxygen therapy 1:

Indications for NIV:

• pH <7.35 and PaCO2 >6.5 kPa despite maximal medical therapy 1
• Respiratory distress with increased work of breathing
• No contraindications to NIV

Initial ventilator settings for bi-level pressure support:

• IPAP (inspiratory positive airway pressure): 12-15 cmH2O
• EPAP (expiratory positive airway pressure): 3-5 cmH2O
• Adjust based on patient comfort and response

Monitoring during NIV:

• Continuous monitoring of vital signs and oxygen saturation
• Arterial blood gas analysis at 1-2 hours after initiation 1
• If pH and PaCO2 deteriorate after 1-2 hours on optimal settings, consider alternative management plan 1

Contraindications to NIV:

• Inability to protect airway
• Severe agitation
• Hemodynamic instability
• Recent facial or upper airway surgery 2

4. Invasive Mechanical Ventilation

Consider invasive mechanical ventilation if:

• NIV fails (pH continues to deteriorate despite optimal NIV)
• pH <7.25 with severe respiratory distress 1, 2
• Contraindications to NIV exist
• Patient has decreased consciousness or inability to protect airway

5. Emerging Therapies

• Extracorporeal CO2 removal (ECCO2R) may be considered in specialized centers for patients who fail NIV but are not suitable for invasive ventilation 1
• This technique is still being evaluated and should only be used in centers with appropriate expertise 4

Special Considerations for Specific Conditions

COPD

• Use longer expiratory times and shorter inspiratory times to avoid further hyperinflation 1
• Monitor for auto-PEEP (intrinsic PEEP)
• NIV has strong evidence for reducing mortality and need for intubation 1

Neuromuscular Diseases and Chest Wall Disorders

• NIV should be initiated early, even before acidosis develops 1
• Lower pressure support is typically needed for neuromuscular diseases (8-12 cmH2O)
• Higher pressures may be needed for chest wall disorders (up to 20-30 cmH2O) 1
• Consider 1:1 inspiratory to expiratory ratio 1

Obesity Hypoventilation Syndrome

• Higher EPAP may be needed to overcome upper airway obstruction
• Address sleep-disordered breathing component

Prognosis and Outcomes

AHRF signals advanced disease and limited long-term prognosis in conditions like COPD, with median survival following recovery from AHRF reported as approximately 1 year 1. Mortality risk factors include:

• Older age
• Severe acidosis (pH <7.25)
• Hypotension
• Elevated urea 5
• Need for invasive mechanical ventilation

Common Pitfalls to Avoid

1. Excessive oxygen administration in patients with chronic CO2 retention, which can worsen hypercapnia
2. Delayed initiation of NIV when indicated
3. Failure to recognize NIV failure and need for escalation to invasive ventilation
4. Inadequate monitoring during NIV treatment
5. Inappropriate patient selection for NIV (patients with contraindications)
6. Neglecting the underlying cause while focusing only on ventilatory support

Early recognition and appropriate management of AHRF are essential to improve outcomes and reduce mortality in this critical condition.