What is causing my patient's hypercapnia?

Causes of Hypercapnia in Patients

Hypercapnia is primarily caused by alveolar hypoventilation or ineffective ventilation, with common underlying conditions including COPD, neuromuscular disorders, chest wall deformities, central nervous system depression, and obesity hypoventilation syndrome. 1

Definition and Diagnosis

Hypercapnia is defined as a PaCO₂ above the normal range of 4.6-6.1 kPa (34-46 mmHg) and indicates type 2 respiratory failure, even if oxygen saturation remains normal. 2, 1

The four primary mechanisms leading to hypercapnia are:

1. Alveolar hypoventilation or ineffective ventilation - Most common cause
2. Increased concentration of CO₂ in inspired gas - Iatrogenic cause, particularly in patients breathing from external equipment
3. Increased carbon dioxide production - Especially when minute ventilation is fixed
4. Increased external dead space - Including V/Q mismatch 2, 1

Common Causes by Mechanism

1. Alveolar Hypoventilation/Ineffective Ventilation

• Airway disorders: COPD, asthma, bronchiectasis
• Neuromuscular weakness: Guillain-Barré syndrome, myasthenia gravis, ALS
• Chest wall disorders: Kyphoscoliosis, flail chest, obesity
• Central nervous system depression: Sedatives, opioids, stroke, brain injury
• Obesity hypoventilation syndrome: Reduced chest wall compliance 1

2. Increased CO₂ in Inspired Gas

• Rebreathing from external equipment
• Incorrectly configured ventilator circuits
• Exhausted CO₂ absorber in anesthesia circuit 2

3. Increased CO₂ Production

• Sepsis
• Increased work of breathing
• Fever
• Hyperthyroidism
• Malignant hyperthermia
• Excessive carbohydrate feeding 1

4. Increased Dead Space Ventilation

• Pulmonary embolism
• Pulmonary hypertension
• Emphysema
• V/Q mismatch 1

Clinical Manifestations

Hypercapnia can cause:

• Cerebral effects: Headache, confusion, drowsiness, progressing to coma
• Cardiovascular effects: Flushed appearance, dilated peripheral veins, bounding pulse
• Respiratory effects: Rapid, shallow breathing pattern 1

Special Considerations

Oxygen-Induced Hypercapnia

20-50% of patients with AECOPD or obesity-hypoventilation syndrome are at risk of CO₂ retention with excessive oxygen therapy. This occurs through several mechanisms:

• Loss of hypoxic drive
• Loss of hypoxic pulmonary vasoconstriction
• Absorption atelectasis
• Haldane effect 2, 1

For these patients, maintaining an oxygen saturation target of 88-92% is recommended to prevent worsening hypercapnia. 2

Compensated Respiratory Acidosis

When hypercapnia has been present for more than a few hours, the kidneys retain bicarbonate to buffer acidity, resulting in high PaCO₂ with high bicarbonate and normal pH. 2

Diagnostic Approach

1. Measure arterial blood gases to confirm hypercapnia (PaCO₂ > 6.1 kPa or 46 mmHg)
2. Assess for signs of respiratory acidosis (pH < 7.35)
3. Evaluate respiratory rate and pattern
4. Consider underlying causes based on clinical presentation
5. Rule out iatrogenic causes (e.g., excessive oxygen therapy in vulnerable patients) 2, 1

Management Considerations

• For patients at risk of hypercapnic respiratory failure, maintain oxygen saturation target of 88-92%
• Consider non-invasive ventilation (NIV) if pH <7.35, PaCO₂ ≥6.5 kPa, and respiratory rate >23 breaths/min persist after optimal medical therapy
• In patients with neuromuscular or chest wall disease, consider NIV when vital capacity is <1L and respiratory rate >20, even if normocapnic
• If respiratory rate exceeds 30 breaths/min with severe acidosis (pH <7.25), consider intubation 1

Remember that even minor infections can cause progressive hypercapnia in vulnerable patients, and bulbar dysfunction can impair effective coughing and worsen CO₂ retention. 1