What is the term for elevated carbon dioxide (CO2) in the blood?

Elevated Carbon Dioxide: Hypercapnia

Hypercapnia is the medical term for elevated carbon dioxide levels in the blood, defined as a PaCO₂ above the normal range of 4.6-6.1 kPa (34-46 mmHg). 1

Definition and Measurement

Hypercapnia occurs when the partial pressure of carbon dioxide (PaCO₂) in the arterial blood exceeds 6.1 kPa or 46 mmHg. This elevated level of carbon dioxide is a key indicator of type 2 respiratory failure, even if oxygen saturation remains within normal range. 1

Carbon dioxide in the blood is carried in three forms:

• 70-85% as bicarbonate
• 5-10% as dissolved carbon dioxide
• 10-20% bound to hemoglobin 1

Unlike oxygen, carbon dioxide carriage is not expressed as a saturation but rather by its partial pressure in the blood.

Causes of Hypercapnia

The four primary mechanisms that lead to hypercapnia are:

1. Alveolar hypoventilation or ineffective ventilation - By far the most common cause 1, 2

   • COPD is the most frequent disease producing hypercapnia
   • Rapid, shallow breathing increases dead space to tidal volume ratio
   • Respiratory muscle weakness

2. Increased concentration of CO₂ in inspired gas 1, 2

   • Iatrogenic cause - uncommon but should be excluded first
   • Can occur with poorly configured breathing circuits

3. Increased carbon dioxide production 1, 2

   • Usually only causes hypercapnia if minute ventilation is fixed
   • Can occur with sepsis or increased work of breathing

4. Increased external dead space 1, 2

   • Commonly seen in patients breathing through artificial circuits

Clinical Significance and Complications

Hypercapnia can lead to several physiological effects:

• Respiratory acidosis: When pH falls below 7.35 in the presence of elevated CO₂ 1
• Compensated respiratory acidosis: When the kidneys retain bicarbonate to buffer acidity, resulting in high PaCO₂ with high bicarbonate and normal pH 1
• Cerebral effects: Carbon dioxide is a vasodilator that can cause headache due to cranial vasodilation 1
• Neurological effects: High concentrations of CO₂ have hypnotic effects, potentially progressing from drowsiness to confusion to coma 1
• Cardiovascular effects: Patients may appear flushed with dilated peripheral veins and bounding pulse 1

Measurement Considerations

While arterial blood gas analysis is the gold standard for measuring PaCO₂, transcutaneous carbon dioxide monitoring (PtcCO₂) is increasingly used. However, it's important to note that PtcCO₂ tends to underestimate PaCO₂ levels, especially in cases of severe hypercapnia. 3

Management Principles

For patients at risk of hypercapnia:

• Oxygen therapy: Maintain oxygen saturation target of 88-92% to avoid worsening hypercapnia 1, 2
• Non-invasive ventilation: Consider when pH <7.35, PaCO₂ ≥6.5 kPa, and respiratory rate >23 breaths/min persist after optimal medical therapy 2
• Address underlying cause: Treatment should be directed at the primary cause of hypercapnia 4

Clinical Pitfalls

• Oxygen-induced hypercapnia: Between 20-50% of patients with AECOPD or obesity-hypoventilation syndrome are at risk of carbon dioxide retention if given excessively high oxygen concentrations 1
• Masking deterioration: Excessive oxygen therapy may delay recognition of clinical deterioration 1
• Severe acidosis: If respiratory rate exceeds 30 breaths/min with severe acidosis (pH <7.25), intubation should be considered 2

Remember that hypercapnia is not just a laboratory finding but a significant physiological disturbance that requires prompt recognition and appropriate management to prevent complications and improve outcomes.