What are the effects of hypercarbia (elevated carbon dioxide levels)?

From the Guidelines

Hypercarbia should be managed by addressing the underlying cause and supporting respiration, with a focus on maintaining an oxygen saturation of 88-92% to avoid worsening respiratory acidosis, as recommended by the BTS guideline for oxygen use in adults in healthcare and emergency settings 1.

Causes and Symptoms of Hypercarbia

Hypercarbia is a condition characterized by elevated levels of carbon dioxide (CO2) in the blood, typically defined as a partial pressure of CO2 (PaCO2) greater than 45 mmHg. It occurs when the body's respiratory system cannot effectively eliminate CO2 produced during metabolism. Common causes include:

• Respiratory depression from medications (like opioids or sedatives)
• Chronic obstructive pulmonary disease (COPD)
• Severe asthma
• Neuromuscular disorders affecting breathing
• Sleep apnea Symptoms may include:
• Headache
• Confusion
• Drowsiness
• Flushed skin
• In severe cases, altered consciousness or coma

Treatment and Management

Treatment focuses on addressing the underlying cause while supporting respiration. This might involve:

• Bronchodilators like albuterol (2-4 puffs every 4-6 hours) for bronchospasm
• Antibiotics for respiratory infections
• Non-invasive ventilation such as CPAP or BiPAP to support breathing
• In severe cases, mechanical ventilation For medication-induced hypercarbia, reducing or discontinuing the offending agent and possibly administering reversal agents like naloxone (0.4-2 mg IV for opioid reversal) may be necessary.

Importance of Oxygen Saturation

Maintaining an oxygen saturation of 88-92% is crucial to avoid worsening respiratory acidosis, as high concentrations of oxygen can lead to increased carbon dioxide levels and acidosis in patients with COPD or other respiratory conditions 1.

Risks of Hypercapnia

Hypercapnia and respiratory acidosis are inextricably linked and can lead to significant organ dysfunction if not promptly addressed. The risks of hypercapnia include:

• Cerebral vasodilation and increased intracranial pressure
• Compromised myocardial contractility
• Ventilator-induced lung injury in patients with ARDS

Guideline Recommendations

The BTS guideline for oxygen use in adults in healthcare and emergency settings recommends:

• Targeting an oxygen saturation of 88-92% in patients with COPD or other respiratory conditions
• Using Venturi masks or nasal cannulae to deliver targeted oxygen therapy
• Avoiding excessive oxygen use in patients with COPD or other respiratory conditions
• Monitoring blood gases regularly to check for rising PCO2 or falling pH 1

From the Research

Hypercarbia Overview

• Hypercarbia, also known as hypercapnia, is a condition characterized by an excess of carbon dioxide in the blood [(2,3,4,5,6)].
• It can occur in patients with chronic obstructive pulmonary disease (COPD) who receive supplemental oxygen therapy [(2,3,4,5,6)].

Causes of Hypercarbia

• The abolition of 'hypoxic drive' is a key mechanism leading to the development of oxygen-induced hypercapnia 3.
• Loss of hypoxic vasoconstriction and absorption atelectasis can also contribute to the increase in dead-space ventilation and the Haldane effect, leading to hypercapnia [(3,4)].
• Changes in physiologic deadspace can account for the hypercarbia developed by patients with acute exacerbations of COPD when treated with supplemental oxygen 4.

Clinical Implications

• The risk of hypercapnia should not be a deterrent to oxygen therapy in hypoxemic patients with chronic lung diseases, as hypoxemia may lead to life-threatening cardiovascular complications 3.
• A target oxygen saturation of 88% to 92% is recommended in patients with acute exacerbations of chronic obstructive pulmonary disease (AECOPD) and other chronic lung diseases at risk of hypercapnia 3.
• Oxygen should be administered only when oxygen saturation is below 88% 3.
• Unsupervised use of supplemental oxygen can lead to hypercapnic respiratory failure and severe respiratory acidosis, emphasizing the need for cautious oxygen supplementation in COPD 5.

Mechanisms of Oxygen-Induced Hypercarbia

• Oxygen-induced hypercarbia can occur due to the suppression of hypoxic respiratory drive [(2,6)].
• The CO2 recruitment threshold (PCO2 RT) can increase after oxygen supplementation, indicating a suppression of hypoxic respiratory drive 6.
• The dead space to tidal volume ratio (VD/VT) can rise after oxygen supplementation, but CO2 elimination (VCO2) remains constant 6.