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

From the Guidelines

Hypercarbia, characterized by elevated carbon dioxide levels in the blood, can lead to respiratory acidosis and has severe consequences, including headache, confusion, and coma, emphasizing the need for prompt treatment focusing on improving ventilation. The effects of hypercarbia are multifaceted, involving not just the direct consequences of elevated CO2 levels but also the body's compensatory mechanisms and the potential for acidosis. According to the BTS guideline for oxygen use in adults in healthcare and emergency settings 1, hypercapnia and respiratory acidosis are closely linked, with the former potentially leading to the latter, especially in cases of acute elevation of blood carbon dioxide levels.

Causes and Consequences of Hypercarbia

The development of hypercapnia can be attributed to various factors, including depressed respiration due to opioid overdoses, obesity with hypoventilation, and neuromuscular disorders affecting the muscles of respiration 1. The consequences of hypercarbia can be severe, ranging from mild symptoms such as headache and drowsiness to more severe manifestations like confusion and coma. The vasodilatory effect of carbon dioxide can lead to flushed skin, dilated peripheral veins, and a bounding pulse, while cranial vasodilation may cause headache.

Treatment Approaches

Treatment of hypercarbia depends on the underlying cause but generally involves improving ventilation. For acute severe hypercarbia, immediate intervention with supplemental oxygen and possibly mechanical ventilation is necessary. In chronic cases, addressing the underlying condition is paramount, which may include the use of bronchodilators, antibiotics for respiratory infections, or non-invasive positive pressure ventilation such as CPAP or BiPAP. The BTS/ICS guideline for the ventilatory management of acute hypercapnic respiratory failure in adults 1 provides guidance on initial settings for ventilation, including oxygenation targets, tidal volumes, respiratory rates, and inspiratory to expiratory ratios, highlighting the importance of individualized adjustment and regular review of settings.

Considerations for Management

In managing hypercarbia, especially in conditions like acute respiratory distress syndrome (ARDS) and acute exacerbations of chronic obstructive pulmonary disease (AECOPD), the concept of permissive hypercapnia is crucial. Allowing for a higher than normal CO2 level (permissive hypercapnia) can be beneficial in reducing mortality by avoiding ventilator-induced lung injury, although it requires careful consideration of the potential effects on cerebral vasodilation and myocardial contractility. The guideline suggests that in ARDS, a peak airway pressure of 30 cmH2O is a trigger for employing permissive hypercapnia, aiming for a pH above 7.2.

Key Recommendations

• For acute severe hypercarbia, immediate intervention with supplemental oxygen and possibly mechanical ventilation is necessary.
• In chronic cases, addressing the underlying condition is paramount.
• Patients with chronic hypercarbia should avoid excessive oxygen therapy as it can worsen CO2 retention in some cases.
• The use of non-invasive positive pressure ventilation such as CPAP or BiPAP may be beneficial in certain cases, starting with inspiratory pressures of 8-12 cmH2O and expiratory pressures of 3-5 cmH2O.
• In ARDS, a strategy of permissive hypercapnia, targeting a pH above 7.2, can reduce mortality by minimizing ventilator-induced lung injury.

From the Research

Effects of Hypercarbia

The effects of hypercarbia, or elevated carbon dioxide levels, can be severe and far-reaching. Some of the key effects include:

• Increased intracranial pressure 2
• Induction or worsening of right heart failure 2
• Respiratory acidosis, characterized by increased PaCO2, decreased pH, and compensatory increases in blood HCO3- concentration 3
• Alveolar hypoventilation, which can lead to hypoxemia, especially in patients inspiring room air 3
• Decreased efficacy of non-invasive ventilation (NIV) and increased risk of NIV failure 2
• Dynamic hyperinflation and intrinsic positive end-expiratory pressure, which can contribute to increasing dead space 2

Consequences of Hypercarbia in Specific Conditions

In certain conditions, such as chronic obstructive pulmonary disease (COPD) exacerbation, hypercarbia can have particularly severe consequences, including:

• Increased morbidity and mortality rate due to mechanical ventilation 4
• Need for invasive ventilation, which can lead to further complications 4
• Decreased gas exchange and worsening respiratory acidosis 5

Treatment of Hypercarbia

Various treatments have been proposed to manage hypercarbia, including:

• Extracorporeal carbon dioxide removal (ECCO2R) to eliminate blood carbon dioxide and reduce adverse effects of hypercapnia 2
• Use of soda lime to absorb CO2 from the mask and ventilation tube, which can help reduce PaCO2 levels without the need for invasive ventilation 4
• Heliox administration during high-frequency jet ventilation to augment carbon dioxide clearance 5