Hypercapnia

Hypercapnia is the elevation of arterial carbon dioxide tension (PaCO2) above normal levels, typically defined as PaCO2 greater than 45 mmHg. It most often results from inadequate ventilation that fails to remove CO2, but can also occur with increased CO2 production or impaired gas exchange. Acute hypercapnia causes respiratory acidosis, while chronic hypercapnia may show partial metabolic compensation by bicarbonate.

Common causes include respiratory depression from sedative or opioid drugs, central nervous system disorders, neuromuscular diseases such as amyotrophic lateral sclerosis or Guillain–Barré syndrome, airway obstruction, COPD with hypoventilation, obesity hypoventilation syndrome, and chest wall or restrictive lung diseases. Sleep-disordered breathing can contribute to daytime hypercapnia in some individuals. Environmental factors, such as high altitude, may worsen hypercapnia in susceptible patients.

Symptoms range from headaches and mild confusion to somnolence and reduced consciousness in more severe cases. Signs may include tachycardia or other signs of hypoxemia depending on the underlying disorder. The clinical presentation often reflects the balance between CO2 retention and the body’s compensatory responses.

Diagnosis relies on arterial blood gas analysis showing elevated PaCO2 and often a lowered pH in acute cases or metabolic compensation in chronic cases. Evaluation may include pulse oximetry, capnography, spirometry, chest imaging, and assessment of ventilation-perfusion status to identify the cause.

Treatment targets the underlying cause and the correction of ventilation. Mild hypercapnia may respond to optimization of airway clearance and nocturnal noninvasive ventilation; acute or severe hypercapnia often requires urgent airway support, frequently noninvasive ventilation followed by invasive ventilation if needed. Oxygen should be carefully titrated to avoid worsening hypercapnia. Prognosis depends on etiology and comorbidity; acute hypercapnia is a medical emergency with risk of respiratory failure.