From the Guidelines
Low CO2 on a CMP is most likely caused by hyperventilation, which can occur during anxiety attacks or physiological hyperventilation, leading to hypocapnia. This condition can be physiological or pathological, depending on the underlying cause. According to the British Thoracic Society guideline for oxygen use in adults in healthcare and emergency settings 1, hyperventilation for any reason will produce hypocapnia. Some possible causes of low CO2 include:
- Hyperventilation during anxiety attacks
- Physiological hyperventilation
- Pathological hypoventilation, although this would typically cause hypercapnia, not hypocapnia
- Metabolic acidosis, although this is not directly mentioned in the provided study, it is a common cause of low CO2 It's essential to evaluate other laboratory values like anion gap, pH, and electrolytes to determine the specific type of acidosis or alkalosis. Clinical symptoms may include fatigue, confusion, rapid breathing, nausea, and in severe cases, shock or coma. Treatment depends on addressing the underlying cause while sometimes providing bicarbonate supplementation for severe acidosis. The study by 1 highlights the importance of understanding the physiological principles behind hypoventilation and hyperventilation in order to diagnose and treat the underlying cause of low CO2.
From the Research
Causes of Low CO2 on a CMP
- Metabolic acidosis can lead to a decrease in arterial partial pressure of carbon dioxide (PaCO2) as the body tries to compensate for the acidosis 2
- The decrease in PaCO2 is approximately 1 mmHg for every 1 mmol/l fall in serum bicarbonate (HCO3-) concentration 2
- Certain medications, such as acetazolamide, can worsen acidosis and potentiate clinical deterioration, leading to a decrease in CO2 levels 3
- Acetazolamide can cause severe acidosis and deterioration of clinical status in severe COPD cases, resulting in low CO2 levels 3
- Overproduction of organic acids, such as ketoacids or lactic acid, can lead to acute metabolic acidosis and a subsequent decrease in PaCO2 2
- Bicarbonate wasting and/or impaired renal acidification can lead to chronic metabolic acidosis and a decrease in PaCO2 2