What causes hypocapnia (low carbon dioxide levels in the blood)?

From the Guidelines

Hypocapnia is primarily caused by hyperventilation, which leads to excessive elimination of carbon dioxide from the blood, and can be triggered by various factors including anxiety and panic attacks, high altitude exposure, mechanical ventilation, and certain metabolic disorders. This condition occurs when breathing becomes too rapid or deep, resulting in carbon dioxide levels falling below the normal range (typically below 35 mmHg) 1. According to the BTS guideline for oxygen use in adults in healthcare and emergency settings, hyperventilation for any reason will produce hypocapnia, which may occur during pure hyperventilation during an anxiety attack or during physiological hyperventilation 1.

Common Causes of Hypocapnia

• Anxiety and panic attacks, which trigger rapid breathing
• High altitude exposure where the body compensates for lower oxygen levels
• Mechanical ventilation when settings are too aggressive
• Certain metabolic disorders like diabetic ketoacidosis
• Pain or fever which increase respiratory rate
• Central nervous system disorders affecting respiratory control
• Pulmonary conditions like asthma or pneumonia
• Pregnancy due to hormonal changes
• Medication side effects, particularly from salicylates or stimulants

Clinical Implications

Hypocapnia leads to respiratory alkalosis and can cause symptoms including lightheadedness, numbness in extremities, muscle twitching, and in severe cases, tetany or seizures 1. The body typically compensates for acute hypocapnia through renal mechanisms that excrete bicarbonate to normalize blood pH. It is essential to avoid routine hyperventilation with hypocapnia, especially in post-cardiac arrest patients, as it may worsen global brain ischemia by excessive cerebral vasoconstriction 1.

Ventilation Management

Ventilation rate and volume may be titrated to maintain high-normal Pa CO2 (40 to 45 mm Hg) or P ETCO2 (35 to 40 mm Hg) while avoiding hemodynamic compromise 1. Normocapnia should be considered the standard, and hyperventilation should be avoided, especially in hypotensive patients 1.

From the Research

Causes of Hypocapnia

• High intensity exercise can lead to depletion of CO2 from the body, resulting in hypocapnia 2
• Hyperventilation can cause a reduction in cerebral perfusion, leading to hypocapnia 3
• Respiratory alkalosis, or primary hypocapnia, occurs when alveolar ventilation exceeds that required to eliminate the carbon dioxide produced by tissues 4

Associated Conditions

• Acute heart failure (AHF) has been associated with hypocapnia, which is an independent predictor of in-hospital mortality in AHF patients 5
• Hypocapnia can cause or worsen cerebral ischemia, and may also injure other organs 6
• Hypocapnia can be used in the management of acute brain injury, but it can produce neuronal ischemia and injury, potentially worsening outcome 6

Mechanisms

• The cerebral vasculature is sensitive to changes in both the arterial carbon dioxide (PaCO2) and oxygen (PaO2) partial pressures, so that hypercapnia/hypoxia increases and hypocapnia/hyperoxia reduces global cerebral blood flow 3
• Sustained hypocapnia can cause a decrease in cerebral blood flow, and subsequent normocapnia can cause rebound cerebral hyperemia and increase intracranial pressure 6