What causes low carbon dioxide levels in the blood, also known as hypocapnia?

Causes of Low Carbon Dioxide in Blood (Hypocapnia)

Hyperventilation is the primary cause of low carbon dioxide levels in the blood, occurring when breathing exceeds metabolic needs and eliminates more CO2 than the body produces. 1

Primary Mechanism

The fundamental cause of hypocapnia is hyperventilation—breathing in excess of metabolic requirements—which eliminates carbon dioxide faster than it is produced, resulting in respiratory alkalosis. 1, 2

• The British Thoracic Society guidelines explicitly state that "hyperventilation for any reason will produce hypocapnia" 1
• This can occur during pure hyperventilation (such as anxiety attacks) or during physiological hyperventilation 1
• Normal blood CO2 range is 4.6-6.1 kPa (34-46 mm Hg), and hyperventilation drives levels below this 1

Clinical Causes of Hyperventilation Leading to Hypocapnia

Psychological/Anxiety-Related Causes

• Panic disorder and anxiety attacks are major causes of hyperventilation-induced hypocapnia 2, 3, 4
• Hyperventilation syndrome produces a broad spectrum of symptoms through low PaCO2 and increased sympathetic tone 2
• Patients with panic disorder chronically hyperventilate to keep pCO2 low, potentially due to hypersensitive CO2 chemoreceptors 3
• Premenstrual dysphoric disorder (PMDD) can also trigger hyperventilation and panic responses similar to panic disorder 5

Physiological Compensation

• Metabolic acidosis triggers compensatory hyperventilation to lower CO2 and normalize pH 1
• The body's chemoreceptors sense pH changes and stimulate increased ventilation to blow off CO2 1
• This is a normal physiological response but results in hypocapnia 1

Respiratory Conditions

• Asthma patients may develop hypocapnia through hyperpnea and hyperventilation 4
• Hypocapnia in asthma is linked to bronchoconstriction and symptom exacerbation 4
• Any condition causing increased respiratory drive without corresponding metabolic need can produce hypocapnia 1

High Altitude

• Reduced atmospheric pressure at altitude triggers hyperventilation as a compensatory mechanism 1
• The alveolar gas equation shows that decreased inspired PO2 at altitude leads to compensatory hyperventilation 1
• This physiological response lowers PACO2, resulting in hypocapnia 1

Clinical Consequences

• Chronic hyperventilation may cause cerebral hypoxia through vasoconstriction, perpetuating anxiety symptoms 3
• Hypocapnia produces respiratory alkalosis and elevated blood pH 2
• Symptoms can include dizziness, paresthesias, chest tightness, and dyspnea 2
• In panic disorder, baseline end-tidal CO2 levels are significantly lower in patients who subsequently panic 5

Important Clinical Pitfall

Do not confuse hypocapnia (low CO2) with hypercapnia (high CO2)—they have opposite causes and require opposite management approaches. While hypercapnia results from hypoventilation or ineffective ventilation (common in COPD) 1, 6, hypocapnia results from hyperventilation 1. Treating hyperventilation-induced hypocapnia requires addressing the underlying cause of excessive breathing, not increasing ventilation further.