Causes of Low CO2 (Hypocapnia)
Low CO2 levels (hypocapnia) are primarily caused by hyperventilation, which eliminates more carbon dioxide than the body produces, resulting in respiratory alkalosis. 1
Primary Mechanism
Hyperventilation is the fundamental cause of hypocapnia, defined as breathing in excess of metabolic needs and eliminating more CO2 than produced. 2 This drives PaCO2 below the normal physiological range of 4.6-6.1 kPa (34-46 mm Hg). 1
Clinical Causes
Compensatory Hyperventilation
- Metabolic acidosis triggers compensatory hyperventilation to lower CO2 and normalize pH—this is a normal physiological response but results in hypocapnia. 1
- The body increases ventilation to "blow off" CO2 in an attempt to raise blood pH when metabolic acids accumulate. 3
Respiratory Conditions
- Any condition causing increased respiratory drive without corresponding metabolic need produces hypocapnia. 1
- This includes anxiety-related hyperventilation, pain, fever, or central nervous system disorders affecting respiratory control. 2
High Altitude Exposure
- Reduced atmospheric pressure at altitude triggers hyperventilation as a compensatory mechanism for hypoxemia, leading to hypocapnia. 1
- The hypoxic stimulus drives increased ventilation, which secondarily lowers CO2 levels. 4
Hyperventilation Syndrome
- Characterized by physiologically inappropriate hyperventilation producing a variety of somatic symptoms. 2
- Symptoms stem from low PaCO2 itself and increased sympathetic adrenergic tone. 2
- The hypocapnia can be maintained even when the original trigger is no longer present. 2
Exercise-Related Hypocapnia
- High-intensity exercise can lead to CO2 depletion from the body, particularly noticeable during recovery periods. 5
- Post-exercise hypocapnia can result in a neural fatigue state. 5
Critical Clinical Distinction
Do not confuse hypocapnia (low CO2) with hypercapnia (high CO2), as they have opposite causes and require opposite management approaches. 1 Hypocapnia results from hyperventilation, while hypercapnia results from hypoventilation or ineffective ventilation. 6
Physiological Consequences
- Respiratory alkalosis develops with concurrent decreases in PaCO2, increases in pH, and compensatory decreases in blood HCO3- levels. 3
- Acute hypocapnia initially triggers cellular uptake of HCO3- and buffering by intracellular phosphates and proteins. 3
- Chronic hypocapnia results in longer-lasting decreases in renal reabsorption of HCO3-, with arterial pH approaching near-normal values. 3
Individual Variability
The degree of ventilatory inhibition by falling CO2 varies between individuals and depends on individual ventilatory responsiveness to CO2. 4 Those with higher CO2 sensitivity experience greater inhibition of their hypoxic ventilatory response when CO2 falls. 4