Hypocapnia

Hypocapnia is a state in which arterial carbon dioxide tension (PaCO2) falls below the normal range, typically defined as a PaCO2 under 35 mmHg. It most commonly results from hyperventilation and is often associated with respiratory alkalosis. By lowering paCO2, hypocapnia reduces cerebral blood flow through cerebral vasoconstriction and can produce symptoms such as lightheadedness, paresthesias, and, in some cases, fainting.

Pathophysiology involves a rise in blood pH due to loss of CO2, a key component of the body's acid-base balance. The alkalemic state shifts the oxygen-hemoglobin dissociation curve and increases the binding of calcium to albumin, which can transiently reduce ionized calcium and contribute to tingling, muscle cramps, or carpopedal spasm. The degree and duration of hypocapnia influence whether compensation by the kidneys alters bicarbonate levels over time.

Common causes include acute or deliberate overbreathing during anxiety or panic, pain, fever, sepsis, pregnancy, altitude exposure, and certain pulmonary or neurologic conditions that increase respiratory rate. It can also occur iatrogenically during mechanical ventilation when ventilatory settings over-ventilate the patient.

Clinical features range from a sense of dizziness and tingling around the mouth to tetany or syncope in more severe cases. Since symptoms are nonspecific, diagnosis relies on laboratory testing.

Diagnosis is based on arterial blood gas or capnography showing low PaCO2, often with a concurrent increase in pH (respiratory alkalosis). Serum bicarbonate may be reduced with acute hypocapnia and shows metabolic adjustment with chronic hypocapnia.

Management focuses on addressing the underlying cause and reversing hyperventilation when appropriate. Reassurance and breathing techniques (slow, controlled ventilation, paced breathing) are preferred; addressing anxiety or pain is important. Rebreathing into a bag is not routinely recommended due to risks of hypoxemia or unrecognized hypoventilation.