Hyperventilation is abnormally rapid or deep breathing that eliminates carbon dioxide from the blood faster than it is produced by the body. As blood carbon dioxide levels fall, the blood becomes more alkaline (respiratory alkalosis), which causes a cascade of physiological effects including narrowing of blood vessels in the brain, altered nerve excitability, and shifts in calcium and potassium binding.
Symptoms of hyperventilation include tingling or numbness around the mouth and in the hands and feet, light-headedness, dizziness, chest tightness, palpitations, and a paradoxical feeling of being unable to get enough air despite breathing rapidly. In severe cases it can lead to fainting (syncope) as cerebral blood flow is reduced by vessel constriction.
Hyperventilation has several causes, the most common in otherwise healthy people being acute anxiety or panic. It is particularly relevant after cardiac arrest, where survivors may experience panic responses or PTSD that trigger episodes. However, hyperventilation in a cardiac arrest survivor should not automatically be attributed to anxiety without medical assessment, as it can also result from medical causes including pulmonary embolism, metabolic acidosis, or severe anaemia.
In the context of resuscitation and critical care, avoiding hyperventilation is a specific clinical concern: excessive ventilation of a patient in cardiac arrest or immediately afterwards causes raised intrathoracic pressure that impairs venous return and cardiac output, and causes harmful cerebral vasoconstriction in a brain already at risk from hypoxic injury. Current resuscitation guidelines specify target ventilation rates to avoid this.
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