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Respiratory alkalosis is a medical condition in which increased respiration elevates the blood pH (a condition generally called alkalosis). It is one of four basic categories of disruption of acid-base homeostasis. This condition is commonly associated with a decrease in PaCO2 (hyperventilation).
- Alkalosis refers to disorders that elevate cellular pH to > 7.45.
- Alkalemia refers to a arterial pH > 7.45.
- Acute respiratory alkalosis occurs rapidly. For every 10 mmHg drop in PCO2 in arterial blood, there is a corresponding 2 mEq/L drop in bicarbonate ion due to acute compensation. During acute respiratory alkalosis, the person may lose consciousness where the rate of ventilation will resume to normal.
- Chronic respiratory alkalosis is a more long-standing condition. For every 10 mmHg drop in PCO2 in arterial blood, there is a corresponding 5 mEq/L drop in bicarbonate ion. The drop of 5 mEq/L of bicarbonate ion is a compensation effect which reduces the alkalosis effect of the drop in PCO2 in blood. This is termed metabolic compensation.
Respiratory alkalosis generally occurs when some stimulus (see "Causes" below) makes a person hyperventilate. The increased breathing produces increased alveolar respiration, expelling CO2 from the circulation. This alters the dynamic chemical equilibrium of carbon dioxide in the circulatory system, and the system reacts according to Le Chatelier's principle. Circulating hydrogen ions and bicarbonate are shifted through the carbonic acid (H2CO3) intermediate to make more CO2 via the enzyme carbonic anhydrase according to the following reaction:
- psychiatric causes: anxiety, hysteria and stress
- CNS causes: stroke, subarachnoid haemorrhage, meningitis
- drug use: doxapram, aspirin, caffeine and coffee abuse
- moving into high altitude areas, where the low atmospheric pressure of oxygen stimulates increased ventilation
- lung disease such as pneumonia, where a hypoxic drive governs breathing more than CO2 levels (the normal determinant)
- fever, which stimulates the respiratory centre in the brainstem
- high levels of NH4+ leading to brain swelling and decreased blood flow to the brain
Symptoms of respiratory alkalosis are related to the decreased blood carbon dioxide levels, and include peripheral paraesthesiae. In addition, the alkalosis may disrupt calcium ion balance, and cause the symptoms of hypocalcaemia (such as tetany and fainting) with no fall in total serum calcium levels.
In popular culture
In The Andromeda Strain, Michael Crichton's first novel published under his real name, only two people exposed to a pathogenic extraterrestrial microbe survive. Scientists investigating these survivors discover that each had abnormal blood pH. One, a baby, had respiratory alkalosis due to constant crying; the other, an old man, drinks Sterno. As a result, it becomes clear that the microbe cannot survive outside a narrow pH range.