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Respiratory alkalosis is a medical condition in which increased respiration elevates the blood pH beyond the normal range (7.35-7.45) with a concurrent reduction in arterial levels of carbon dioxide. This condition is one of the four basic categories of disruption of acid-base homeostasis.
- Alkalosis refers to the process due to which there is elevation of blood pH more than 7.45.
- Alkalemia refers to an arterial blood pH of greater than 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.
Signs and symptoms
Signs and symptoms of respiratory alkalosis are related to the decreased blood carbon dioxide levels, and include: peripheral paraesthesia. In addition, the alkalosis may disrupt calcium ion balance, and cause the symptoms of hypocalcaemia (such as tetany, hyperreflexia, muscle cramps, muscle twitching and fainting secondary to hypotension) with no fall in total serum calcium levels. In addition, hypokalemia results due to the shift of hydrogen ions from the intracellular fluid to the extracellular fluid. In contrast, chronic respiratory alkalosis leads to hyperphosphatemia and hypocalcemia, by inducing renal PTH-resistance.
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:
Respiratory alkalosis may be produced as a result of medical treatment (iatrogenically) during excessive mechanical ventilation, hyperventilation, or respiratory overstimulation . Other causes include:
- 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 or sepsis, which stimulates the respiratory centre in the brainstem
- Pain (due to hyperventilation)
- high levels of NH4+ leading to brain swelling and decreased blood flow to the brain
- vocal cord paralysis, compensation for loss of vocal volume results in over-breathing and breathlessness.
- compensation for a co-existing metabolic alkalosis
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.
- Arterial blood gas
- Chemical equilibrium
- Metabolic acidosis
- Metabolic alkalosis
- Respiratory acidosis
- Adrogué, HJ; Madias, NE (June 2010). "Secondary responses to altered acid-base status: the rules of engagement.". Journal of the American Society of Nephrology : JASN 21 (6): 920–3. doi:10.1681/ASN.2009121211. PMID 20431042.
- Arbus, GS; Herbert, LA; Levesque, PR; Etsten, BE; Schwartz, WB (16 January 1969). "Characterization and clinical application of the "significance band" for acute respiratory alkalosis.". The New England Journal of Medicine 280 (3): 117–23. doi:10.1056/NEJM196901162800301. PMID 5782512.
- Krapf, Reto; Jaeger, Philippe; Hulter, Henry N (10 April 1992), "Chronic respiratory alkalosis induces renal PTH-resistance, hyperphosphatemia and hypocalcemia in humans", Kidney International 42 (42): 727–734, doi:10.1038/ki.1992.340, ISSN 0085-2538
- "Medscape: Medscape Access". medscape.com.