Respiratory alkalosis

Respiratory alkalosis
Classification and external resources
Davenport diagram
ICD-10	E87.3
ICD-9	276.3
DiseasesDB	406
MedlinePlus	000111
eMedicine	med/2009
MeSH	D000472

Respiratory alkalosis is a medical condition in which increased respiration (hyperventilation) elevates the blood pH (a condition generally called alkalosis). It is one of four basic categories of disruption of acid-base homeostasis.

Terminology

• Alkalosis refers to a high pH in tissue.
• Alkalemia refers to a high pH in the blood.

Types

There are two types of respiratory alkalosis: chronic and acute.

• Acute respiratory alkalosis occurs rapidly. 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 mM drop in pCO₂ in blood, there is a corresponding 5 mM of bicarbonate ion drop. The drop of 5 mM of bicarbonate ion is a compensation effect which reduces the alkalosis effect of the drop in pCO₂ in blood. This is termed metabolic compensation.

Mechanism

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:

$\rm HCO_3^- + H^+ \rightarrow H_2CO_3 \rightarrow CO_2 + H_2O$

The net result of this is decreased circulating hydrogen ion concentration, and thus increased pH (alkalosis). There is also a decrease in ionized blood calcium concentration.

Causes

Respiratory alkalosis may be produced accidentally (iatrogenically) during excessive mechanical ventilation. 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, which stimulates the respiratory centre in the brainstem
• pregnancy
• high levels of NH4+ leading to brain swelling and decreased blood flow to the brain
• sexual activity, which may induce excessive breathing due to excitation

Symptoms

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 some cases, from someone who has had acute respiratory alkalemia caused by chronic hyperventilation syndrome, this can sometimes result in massive amounts of pain and disorientation. The pH levels change dramatically in order to compensate for the lactic acid [which the muscles under normal fight or flight circumstances would be pumping out] it believes to be entering the blood stream. The dramatic shift causes a 'physical confusion' which translates to a burning pain that comes from everywhere, and can also alter the brain chemistry which leads to confusion at best and feeling like 'I was in the wrong body" for anywhere from five minutes to a half an hour as the body reorients itself. Note: this is in extreme cases.

In popular culture

In The Andromeda Strain, Michael Crichton's first novel, scientists instruct a town's sole adult survivor of an extraterrestrial-pathogenic epidemic to hyperventilate in order to raise his blood pH to a level inhospitable to the pathogen (the "strain" in the title).

See also

• Metabolic acidosis
• Metabolic alkalosis
• Respiratory acidosis
• Hypocalcemia