Respiratory alkalosis

From Wikipedia, the free encyclopedia

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 disorders that elevate cellular pH to > 7.45.
Alkalemia refers to a arterial pH > 7.45.

Types

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

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.

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:

${\rm {HCO_{3}^{-}+H^{+}\rightarrow H_{2}CO_{3}\rightarrow CO_{2}+H_{2}O}}$

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

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 popular culture

In The Andromeda Strain, Michael Crichton's first novel, 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.