Respiratory alkalosis

Respiratory alkalosis is an acid-base disorder characterised by a respiratory pathological process which increases the pH of the blood. If the alkalosis is severe, it can overcome the body's defense against alkalosis, causes the pH in the blood to increase beyond 7.45. This is called alkalaemia.

Metabolic alkalosis is similar but is rather due to a metabolic problem. Respiratory acidosis is sort of the opposite of respiratory alkalosis.

Respiratory alkalosis occurs due to hyperventilation, which causes pCO2 to decrease (hypocapnia). Respiratory alkalosis is rarely as clinically significant as the other acid-base disorders.

Etiology

Any condition which causes hyperventilation, i.e. minute ventilation beyond what is necessary for the body, can cause respiratory alkalosis. This is due to the "washing out" of CO₂ that occurs.

• Intentional hyperventilation
• Anxiety
• Pain or other distressing stimuli
• Pregnancy
• High altitude

Hypoxaemia causes hyperventilation as a compensatory reaction. If the arterial pO2 falls below 60 mmHg ventilation will be stimulated, which may normalise oxygen levels but wash out CO₂.

Pathophysiology

CO₂ is a cerebral vasodilator, so hypocapnia causes cerebral vasoconstriction, which reduces cerebral blood flow. This may cause syncope.

Alkalaemia may cause plasma proteins to bind more free calcium ions, causing the level of free calcium in the serum to decrease, effectively causing hypocalcaemia.

Alkalaemia also shifts the haemoglobin-oxygen dissociation curve to the left, which decreases tissue oxygenation.

Compensation

The initial compensation occurs to intracellular buffers like the haemoglobin buffers, and is relatively modest.

The healthy kidney can compensate for respiratory alkalosis by increasing its excretion of bicarbonate to a large degree. Renal compensation begins soon after the alkalosis, but it takes up to five takes to become complete.

Clinical features

Respiratory alkalosis is rarely clinically significant and rarely causes symptoms. It may cause symptoms like dizziness and syncope.

Diagnosis and evaluation

Arterial blood gas is essential in the evaluation of acid-base disorders. It will give the pH, bicarbonate level, pCO2, pO2, and lactate levels. In respiratory alkalosis, the pCO₂ is low (< 35 mmHg). If there is alkalaemia, the pH is > 7.45.

The renal compensation to respiratory alkalosis reduces bicarbonate by 2-5 units for every 10 mmHg reduction in pCO₂.

The cause of respiratory alkalosis is often apparent by the anamnesis.

Management

Treatment should be directed toward the underlying cause. If caused by anxiety, breathing into a bag may help.