Alkalosis

Respiratory Alkalosis

Respiratory alkalosis is caused by excess CO₂ removal (essentially the opposite of respiratory acidosis). This shifts the buffer equation to produce more CO₂ which uses up H⁺ ions, resulting in an increase in pH of the blood.

We would therefore expect to see a high pH with a low PaCO₂ – if you see this you know there is a respiratory alkalosis.

Compensation

In chronic respiratory alkalosis the kidneys retain more H⁺ ions in an attempt to correct the plasma pH. This takes a few days to occur, so you won’t see any compensation in the acute setting.

The increase in H⁺ ions are mopped up by HCO₃^-, so in a prolonged respiratory alkalosis you may see a low PaCO₂, a low HCO₃^-, and a high or normal pH.

Common Causes

Some common causes of respiratory alkalosis include:

• Anxiety
• Pain
• Febrile illness
• Hypoxia
• Pulmonary embolism
• Sepsis
• Pregnancy

Metabolic Alkalosis

Metabolic alkalosis is caused by an increase in HCO₃^-, which will mop up H⁺ ions in the plasma leading to an increase in pH. The respiratory system is (initially) unaffected so if you see a high pH with a high HCO₃^- and a normal PaCO₂ you know there is a metabolic alkalosis.

Compensation

To correct this imbalance the body may start to retain CO₂ in an attempt to shift the buffer equation in favour of producing more carbonic acid which will be converted to H⁺ and HCO₃^- ions, increasing the overall number of H⁺ ions and therefore reducing the pH.

This is respiratory compensation, and you would expect to see a high (or normal) pH, a high HCO₃^-, and a high PaCO₂.

Common Causes

Some common causes of metabolic alkalosis include:

• Vomiting
• Overdose of bicarbonate-containing compounds (e.g. antacids)
• Prolonged use of potassium-wasting diuretics
• Prolonged use of steroids
• Cushing’s disease