Acidosis

Respiratory Acidosis

Respiratory acidosis is caused by CO₂ retention which causes an increase in carbonic acid in the blood, resulting in a greater number of H⁺ ions and therefore a lower pH.

So if a patient has a respiratory acidosis we would expect to see a low pH and an increased PaCO₂. If you see this, you know there is a respiratory acidosis – there is no other reason for this combination of results.

Compensation

If respiratory acidosis is prolonged, the body may try to compensate by retaining/producing more bicarbonate in an attempt to mop up the extra H⁺ ions.

This is metabolic compensation and results in an increased HCO₃^-. Remember it takes days for this to occur, so you won't see metabolic compensation in acute respiratory acidosis.

Common Causes

Some common causes of respiratory acidosis include:

• Chronic obstructive pulmonary disease
• Drug induced respiratory depression
• Pulmonary oedema
• Severe asthma

Metabolic Acidosis

Metabolic acidosis can be caused by a variety of conditions, for example excess lactic acid production or excess ketones as in the case of diabetic ketoacidosis. The result is an increase in H⁺ ions in the blood, and therefore a lower pH.

The difference between a metabolic acidosis and a respiratory acidosis is that HCO₃^- will be reduced in metabolic acidosis.

This is because HCO₃^- will immediately start mopping up excess H⁺ ions to become carbonic acid, which will be converted to H₂O and CO₂ which is then excreted via the lungs (a route that is unavailable in respiratory acidosis).

Compensation

Because there is an increase in plasma CO₂ production due to the buffering equation, the respiratory centres may increase ventilation (increased respiratory rate, volume, or both) to compensate for the extra CO₂.

This is respiratory compensation and results in an overall reduction in PaCO₂.

Common Causes

There are lots of potential causes of a metabolic acidosis, so to help us narrow it down we need to discuss the anion gap.