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Acidosis & alkalosis

Acidosis and Alkalosis is a difficult topic to approach, but with a little understanding it is actually fairly simple.


  • Acidosis - The pH of the blood is low. This means there are a lot of H+ ions.
  • Alkalosis - The pH of the blood in high. This means there aren't many H+ ions.
  • Respiratory - The cause of the pH change is respiratory.
  • Metabolic - The cause of this pH change is non respiratory (usually kidney)
  • Compensatory means that there is a change to the body's acid-base balance but the buffer systems are attempting to deal with it.

Working out what's going on

There are basically four abnormalities with regards to the acid-base balance in blood. You should ask yourself three questions:

  1. Is the pH high or low? - low pH= acidosis and high pH = alkalosis.
  2. Is CO2 abnormal? - if so, is this in keeping with the pH change? - i.e. when pH↑, CO2 or pH↓, CO2. These changes point to a respiratory acidosis/alkalosis. If there is no abnormality in CO2, it may be there has been a compensatory change.
  3. Is the HCO3- abnormal? - if so, is this in keeping with the pH change? - i.e. (opposite to CO2 changes) when pH↑, HCO3- OR pH↓ HCO3- then the change is metabolic. Otherwise, there may again have been a compensatory change.

The types of imbalance

At the end of asking these questions you should end with one of the five options below:

  • Metabolic acidosis with a high anion gap
  • Metabolic acidosis with a normal anion gap
  • Metabolic alkalosis
  • Respiratory acidosis with a high anion gap
  • Respiratory alkalosis

Metabolic acidosis

Unfortunately, life as ever, is not so simple. There are two types of metabolic acidosis - with or without an anion gap. Essentially, this is the difference in concentrations between cations (+ve ions) and anions (-ve ions). You calculate it using the following formula

Na+ - (Cl- + HCO3-) = 8-12mEq/l (that's the number of charges)

We then split metabolic acidosis into types of anion gap:

High anion gap

This usually occurs due to an increase in non-'measured' acids such as. The main causes are:

Normal anion gap

Essentially, what happens is that bicarbonate is lost or H+ ions are ingested (though Cl- is retained). This decreases blood pH.

Metabolic alkalosis

Essentially, there are lots of causes but basically the H+ ions go somewhere other than the blood.

Hypokalaemia causes a shift of K+ ions from intra- to extracellular space. To maintain electrical neutrality, cells shift H+ into the cells. Burns and the ingestion of a basic substance can also cause metabolic alkalosis.

Respiratory acidosis

The first thing to do is confirm respiratory failure by checking the PaO2 is low. This is important because they may need non-invasive ventilation if they are becoming acidotic.

Other causes are rare.

Respiratory alkalosis

  • Most common: hyperventilation.

There are numerous causes to hyperventilation: CNS - stroke, subarachnoid haemorrhage, meningitis; others - anxiety, altitude, hyperthermia, pregnancy, PE, drugs (e.g. salicylates).