Acidosis & alkalosis

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

Glossary

• 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:

• lactic acidosis (increased lactic acid) (due to shock, infection or hypoxia, leading to increase in breathing muscles, producing acid).
• renal failure (increased uric acid or urate)
• diabetic ketoacidosis (note - alcohol can also cause ketoacidosis)
• drugs or toxins (salicylates, biguanides like metformin, ethylene glycol and methanol which are found in antifreeze).

Normal anion gap

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

• Most common: diarrhoea
• renal tubular acidosis (failure of the kidneys to transport H⁺ ions into urine);
• drugs, particularly acetazolamide;
• Addison's disease;
• pancreatic fistulae;
• and ammonium chloride ingestion.

Metabolic alkalosis

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

• Most common: vomiting

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

• Most common: respiratory failure.

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).