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:
- Is the pH high or low? - low pH= acidosis and high pH = alkalosis.
- 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.
- 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
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.
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.
- 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.
- Most common: hyperventilation.