Normal values:
pH = 7.35-7.45
pO 2 = 12-15 kPa
pCO 2 = 4.4 – 5.6 kPa
Bicarb = 21-28 mmol/L

Four Steps:

  1. 1. Assess oxygenation – pO 2 .
  2. 2. Assess pH
  3. 3. Determine primary problem by comparing direction of change of pCO 2 and pH.
    Opposite directions (i.e. a falling pH with a rising pCO 2 ) indicates primary respiratory problem.
    Same direction indicates primary metabolic problem.
  4. 4. Assess compensation by comparing direction of change of pCO 2 and bicarbonate and applying a compensation rule. If no rule fits, then two problems are probably occurring simultaneously – see later.

Four compensation rules

  1. 1. Acute respiratory acidosis: bicarbonate increases roughly 1mmol/L for every 1kPa elevation of pCO 2 above 5kPa.
  2. 2. Chronic respiratory acidosis: bicarbonate increases roughly 3mmol/L for every 1kPa elevation of pCO 2 above 5kPa.
  3. 3. Acute respiratory alkalosis: bicarbonate will decrease by 1.5 mmol/L for every 1kPa decrease in pCO 2 below 5kPa.
  4. 4. Metabolic acidosis – predicted pCO2 = (bicarbonate/5) + 1

Understanding Compensation

Campbell 's simplification, simplified (the Sinclair equation):

The aim of compensation is to return the levels of H + to normal – either by varying carbon dioxide or bicarbonate levels: the component varied will be the opposite to the one causing the problem in the first place.

i.e.
In a respiratory problem (CO 2 levels causing imbalance) – compensate with bicarbonate .
In a metabolic problem (bicarbonate levels causing imbalance) – compensate with CO 2 .

e.g.
In metabolic acidosis – increased PaCO 2 causes a rise in H+. Therefore the body compensates by raising bicarbonate levels, thus lowering H+.

The amount by which the body has compensated will depend on the amount of time the compensation process has occurred over – so in acute acid-base imbalance, there will only be a small degree of compensation, but in chronic imbalances, full compensation will have taken place.
As a rule of thumb – if PaCO 2 and bicarbonate move in the same direction, compensation is occurring. Although note that if they move in opposite directions, more than one pathology may be present.

If you Didn't Get that...

Consider the following chemical equation:
CO2 + H2O <-> H2CO3 <-> H+ + HCO3-

Acidity is defined as the concentration of hydrogen ions. Remembering the principles of equilibrium from A-level chemistry, an equilibrium shift to the right will produce more hydrogen ions and so increase acidity – or a shift to the left will decrease the numbers of hydrogen ions and so make the system more alkaline. A shift to the right would be produced by increasing carbon dioxide or decreasing bicarbonate levels, while a shift to the left would be produced by decreasing carbon dioxide or increasing bicarbonate levels.

Anion Gap


Normal value approximately 12.

Increased gap indicates metabolic acidosis – this helps detect acidosis from a venous sample.

In metabolic acidosis, the HCO 3 - is reduced, so the concentration of other anions (either Cl - or an unmeasured anion) must increase so that an excess positive charge does not accumulate. The anion gap effectively measures these other anions, so an increase indicates metabolic acidosis – this is referred to as high anion gap metabolic acidosis, or HAGMA. It can be caused by:
Ketoacidosis, lactic acidosis, chronic renal failure, salicylate poisoning, methanol poisoning, ethylene glycol poisoning.

If it is Cl - that increases, then the anion gap will not rise – this is known as normal anion gap metabolic acidosis, or NAGMA. Causes include:
Diarrhoea, renal tubular acidosis, acetazolamide use (carbonic anhydrase inhibitor)

Body Defences Against Acid-Base Imbalance

  1. 1. The chemical acid-base buffer system – this acts more or less immediately as it is essentially a chemical reaction and compensates for minor changes by locking up hydrogen ions – however it cannot add or remove hydrogen ions from the body.
  2. 2. The respiratory system – this acts within minutes to eliminate CO 2 or allow it to accumulate. (CO 2 combines with water to form H 2 CO 3 , an acid)
  3. 3. The kidneys – these can eliminate excess acid or base from the body, but act much more slowly (over hours to days).