22nd June 2023

Metabolic acidosis

Metabolic acidosis is defined as a reduced serum pH (<7.35) with a reduced bicarbonate (<22 mmol/L).

In mild to moderate cases, there may be no symptoms of metabolic acidosis itself, but there may be symptoms of its underlying aetiology present, such as dehydration, vomiting or diarrhoea, lethargy or drowsiness in the case of poisoning, or signs of uraemia and fluid overload in the case of renal failure.

In severe cases, there may be evidence of attempted respiratory compensation with Kussmaul breathing (fast, deep breaths to offload CO₂ ), as well as cardiovascular effects such as cardiac arrythmias due to hyperkalaemia, peripheral venoconstriction and reduction in myocardial contractility in turn leading to decreased cardiac output and decrease in systemic blood pressure.

Anion Gap

The anion gap is the difference between the positive and the negative anions found in the body. This is calculated as follows:

Anion Gap (AG) = [Na⁺] – [Cl⁻ + HCO₃⁻]

All units in mmol/L
Normal range = 8-12 mmol/L

Some sources may include potassium in the calculation, but this is not something that has been traditionally included. The role of other anions and cations such as phosphate, sulphate, calcium and magnesium in the deriving of the formula have been left out here as beyond the scope of what is required for MRCP.

Patients who are known to have hypoalbuminaemia may have a falsely lowered anion gap on calculation. However, this can be corrected as follows:

Corrected Anion Gap (cAG) = AG + [0.25 x (40 – measured albumin g/L)]

Normal range = 8-12 mmol/L

Delta ratio

The delta gap can be used in the presence of a high anion gap metabolic acidosis (HAGMA) to assess whether the anion gap is in keeping with pure HAGMA or if there is evidence of co-existing normal anion gap metabolic acidosis or metabolic alkalosis (for example, chronic compensation of COPD).

The delta ratio can be calculated as follows:

Delta ratio =   [AG]-12 / 24-[HCO₃⁻]

Winter’s formula

Winter’s formula can be used in metabolic acidosis to calculate the expected pCO₂ and therefore ascertain if there is also a concurrent respiratory acidosis. If the measured pCO₂ is higher than the calculated pCO₂ , then it suggests that there is also respiratory acidosis present:

Calculated pCO₂ (mmHg) = [1.5 x HCO₃⁻ (mmol/L)] + 8 ± 2

mmHg can be converted to kPa as follows:

kPa = mmHg x 0.1333

The differentials for metabolic acidosis can be broadly separated into two categories: high anion gap and normal anion gap metabolic acidosis.