Published on 09/04/2015 by admin
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Last modified 09/04/2015
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1.5 Disorders of acid–base balance
A metabolic acidosis is any process, other than a rise in PaCO2, that acts to lower blood pH. It may occur through accumulation of metabolic acids (excess ingestion, increased production or reduced renal excretion) or through excessive loss of base (HCO3). Calculating the anion gap (see over) may help to establish the cause of a metabolic acidosis.
Metabolic acidosis is recognised on an ABG by low HCO3 (and negative base excess (BE)). There is normally a compensatory increase in alveolar ventilation to lower PaCO2. If respiratory compensation is overwhelmed, an acidaemia will result. Severity must be judged according to both the underlying process and the resulting acidaemia. An HCO3 < 15 mmol/L (or BE < 10) indicates a severe acidotic process whereas a pH < 7.25 (H+ > 55) constitutes serious acidaemia.
The dominant symptom in metabolic acidosis is often hyperventilation (Kussmaul’s respiration) owing to the respiratory compensation. Other signs are fairly non-specific or related to the underlying cause. Profound acidaemia (pH < 7.15; H+ > 70) may lead to circulatory shock, organ dysfunction, and, ultimately death.
Specific causes of metabolic acidosis are discussed in greater detail in the relevant cases in Part 2.
The anion gap
In blood, positively charged ions (cations) must be balanced by negatively charged ions (anions) to maintain electroneutrality. But when the main cations (Na+ + K+) are compared with the main anions (Cl− + HCO3−), there appears to be a shortage of anions or an anion gap.
The gap is made up of unmeasured anions such as phosphate and sulphate and negatively charged proteins (these are difficult to measure).
A raised anion gap (>18 mmol/L) therefore indicates the presence of increased unmeasured anions, e.g. lactate, salicylate.
A note on … lactic acidosis
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