Mixed Acid-base Picture

A mixed acid-base disturbance is where there is more than one primary disorder at a time. This often occurs in acutely unwell patients.

A mixed disorder should be suspected in several circumstances:

• Where the compensatory response is more or less than expected or does not appear to occur at all
• It is rare for compensation to achieve a completely normal pH, so if the pH is within normal limits, whilst CO₂ or HCO₃^– are not, then a mixed disorder is likely
• The proportionality of the response – if the change in HCO₃^– is not proportional to the change in anion gap in a metabolic acidosis, again, a mixed picture is likely
• Usually, in simple disorders, the compensatory response (of CO₂ or HCO₃^–) is in the same direction as the initial abnormal change (CO₂ or HCO₃^–). Therefore, when CO₂ or HCO₃^– move in the opposite direction to each other, a mixed picture is the usual explanation

NB it is impossible to have more than one respiratory disorder in a mixed picture. i.e. a metabolic acidosis and alkalosis can co-exist, but not a respiratory acidosis and alkalosis.

When considering all mixed disturbances, the clinical picture will usually indicate the underlying problem – the blood gas results should always be put in a clinical context.

Learning bite

If the results of the blood gas analysis appear conflicting or inconsistent with a single diagnosis, consider a mixed acid-base disturbance – always put the results in the clinical context of your patient.

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