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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