Mixed acid base disturbances: a clinical approach.

The analysis of a mixed acid-base disturbance begins with the history and physical examination from which data can be derived that make the clinician suspect a specific disturbance(s). The electrolytes are then evaluated with emphasis on the meaning of the values for serum bicarbonate, potassium and chloride concentration and on the level of the anion gap. Other laboratory data such as serum creatinine or glucose concentrations, blood cultures, and so forth, should also be reviewed for further clues to a possible disturbance(s). When it is clinically indicated, values for pH and Pco2 are obtained by blood gas determination. If the evidence up to this point indicates the presence of at least one disturbance, the data are examined to see if compensation for this disturbance is appropriate. If not, a mixed disturbance must be present. A normal pH in the setting of an abnormal serum HCO3(-) concentration or Pco2 also suggests a mixed disturbance since compensation rarely corrects the pH back to normal. Of course, a pH deviated in the opposite direction than that expected for a known primary disturbance makes the diagnosis of a mixed disturbance certain. The diagnosis of a mixed acid-base disturbance is therefore based on an analysis of all the clinical data and not just the blood gas measurements. Treatment of the disorders should be directed at maintaining a normal or near normal pH. Some combined acid-base disorders are important to recognize because they can result in a severe deviation in blood pH that demands immediate, specific therapy. Other mixed disturbances result in a pH which is near normal but are important to recognize since they can alert the clinician to the possibility of certain clinical derangements such as septic shock or drug ingestion. Careful analysis of mixed acid-base disturbances in this way is not peutic information to be used in caring for his (her) patients.