MIXED ACID–BASE DISORDERS
Sometimes patients may present with more than one acid–base disorder and this is known as a mixed acid–base disorder. These may present as (i) severe acidemia, that is a low blood pH, (ii) with a normal or near normal pH or (iii) with alkalemia, that is, a high blood pH. Whatever the underlying cause, all mixed acid–base disorders are associated with abnormal levels of blood PCO2 and HCO3–.
For example, a patient with chronic bronchitis may also have renal failure. Both these disorders increase the concentration of H+ in the blood. Chronic bronchitis leads to respiratory acidosis while the renal failure causes metabolic acidosis. This patient will therefore present with a mixed acid–base disorder with a high blood PCO2 and H+ concentration but a low concentration of HCO3–. In some cases, however, the two disorders in a mixed acid–base disorder can be antagonistic, that is, have opposing effects on the concentration of H+ in blood. In this case the blood H+ concentration may be near normal although the PCO2 and HCO3– concentration will both be abnormal. For example, a patient with salicylate poisoning may have a metabolic acidosis together with a respiratory alkalosis. Patients may also present with metabolic and respiratory alkaloses. This could occur in someone with congestive cardiac failure who is on diuretic therapy. The former will cause a respiratory alkalosis and the latter a metabolic alkalosis. Such individuals will usually have a high blood pH and increased HCO3– but the PCO2 will be decreased.