Mixed Acid–Base Disorders and Compensation

MIXED ACID–BASE DISORDERS

At times patients can simultaneously experience two or more in-dependent acid–base disorders. A normal pH in the presence of changes in the PaCO₂ and plasma HCO₃^− concentration immediately suggests a mixed disorder. The only mixed disorder that cannot occur is a mixed respiratory acidosis and alkalosis, because it is impossible to have alveolar hypoventilation and hyperventila-tion at the same time. An example of a mixed disorder is the simul-taneous occurrence of metabolic acidosis and respiratory acidosis during respiratory and cardiac arrest.

COMPENSATION

Generally, the pulmonary and renal systems compensate for each other to return the pH to normal. In a single acid–base disorder, the system not causing the problem will try to compensate by re-turning the ratio of bicarbonate to carbonic acid to the normal 20 1. The lungs compensate for metabolic disturbances by changing CO₂ excretion. The kidneys compensate for respiratory disturbances by altering bicarbonate retention and H⁺ secretion.

In respiratory acidosis, excess hydrogen is excreted in the urine in exchange for bicarbonate ions. In respiratory alkalosis, the renal excretion of bicarbonate increases, and hydrogen ions are retained. In metabolic acidosis, the compensatory mecha-nisms increase the ventilation rate and the renal retention of bicarbonate.

In metabolic alkalosis, the respiratory system compensates by decreasing ventilation to conserve CO₂ and raise the PaCO₂. Be-cause the lungs respond to acid–base disorders within minutes, compensation for metabolic imbalances occurs faster than com-pensation for respiratory imbalances. Table 14-7 summarizes compensation effects.