The kidneys excrete endogenous acids. An adult normally produces about 1 to 2 mEq/kg/day of hydrogen ions, whereas a child produces 2 to 3 mEq/kg/day. The hydrogen ions from endogenous acid production are neutralized by bicarbonate, potentially causing the bicarbonate concentration to fall. The kidneys regenerate this bicarbonate by secreting hydrogen ions, maintaining the serum bicarbonate concentration in the normal range (20 to 28 mEq/L).
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Acidemia is a pH below normal (<7.35), and alkalemia is a pH above normal (>7.45). Acidosis is a pathologic process thatcauses an increase in the hydrogen ion concentration, and alkalosis is a pathologic process that causes a decrease in the hydrogen ion concentration. A simple acid-base disorder is a singleprimary disturbance. During a simple metabolic disorder, there is respiratory compensation; the Pco2 decreases during a metabolic acidosis and increases during a metabolic alkalosis. Withmetabolic acidosis, the decrease in the pH increases the ventilatory drive, causing a decrease in the Pco2. The fall in the CO2 concentration leads to an increase in the pH. This appropriate respiratory compensation for a metabolic process happensquickly and is complete within 12 to 24 hours.
During a primary respiratory process, there is metaboliccompensation mediated by the kidneys. The kidneys respondto a respiratory acidosis by increasing hydrogen ion excretion, increasing bicarbonate generation, and raising the serumbicarbonate concentration. The kidneys increase bicarbonateexcretion to compensate for a respiratory alkalosis; the serum bicarbonate concentration decreases. In contrast to a rapidrespiratory compensation, it takes 3 to 4 days for the kidneysto complete appropriate metabolic compensation. However, there is a small and rapid compensatory change in the bicarbonate concentration during a primary respiratory process. Theexpected appropriate metabolic compensation for a respiratorydisorder depends on whether the process is acute or chronic.
A mixed acid-base disorder is present when there is morethan one primary acid-base disturbance. An infant with bronchopulmonary dysplasia may have a respiratory acidosis fromchronic lung disease and a metabolic alkalosis from a diureticused to treat the chronic lung disease. Formulas are available for calculating the appropriate metabolic or respiratory compensation for the six primary simple acid-base disorders (Table 37-1). Appropriate compensation is expected in a simple disorder; it is not optional. If a patient does not have appropriatecompensation, a mixed acid-base disorder is present.
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Metabolic acidosis occurs frequently in hospitalized children; diarrhea is the most common cause. For a patient with
Metabolic acidosis PCO2 = 1.5 × [HCO3–] + 8 ± 2 Metabolic alkalosis PCO2 increases by 7 mm Hg for each 10-mEq/L increase in the serum [HCO3–] Respiratory acidosis Acute [HCO3–] increases by 1 for each 10-mm Hg increase in the PCO2 Chronic [HCO3–] increases by 3.5 for each 10-mm Hg increase in the PCO2 Respiratory alkalosis Acute [HCO3–] falls by 2 for each 10-mm Hg decrease in the PCO2 Chronic [HCO3–] falls by 4 for each 10-mm Hg decrease in the PCO2 an unknown medical problem, the presence of a metabolic acidosis is often helpful diagnostically because it suggests a relatively narrow differential diagnosis (Table 37-2).