Acid-base disorders in children

Every blood gas is a story about two variables working in opposition. The Henderson-Hasselbalch equation sets the pH from the ratio of bicarbonate to dissolved carbon dioxide, and the body defends that ratio with two organs: the kidney controls bicarbonate over hours to days, and the lung controls carbon dioxide over seconds to minutes. When one variable moves abnormally, the other shifts in the same direction to pull the pH back toward normal, and that is called compensation. A disorder is "simple" when the compensation is appropriate and "mixed" when a second process is also at work, and the whole skill of acid-base interpretation is telling those two apart with a small set of calculations rather than guesswork. ^{[1] [11]}

In children, three things make the gas harder to read than in adults. Normal values shift with age, so a neonate runs a lower bicarbonate and carbon dioxide tension than a teenager. The common disorders differ, with diabetic ketoacidosis, bronchiolitis, asthma and inherited metabolic disease dominating rather than the chronic renal and respiratory disease of adults. And the child decompensates faster, because the smaller body buffers less acid and the airway gives way more quickly, so the gas is both a diagnostic and an emergency tool. This page builds the model around the systematic approach, then applies it to the four primary disorders and their paediatric causes. ^{[2] [7]}

Overview & Definition

Acid-base homeostasis keeps the arterial pH between 7.35 and 7.45 by balancing acid generation against excretion. The body produces acid constantly from protein metabolism and carbon dioxide load, and it disposes of it through the lung (as carbon dioxide) and the kidney (as titratable acid and ammonium). A disorder arises when generation overwhelms excretion, or when excretion fails, and the result is a pH that drifts outside the normal range. The venous or arterial blood gas measures the three numbers that define the disturbance: pH, the carbon dioxide tension, and the bicarbonate concentration. ^{[1] [11]}

The first decision on any gas is the pH itself. Acidemia is a pH below 7.35 and alkalemia is a pH above 7.45; a pH within the normal range with abnormal carbon dioxide and bicarbonate means a mixed disorder or a fully compensated single disorder. The pH names the dominant process and sets the direction of the analysis, because the body never over-compensates. If the pH is acidemic, the primary process is an acidosis; if it is alkalemic, the primary process is an alkalosis, and only then do the carbon dioxide and bicarbonate reveal whether that process is metabolic or respiratory. ^{[1] [2]}

Four primary disorders cover almost everything a clinician meets. Metabolic acidosis is a fall in bicarbonate from acid gain or bicarbonate loss. Metabolic alkalosis is a rise in bicarbonate from acid loss or bicarbonate gain. Respiratory acidosis is a rise in carbon dioxide tension from hypoventilation. Respiratory alkalosis is a fall in carbon dioxide tension from hyperventilation. Each has a characteristic compensation, and the task of the systematic approach is to confirm that the compensation is appropriate, because an inappropriate compensation unmasks a second disorder. ^{[1] [3]}

References17ShowHide

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