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Acidosis is a common finding in many childhood disease states, ranging from self-resolving diarrheal illnesses to progressively degenerative mitochondrial disorders to acute septicemia and shock. Given the broad range of underlying illnesses and their disparate clinical consequences, identifying the specific etiology of acidosis is often crucial to a child’s hospital care. In circumstances where there is relative diagnostic certainty, appropriate identification and characterization of acidosis can contribute important clinical evidence for or against a prevailing diagnostic theory or management strategy. For patients whose illnesses have not been fully characterized, a rational approach to the interpretation of acidosis can help direct more specific diagnostic testing and further clinical evaluation.
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In this chapter, we discuss the pathophysiology, differential diagnoses, and workup strategies for acidosis, focusing primarily on metabolic acidosis. We include a section on respiratory acidosis in Special Considerations.
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Acidosis is a metabolic state characterized by a relative excess of hydrogen ions (H+). This relative excess can result from a number of processes: (1) an increase in H+ generation, (2) a decrease in H+ elimination via the kidneys, (3) a decrease in basic buffer molecules, or (4) a combination of the three. These processes reflect imbalances in the various counter-regulatory systems involved with acid–base homeostasis.
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A growing child generates a net of 1–3 mEq/kg/day of acid through normal metabolic processes. To maintain the optimal conditions for enzymatic activity (a pH between 7.35 and 7.45), the body must efficiently eliminate this and any additional acid load. Three primary mechanisms are responsible for maintaining this balance: the blood buffer system, respiratory ventilation, and renal excretion. Immediately upon exposure to excess acid, circulating buffers, including bicarbonate (HCO3–), bind excess H+, diminishing its concentration in the extracellular space. The excess H+ that binds to bicarbonate forms the “volatile” carbonic acid, which can then dissociate into carbon dioxide (CO2) and water. Assuming sufficient H+ excess and effective pulmonary ventilation, excess CO2 is then removed from the circulation through the pulmonary capillaries and alveoli. In response to persistently diminished pH, peripheral chemoreceptors and central respiratory centers increase minute ventilation to augment this process. Lastly, the proximal tubules in the kidney respond to excess H+ by increasing bicarbonate reabsorption and ammonia synthesis, the latter of which assists with renal hydrogen ion elimination in the form of ammonium ions (NH4+).
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Acidemia, defined as an arterial pH of less than 7.35, occurs when the body’s numerous compensatory mechanisms to address an acid load are overwhelmed or compromised. The compensatory mechanisms can be overwhelmed in the setting of excess “non-volatile” acid production, as occurs acutely in lactic acidosis, diabetic ketoacidosis (DKA), or more indolently in the organic acidopathies. The body’s buffer system can be compromised when there is a reduction in circulating bicarbonate from gastrointestinal losses (e.g. prolonged diarrheal ...