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BACKGROUND

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Metabolic acidemia is a common laboratory finding, and most cases are caused by conditions such as hypovolemia, systemic infection, and acute diarrhea. The challenge for the physician is to confirm that the acidemia has a presentation, history, and response to treatment that are consistent with one of these common illnesses rather than an alternate heritable or acquired disorder predisposing to acidemia. Specifically, a number of inborn errors of metabolism present with severe metabolic acidemia. This chapter presents a set of principles by which clinicians can recognize signs and symptoms of severe metabolic acidemia, treat severe acidemia if needed, and broaden the differential diagnosis when an acidemia is not adequately explained by the clinical presentation and laboratory workup.

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PHYSIOLOGY

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The term acidemia, or “acid in the blood,” is used to denote a state of acidic pH measured by a blood gas analysis or a serum bicarbonate level. The term acidosis is reserved for cases in which a specific acidic substance is suspected or has been identified in the blood (e.g. lactic acidosis, ketoacidosis).

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ACID BUFFERING

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Buffers in the blood and in the extracellular and intracellular fluid maintain normal body pH. In the blood, both fast-acting mechanisms (buffering by proteins and the bicarbonate–carbonic acid system) and slow-acting mechanisms (modulation of renal bicarbonate reabsorption) contribute to acid–base homeostasis. Quantitatively, proteins such as albumin and hemoglobin form the largest reserve of immediate buffer. The bicarbonate–carbonic acid system has the largest capacity to alter its flux in response to an acid challenge. Impairment of any of these mechanisms can decrease the body’s ability to respond to a perturbation in acid–base status.

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ACID PRODUCTION AND EXCRETION

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Acidemia of any type represents an imbalance between acid production and acid excretion. Physiologic acid production can generally be categorized into volatile (mostly carbon dioxide) and nonvolatile components. The body generates approximately 15,000 mmol of carbon dioxide per day—an amount equivalent to twenty 2-liter bottles of carbonated beverage.1,2 Pulmonary ventilation excretes most of this excess carbon dioxide. The majority of nonvolatile acids are generated by dietary acid intake, amino acid catabolism, and fatty acid oxidation (which produces ketones and lactic acid). An adult generates 50 to 70 mEq/day of nonvolatile acid.3 Most nonvolatile acid secretion occurs through renal mechanisms, including bicarbonate reclamation and ammoniagenesis.4

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ACIDEMIA SECONDARY TO BICARBONATE LOSS

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Because the bicarbonate–carbonic acid system is a critical buffering mechanism, metabolic acidemia may be caused solely by excessive loss of bicarbonate rather than by increased acid production. Bicarbonate losses may be either gastrointestinal or renal. Renal tubular acidosis is a relatively common cause of chronic acidemia and is due to renal tubular dysfunction. The anion gap is usually normal, owing to a compensatory increase in the plasma chloride (Cl) concentration (see later). Injury to the proximal renal tubules ...

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