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Metabolic acidosis in the neonate can be caused by several reasons, including increased acid intake from exogenous sources; increased endogenous production of an acid, such as seen in an inborn error of metabolism (IEM); inadequate excretion of acid by the kidneys; or excessive loss of bicarbonate in urine or stool. Presence or absence of an anion gap (AG) can help to distinguish the underlying etiology.


In general, with a pure or uncompensated metabolic acidosis, every 10 mEq/L fall in bicarbonate (HCO3) results in an average pH fall of 0.15. Neonates have an average arterial pH of 7.37 (range of 7.35–7.45). The average bicarbonate level in a neonate is 20 mEq/L. A diagnosis of metabolic acidosis can be made when the pH is less than 7.35 and a base deficit greater than 5 exists.1


The AG is calculated by subtracting the serum concentrations of the measured anions (bicarbonate and chloride) from the cation sodium (Figure 60-1). The AG equation can be written as AG = ([Na+]) − ([Cl] + [HCO3]). A normal AG is typically less than 12 mEq/L.1 If the AG is elevated (ie, > 15 mEq/L), then there are anions that have not been accounted for, and an investigation must be performed to search for the culprit. Common anions that result in an elevated AG include lactate and the ketone bodies β-hydroxybutyrate and acetoacetate. Neonates with intoxication IEMs usually have elevated AGs from accumulation of the toxic organic acid, such as isovaleric acid in isovaleric acidemia (IVA), in addition to lactate and ketone bodies secondary to clinical decompensation.


Anions are molecules that contain a negative charge. Cations are molecules that contain a positive charge. Generally, the total amount of anions and cations in plasma, serum, or urine are equivalent. The anion gap (AG) is the difference between the measured anions and cations and can be calculated by subtracting the concentrations of bicarbonate and chloride from sodium. The AG equation can be written as AG = ([Na+]) − ([Cl] + [HCO3]). A normal AG is less than 12 mEq/L. If the AG is elevated, then there are anions in the bodily fluid that have not been accounted for in the equation. Common anions that result in an elevated AG include lactate and the ketone bodies β-hydroxybutyrate and acetoacetate. Organic acids that accumulate in inborn errors of metabolism (IEMs) can also lead to an increased anion gap.

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Infants will try to correct metabolic acidosis by a reflex respiratory alkalosis using hyperventilation and Kussmaul respirations. More severe uncompensated acidosis can decrease peripheral vascular resistance and cardiac ventricular function, leading to hypotension, pulmonary edema, and tissue hypoxia, which will further complicate the picture by increasing lactate production because ...

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