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Although not directly related to fluid and electrolyte management, acidв base disorders are important to consider in the care of surgical infants and children because shifts in hydrogen or bicarbonate will effect distribution and total body content of the major electrolytes. Acid base disorders are termed either metabolic or respiratory, based on the pathogenesis of the imbalance. Metabolic acidosis is either from excess acid production or administration, or renal losses of bicarbonate. Metabolic alkalosis is usually from volume contraction or loss of hydrogen ions. Respiratory acidosis results from carbon dioxide retention, whereas alkalosis is the consequence of hyperventilation. Disorders are termed simple if there is only one primary disorder, or mixed if two or more are involved. The body attempts to correct metabolic disorders in the short term by altering respiration; however, respiratory defects can only be compensated by metabolic processes over longer periods of time. Compensatory mechanisms never overcorrect the primary derangement.

Metabolic acidosis occurs when exogenous acid is administered, endogenous acids are produced, or bicarbonate is lost in either gastrointestinal fluid or in the urine. It is defined by a plasma pH less than 7.35. The body attempts to compensate by increasing minute ventilation in an effort to lower dissolved carbon dioxide content. There are two major types of metabolic acidosis, anion gap and nonanion gap acidosis. The anion gap is the difference between unmeasured cations and anions and is usually around 8 to 16 meq per L. It is estimated by the formula:

Anion gap (meq/L) = sodium concentration - (chloride + bicarbonate concentrations)

When the calculated gap is in the normal range, it is termed nonanion gap acidosis, which is usually related to bicarbonate loss. If the anion gap is above the normal range, it is due to excess acid production or administration.