A Diagnostic Approach to Metabolic Acidosis in Children

¹ Departments of Pediatrics and Family Practice, University of Arizona College of Medicine, Tucson

Accurate evaluation of extracellular fluid acid-base status used to be a rather formidable task. However, with the availability of automated clinical testing, this evaluation is now readily performed on any hospitalized child. The following discussion will be limited to various aspects of metabolic acidosis with a brief review of normal and abnormal physiology. A practical approach to the diagnostic work-up of an acidotic child will also be discussed. Several more detailed discussions of acid-base homeostatic mechanisms are available for the reader who is interested in pursuing this subject.^1-5

Acidosis, as used in this article, will refer to an abnormal increase in circulating acid with an accompanying decrease in the buffering capacity of the blood as manifest by a low serum bicarbonate level. Acidosis that ultimately exceeds the body's compensatory buffering mechanisms leads to an increase in extracellular fluid (ECF) H⁺ concentration such that pH is reduced below 7.35. This state is referred to as acidemia.

NORMAL ACID-BASE PHYSIOLOGY

The normal arterial blood pH is maintained at 7.40 (H⁺ = 39.8 mEq/liter) and ranges between 7.35 and 7.45. When this pH value drops below the normal range, acidemia is present.⁶ Acidosis (excess circulating acid) may or may not be accompanied by a fall in pH (acidemia) because the body can compensate for a drop in blood pH by combining some of the excess H⁺ with bicarbonate to form carbonic acid and then CO₂, which is eliminated through the lungs by increased ventilation. Since this compensatory process is limited by the amount of bicarbonate available for the reaction with H⁺, only prolonged or severe acidosis may lead to acidemia.