Anion Gap

Some causes of metabolic acidosis release anions into the ECF that are not normally measured. When this occurs, there is an unexpected discrepancy between the sums of the principal cations and anions. The usual sum is shown in the following equation:

Na⁺ + K⁺ = Cl⁻ + HCO₃ ⁻ + Gap

140 + 5 = 105 + 25 + (−15)

When there are any additional, unmeasured anions, they become part of the gap, which is then correspondingly larger. A gap greater than 30 suggests that there is an increase in the concentration of the unmeasured anions. Unfortunately, this method relies on the accuracy of the other measurements. Small errors in these large numbers cause a proportionately greater error in the result. If information is required about these anions, it is more appropriate to measure their concentration. In practice, it suffices to analyze lactate in cases of tissue hypoxia, 3-hydroxybutyrate in diabetic ketosis, and phosphate or sulfate in renal failure.

The common causes of perioperative metabolic alkalosis include antacid therapy, incidental administration of citrate with blood products, sodium bicarbonate administration, gastric drainage, or renal bicarbonate retention caused by diuresis or in compensation for respiratory acidosis. Sodium chloride (NaCl) or potassium chloride (KCl) can be administered orally or through a peripheral intravenous catheter, or 0.1 N HCl may be administered

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