Strong ions are cations and anions that exist as charged particles dissociated from their partner ions at physiologic pH. The SID (Strong Ion Difference) is the difference between the positively- and negatively-charged strong ions in plasma. This method of evaluating acid-base disorders was developed to help determine the mechanism of the disorder rather than simply categorizing them into metabolic vs. respiratory acidosis/alkalosis as with the Henderson-Hasselbalch equation. Strong cations predominate in the plasma at physiologic pH leading to a net positive plasma charge of approximately +40: SID = [strong cations] – [strong anions] = [Na+ + K+ + Ca2+ + Mg2+] – [Cl- + lactate- + SO42-] Disturbances that increase the SID increase the blood pH while disorders that decrease the SID lower the plasma pH. According to the law of electroneutrality the sum of positive charges is equal to the sum of negative charges. Therefore the SID must be equal to the sum of weak anions in the body (such as bicarbonate, albumin, and phosphate). Hyperchloremic acidosis may result from chloride replacing lost bicarbonate. Such bicarbonate-wasting conditions may be seen in the kidneys (renal tubular acidosis) or the GI tract (diarrhea). This may also occur with aggressive volume resuscitation with normal saline (>30cc/kg/hr) due to excessive chloride administration impairing bicarbonate resorption in the kidneys. The strong ion difference of normal saline is 0 (Na+ = 154mEq/L and Cl- = 154mEq/L → SID = 154 – 154 = 0). Therefore, aggressive administration of NS will decrease the plasma SID causing an acidosis. Administering a solution with a high SID such as sodium bicarbonate should be expected to treat this strong ion acidosis.
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