Chapter 43: Strong Ion Difference

INTRODUCTION

A strong ion describes the complete dissociation of a molecule in solution. Its clinical application explains the acid–base abnormality associated with administration of intravenous normal saline crystalloid fluids. Understanding how strong ion chemistry affects blood pH is essential in understanding nongap metabolic acidosis. Many clinicians feel that this approach to understanding acid-base disorders is more complete that using the more traditional Henderson–Hasslebach method. Peter Stewart first popularized this approach in 1981 by the publication of his textbook and two years later in a journal article.

Sodium chloride represents the prototypical strong ion. In solution or plasma, the molecule completely dissociates into its constituent ions:

The metabolic derangement occurs because 0.9% saline solution or normal saline contains 154 mEq/L of sodium and 154 mEq/L of chloride. While this is a moderate increase in the normal blood concentration of sodium (140 mEq/L), it is a significant increase to the blood chloride levels (102 mEq/L). Administering large quantities of 0.9% saline solution creates a relative increase in chloride ions, or negatively charged anions. The law of electrochemical neutrality mandates that charges balance. The body’s adaptive response is to produce a cation in the form of a proton (H⁺). This intravascular increase in protons leads to a “nongap” metabolic acidosis also known as hyperchloremic acidosis. This relationship is best visualized on a gamblegram (Figure 43-1).

The metabolic acidosis generated is considered “nongap” because the chloride concentration is listed on routine chemistry panels and not an unaccounted for acid such as lactate, ketones, or albumen.

To evaluate a patient with a metabolic acidosis for a strong ion effect, the strong ion difference (SID) is first calculated. This is also known as the Stewart approach to analyzing acid–base disorders. While there are multiple strong ions in plasma (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^–, and lactate), Na⁺ and Cl^– are used to calculate the simplified SID. The normal or “apparent” SID (SID_a) is positive 38, calculated by subtracting the normal sodium from chloride (140–102). Next, calculate the patient’s SID, or “effective” SID (SID_e). Finally, determine the strong ion gap (SIG) by subtracting the apparent from effective SID.

• Example:

• Patient’s chemistry: Na⁺ 142, K⁺ 4.3, Cl^– 110, HCO₃ 22

• SIDa 38

• SIDe 142 mEq/L – 110 mEq/L = 32

• SIG 38 – 32 = 6

Because the SIG is positive, strong ion forces are involved in the patient’s metabolic acidosis. It is uncommon for these forces to be the sole contribution to the patient’s acid–base disorder. The contribution may ...