What is the Strong Ion Theory of the pathophysiology of metabolic acidosis?

Updated: Oct 05, 2018
  • Author: Antonia Quinn, DO; Chief Editor: Romesh Khardori, MD, PhD, FACP  more...
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These classical descriptions of acid/base physiology often failed to account for acid/base findings in critically ill patients. An alkalosis was often noted in critically ill patients as their serum albumin level decreased, which could not be quantified by Henderson Hasselbalch or BE. Also, the "dilutional" acidosis frequently encountered after a large infusion of normal saline could not be explained by either of these 2 approaches to acid/base balance.

Both Henderson Hasselbalch and BE assume that the cations (Ca2+, Mg2+) and anions (Cl-, albumin, PO4-) in plasma remain unchanged in a patient with metabolic acidosis. Yet, in critically ill patients, these ions are known to be in dynamic flux. During the 1980s, Dr. Peter Stewart developed an acid/base theory (Strong Ion) using quantitative chemistry, which accounted for fluctuations of all the ions dissolved in plasma. Based on the requirements for electrical neutrality in any solution as any one of the concentrations of these ions changes, water must dissociate into H+ or OH- to balance the charge. The pH in this scheme is not a consequence of the ratio of acid to base in solution but determined by 3 independent variables:

  • Strong ion difference (SID) – Ions almost completely dissociated at physiologic pH.

SID = [Na+ + K+ + Ca2+ + Mg2+] – [Cl- + Lactate-]

(Ca2+ and Mg2+ are the concentrations of their ionized forms, Mg2+ X 0.7 = ionized Mg2+ concentration)

  • Total weak acid concentration (Atot) – Ions that can exist dissociated (A-) or associated (AH) at physiologic pH (buffers)

Atot = 0.325275 X [albumin] + 2 X [phosphate]

  • pCO 2 (mm Hg)

The Henderson Hasselbalch equation can be reformulated with variables from the Strong Ion Theory to give a more generalizeable solution to pH.

pH = pK1 ’ + log [SID] – Ka – [ATOT]/[Ka + 10–pH]


(K1’ is the equilibrium constant for the Henderson-Hasselbalch equation, Ka is the weak acid dissociation constant, and S is the solubility of CO2 in plasma.) See the image below.

Approach for evaluating metabolic acidosis. Approach for evaluating metabolic acidosis.

Once a metabolic acidosis is suspected by low bicarbonate concentration, an arterial blood gas analysis should be obtained. The low HCO3 level can be caused either by a primary metabolic acidosis or as the metabolic compensation for a respiratory alkalosis. The direction of the pH will separate metabolic acidosis (pH < 7.35) from a respiratory alkalosis (pH > 7.45).

The normal respiratory response (Kussmaul breathing) to a metabolic acidosis is a decrease in pCO2. This is given by the Winter’s equation:

PCO2 = 1.5 X (observed HCO3) + 8±2

(A quick rule of thumb: The PCO2 should approximate the last two digits of pH. For example, pH 7.25, PCO2 should be close to 25 mm Hg.)

Failure to have an appropriate respiratory response to metabolic acidosis represents a failure of airway and/or breathing, which must be addressed before any other workup commences.

Once an appropriate respiratory response for a metabolic acidosis has been established, the workup for the presence of unmeasured anions can progress by using the traditional anion gap, the delta-delta approach, or the strong ion gap. This allows the differential of metabolic acidosis to be narrowed and the appropriate therapy applied.

To differentiate between the causes of metabolic acidosis, one traditionally calculates the anion gap (AG), corresponding the presence of unmeasured anions. [2]

AG = (Na+) - ([Cl-] + [HCO3-])

The anion gap allows for the differentiation of 2 groups of metabolic acidosis. Metabolic acidosis with a high AG is associated with the addition of endogenously or exogenously generated acids. Metabolic acidosis with a normal AG is associated with the loss of HCO3 from the kidney or GI tract, or the failure of the kidney to excrete H+.

The delta/delta concept allows for the partitioning of metabolic acidosis into an anion gap and a non-anion gap component, which can occur contemporaneously. The concept behind delta/delta is based on the assumption that for every increase in anion gap of 1 mmol/L above normal (12 mmol), serum HCO3- will drop by an equal amount. [3]

Δ anion gap = Δ HCO3

If the delta HCO3- is greater than the delta anion gap, then a concomitant non-anion gap acidosis must exist along side the anion gap acidosis. One example would be a patient with a congenital renal tubular acidosis in diabetic ketoacidosis (DKA).

Stewart provides a replacement for the standard anion gap and delta/delta, which allows one to directly measure the amount of unmeasured anions in solution corrected for changes from normal of Ca2+, Mg2+, albumin, and phosphate. [4] This is the Strong Ion Gap (SIG). All the strong ions are expressed in mEq/L, and only the ionized portions of Mg2+ and Ca2+ are considered (to convert total to ionized Mg2+, multiply by 0.7). Because of the complexity of the equation, several Internet resources are available to calculate the SIG. For example, The Stewart approach to acid-base is a good resource. The normal SIG is between 0 and 2. SIG has been shown to be better than blood lactate, pH, or injury severity scores in trauma patients and pediatric surgery patients as a predictor of mortality.

A report by Masevicius and Dubin stated, however, that the variables presumed to be independent in the Stewart approach are actually interdependent in various situations and that there is a lack of experimental evidence to show that water dissociates in response to SID changes. The report also contends that severe methodologic drawbacks exist in studies that have attempted to demonstrate that the Stewart approach is clinically superior to conventional methods for analyzing acid-base disorders, while the largest such study indicated that the Stewart approach can be used interchangeably with conventional techniques. Masevicius and Dubin concluded that the Stewart approach does not offer a significantly better method of understanding, diagnosing, and treating acid-base changes in critically ill individuals. [5]

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