Strong Ion Difference in Critically Ill Patients

Calculation and Clinical Management

The World Journal of Emergency Surgery recommends using balanced crystalloids, such as lactated Ringer's, rather than 0.9% saline to avoid decreasing Strong Ion Difference (SID) and worsening acidosis in critically ill patients 1
The American College of Cardiology suggests correcting potassium > 4.0 mEq/L and magnesium > 2.0 mg/dL to optimize SID and prevent arrhythmias 2
The Journal of Infection notes that assessing for other organ dysfunction, such as renal or hepatic dysfunction, is crucial in managing strong ion balance in critically ill patients 3
The World Journal of Emergency Surgery advises against excessive saline resuscitation, as it decreases SID through hyperchloremia and worsens metabolic acidosis independent of tissue perfusion 1

The World Journal of Emergency Surgery recommends targeting a postoperative fluid balance of 0-2 L to avoid volume overload while maintaining adequate SID 1
The Journal of the American College of Cardiology suggests monitoring for acid-base disturbances during electrolyte correction, as electrolyte shifts affect SID 2

The apparent Strong Ion Difference (SID) is calculated as the difference between measured strong cations and measured strong anions in plasma, representing the charge difference between the sum of measured strong cations (sodium, potassium, calcium, and magnesium) and measured strong anions (chloride and lactate) 4, 5
A simplified version of the apparent SID uses only the major strong ions: SIDa = [Na+] - [Cl-], which represents the most quantitatively important contributors to the strong ion difference 4, 5

A decrease in SID results in an acidifying effect on plasma, while an increase in plasma chloride relative to sodium decreases the plasma SID and lowers pH 4, 5
The anion gap may overestimate severity with concomitant acute kidney injury or ketoacidosis, or underestimate severity with hypoalbuminemia 6, 7

The World Journal of Emergency Surgery recommends avoiding excessive crystalloid overload to optimize bowel perfusion in critically ill patients, with a target postoperative fluid balance of 0-2L 8
The American College of Critical Care Medicine suggests that sodium bicarbonate increases SID and thus pH, but its therapy is not recommended for hypoperfusion-induced lactic acidemia with pH ≥ 7.15, as it provides no hemodynamic benefit and causes sodium/fluid overload 9

The Society of Critical Care Medicine recommends using balanced crystalloids, such as lactated Ringer's or Plasma-Lyte, rather than 0.9% saline to avoid decreasing SID and worsening acidosis, as normal saline contains equal sodium and chloride, which is non-physiological and decreases SID by increasing chloride relative to sodium 8
The European Society of Intensive Care Medicine suggests limiting normal saline to a maximum of 1-1.5L if it must be used, as excessive saline causes hyperchloremic metabolic acidosis independent of tissue perfusion 8

The National Kidney Foundation recommends assessing renal function and urine output, with a negative urinary SID ([Na+]urine + [K+]urine - [Cl-]urine < 0) indicating appropriate renal response in metabolic acidosis 8
The International Society of Nephrology suggests that patients with positive urinary SID have more severe acidosis, with lower bicarbonate, lower base excess, and higher plasma chloride, indicating the kidneys are retaining chloride relative to sodium, worsening the acidosis 8