24 Oct 07
Stewart has come to the bedside: Is it time to use the Stewart methodology routinely?
By H Morimatsu
Critical Care Medicine 2007;35:2456-2457
Fluid resuscitation is a vital part of critical care. We use massive amounts of fluids in our daily critical care practice. Almost all of these fluids contain strong ions (such as sodium and chloride) and, sometimes, weak acids (such as albumin). Thus, it is easy to imagine that massive fluid resuscitation can result in metabolic changes that alter a patient’s acid-base status. Saline-induced metabolic acidosis has been well described; it has been explained by so-called dilution of the bicarbonate concentration. Recently, however, the Stewart methodology has been applied to critical care medicine; it has revealed that this acidosis is primarily due to a decreased strong ion difference (SID) induced by hyperchloremia.
According to Stewart’s concept, neither bicarbonate nor hydrogen ion is an independent determinant of the acid-base status, whereas strong ion difference and total weak acid (ATOT) are independent variables. Thus, the balance between strong ions and weak acids, including albumin, phosphate, and unmeasured anions, plays an important role in acid-base physiology. Using this methodology, studies have shown that metabolic acid-base disorders have a complex etiology, which could not be previously understood using bicarbonate-oriented methodology.