E. Antonogiannaki, E. Lilitsis, D. Georgopoulos
Intensive Care Unit, Heraklion University Hospital, Crete, Greece
Key words: acid base balance, physicochemical approach, Stewart approach, metabolic disturbances
Τhe assessment of acid base balance in blood plasma may serve as an invaluable adjunct in clinical medicine, since it may provide substantial information about disease severity and pathophysiology. Traditionally, the interpretation of acid base balance is based either on plasma bicarbonate concentration ([HCO3-]) and anion gap (AG), or on the base excess/deficit (BE).
Physicochemical approach represents an alternative method of evaluation of acid base status. A fundamental concept of this approach is the distinction of the system variables as independent and dependent. Independent variables are those that can change primarily and independently of one another. In blood plasma in vivo, the independent variables are: 1) the partial pressure of arterial CO2 (PaCO2), 2) the “strong ion difference” (SID) which is the difference between the sum of all the strong (fully dissociated, chemically non-reacting) cations ([Na+], [K+], [Ca2+], [Mg2+]) and all the strong anions ([Cl-] and other strong anions including lactate) and 3) the total concentration (in dissociated and un-dissociated forms) of nonvolatile weak acids (Atot, albumin and inorganic phosphate). According to this approach, dependent variables such as pH and [HCO3-] cannot be changed either primarily or individually. All the dependent variables vary simultaneously if, and only if, one or more of the independent variables change.
The utility of the physicochemical approach has been extensively studied, especially in critically ill patients as diagnostic and prognostic tool. The physicochemical approach can detect and assess every metabolic component, even if there are complex cases and provide information about the pathogenesis of metabolic disturbances.