REVIEW
Physicochemical
approach to acid base balance
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.