Strong ion difference: a new paradigm or new clothes for the Acid-base emperor.
OBJECTIVE
To review and compare the 'metabolic' component of an acid-base abnormality by assessing the arterial blood bicarbonate and the 'strong ion difference'.


METHODS
A review of published peer-review articles and studies reported from 1983 to 1999 and identified through a MEDLINE search on 'strong ion difference'.


RESULTS
The Henderson-Hasselbalch equation describes the simple relationship between the arterial pH, PaCO(2) and bicarbonate concentration (HCO(3)(-)), and has been used by clinicians to classify acid-base abnormalities as either respiratory or a non-respiratory (i.e. metabolic). However, as the HCO(3)(-) concentration cannot be measured directly and as it can also be altered by an alteration in the PaCO(2), derived values such as the standard bicarbonate, buffer base, base excess and standard base excess have been proposed to assess the true 'metabolic' acid-base component. Recently, an analysis of acid-base has been reported based on the Law of electroneutrality in aqueous solutions, in which it is proposed that the independent variables of 'strong ions' (e.g. sodium, potassium, calcium, magnesium, chloride and organic anions), CO(2) and non volatile weak acids (i.e. A(TOT)) alter the dependent variables of pH and HCO(3)(-). The concept of 'strong ion difference' (SID) is used to help explain 'metabolic' acid base abnormalities, particularly those associated with saline infusions. The relationship between the HCO(3)(-) ion and the SID can be represented as HCO(3)(-) = (SID - A(-)) and the Henderson Hasselbalch equation can be written as pH infinity (SID - A(.))/PaCO(2) although, the body regulates pH by regulating the PaCO(2) and HCO(3)(-), rather than by regulating the SID or A(TOT).


CONCLUSIONS
In man the renal and respiratory systems regulate acid-base homeostasis by modifying the bicarbonate buffer pair (i.e. PCO(2) and HCO(3)(-)), with all other body buffer systems adjusting to alterations in this pair. To maintain electrical neutrality there is a change in cation concentration commensurate with the change in bicarbonate concentration.
Worthley
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Department of Critical Care Medicine, Flinders Medical Centre, Adelaide, SA lindsay.Worthley@flinders.edu.au Australia.