Strong ion calculator--a practical bedside application of modern quantitative acid-base physiology.
OBJECTIVE
To review acid-base balance by considering the physical effects of ions in solution and describe the use of a calculator to derive the strong ion difference and Atot and strong ion gap.


METHODS
A review of articles reporting on the use of strong ion difference and Atot in the interpretation of acid base balance.


RESULTS
Tremendous progress has been made in the last decade in our understanding of acid-base physiology. We now have a quantitative understanding of the mechanisms underlying the acidity of an aqueous solution. We can now predict the acidity given information about the concentration of the various ion-forming species within it. We can predict changes in acid-base status caused by disturbance of these factors, and finally, we can detect unmeasured anions with greater sensitivity than was previously possible with the anion gap, using either arterial or venous blood sampling. Acid-base interpretation has ceased to be an intuitive and arcane art. Much of it is now an exact computation that can be automated and incorporated into an online hospital laboratory information system.


CONCLUSIONS
All diseases and all therapies can affect a patient's acid-base status only through the final common pathway of one or more of the three independent factors. With Constable's equations we can now accurately predict the acidity of plasma. When there is a discrepancy between the observed and predicted acidity we can deduce the net concentration of unmeasured ions to account for the difference.
Anaesthetic Department, Hawke's Bay Regional Hospital, Hastings, New Zealand. peter.lloyd@hawkesbaydhb.govt.nz
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