Using Stewart for clinical gain!

The traditional approach to understanding acid-base is based on use of the Henderson-Hasselbalch equation. This approach is only a partial solution to the problems of acid-base, and therefore breaks down in certain circumstances. The following paper explores some of these circumstances, and how the physico-chemical (Stewart) approach helps us to explain clinical anomalies that are not resolved using a traditional attack. This document is entirely based on an excellent presentation supplied by Jon Waters .

A general approach

The physico-chemical acid-base approach is different from the conventional approach based on the Henderson-Hasselbalch equation, and requires a new way of approaching acid-base problems. At any one point in time, the [H +] is determined by the composition of the electrolytes and the PCO 2 of the solution. Mathematical analysis shows that it is not absolute concentrations of almost totally dissociated ("strong") ions that influence hydrogen ion concentration, but the difference between the activities of these strong ions (This "strong ion difference" is commonly abbreviated "SID"). We have already displayed the relationship between SID and [H +] as well as [OH -], but let's sketch the two on the same set of axes:

We are now ready to approach acid-base using a physico-chemical model. A very simplistic way of approaching acid-base problems is to think of H + and OH- as charge buffers. Any change in the charge composition of a solution will result in a change in H + or OH- to maintain electroneutrality. For instance, an increase in the negatively charged chloride will result in an increase in H + to maintain electroneutrality. This increased [H +] we call 'acidosis'. Because of the inverse relationship between H + and OH-, it is sometimes easier to assess pH changes through changes in the basic OH-. Increased OH- leads to alkalosis, decreased OH- results in acidosis. For example, the problem of hyperchloremia can be looked at in another way using OH-. The increased Cl- will decrease the SID. The SID is normally positively charged so that the decreased SID would result in fewer OH-. Fewer of the basic OH- results in acidosis. The following picture is a simplified rendering of normal acid-base status in plasma. We will explore changes in acid-base balance using this illustration:

Note that the above picture is used to provide a conceptual framework for discussion, and is not intended to replace the more detailed analysis of acid base that we previously attempted.

Specific Metabolic abnormalities

From the above general approach more specific metabolic problems can be addressed. Metabolic problems arise from abnormalities in either

  1. SID, or
  2. weak acid.
The weak acids are primarily composed of proteins and phosphate. These weak electrolytes are partially charged thus any change in pH is brought about only by the charged portion of that change. Thus, change in protein or phosphate is tempered by the partial contribution that they make to the electroneutrality equation. This means that most acute acid-base metabolic changes are a result of change in the SID. There are three general mechanisms by which SID changes:
  1. changing the water content of plasma (contraction alkalosis and dilutional acidosis)
  2. changing the Cl- (hyperchloremic acidosis and hypochloremic alkalosis), and
  3. increasing the concentration of unidentified anions (organic acidosis).
Let's systematically explore the ways that SID changes.

  1. Free water change - Dilutional acidosis and contraction alkalosis

  2. Chloride changes

  3. Unidentified anions

    SID can also be affected by the presence of organic acids such as lactate or ketoacids. Again, because these negatively charged molecules lower the SID, they result in an acidosis. Treatment is usually focused on stopping the production of acid. Resolution of the abnormal H + can also be achieved by increasing the SID using NaHCO3.

A Point to Ponder

Above, we considered the effect on SID of adding or removing free water to/from plasma. Use your knowledge of the Stewart approach to predict what effect these changes in free water will have on pH by virtue of changes in albumin concentration!

Many thanks to Amit Rajguru , MD for his incisive comments on the above page - he pointed out several errors in calculation (which do not however affect the conceptual framework on which this page is based) and also an area where further explanation was required.