Non-traditional approach
The non-traditional approach to acid-base status involves independent and dependent variables. This differs from traditional approaches to acid-base interpretation as it contends that any changes in bicarbonate are secondary to changes in plasma proteins, respiration and electrolytes. In theory, it is a more physiologic approach to acid-base abnormalities, however, in practice, using non-traditional approaches does not alter interpretations gleaned from traditional approaches.

The non-traditional approach to acid-base disturbances involves 3 rules:
  1. Electroneutrality must be maintained. Acid-base homeostasis is maintained by the lungs and the kidneys. The kidneys alter the strong ion difference.
  2. Independent variables are pCO2, strong ion difference and total concentraton of non-volatile weak acids (plasma proteins + inorganic phosphate). These are altered from outside without affecting each other and alter the dependent variables.
  3. Dependent variables are bicarbonate and hydrogen and are dependent on changes in the independent variables.
Respiratory component

Like the traditional approach, hypercapnea indicates a respiratory acidosis and hypocapnea indicates a respiratory alkalosis.

Strong ions

These include Na+, K+, Ca, Mg, Cl, lactate, ketones, sulfates. The strong ion difference is calculated using the following formula:

SID = Na + H = Cl - OH

Rearrangement of this equation yields:

Na - Cl = OH - H, and therefore

SID = Na - Cl
  • An increased SID indicates that OH is > than H and is compatible with an alkalosis. This can be due a decrease in free water, resulting in increased Na or a so-called contraction alkalosis. A contraction alkalosis can also be due to loss of a strong anion such as chloride, as occurs with diuretic therapy.

  • A decreased SID indicates that OH is < H and is compatible with an acidosis. This can be due to an increase in free water, with a decreased Na or so-called dilutional acidosis or an increase in anions. If the anion is chloride, there will be no change in the anion gap, resulting in a hyperchloremic metabolic acidosis. If the anions are unmeasured anions, this will result in a high anion gap acidosis.
Non-volatile weak acids

An increase in these weak acids indicates an acidosis (hyperphosphatemic acidosis or hyperproteinemic acidosis).
A decrease in these weak acids indicates an alkalosis (hypoproteinemic alkalosis).

Summary
  • An increased SID indicates:
    1) Metabolic alkalosis. Anion gap may be slightly high or normal.
    2) High anion gap - A mixed metabolic alkalosis and high anion gap metabolic acidosis.

  • A decreased SID indicates:
    1) Hyperchloremic metabolic acidosis (normal anion gap).
    2) High anion gap - A normal and high anion gap metabolic acidosis.
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