Acid base balance: A complete explanation for Students, postgraduate Physicians especially Anesthesia Students

Definitions

The definitions of the terms used here to describe acid-base disorders are those suggested by the Ad-Hoc Committee of the New York Academy of Sciences in 1965. Though this is over 35 years ago, the definitions and discussion remain valid today.

Basic Definitions

• Acidosis - an abnormal process or condition which would lower arterial pH if there were no secondary changes in response to the primary aetiological factor.
• Alkalosis - an abnormal process or condition which would raise arterial pH if there were no secondary changes in response to the primary aetiological factor.
• Simple (Acid-Base) Disorders  are those in which there is a single primary aetiological acid-base disorder.
• Mixed (acid-Base) Disorders are those in which two or more primary aetiological disorders are present simultaneously.
• Acidaemia - Arterial pH < 7.36 (ie [H⁺] > 44 nM )
• Alkalaemia - Arterial pH > 7.44 (ie [H⁺] < 36 nM )

An acidaemia of course must be due to an acidosis so is an indicator of the presence of this disorder. In mixed acid-base disorders, there may be co-existing disorders each having opposite effects on the ECF pH so a quick check of the arterial pH is insufficient to fully indicate all primary acid-base disorders. In mixed disorders, it does indicate in general terms the most severe disorder. That is, if the arterial pH is 7.2 (an acidaemia), there must be an acidosis present, and any alkalosis present must be of lesser magnitude. (This idea is the basis of an initial step in the systematic approach to analysis of arterial blood gas results).

The Disorders

The 4 simple acid base disorders are:

• Respiratory acidosis
• Respiratory alkalosis
• Metabolic acidosis
• Metabolic alkalosis.

Respiratory disorders are caused by abnormal processes which tend to alter pH because of a primary change in pCO₂ levels.
Metabolic disorders are caused by abnormal processes which tend to alter pH because of a primary change in [HCO₃^-].

Correct Termin