An introduction to acid-base disorders and interpretation of blood gases

Arterial blood gas analysis provides the means for assessment of two related physiological functions that are disturbed in a wide range of medical conditions. 

The first is the facility of the lungs to simultaneously add inspired oxygen to blood and remove carbon dioxide from blood, prior to excretion in expired air (the dual process called pulmonary gas exchange) and the second is the ability of the body to maintain the pH of blood within very narrow limits (called acid-base homeostasis). 

A recently published article provides a basic introduction to understanding disturbances of the second of these two functions, acid-base homeostasis, and how the results of arterial blood gas analysis (pH, pCO₂ and bicarbonate) help to identify those disturbances. 

The authors assume little knowledge of the topic on behalf of the reader, and begin with some basic definitions of an acid, a base and a buffer, before introduction of the all-important Henderson-Hasselbalch equation that defines the relationship between pH, bicarbonate and pCO₂. 

Then follows a brief discussion of the role of the kidneys in maintaining normal blood pH. Most of the rest of the article is devoted to discussion in turn of each of the four classes of acid-base disturbance: metabolic acidosis, metabolic alkalosis, respiratory acidosis and respiratory alkalosis. 

For each of these, the blood gas values (pH, pCO₂ and bicarbonate) that define them are described, as are the compensatory changes in pCO₂ or bicarbonate that they induce. The most common causes of each of these four classes of disturbance are listed and discussion of each class of disturbance ends with an illustrative case history. 

There is a further brief section devoted to interpretation of blood gas results and another that expands on the mechanisms of compensation in acid–base disturbances. This educative article provides a helpful introduction to what is a complex topic that many students find difficult to understand.