Читать книгу Respiratory Medicine - Stephen J. Bourke - Страница 93

The three variables pH, PCO₂ and bicarbonate are yoked together as just described. Analysis of their values provides information on the acid/base balance of the body and the broad nature of its cause. It may also provide information about the chronicity of an abnormality.

Changes in the acid/base status caused by changes in PCO₂ (hyper‐ or hypoventilation) are termed respiratory. Changes in acid/base status caused by changes in bicarbonate are termed metabolic. A disturbance in one system tends to prompt a compensatory response in the other. When needed, the respiratory system responds promptly, and changes are evident within seconds to minutes. The metabolic system, largely regulated via renal excretion, is much slower, taking between hours and days to equilibrate. In respiratory disturbances, therefore, the degree of correction achieved by the metabolic system can tell us something about the duration of the abnormality.

As a general principle, physiological compensatory mechanisms don’t overcompensate; in fact, they often stop just short of total correction. This is a useful fact to remember when trying to interpret a blood gas result that displays both respiratory and metabolic changes. If the pH is in the normal range, it may be difficult to determine which is the primary abnormality and which the compensatory response. Look again at the pH. If the pH is towards the higher end of the normal range, the primary abnormality is probably an alkalosis; if at the lower end then the primary disturbance is an acidosis.

In reading the following examples of acid/base disturbance, refer to the diagrams in Figs 3.9 and 3.10.