Acid–base and blood gas analysis

Abstract 

The concentration of hydrogen ions is one of the most tightly controlled systems in the body. Defence of normal pH is thought to be from three basic mechanisms: respiratory control of carbon dioxide, renal excretion of acids, and plasma buffering systems. The traditional approach to acid–base control centres on the Henderson–Hasselbalch equation, in which pH can be defined as the ratio of bicarbonate to carbon dioxide. Alterations in pH result from changes in carbon dioxide (respiratory) or bicarbonate (metabolic). Most pH disturbances can be classified into one of four main types: respiratory acidosis; respiratory alkalosis; metabolic acidosis; metabolic alkalosis. The Stewart hypothesis is an alternative approach to acid–base analysis. It challenges the concept that changes in bicarbonate concentration can alter pH. This theory, based on mathematical solution, is that only three things, alone or in combination, can determine the hydrogen ion concentration: strong ion difference (net charge balance of dissociated ions in plasma); partial pressure of carbon dioxide; and the sum of acids present.

Keywords: Henderson–Hasselbalch equation, pH, Stewart hypothesis, strong ion difference