ICU · Acid–base

Acid–Base Physiology and Blood-Gas Interpretation

Also known as Acid-base · Blood gas interpretation · Metabolic acidosis · Metabolic alkalosis · Stewart approach · Strong ion difference · Anion gap

Acid–base homeostasis is the chemistry of the hydrogen ion, and its disturbances are among the commonest and most informative derangements in critical illness. This topic builds the examiner's framework on four ideas. First, the physiology — the bicarbonate buffer system described by Henderson and Hasselbalch, and the more complete Stewart view in which pH is set by the carbon dioxide tension, the strong ion difference (largely sodium minus chloride) and the total concentration of weak acids (albumin and phosphate). Second, a systematic method for reading a blood gas — the pH, the pattern (metabolic versus respiratory), the appropriateness of compensation, the anion gap and the delta-delta, and the osmolar gap when a toxin is suspected. Third, the clinical disorders — the high anion-gap and normal anion-gap acidoses (lactic acidosis, ketoacidosis, the toxic alcohols, and renal tubular acidosis), the metabolic alkaloses, and the respiratory acid–base disturbances. Fourth, the evidence that has reshaped management — the SMART and SALT-ED trials showing balanced crystalloids modestly outperform saline, BICAR-ICU showing bicarbonate confers no overall benefit in severe acidaemia (with a signal in the acute-kidney-injury subgroup), and the principles of treating the cause rather than the number. Built entirely on these verified landmark trials.