Arterial pH is tightly regulated (Box 37.1). Disorders of arterial pH are commonly encountered in acute medicine; arterial blood gases and pH should be measured in any patient with critical illness. Abnormalities of acid-base balance can be identified as acidosis or alkalosis, noting the severity and potential adverse features (Tables 37.1, 37.2, 37.3; Figure 37.1). The physiological and pathological consequences of acid-base disorders are summarized in Table 37.4.

• An effective approach to understanding an acid-base disorder is to look at the relationship between arterial pH (or hydrogen ion concentration) and PaCO₂ (Figure 37.1). When the primary disturbance is metabolic, the PaCO₂ will generally be either normal or out of keeping with the pH, that is, low in metabolic acidosis and high in metabolic alkalosis. When the primary disturbance is respiratory, the PaCO₂ will be in keeping with the pH, that is, high in respiratory acidosis, and low in respiratory alkalosis.
• Note that hyperkalaemia commonly accompanies both acute and chronic extracellular acidosis, so the plasma potassium concentration should be measured early in acidotic patients. Hyperkalaemia often results from impaired potassium excretion in renal failure, but extracellular acidosis of any cause may also lead to hyperkalaemia through the uptake of hydrogen ions into cells, in exchange for potassium. In extracellular alkalosis, the direction of exchange is reversed, so hypokalaemia may result.
• Common causes of the four primary acid-base disturbances are given in Tables 37.5, 37.6, 37.7, 37.8.
• Causes of metabolic acidosis can be subdivided according to the size of the so-called ‘anion gap’. This value represents the difference between the sum of the concentration of major plasma cations (sodium and potassium), and the sum of the concentration of major plasma anions (chloride and bicarbonate):
  Anion gap = ([Na⁺]+[K⁺])–([Cl^–]+[HCO₃^–])
  A normal anion gap is around 10–18 mmol/L, but the reference range can vary considerably according to laboratory. It is an estimate of the unmeasured anions in the plasma. A high anion gap implies excess unmeasured plasma anions, which may be the cause of metabolic acidosis.
• When the primary pathology is not clear, consider a mixed disturbance, with two or more contributing factors (Table 37.2). This is distinct from a single disturbance with compensation (Box 37.1). Common examples include respiratory and metabolic acidosis in the setting of severe pneumonia and sepsis, or combined metabolic alkalosis and metabolic acidosis in a patient with vomiting and chronic renal failure.