Classify the acid–base disorders. Describe their causes, compensation and the use of the anion gap.

Normal Values

• The kidney controls the numerator (bicarbonate); the lung controls the denominator (pCO2). Every acid–base disorder is a disturbance of one, compensated by the other

The Four Primary Disorders

• Compensation never fully corrects the pH — if the pH is normal with abnormal values, suspect a mixed disorder
• Respiratory compensation is fast; renal compensation is slow but far more powerful

Metabolic Acidosis and the Anion Gap

• Mnemonic mudpiles for the raised gap causes
• Normal anion gap acidosis is also called hyperchloraemic — chloride rises to replace the lost bicarbonate
• The gap must be corrected for albumin — add 2.5 mEq/L for every 1 g/dL that albumin falls below 4

Causes of the Other Disorders

Approach to Interpretation

• 1. Look at the pH — acidaemia or alkalaemia? → 2.
• Which value explains it — HCO3 (metabolic) or pCO2 (respiratory)? → 3.
• Is compensation appropriate? Use the expected formulas → 4.
• If metabolic acidosis, calculate the anion gap → 5.
• If the gap is raised, check the delta ratio for a second disorder → 6.
• Interpret alongside the clinical picture
• Winter formula for metabolic acidosis: expected pCO2 = 1.5 × HCO3 + 8 ± 2. A higher value means a coexisting respiratory acidosis; a lower one, a respiratory alkalosis

Clinical Consequences

• Acidosis — Kussmaul breathing, reduced myocardial contractility, vasodilatation, arrhythmias, hyperkalaemia, insulin resistance, confusion
• Alkalosis — tetany (ionised calcium falls), paraesthesiae, confusion, hypokalaemia, arrhythmias; the oxygen dissociation curve shifts left
• Acidosis raises serum potassium by shifting it out of cells; alkalosis lowers it — a link that must be remembered when correcting either

Compensation Formulas

Applied Aspects

• Salicylate poisoning gives a mixed picture — direct stimulation of the respiratory centre causes respiratory alkalosis, while uncoupling and lactate cause metabolic acidosis. A normal pH here is ominous
• Treat the cause, not the number — bicarbonate in lactic acidosis and DKA is reserved for severe acidaemia, and may worsen intracellular pH
• Correcting acidosis unmasks hypokalaemia and can precipitate arrhythmia; potassium must be replaced alongside
• In chronic COPD the bicarbonate is high from renal compensation; giving too much oxygen removes the hypoxic drive and worsens CO2 retention
• Venous blood gases suffice for pH and bicarbonate in most non-respiratory situations, sparing an arterial puncture