Outline the way in which you would evaluate the aetiology of metabolic alkalosis in the critically ill.
Evaluation of causes of metabolic alkalosis requires a systematic approach involving history, examination and some specific investigations. Categories of aetiology include loss
of hydrogen ions (gastrointestinal, renal), intracellular shift of hydrogen ions, administration of alkali, and contraction alkalosis. History and examination will reveal, documented fluid losses (vomiting & gastric losses, laxative induced diarrhoea), volume depletion (loss of bicarbonate free fluids), administered drugs (mineralocorticoids, diuretics, and antacids in renal failure), alkali (bicarbonate, lactate, citrate etc) and recent hypercapnia. Investigations may reveal hypokalemia (with hydrogen shifting into cells), hypochloremia and urinary findings may include excessive potassium excretion (reabsorbing hydrogen), alkaline pH (increased bicarbonate) and inappropriately elevated chloride excretion (diuretic therapy, hypokalaemia).
This question closely resembles Question 7 from the first paper of 2008.