Metabolic Alkalosis Evaluation

OVERVIEW

• Evaluation of causes of metabolic alkalosis requires a systematic approach involving history, examination and some specific investigations.

CAUSES

Initiating Process

• gain of HCO3- — endogenous: metabolism of ketoacids — exogenous: citrate, NaHCO3, lactate, antacid
• loss of H+ — Renal: diuretics — GI: vomiting, nasogastric losses

Maintenance Process

• hypochloraemia
• hypokalaemia
• hypomagnasaemia
• volume contraction
• increased adrenocorticoids (endogenous or exogenous)

Stewart approach

• elevation in SID: plasmalyte or NaHCO3 use
• reduction in ATOT: hypoalbuminaemia

HISTORY AND EXAMINATION

• vomiting & gastric losses, laxative induced diarrhoea
• signs of volume depletion (loss of bicarbonate free fluids)
• administered drugs (mineralocorticoids, diuretics and antacids in renal failure)
• administration of alkali (bicarbonate, lactate, citrate etc)
• recent hypercapnia

INVESTIGATIONS

Plasma

• hypokalemia (with hydrogen shifting into cells)
• hypochloremia
• hypomagnesaemia

Urinary

• high potassium excretion (reabsorbing hydrogen)
• alkaline pH (increased bicarbonate)
• high chloride excretion (diuretic therapy, hypokalaemia)

MANAGEMENT

• treat cause!
• K+ and Mg2+ replacement
• correct hypovolaemia with 0.9% NaCl
• consider acetazolamide
• consider spinolactone to antagonize hyperaldosteronism
• consider drugs to reduce GI acid secretion – H2-blockers, PPIs, octreotide
• consider administration of acid – lysine or arginine HCl or HCl (CVL and monitor K+)
• if the diagnosis is not obvious, spot urine chloride is useful: low levels suggest Cl- depletion and need for replacement; high levels suggest adrenocortical excess and need for K+ replacement

References and Links

• Acid-Base: ABG analysis – Anion Gap – SID – NAGMA
• Metabolic acidosis: Overview – Evaluation – DDx
• Metabolic alkalosis: Overview – Evaluation – DDx
• Respiratory acidosis: Overview – DDx
• Respiratory alkalosis: Overview – DDx