How do the listed conditions (vomiting, nasogastric suction, Bartter syndrome, Gitelman syndrome, loop diuretics, thiazide diuretics, primary aldosteronism, Cushing syndrome, Liddle syndrome, apparent mineralocorticoid excess, and 11β‑hydroxylase or 17α‑hydroxylase deficiencies) lead to metabolic alkalosis?

Mechanisms of Metabolic Alkalosis in These Conditions

These conditions cause metabolic alkalosis through three primary mechanisms: loss of gastric acid (vomiting/NG suction), renal salt-wasting with secondary hyperaldosteronism (Bartter/Gitelman syndromes, loop/thiazide diuretics), or primary excess mineralocorticoid activity (primary aldosteronism, Cushing syndrome, Liddle syndrome, AME, and enzyme deficiencies).

Gastric Acid Loss

Vomiting and nasogastric suction directly remove hydrochloric acid from the stomach, creating a net gain of bicarbonate in the extracellular fluid. The resulting volume contraction triggers renal bicarbonate retention and prevents correction of the alkalosis through decreased glomerular filtration rate, hypochloremia, and secondary hyperaldosteronism with potassium wasting.

Renal Salt-Wasting Disorders

Bartter Syndrome

Bartter syndrome causes metabolic alkalosis through impaired salt reabsorption in the thick ascending limb of the loop of Henle 1. The molecular defects (affecting NKCC2, ROMK, ClC-Kb, barttin, or MAGE-D2) result in:

• Renal tubular salt wasting → volume contraction
• Activation of renin-angiotensin system → hyperaldosteronism
• Increased distal sodium delivery → enhanced potassium and hydrogen ion secretion
• Consequent hypokalemic hypochloremic metabolic alkalosis 1

The tubuloglomerular feedback at the macula densa senses low tubular chloride, activating COX-2 to produce prostaglandins that further stimulate renin secretion and aldosterone production 1.

Gitelman Syndrome

Gitelman syndrome mimics thiazide diuretic effects with defective sodium-chloride reabsorption in the distal convoluted tubule, producing similar metabolic alkalosis through volume contraction and secondary hyperaldosteronism 2, 3. Unlike Bartter syndrome, it presents with hypocalciuria and hypomagnesemia.

Loop and Thiazide Diuretics

These medications pharmacologically replicate Bartter and Gitelman syndromes respectively. Loop diuretics block NKCC2 in the thick ascending limb, while thiazides block the sodium-chloride cotransporter in the distal tubule, both causing salt wasting, volume contraction, and compensatory hyperaldosteronism with resultant metabolic alkalosis 4, 5.

Primary Mineralocorticoid Excess States

Primary Aldosteronism

In primary aldosteronism, excess aldosterone increases sodium reabsorption through mineralocorticoid receptor activation and enhanced ENaC expression in the distal nephron 6. This leads to:

• Volume expansion (though often subtle)
• Increased potassium excretion → hypokalemia
• Enhanced hydrogen ion secretion → metabolic alkalosis
• Suppressed renin (distinguishing it from Bartter/Gitelman syndromes) 4

Cushing Syndrome

Excess cortisol has mineralocorticoid activity at high concentrations, overwhelming the protective 11β-hydroxysteroid dehydrogenase type 2 enzyme, leading to mineralocorticoid receptor activation and metabolic alkalosis 7.

Liddle Syndrome

Liddle syndrome involves constitutive activation of ENaC channels independent of aldosterone, causing excessive sodium reabsorption and potassium/hydrogen ion loss, producing metabolic alkalosis with suppressed renin and aldosterone 4, 7.

Apparent Mineralocorticoid Excess (AME)

AME results from deficiency of 11β-hydroxysteroid dehydrogenase type 2, which normally inactivates cortisol to cortisone in the kidney. Cortisol then inappropriately activates mineralocorticoid receptors, mimicking aldosterone excess despite low aldosterone levels 7.

11β-Hydroxylase and 17α-Hydroxylase Deficiencies

These congenital adrenal hyperplasia variants cause accumulation of mineralocorticoid precursors (deoxycorticosterone and corticosterone) that activate mineralocorticoid receptors, producing hypertension and metabolic alkalosis with suppressed renin and aldosterone 7.

Key Diagnostic Distinctions

Critical pitfall: Metabolic alkalosis with hypokalemia and elevated aldosterone can represent either:

• Bartter/Gitelman syndromes: Normal blood pressure, elevated renin (secondary hyperaldosteronism), urinary chloride >20 mEq/L
• Vomiting: Normal blood pressure, elevated renin, urinary chloride <10 mEq/L 8
• Primary aldosteronism/other mineralocorticoid excess: Hypertension, suppressed renin 4

The urine chloride and renin levels are essential for distinguishing these mechanisms in clinical practice.