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Rationale Thiazides as antihypertensive Spironolactone in ascites associated with liver cirrhosis Chlorthiazide combined with spironolactone in hypertension Terlipressin in bleeding esophageal varices Mannitol in cerebral edema Mannitol is an osmotic diuretic. By its osmotic action, it encourages the movement of water from brain parenchyma and CSF. Amiloride in Lithium induced diabetes insipidus Amiloride is a renal epithelial sodium channel blocker. Thus it can block the entry of Lithium through sodium channels. Furosemide in acute pulmonary edema/ acute LVF

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Rationale of Diuretics and Related Drugs

1. Thiazides as Antihypertensives

Thiazides (e.g., hydrochlorothiazide, chlorthalidone) act primarily in the distal convoluted tubule (DCT) by inhibiting the Na+/Cl- cotransporter (NCC / SLC12A3), reducing sodium and water reabsorption. This produces two sequential effects:
Acute phase: Reduced extracellular volume (ECV) lowers cardiac output, which drops BP.
Chronic phase: Cardiac output returns to near-normal via compensatory RAAS activation, but BP remains lowered because of a sustained reduction in peripheral vascular resistance. This long-term vasodilation is an indirect consequence of persistent negative Na+ balance - when a high-salt diet reverses the natriuretic effect, the antihypertensive effect is also reversed. Since SLC12A3 is expressed in the DCT rather than in vascular smooth muscle, the vasodilatory effect is mediated through the kidney, not directly on vessels.
Thiazides are "low-ceiling" diuretics - increasing the dose beyond therapeutic range does not increase diuretic effect. They lose antihypertensive efficacy when GFR falls significantly (e.g., GFR < 30 mL/min), consistent with a renal mechanism of action.
Goodman & Gilman's Pharmacological Basis of Therapeutics; Lippincott Illustrated Reviews Pharmacology

2. Spironolactone in Ascites Associated with Liver Cirrhosis

In cirrhosis, portal hypertension leads to splanchnic vasodilation, reduced effective arterial blood volume, and compensatory activation of the RAAS, resulting in secondary hyperaldosteronism. Elevated aldosterone promotes Na+ retention in the collecting duct (via ENaC upregulation and Na+/K+-ATPase synthesis), driving the formation and persistence of ascites.
Spironolactone is a competitive antagonist of aldosterone at mineralocorticoid receptors in the principal cells of the collecting duct. It blocks aldosterone-mediated Na+ reabsorption and K+ secretion, promoting natriuresis while preserving potassium. Since hyperaldosteronism is the core driver, spironolactone is the first-line diuretic for cirrhotic ascites (starting at 100 mg/day). Its mechanism of action is slow (effect takes 72+ hours to manifest), so dose escalation should not occur sooner than 72 hours after the previous change.
Sleisenger & Fordtran's Gastrointestinal and Liver Disease; Barash Clinical Anesthesia; Comprehensive Clinical Nephrology

3. Chlorothiazide Combined with Spironolactone in Hypertension

This combination addresses two complementary issues:
  • Spironolactone alone can cause sodium retention (modest natriuretic effect alone) but does prevent the K+ wasting that thiazides produce.
  • Thiazides cause hypokalaemia as a significant adverse effect (enhanced K+ secretion in the DCT due to increased Na+ delivery distally, and RAAS activation).
When combined:
  1. Thiazide provides robust natriuresis and BP reduction via DCT NCC inhibition.
  2. Spironolactone counters thiazide-induced hypokalaemia by blocking aldosterone-mediated K+ excretion in the collecting duct.
  3. The two drugs act at different nephron segments (DCT vs. collecting duct), giving an additive diuretic and antihypertensive effect.
  4. Spironolactone's vasodilatory and anti-fibrotic properties (RAAS blockade) add cardiovascular benefit.
This pairing is particularly useful in resistant hypertension.

4. Terlipressin in Bleeding Esophageal Varices

Esophageal varices bleed because of elevated portal pressure (HVPG > 12 mmHg), driven by portal hypertension in cirrhosis. Splanchnic vasodilation (mediated by nitric oxide and glucagon) increases flow into the portal system, raising pressure and risking variceal rupture.
Terlipressin is a synthetic vasopressin analogue (V1 receptor agonist). Its mechanism:
  • Causes splanchnic vasoconstriction by activating V1 receptors on splanchnic arterioles.
  • Reduces mesenteric blood flow, which directly lowers portal pressure and portal-variceal blood flow.
  • This reduces the bleeding pressure across variceal walls, facilitating haemostasis.
Terlipressin is given as 2 mg IV every 4 hours for the first 48 hours, then 1 mg IV every 4 hours. It is used alongside endoscopic therapy (EVL). It is not available in the United States, where octreotide (a somatostatin analogue with similar splanchnic vasoconstrictor effect) is used instead.
Yamada's Textbook of Gastroenterology

5. Mannitol in Cerebral Edema

Mannitol is a non-reabsorbable, freely filtered sugar alcohol that acts as an osmotic diuretic.
Its mechanism in cerebral edema has two components:
A. Osmotic gradient across the blood-brain barrier (BBB): Mannitol, given IV in doses of 0.25-1.0 g/kg, remains in the plasma and creates a hyperosmolar state. Since an intact BBB is relatively impermeable to mannitol, an osmotic gradient is established drawing water out of brain parenchyma and interstitial fluid into the vascular compartment, thereby reducing brain volume and ICP.
B. Osmotic diuresis: Mannitol is filtered at the glomerulus and not reabsorbed. It retains water in the tubular lumen by osmosis, increasing urine output and further lowering plasma osmolality load on the brain over time.
The net result: reduced cerebral oedema, decreased ICP, and improved cerebral perfusion pressure (CPP). ICP above 22 mmHg carries increased mortality; mannitol is a mainstay of acute ICP management.
Rosen's Emergency Medicine; Morgan & Mikhail's Clinical Anaesthesiology; Bradley & Daroff's Neurology

6. Amiloride in Lithium-Induced Diabetes Insipidus

Lithium enters principal cells of the collecting duct via epithelial sodium channels (ENaC) - the same channels responsible for Na+ entry. Once inside the cell, lithium impairs the responsiveness of adenylyl cyclase to ADH (vasopressin), reducing cAMP generation. This prevents aquaporin-2 (AQP2) insertion into the apical membrane, causing nephrogenic diabetes insipidus (NDI) - polyuria and polydipsia resistant to exogenous vasopressin/desmopressin.
Amiloride is a potassium-sparing diuretic that directly blocks ENaC in the collecting duct principal cells. By blocking ENaC, amiloride prevents lithium from entering the cells in the first place, thereby reducing intracellular lithium accumulation and restoring some ADH responsiveness.
This is a pharmacokinetic rather than pharmacodynamic antagonism - amiloride does not compete with lithium at adenylyl cyclase, but blocks its cellular entry via ENaC.
Amiloride (5-10 mg/day) is the preferred treatment for lithium-induced NDI when lithium cannot be stopped.
Goldman-Cecil Medicine; Katzung's Basic & Clinical Pharmacology; Lippincott Pharmacology

7. Furosemide in Acute Pulmonary Edema / Acute LVF

Furosemide (a loop diuretic) acts on the thick ascending limb of the loop of Henle, inhibiting the Na+/K+/2Cl- (NKCC2) cotransporter. This is the most powerful nephron site for diuresis (up to 25% of filtered Na+ can be reclaimed here). The effects in acute pulmonary edema are twofold:
A. Venodilation (early - within minutes): Before significant diuresis begins, furosemide promotes prostaglandin-mediated venodilation, reducing venous return (preload) to the heart. This rapidly decreases pulmonary capillary wedge pressure and relieves pulmonary congestion.
B. Diuresis (delayed - within 30-60 min): Inhibition of NKCC2 causes massive natriuresis and water loss, reducing circulating volume, lowering both cardiac preload and afterload, and further resolving pulmonary oedema.
The combination of rapid venodilation and sustained diuresis makes furosemide uniquely effective in acute LVF. It is given IV for fastest onset.
Katzung's Basic & Clinical Pharmacology; Sabiston Textbook of Surgery

Summary Table

DrugSite of ActionKey MechanismClinical Use
ThiazidesDCT - NCC inhibition↓ ECV → chronic ↓ PVRHypertension
SpironolactoneCollecting duct - aldosterone antagonist↓ Na+ retention in hyperaldosteronismCirrhotic ascites, hypertension
Chlorothiazide + SpironolactoneDCT + Collecting ductAdditive natriuresis; prevents K+ lossResistant hypertension
TerlipressinV1 receptors - splanchnic arteriesSplanchnic vasoconstriction → ↓ portal pressureVariceal bleeding
MannitolBBB osmotic gradient + renal tubuleDraws water out of brain; osmotic diuresisCerebral edema, ↑ ICP
AmilorideENaC - collecting ductBlocks Li+ entry via ENaCLithium-induced NDI
FurosemideLoop of Henle - NKCC2 inhibitionVenodilation + massive natriuresisAcute pulmonary edema/LVF
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