Diuretics are medicines that help the body remove excess salt and water through urine. They are commonly used to treat high blood pressure, swelling (oedema), heart failure, kidney problems, and certain electrolyte imbalances. Each type of diuretic works at a different part of the kidney tubule and has different levels of strength.
Classification of Diuretics
Diuretics are usually classified based on the part of the kidney where they act:
- Carbonic anhydrase inhibitors – act at the proximal tubule
- Loop diuretics – act at the thick ascending limb of loop of Henle
- Thiazide diuretics – act at the early distal tubule
- Potassium-sparing diuretics – act at the late distal tubule and collecting duct
- Osmotic diuretics – act on the entire nephron
1. Carbonic Anhydrase Inhibitors
Example: Acetazolamide
These drugs reduce the reabsorption of bicarbonate, sodium, and water. Because their diuretic effect is weak, they are not used routinely for oedema.
Uses
- To treat glaucoma (reduces eye pressure)
- To make urine alkaline in certain poisonings
- Acute mountain sickness (prevention and relief)
Side Effects
- Metabolic acidosis
- Low potassium levels
- Drowsiness, tingling sensation
2. Osmotic Diuretics
Examples: Mannitol, glycerol, isosorbide
These drugs increase the osmotic pressure of the blood, pulling water out of tissues and increasing urine output.
Uses
- To reduce brain swelling (intracranial pressure)
- To reduce eye pressure in acute glaucoma
- To prevent kidney shutdown in shock or surgery
Side Effects
- Headache, nausea
- Risk of fluid overload leading to pulmonary oedema
Contraindications
Not used in heart failure and pulmonary oedema because they increase blood volume.
3. Loop Diuretics (High-Ceiling Diuretics)
Examples: Furosemide, bumetanide, torsemide
These are the most powerful diuretics. They block sodium, potassium, and chloride reabsorption at the loop of Henle, causing large amounts of urine formation.
Uses
- Oedema due to heart failure, kidney disease, or liver cirrhosis
- Acute pulmonary oedema (emergency)
- High calcium levels in blood
- High blood pressure with kidney problems
Side Effects
- Low potassium (hypokalaemia)
- Low sodium and magnesium
- Dehydration
- Hearing problems (ototoxicity)
- High uric acid, high glucose
Important Points
- Furosemide acts quickly—within minutes when given IV.
- Bumetanide and torsemide are more potent and have better absorption.
4. Thiazide Diuretics
Examples: Hydrochlorothiazide, chlorthalidone, indapamide, metolazone
These are medium-strength diuretics and are widely used for treating high blood pressure.
Mechanism
They block sodium and chloride reabsorption in the early distal tubule, causing moderate diuresis.
Uses
- First-line treatment for high blood pressure
- Oedema in heart failure (often combined with loop diuretics)
- Calcium kidney stones (they reduce calcium loss in urine)
- Diabetes insipidus
Side Effects
- Low potassium
- Low sodium
- High calcium levels
- High uric acid, high sugar, high lipids
- Skin rashes and photosensitivity
5. Potassium-Sparing Diuretics
These diuretics help retain potassium, making them useful when low potassium is a concern.
Types
- Aldosterone antagonists: Spironolactone, eplerenone
- Direct sodium channel blockers: Amiloride, triamterene
Uses
- Used along with loop or thiazide diuretics to prevent potassium loss
- Spironolactone is helpful in heart failure and liver cirrhosis
- Amiloride is useful in lithium-induced diabetes insipidus
Side Effects
- High potassium (dangerous)
- Hormonal effects with spironolactone: breast enlargement in males, menstrual changes
- Nausea, dizziness
Diuretic Resistance
Sometimes the body stops responding to diuretics, especially with long-term use. This is known as diuretic resistance and may occur in elderly patients or those with heart, liver, or kidney problems.
How to Overcome Resistance
- Increase dose or frequency
- Take before meals
- Use combination of diuretics
- Restrict salt intake
Detailed Notes:
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