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Pharmacology: Diuretics | Respiratory System | Blood | Gut

Complete Concept Map + Case-Based Interlinked Q&A

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PART 1: DIURETICS

Core Concepts You Must Know

Nephron Site Map

PCT → Loop of Henle → DCT → Collecting Duct
 ↑           ↑          ↑          ↑
CAI       Furosemide  Thiazides  Spironolactone
(Glaucoma) (Pulm edema) (HTN)   (Hyperaldo)

CASE 1 - "The Swollen Heart Patient"

A 65-year-old man with chronic heart failure presents with bilateral pitting edema up to the knees, dyspnea at rest, JVP raised. BP = 150/90 mmHg. Creatinine = 1.4 mg/dL.

• Furosemide (Loop diuretic) - inhibits Na+/K+/2Cl- transporter in thick ascending limb. Most potent, works even in reduced GFR. IV furosemide also causes venodilation within minutes (before diuresis begins), reducing preload acutely.

• Spironolactone - aldosterone antagonist, K+-sparing. Also shown in RALES trial to reduce mortality in severe CCF by blocking cardiac fibrosis and remodeling (beyond its diuretic effect).

• Thiazides (hydrochlorothiazide, chlorthalidone) lose efficacy when GFR < 30 mL/min. They work on DCT NaCl cotransporter. Ineffective in significant renal impairment.

• Spironolactone 100 mg + Furosemide 40 mg (ratio 100:40 maintained to preserve K+ balance). Spironolactone is preferred in cirrhotic ascites because secondary hyperaldosteronism is the primary mechanism.

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CASE 2 - "The Hypertensive Diabetic"

A 52-year-old diabetic woman on metformin has BP = 158/96 mmHg. Serum K+ = 4.1, Creatinine normal.

• Chlorthalidone (thiazide-like) - superior to HCTZ for 24-hour BP control due to longer half-life. First-line in JNC/guidelines for uncomplicated HTN.

• Thiazides → ↓ insulin secretion (hypokalemia inhibits insulin release) + ↑ insulin resistance → hyperglycemia.
• Thiazides → ↑ proximal tubule uric acid reabsorption → hyperuricemia/gout flare.

• Diuretics → hemoconcentration → further ↑ blood viscosity - dangerous in polycythemia. Adequate hydration + cautiuos dosing required. Hydroxyurea (cytoreductive) remains the cornerstone for PV.

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CASE 3 - "Acute Mountain Sickness"

A 28-year-old trekker at 4500m develops headache, nausea, and confusion.

• Acetazolamide (carbonic anhydrase inhibitor). Inhibits CA in PCT → ↓ HCO3- reabsorption → metabolic acidosis → stimulates ventilation → compensates for high-altitude respiratory alkalosis.

• At altitude, hypoxia → hyperventilation → respiratory alkalosis → blunts respiratory drive. Acetazolamide-induced metabolic acidosis counteracts this, maintaining ventilatory drive. Also used in central sleep apnea for same reason.

• Glaucoma (↓ aqueous humor formation), epilepsy (petit mal), periodic paralysis, alkalinize urine (salicylate poisoning).

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PART 2: RESPIRATORY SYSTEM PHARMACOLOGY

Core Concepts You Must Know

CASE 4 - "The Wheezing Child"

A 9-year-old boy wakes at night with wheeze and cough. Spirometry shows FEV1/FVC = 68% (obstructive). He uses salbutamol inhaler twice weekly.

• Mild intermittent asthma (symptoms < 2 days/week, nocturnal < 2x/month, no interference with activity, FEV1 ≥ 80%). Step 1 treatment.

• Low-dose inhaled corticosteroid (ICS) - e.g., budesonide 200 mcg/day. This is Step 2. ICS is the cornerstone controller - reduces airway inflammation, hyperresponsiveness, and exacerbation frequency.

• Furosemide IV can paradoxically cause bronchospasm by stimulating prostaglandin release. Also, loop diuretics cause hypokalemia - hypokalemia potentiates β2 agonist-induced hypokalemia (salbutamol also lowers K+) → dangerous arrhythmia risk.

• Anemia → ↓ O2 carrying capacity → tissues hypoxic → compensatory tachypnea → may worsen dyspnea perception. Iron deficiency specifically also affects respiratory muscle mitochondrial function. Treat the anemia with oral iron.

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CASE 5 - "The COPD Exacerbation"

A 68-year-old ex-smoker, known COPD (GOLD Stage III), presents with increased breathlessness, yellow sputum, SpO2 = 86% on room air.

• Controlled O2 (target SpO2 88-92% - hypercapnic drive risk)
• Nebulized salbutamol + ipratropium (SABA + SAMA combination)
• Systemic corticosteroids (prednisolone 40 mg x 5 days - shortens recovery)
• Antibiotics if bacterial trigger (amoxicillin / doxycycline / azithromycin)

• Chronic hypercapnia → central chemoreceptors desensitized to CO2 → rely on peripheral chemoreceptors (hypoxic drive). High O2 → abolishes hypoxic drive → apnea/CO2 narcosis.

• Furosemide. In cor pulmonale, right heart failure leads to systemic venous hypertension and edema. Loop diuretic reduces preload. Caution: aggressive diuresis can cause metabolic alkalosis → compensatory hypoventilation → worsens CO2 retention in a COPD patient. A connection between diuretics and respiratory system!

• Critical illness → ↑ cortisol + ↑ sympathetic tone → ↓ gastric mucosal blood flow + ↑ acid secretion. Also systemic steroids used for COPD exacerbation → further mucosal damage.
• Prophylaxis: PPI (pantoprazole IV) or H2 blocker (ranitidine). This links the respiratory chapter to the gut chapter.

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PART 3: BLOOD PHARMACOLOGY

Core Concepts You Must Know

CASE 6 - "The DVT Patient"

A 45-year-old woman post-orthopedic surgery develops swollen left leg. Doppler confirms deep vein thrombosis. She is 8 weeks pregnant.

• Low molecular weight heparin (LMWH) - enoxaparin. Warfarin crosses placenta → embryopathy (esp. 6-12 weeks), fetal hemorrhage. DOACs are contraindicated in pregnancy. LMWH does NOT cross placenta (large molecule).

• Both potentiate antithrombin III. UFH inactivates both thrombin (IIa) AND Xa. LMWH primarily inhibits factor Xa (anti-Xa effect). LMWH has more predictable PK, no need for aPTT monitoring (use anti-Xa levels if needed).

• Active GI bleeding is a relative contraindication. Anticoagulation → risk of major GI bleed from pre-existing ulcer. Must treat ulcer first with PPI. LMWH preferred over warfarin as effect more predictable and reversible with protamine.

• Heparin → Protamine sulfate (1 mg per 100 U heparin)
• Warfarin → Vitamin K (slow, 6-24h) + FFP/PCC (immediate)
• Dabigatran → Idarucizumab (specific antidote)
• Rivaroxaban/Apixaban → Andexanet alfa

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CASE 7 - "The Atrial Fibrillation Stroke Prevention"

A 72-year-old man with AF and CHA2DS2-VASc score = 4. His INR on warfarin = 1.5 (subtherapeutic).

• Target INR = 2.0-3.0 for AF. INR is affected by diet (Vit K intake), drugs (many interactions), alcohol, illness. Hence patient compliance and regular monitoring essential.

• Enhancers (↑ INR): Amiodarone, metronidazole, fluconazole, aspirin, NSAIDs, broad-spectrum antibiotics (kill Vit K-producing gut flora)
• Reducers (↓ INR): Rifampicin, carbamazepine, St. John's Wort (CYP450 inducers)

• Rifampicin = potent CYP2C9 inducer → ↑ warfarin metabolism → ↓ INR → subtherapeutic → stroke risk. INR must be monitored much more frequently. DOAC (rivaroxaban) would also be affected via CYP3A4 induction.

• Furosemide-induced hypovolemia → ↑ warfarin concentration (less volume of distribution) → ↑ bleeding risk. Also acidosis from loop diuretics can displace warfarin from albumin binding.

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CASE 8 - "Iron Deficiency Anemia in Pregnancy"

A 26-year-old woman, 28 weeks pregnant, Hb = 7.2 g/dL, MCV = 68, serum ferritin = 4 ng/mL.

• Ferrous sulfate 200 mg three times daily (elemental iron ~65 mg per tablet). Ferrous form (Fe2+) absorbed from duodenum via DMT-1 transporter. In the enterocyte, converted to Fe3+ and stored as ferritin or transferred to plasma via ferroportin.

• Enhancers: Vitamin C (reduces Fe3+ → Fe2+), acidic gastric environment, meat factor
• Inhibitors: Phytates (chapati/roti), tannins (tea/coffee), calcium, antacids, PPIs, H2 blockers

• Classic error! Tea (tannins) + antacid → both chelate iron → ↓ absorption drastically. Iron should be taken on empty stomach or with Vit C, at least 2 hours apart from antacids.

• Anemia + anticoagulation = high risk situation. Low Hb means any bleeding (including from anticoagulant) is less tolerated. Correct anemia (consider IV iron - Ferric carboxymaltose - faster repletion) + use LMWH (reversible, no teratogenicity post-delivery).

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PART 4: GUT PHARMACOLOGY

Core Concepts You Must Know

CASE 9 - "The Burning Stomach"

A 48-year-old man with H. pylori positive peptic ulcer disease. On NSAIDs for arthritis. Complains of epigastric pain after meals.

• Standard Triple Therapy (7-14 days):
  • PPI (omeprazole 20mg BD) + Clarithromycin 500mg BD + Amoxicillin 1g BD
• If clarithromycin resistance suspected: Quadruple therapy = PPI + Bismuth + Metronidazole + Tetracycline

• NSAIDs inhibit COX-1 → ↓ prostaglandin synthesis → ↓ mucus and HCO3- secretion + ↓ mucosal blood flow → loss of cytoprotection → ulcer.
• Prevention: Add PPI when NSAIDs are unavoidable. Switch to selective COX-2 inhibitor (celecoxib) - spares COX-1, less GI risk (but ↑ CV risk).

• Aspirin + non-selective NSAID = very high GI bleeding risk. Ibuprofen also competes with aspirin at COX-1 binding site → blocks aspirin's antiplatelet effect. Use paracetamol for pain instead. If NSAID is unavoidable, use celecoxib + PPI.

• Furosemide can cause nausea, vomiting, diarrhea as GI side effects. Also, loop diuretics → hypokalemia → ileus (smooth muscle hyperpolarized → decreased gut motility). Severe hypokalemia can mimic functional bowel obstruction.

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CASE 10 - "Chemotherapy-Induced Nausea and Vomiting (CINV)"

A 55-year-old woman with breast cancer starts cisplatin-based chemotherapy. She develops severe nausea and vomiting beginning 1 hour after infusion and continuing for 3 days.

• Acute CINV (0-24h): 5HT3 surge from damaged enterochromaffin cells → stimulates vagal afferents → CTZ → vomiting center.
  • Drug: Ondansetron (5HT3 antagonist) + Dexamethasone (synergistic - mechanism unclear)
• Delayed CINV (24h-5 days): NK1 receptor (substance P) mediated.
  • Drug: Aprepitant (NK1 antagonist) - standard for highly emetogenic chemo

• Cisplatin → tubular necrosis → ↓ GFR → Hypomagnesemia (Mg2+ wasting) + Anemia (↓ EPO production from damaged tubular cells).
• Furosemide is used during cisplatin infusion to force diuresis and reduce nephrotoxicity. But forced diuresis → further Mg2+ and K+ loss. Must pre-hydrate and supplement electrolytes.

• Cisplatin can cause interstitial pneumonitis/fibrosis. Symptoms: dry cough, progressive dyspnea. Management: stop cisplatin, high-dose corticosteroids. This connects respiratory toxicity of drugs to the GI/blood chapter.

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MASTER INTERLINK TABLE

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HIGH-YIELD EXAM TRIGGERS (One-liners)

1. Furosemide + gentamicin = synergistic ototoxicity (both damage cochlear hair cells)
2. Spironolactone ADR = gynecomastia (antiandrogen effect), hyperkalemia
3. Thiazides = useful in nephrolithiasis (hypercalciuria) by ↑ Ca2+ reabsorption in DCT
4. Salbutamol causes hypokalemia (K+ shifts into cells via Na/K ATPase) - dangerous in asthma + diuretic combo
5. Theophylline toxicity = seizures, arrhythmias - narrow therapeutic index (5-15 mcg/mL)
6. Warfarin + cranberry juice = ↑ INR (CYP2C9 inhibition)
7. Metoclopramide crosses BBB → extrapyramidal effects (Parkinsonism, tardive dyskinesia)
8. Domperidone does NOT cross BBB → no EPS, preferred in elderly
9. Misoprostol (PGE1 analogue) = gastroprotection + used for medical abortion + cervical ripening
10. Omeprazole (PPI) + clopidogrel = reduced antiplatelet effect (CYP2C19 competition)

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QUICK CONCEPT SUMMARY (For Revision)

DIURETICS
├── Loop (Furosemide) → CCF, Pulmonary edema, Hypercalcemia
├── Thiazide (HCTZ) → HTN, Nephrolithiasis (hypercalciuria)
├── K+-sparing (Spiro) → Hyperaldosteronism, Cirrhosis, CCF
└── CAI (Acetazolamide) → Glaucoma, Altitude sickness, Alkalinize urine

RESPIRATORY
├── Asthma → SABA + ICS (controller) → LABA for persistent
├── COPD → LAMA (Tiotropium) first + LABA, ICS if exacerbations
└── Severe → Biologics (Omalizumab for IgE, Mepolizumab for eosinophilic)

BLOOD
├── DVT/PE → LMWH acute → DOAC long term (warfarin in valves/pregnancy)
├── ACS → Aspirin + Clopidogrel (dual antiplatelet)
└── Anemia → Iron (microcytic) / B12+Folate (macrocytic) / EPO (renal)

GUT
├── PUD → PPI + H.pylori eradication triple therapy
├── GERD → Lifestyle + PPI
└── CINV → Ondansetron (acute) + Aprepitant (delayed)

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