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🔴 MASTER COMPARISON — ANTIHYPERTENSIVES & MgSO₄ IN OBSTETRICS

1️⃣ Molecular class, receptor targets & signaling pathways

Feature	Labetalol	Methyldopa	Nifedipine	Hydralazine	Magnesium sulfate (MgSO₄)
Chemical class	Mixed α₁ + β₁/β₂ blocker	α-methyldopa (false neurotransmitter)	Dihydropyridine CCB	Direct arteriolar vasodilator	Inorganic salt (Mg²⁺)
Primary receptor / target	α₁ blockade + β blockade	Central α₂ agonist (via α-methylnorepinephrine)	L-type Ca²⁺ channels (vascular smooth muscle)	NO/cGMP-mediated smooth muscle relaxation	NMDA receptor, Ca²⁺ channels, neuromuscular junction
Site of action	Peripheral vasculature + heart	CNS (vasomotor center)	Peripheral arterioles > myocardium	Arterioles (not veins)	CNS + NMJ + uterus
Second messenger pathway	↓ IP₃-Ca²⁺ (α₁ block) + ↓ cAMP (β block)	↓ sympathetic outflow from brainstem	↓ Ca²⁺ influx → vasodilation	↑ NO → ↑ cGMP → ↓ Ca²⁺	↓ Ca²⁺-mediated neuronal firing
Key physiologic effect	↓ BP without major ↓ uteroplacental flow	↓ sympathetic tone	Potent vasodilation Tocolytic	Rapid BP reduction	Anticonvulsant, mild vasodilation

2️⃣ Pharmacokinetics (PK)

PK Feature	Labetalol	Methyldopa	Nifedipine	Hydralazine	MgSO₄
Routes used in OB	Oral, IV	Oral	Oral (IR / SR)	IV (acute)	IV / IM
Onset	IV: 2–5 min	Slow (hours–days)	Oral IR: 20–30 min	IV: 10–20 min	IV: immediate
Duration	Moderate	Long (central effect)	Short (IR), long (SR)	Short, unpredictable	Depends on serum level
Half-life	~6 h	~2 h (but effect long)	~2 h (IR)	~3 h (variable)	4–5 h
Metabolism	Hepatic	Hepatic → active metabolite	Hepatic (CYP3A4)	Hepatic (acetylation)	Renal excretion only
Placental transfer	Yes (safe)	Yes (safe)	Yes (safe)	Yes (safe)	Yes (used for fetus too)

3️⃣ Pharmacodynamics (PD) & clinical use (EXAM CORE)

Feature	Labetalol	Methyldopa	Nifedipine	Hydralazine	MgSO₄
Primary OB use	1st-line for severe HTN	Chronic HTN in pregnancy	Acute & maintenance HTN	Severe HTN emergency	Eclampsia prevention & treatment
Effect on HR	↓ / neutral	↓	Reflex ↑	Reflex ↑	↓ neuronal firing
Effect on uterine blood flow	Preserved	Preserved	Preserved	Preserved	Preserved
Effect on fetus	Safe	Safe (long history)	Safe	Safe	Neuroprotection (<32 wks)
Special role	Best IV drug in emergencies	Safest long-term	Oral alternative to IV drugs	Rapid BP lowering	Not an antihypertensive

4️⃣ Adverse effects & complications (VERY HIGH-YIELD)

🔹 Maternal adverse effects

Drug	Major adverse effects
Labetalol	Bradycardia, hypotension, fatigue, bronchospasm (β₂ block)
Methyldopa	Sedation, depression, dry mouth, ↑ prolactin
Nifedipine	Headache, flushing, ankle edema, reflex tachycardia
Hydralazine	Headache, flushing, tachycardia, drug-induced lupus
MgSO₄	Loss of reflexes, respiratory depression, cardiac arrest (toxicity)

🔹 Fetal / neonatal effects

Drug	Fetal effects
Labetalol	Neonatal bradycardia, hypoglycemia (rare)
Methyldopa	Minimal (safe)
Nifedipine	Minimal
Hydralazine	Minimal
MgSO₄	Neonatal hypotonia, respiratory depression (high dose)

5️⃣ Contraindications (EXAM FAVORITES)

Condition	Labetalol	Methyldopa	Nifedipine	Hydralazine	MgSO₄
Asthma	❌	✔	✔	✔	✔
Heart block / bradycardia	❌	✔	✔	✔	✔
Depression	✔	❌	✔	✔	✔
Liver disease	✔	❌ (hepatotoxicity)	✔	✔	✔
Renal failure	✔	✔	✔	✔	❌
Myasthenia gravis	✔	✔	✔	✔	❌ ABSOLUTE

6️⃣ Drug interactions & monitoring

Drug	Key interactions	Monitoring
Labetalol	Other β-blockers, Ca-blockers	BP, HR
Methyldopa	Iron ↓ absorption	LFTs, Coombs test
Nifedipine	CYP3A4 inhibitors	BP
Hydralazine	Other vasodilators	BP, ANA (long term)
MgSO₄	CCBs → cardiac collapse	RR, reflexes, urine output, serum Mg²⁺

7️⃣ MgSO₄ — TOXICITY & ANTIDOTE (VIVA GOLD)

Parameter	Normal	Toxic
Serum Mg²⁺	1.5–2.5 mEq/L	>9 mEq/L
Reflexes	Present	Absent first
Respiration	Normal	Respiratory depression
Heart	Normal	Cardiac arrest

🔥 Antidote

➡️ 10 mL of 10% calcium gluconate IV slowly

🧠 ONE-LINE EXAM LOCKS

Labetalol → best IV drug for severe HTN
Methyldopa → safest long-term
Nifedipine → oral rapid alternative
Hydralazine → fast but unpredictable
MgSO₄ → anticonvulsant, NOT antihypertensive

CYP3A4 INHIBITORS — TRANSPOSED TABLE

Category	STRONG inhibitors	MODERATE inhibitors	WEAK inhibitors
Drugs / substances	Grapefruit juiceKetoconazoleItraconazoleVoriconazoleClarithromycinRitonavirIndinavirNefazodoneAmiodarone	ErythromycinDiltiazemVerapamilFluconazoleCimetidineCyclosporine	Oral contraceptivesFluoxetineFluvoxamineIsoniazid
Common classes	Azole antifungalsMacrolides (clarithro)Protease inhibitorsAntiarrhythmicFood	Macrolides (erythro)CCBsAzolesH₂ blockerImmunosuppressant	HormonesSSRIsAntitubercular
Effect on CYP3A4	Marked inhibition	Moderate inhibition	Mild inhibition
Clinical impact	Large ↑ drug levels → high toxicity risk	Moderate ↑ drug levels	Small ↑ drug levels
Classic exam use	Ritonavir used as booster	CCB–statin interactions	Usually side-note
High-risk interactions	Statins, benzodiazepines, CCBs, steroids, cyclosporine	Statins, cyclosporine, warfarin	Usually minimal

✅ 1. LABETALOL

⭐ Class

α + β blocker (combined)

⭐ Mechanism (PD)

β1 block → ↓ heart rate
α1 block → vasodilation
Result → ↓ BP without ↓ placental blood flow

⭐ Pharmacokinetics (PK)

Oral & IV
Hepatic metabolism
Half-life: 6–8 hours

⭐ Use

First-line for acute severe hypertension in pregnancy
Severe pre-eclampsia

⭐ Dose

20 mg IV, then 40 mg → 80 mg every 10 min (max 220 mg)
Or infusion 1–2 mg/min
Oral: 100–400 mg TDS

⭐ Side Effects

Bradycardia
Hypotension
Fatigue
Fetal bradycardia

⭐ Contra

❌ Asthma

❌ Heart block

❌ Bradycardia

🟦🟦🟦

✅ 2. METHYLDOPA — 20% VERSION

⭐ Class

Central α2-agonist

⭐ Mechanism (PD)

Stimulates central α2 receptors → ↓ sympathetic outflow

→ Slow, steady BP reduction

⭐ Pharmacokinetics (PK)

Oral
Liver metabolism
Half-life: 2 hours

⭐ Use

Chronic hypertension in pregnancy (long-term control)

⭐ Dose

250 mg TDS → up to 500 mg QID

⭐ Side Effects

Sedation
Depression
Dry mouth
Hemolytic anemia (rare)
↑ LFTs

⭐ Contra

❌ Liver disease

❌ Depression

🟦🟦🟦

✅ 3. MgSO₄ (MAGNESIUM SULFATE) — 20% VERSION

⭐ Class

Anticonvulsant for eclampsia prevention/treatment

⭐ Mechanism (PD)

Blocks NMDA receptors
Stabilizes neuronal membranes
Competes with calcium → uterine & CNS relaxation

👉 Prevents seizures, NOT a BP-lowering drug.

⭐ Pharmacokinetics (PK)

IV only
Renal excretion
Half-life: 4 hours

⭐ Use

Pre-eclampsia: seizure prophylaxis
Eclampsia: seizure treatment

⭐ Dose (Pritchard or Zuspan)

Loading: 4 g IV over 10 min
Maintenance: 1 g/hour infusion

⭐ Side Effects

Loss of reflexes
Respiratory depression
Hypotension
Cardiac arrest (very high levels)

⭐ Toxicity Monitoring

Reflexes
Respiratory rate
Urine output
Antidote: Calcium gluconate

⭐ Contra

❌ Renal failure

❌ Myasthenia gravis

🟦🟦🟦

✅ 4. NIFEDIPINE — 20% VERSION

⭐ Class

Calcium-channel blocker (dihydropyridine)

⭐ Mechanism (PD)

Blocks L-type calcium channels

→ Vasodilation

→ ↓ BP

→ Also relaxes uterus (tocolysis)

⭐ Pharmacokinetics (PK)

Oral
Liver metabolism
Half-life: 2–5 hours

⭐ Use

Acute severe hypertension
Chronic hypertension
Tocolysis (uterine relaxation)

⭐ Dose

10 mg oral, repeat after 20–30 minutes
Maintenance: 10–20 mg TDS

⭐ Side Effects

Headache
Flushing
Hypotension
Reflex tachycardia

⭐ Contra

❌ Hypotension

❌ With MgSO4 (risk of severe hypotension — relative)

🟦🟦🟦

✅ 5. HYDRALAZINE — 20% VERSION

⭐ Class

Direct arteriolar vasodilator

⭐ Mechanism (PD)

Direct smooth muscle relaxation →

↓ Afterload

↓ BP

↑ Reflex tachycardia

⭐ Pharmacokinetics (PK)

IV
Half-life: 3 hours
Liver metabolism

⭐ Use

Acute severe hypertension in pregnancy

(2nd line after labetalol/nifedipine in many guidelines)

⭐ Dose

5 mg IV bolus, repeat every 20–30 min
Max 20 mg

⭐ Side Effects

Reflex tachycardia
Headache
Hypotension
Nausea
Fetal distress due to maternal hypotension

⭐ Contra

❌ Tachycardia

❌ Heart failure

🔥 ULTRA SUPER-SUMMARY (MEMORISE THESE 10 LINES)

Labetalol – α+β blocker, rapid BP drop, first-line, avoid in asthma.

Methyldopa – central α2 agonist, slow BP drop, chronic use, causes sedation.

MgSO₄ – anticonvulsant for eclampsia, NOT antihypertensive, toxicity → loss of reflexes.

Nifedipine – Ca-channel blocker, fast oral BP control, headaches, avoid with MgSO₄.

Hydralazine – direct vasodilator, reflex tachycardia, used IV in acute crises.

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🔴 MASTER COMPARISON — ANTIHYPERTENSIVES & MgSO₄ IN OBSTETRICS

1️⃣ Molecular class, receptor targets & signaling pathways

Feature	Labetalol	Methyldopa	Nifedipine	Hydralazine	Magnesium sulfate (MgSO₄)
Chemical class	Mixed α₁ + β₁/β₂ blocker	α-methyldopa (false neurotransmitter)	Dihydropyridine CCB	Direct arteriolar vasodilator	Inorganic salt (Mg²⁺)
Primary receptor / target	α₁ blockade + β blockade	Central α₂ agonist (via α-methylnorepinephrine)	L-type Ca²⁺ channels (vascular smooth muscle)	NO/cGMP-mediated smooth muscle relaxation	NMDA receptor, Ca²⁺ channels, neuromuscular junction
Site of action	Peripheral vasculature + heart	CNS (vasomotor center)	Peripheral arterioles > myocardium	Arterioles (not veins)	CNS + NMJ + uterus
Second messenger pathway	↓ IP₃-Ca²⁺ (α₁ block) + ↓ cAMP (β block)	↓ sympathetic outflow from brainstem	↓ Ca²⁺ influx → vasodilation	↑ NO → ↑ cGMP → ↓ Ca²⁺	↓ Ca²⁺-mediated neuronal firing
Key physiologic effect	↓ BP without major ↓ uteroplacental flow	↓ sympathetic tone	Potent vasodilation Tocolytic	Rapid BP reduction	Anticonvulsant, mild vasodilation

2️⃣ Pharmacokinetics (PK)

PK Feature	Labetalol	Methyldopa	Nifedipine	Hydralazine	MgSO₄
Routes used in OB	Oral, IV	Oral	Oral (IR / SR)	IV (acute)	IV / IM
Onset	IV: 2–5 min	Slow (hours–days)	Oral IR: 20–30 min	IV: 10–20 min	IV: immediate
Duration	Moderate	Long (central effect)	Short (IR), long (SR)	Short, unpredictable	Depends on serum level
Half-life	~6 h	~2 h (but effect long)	~2 h (IR)	~3 h (variable)	4–5 h
Metabolism	Hepatic	Hepatic → active metabolite	Hepatic (CYP3A4)	Hepatic (acetylation)	Renal excretion only
Placental transfer	Yes (safe)	Yes (safe)	Yes (safe)	Yes (safe)	Yes (used for fetus too)

3️⃣ Pharmacodynamics (PD) & clinical use (EXAM CORE)

Feature	Labetalol	Methyldopa	Nifedipine	Hydralazine	MgSO₄
Primary OB use	1st-line for severe HTN	Chronic HTN in pregnancy	Acute & maintenance HTN	Severe HTN emergency	Eclampsia prevention & treatment
Effect on HR	↓ / neutral	↓	Reflex ↑	Reflex ↑	↓ neuronal firing
Effect on uterine blood flow	Preserved	Preserved	Preserved	Preserved	Preserved
Effect on fetus	Safe	Safe (long history)	Safe	Safe	Neuroprotection (<32 wks)
Special role	Best IV drug in emergencies	Safest long-term	Oral alternative to IV drugs	Rapid BP lowering	Not an antihypertensive