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hydralazine

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hydralazine heart failure

Not relevant to hydralazine specifically, so no contradicting recent evidence to flag. Here's a comprehensive summary based on the medical library.

Hydralazine

Class: Direct-acting arteriolar vasodilator (hydrazinophthalazine derivative)

Mechanism of Action

Hydralazine directly relaxes arteriolar smooth muscle with little to no effect on venous smooth muscle. The exact molecular mechanism is not fully established, but proposed pathways include:
  • Reduction in intracellular Ca2+ concentration in vascular smooth muscle
  • Inhibition of IP3-induced Ca2+ release from intracellular stores
  • Opening of high-conductance Ca2+-activated K+ channels
  • Activation of arachidonic acid/COX/prostacyclin pathways (explaining NSAID interference)
  • Antioxidant effects that preserve endogenous nitric oxide by inhibiting oxidases that break it down
Because it selectively dilates arterioles (sparing veins), it lowers blood pressure without significant postural hypotension, but the drop in vascular resistance triggers strong baroreceptor-mediated sympathetic reflexes: increased heart rate/contractility, increased renin-angiotensin-aldosterone activity, and fluid retention - all of which blunt its antihypertensive effect over time (tachyphylaxis) - Katzung's Basic and Clinical Pharmacology, p. 279; Goodman & Gilman's Pharmacological Basis of Therapeutics.

Pharmacokinetics

  • Well absorbed orally, but undergoes extensive first-pass N-acetylation in the gut and liver, giving low and variable bioavailability (roughly 16% in fast acetylators, 35% in slow acetylators; Katzung cites ~25% average)
  • Acetylation rate is genetically determined (bimodal - "fast" vs "slow" acetylators, roughly half the US population each), so slow acetylators get more drug effect at a given dose
  • Half-life ~1.5-3 hours (short), but vasodilator effect outlasts plasma levels, likely due to tissue binding
  • Oral tablet, IV, and IM formulations available; the fixed-dose oral combination with isosorbide dinitrate has higher (~90%) bioavailability - Lippincott Illustrated Reviews: Pharmacology, p. 367-368

Clinical Uses

  1. Severe/resistant hypertension - now a later-line agent given reflex tachycardia/tachyphylaxis; most effective combined with a beta-blocker (to blunt reflex tachycardia) and a diuretic (to counter fluid retention)
  2. Heart failure with reduced ejection fraction (HFrEF) - combined with isosorbide dinitrate as an add-on in patients intolerant of ACE inhibitors/ARBs, or added to guideline-directed therapy. The fixed-dose hydralazine/isosorbide dinitrate combination (BiDil) has shown improved symptoms and survival specifically in African-American patients with HFrEF on background beta-blocker + ACE-inhibitor/ARB therapy
  3. Hypertensive emergencies in pregnancy (IV hydralazine), though a meta-analysis of 21 trials found IV hydralazine carried higher risk of maternal hypotension compared with labetalol or nifedipine - Brenner and Rector's The Kidney

Adverse Effects

  • Common: headache, nausea, anorexia, palpitations, sweating, flushing, dizziness, reflex tachycardia, edema
  • In patients with ischemic heart disease: reflex tachycardia/sympathetic stimulation can provoke angina or ischemic arrhythmias
  • Drug-induced lupus-like syndrome: at doses ≥400 mg/day, 10-20% incidence (mainly in slow acetylators) of arthralgia, myalgia, skin rash, and fever resembling lupus erythematosus; this is why usual dosing is kept in the 40-200 mg/day range
  • Should generally be used alongside a beta-blocker and diuretic to offset reflex tachycardia and fluid retention

Related Drug

Minoxidil is a more potent alternative arteriolar vasodilator (K+ channel opener) used when hydralazine is inadequate, particularly in renal failure with severe hypertension, but carries its own toxicity profile including hypertrichosis - Katzung's Basic and Clinical Pharmacology, p. 280.
No recent (2-year) systematic reviews/meta-analyses specific to hydralazine turned up in PubMed that would update or contradict this textbook information - the searches returned unrelated heart failure topics (AML cardiotoxicity, semaglutide, general HF hospitalization studies).
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