Monthly Archives: November 2020

PHENTOLAMINE

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  • Competitive alpha blocker
  • Used in hypertensive emergencies like in pheochromocytoma; also in CRPS
  • MOA: 3 times more specific for alpha 1 receptors than alpha 2. Doesn’t bind covalently to receptors; hence reversible. Also has beta agonistic and anti-serotoninergic activity
  • Comes as pale yellow solution 10 mg/mL; dose is 1–5 mg titrated to effect
  • CVS: Vasodilation and  reduce BP, improve coronary artery perfusion, reduce pulmonary artery pressures. Alpha 2 blockade enhances noradrenaline release causing increase in HR and CO
  • RESPIRATORY SYSTEM: Increase FEV1, increase secretions, prevents bronchospasm caused by histamine release
  • Other Side Effects: Nasal congestion, Hypoglycemia (Causes insulin secretion)

PHENOXYBENZAMINE

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  • Phenoxybenzamine is a haloalkylamine. Used as a vasodilator in hypertensive emergencies like in pheochromacytoma; also used in the management of CRPS. Used in the management of intraarterial injection of thiopentone too.
  • It is a non-selective alpha blocker. It binds covalently to the receptors. So it behaves like a competitive irreversible antagonist.New receptors must be synthesised to overcome drug effect. Blockade of alpha 2 receptors increases the amount of noradrenaline released: produces tachycardia. Partial agonist at 5-HT2 receptors
  • Available as capsules of 10 mg and solution: 50 mg/mL for iv use. Oral dose is 10–60 mg per day in divided doses and IV: 10–40 mg over 1 hour. Effects may lasts for 3–4 days
  • CVS: Vasodilation and fall in BP, but tachycardia increase CO
  • CNS: Sedation. Rapid infusion can cause seizures