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alphablockers

alpha adrenergic blockers

introduction

  • alpha blockers prevent the action of sympathomimetic agents such as adrenaline on the alpha adrenergic receptors

potential pharmacologic effects of alpha-blockers

  • postural hypotension and dizziness +/- reflex tachycardia
  • decreased peripheral vascular resistance and lower diastolic blood pressure
  • improved urine flow in those with benign prostatic hyperplasia (BPH)
  • nasal stuffiness / vasodilatory rhinitis
  • inability to achieve erection
  • reduced libido
  • retrograde ejaculation
  • impaired pupil dilatation causing some blurring of vision
  • decreased sweating may increase risk of heat illness in hot environments
  • decreased saliva production and thus dry mouth
  • fluid retention (may require a concomitant diuretic)
  • impaired cardiac function and possible congestive cardiac failure in those with existing poor ejection fractions

alpha 1a blockers used in Mx of BPH

phentolamine:

  • Classical alpha-1 adrenergic blocker but has alpha-2 antag. as well
    • ⇒ induces additional release of NA → tachycardia;
  • Can be used in HT crisis but tachycardia may be problem in IHD;
  • Uses:
    • To antagonise adrenaline-induced vasoconstriction in cases of inadvertent adrenaline use in end-artery regions;
    • As a diagnostic agent in phaeochromocytoma & as adrenolytic during Sx of this tumour;
    • Severe vasospasm in peripheral vascular disease: 10mg IM/IV 1-2/day;

phenoxybenzamine:

  • Long-acting noncomp. a-blocker,& prevents uptake of amines into neuronal & extraneuronal storage sites thus potentiating the effects of NA & adrenaline on beta-receptors;
  • Also has noncompetitive blockade of histamine, 5HT & muscarinic receptors;
  • Thus, decreases periph.resistance
  • causes significant hypotension, nasal stuffiness, and dizziness as it blocks both a1 and a2 receptors
  • Uses:
    • HT episodes in phaeochromocytoma;
    • Urinary retention due to neuropathic bladder;

urapidil:

  • arterial, venous & pulmonary vasodilatation without signif. tachycardia;
  • similar to nifedipine-type Ca antags. but has greater venous effect;
  • no alpha-2 antagonism;
  • does not increase intracranial pressure;
  • also acts on:
    • CNS - reduces adrenergic activity;
      • ⇒ no associated sustained tachycardia;
    • CNS - serotonin agonist
      • ⇒ induces additional reduction in adrenergic activity;
      • T 1/2 = 3hrs; hepatic metabolism; plasma protein binding 80%;
  • IV dose 0.5 - 2mg/kg bolus for acute elevations in BP;

prazosin:

  • in the 1970's, became the first available selective α1-blocker for the treatment of symptomatic benign prostatic hyperplasia (BPH)
  • short halflife necessitated 3 daily doses, and a high incidence of first-dose syncope resulted in poor compliance
  • Decreases peripheral resistance ? due mainly to block of post-synaptic alpha receptors.
  • Effect mainly at level of arterioles; The effect is more pronounced on diastolic BP;
  • Unlike conventional alpha-blockers, not accompanied by reflex tachycardia!
  • Rebound hypertension does not occur on abrupt withdrawal;
  • May increase plasma renin in CCF;
  • Beneficial in CCF (but effect may rapidly attenuate in some pts) by:
    • reducing LV filling pressure;
    • reducing cardiac impedance;
    • augments cardiac output;
  • Extensively metabolised by liver & bioavailability increases 2-3x in CCF;
alphablockers.txt · Last modified: 2017/06/05 21:40 (external edit)