- Prazosin (Minipress), terazosin
(Hytrin), and doxazosin (Cardura) reduce
arteriolar resistance and increase venous
capacitance as a consequence of alpha1
adrenergic receptor blockade.
- Normal inhibition
of norepinephrine-mediate inhibition
through alpha2
receptors remain-- prazosin (Minipress)
is a selective postsynaptic alpha1 adrenergic receptor blocker
- The short-term increase in
heart rate and plasma renin levels do not
persist although the vasodilation
continues.
- Prazosin (Minipress)
monotherapy --less effective than
thiazide diuretics
- Prazosin (Minipress) in
combination with other agents: quite
effective in young patients with
moderately severe hypertension
- Good patient
compliance
- Cardiovascular
Effects: --
- Prazosin (Minipress)
reduces systemic vascular resistance without:
- causing
reflex-mediated tachycardia
- causing increases
in plasma renin {as seen with minoxidil/hydralazine}
- absence of
changes in plasma renin
reflect continued alpha2
receptor function which
normally inhibits renin
release {recall that
prazosin is an alpha1
selective antagonist}
- Prazosin (Minipress) --
greater affinity for venular alpha
receptors compared to arteriolar alpha
receptors; resultant hemodynamics effect
(orthostatic hypotension) --an action
more similar to nitroglycerin than
hydralazine (Apresoline).
- Renal blood flow is
maintained.
- Retention of salt and
water occurs.
- Alpha1-adrenergic
receptor blockers reduce plasma triglycerides,
total and LDL-cholesterol, and increase HDL-cholesterol.
- Other Therapeutic Uses:
- Congestive heart failure:
valuable for reducing afterload
- Preoperative preparation
of patients with pheochromocytoma
- Treatment of benign
prostatic hypertrophy in older males
(drug decreases prostate size)
- Adverse Effect
- Inital-dose marked
orthostatic hypotension is seen in about 50% of
cases-- (sudden syncope; dosage dependence)
- Fluid retention, vertigo
- dry mouth, urinary
frequency, lethargy, sexual dysfunction, nasal
congestion, nightmares
- Anesthetic
Implications:
- Prazosin (Minipress)-induced alpha1
blockade may cause exaggerated hypotension
during epidural anesthesia (alpha receptor--blockade
prevents compensatory vasoconstriction)
- Prazosin
(Minipress)-exacerbated
hypotension may not be responsive
to typical alpha1
adrenergic agonists (e.g.
phenylephrine) dosage;
epinephrine may be required to
increase systemic vascular
resistance & BP in this
setting
- The combination of prazosin
(Minipress) and a beta-blocker
could result in nearly refractory
hypotension during regional
anesthesia (diminished response
to both b and alpha1 agonists)
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