C H A P T E R 4 3
Drugs affecting blood pressure
677
Therapeutic actions and indications
Sympathomimetic drugs react with sympathetic adren
ergic receptors to cause the effects of a sympathetic
stress response: increased blood pressure, increased
blood volume and increased strength of cardiac muscle
contraction. These actions increase blood pressure and
may restore balance to the cardiovascular system while
the underlying cause of the shock (e.g. volume depletion,
blood loss) is treated.
Adverse effects
The adverse effects related to these drugs are the effects
of stimulation of the sympathetic system; decreased GI
activity with nausea and constipation; increased respira-
tory rate and changes in blood pressure; headache; and
changes in peripheral blood flowwith numbness, tingling
and even gangrene in extreme cases. These drugs should
be used with caution with any disease that limits blood
flow, with tachycardia or with hypertension.
A
lpha
-
specific adrenergic agents
Midodrine (
Gutron
) is an alpha-specific adrenergic
agent used to treat orthostatic hypotension—hypoten-
sion that occurs with position change—that interferes
with a person’s ability to function and has not responded
to any other therapy (Table 43.3). It is currently availa-
ble in New Zealand but not Australia.
Therapeutic actions and indications
Midodrine activates alpha-receptors in arteries and veins
to produce an increase in vascular tone and an increase
in blood pressure. It is indicated for the symptomatic
treatment of orthostatic hypotension in individuals
whose lives are impaired by the disorder and who have
not had a response to any other therapy.
Pharmacokinetics
Midodrine is rapidly absorbed from the GI tract,
reaching peak levels within 1 to 2 hours. It is metabo-
lised in the liver and excreted in the urine with a half-life
of 3 to 4 hours. It should be reserved in pregnancy
for cases in which the benefit to the mother clearly
outweighs the potential risk to the fetus. It is not known
whether midodrine enters breast milk, so caution should
be used during breastfeeding.
Contraindications and cautions
Midodrine is contraindicated in the presence of supine
hypertension, CAD or phaeochromocytoma
because of
the risk of precipitating a hypertensive emergency
; with
acute renal disease,
which might interfere with excretion
of the drug
; with urinary retention
because the stimu-
lation of alpha-receptors can exacerbate this problem
;
and with thyrotoxicosis,
which could further increase
blood pressure.
Caution should be used with pregnancy
and breastfeeding
because of the potential for adverse
effects on the fetus or neonate
; with visual problems,
which could be exacerbated by vasoconstriction
; and
with renal or hepatic impairment,
which could alter the
metabolism and excretion of the drug.
Adverse effects
The most common adverse effects associated with this
drug are related to the stimulation of alpha-receptors
and include piloerection, chills and rash; hypertension
and bradycardia; dizziness, vision changes, vertigo and
headache; and problems with urination.
Clinically important drug–drug interactions
There is a risk of increased effects and toxicity of cardiac
glycosides, beta-blockers, alpha-adrenergic agents and
corticosteroids if they are taken with midodrine. People
who are receiving any of these combinations should be
monitored carefully for the need for a dose adjustment.
TABLE 43.3
DRUGS IN FOCUS Antihypotensive agents
Drug name
Dosage/route
Usual indications
Alpha-specific adrenergic agent
midodrine (Gutron)
10 mg PO t.d.s.
Treatment of orthostatic hypotension in
adults
Sympathetic adrenergic agonists or vasopressors
See Chapter 30
See Chapter 30
First-choice drugs for treatment of
hypotension or shock
Care considerations for people receiving
alpha-specific adrenergic agents
Assessment: History and examination
■
■
Assess for the following conditions,
which could be
contraindications or cautions
: any known allergy
to midodrine
to prevent hypersensitivity reactions
;
impaired kidney or liver function,
which could
interfere with metabolism and excretion of the
