676
P A R T 8
Drugs acting on the cardiovascular system
Lopresor
), nadolol (
Apo-Nadol, Apo-Nadolol
[not available in Australia]), nebivolol (
Nebilet
[not
available in New Zealand]), pindolol (
Barbloc,
Visken
), propranolol (
Deralin, Inderal
) and timolol
(
Timoptol
)
.
• Alpha- and beta-blockers are useful in conjunction
with other agents and tend to be somewhat more
powerful, blocking all of the receptors in the
sympathetic system. People often complain of
fatigue, loss of libido, inability to sleep, and GI and
genitourinary disturbances. As a result, they may be
unwilling to continue taking these drugs. Alpha- and
beta-blockers used to treat hypertension include the
following agents: carvedilol (
Dicarz, Dilasig
) and
labetalol (
Presolol
,
Trandate
)
.
• Alpha-adrenergic blockers inhibit the postsynaptic
alpha
1
-adrenergic receptors, decreasing sympathetic
tone in the vasculature and causing vasodilation,
which leads to a lowering of blood pressure.
However, these drugs also block presynaptic
alpha
2
-receptors, preventing the feedback control of
noradrenaline release. The result is an increase in the
reflex tachycardia that occurs when blood pressure
decreases. These drugs are used to diagnose and
manage episodes of phaeochromocytoma, but they
have limited usefulness in essential hypertension
because of the associated adverse effects. Alpha-
adrenergic blockers include the following agents:
phenoxybenzamine (
Dibenyline
) and phentolamine
(
Regitine
)
.
• Alpha
1
-blockers are used to treat hypertension
because of their ability to block the postsynaptic
alpha
1
-receptor sites. This decreases vascular
tone and promotes vasodilation, leading to a fall
in blood pressure. These drugs do not block the
presynaptic alpha
2
-receptor sites, and therefore the
reflex tachycardia that accompanies a fall in blood
pressure does not occur. Alpha
1
-blockers used to treat
hypertension include the following agents: doxazosin
(
Dosan
[not available in Australia]), prazosin
(
Minipress
) and terazosin (
Hytrin
).
• Alpha
2
-agonists (see Chapter 30) stimulate the
alpha
2
-receptors in the CNS and inhibit the
cardiovascular centres, leading to a decrease in
sympathetic outflow from the CNS and a resultant
drop in blood pressure. These drugs are associated
with many adverse CNS and GI effects, as well as
cardiac dysrhythmias. Alpha
2
-blockers used to treat
hypertension include the following agents: clonidine
(
Catapres
) and methyldopa (
Aldomet
,
Hydopa
).
Moxonidine (
Physiotens
[not available in New
Zealand]) binds to both I
1
-imidazoline receptors
and alpha
2
-adrenoreceptors, reducing sympathetic
nervous system activity, and thus lowering blood
pressure.
■■
Hypertension is a sustained state of higher-than-
normal blood pressure that can lead to blood vessel
damage, atherosclerosis and damage to small vessels
in end organs.
■■
The cause of essential hypertension is unknown;
treatment varies among individuals.
■■
Drug treatment of hypertension aims to change one
or more of the normal reflexes that control blood
pressure.
■■
Sodium levels and fluid volume are decreased by
diuretic agents.
■■
ACE inhibitors prevent the conversion of
angiotensin I to angiotensin II, leading to a fall in
blood pressure.
■■
ARBs prevent the body from responding to
angiotensin II, causing a loss of effectiveness of the
renin–angiotensin system.
■■
Calcium channel blockers interfere with the ability
of muscles to contract, which leads to vasodilation,
which in turn reduces blood pressure.
■■
Other drugs used to treat hypertension include
diuretics, which decrease the sodium content in the
body, and various sympathetic blockers, which block
the blood pressure–raising effects of the sympathetic
system.
ANTIHYPOTENSIVE AGENTS
As mentioned earlier, if blood pressure becomes too
low (hypotension), the vital centres in the brain and the
rest of the tissues of the body may not receive sufficient
oxygenated blood to continue functioning. Severe hypo-
tension or shock puts the body in serious jeopardy; it
is often an acute emergency situation, with treatment
required to save the person’s life. The first-choice drug
for treating shock is usually a sympathomimetic drug.
See Figure 43.3 for sites of action of drugs used to treat
hypotension. Antihypotensive agents are also discussed
in Table 43.3.
S
ympathetic adrenergic agonists or
vasopressors
Sympathomimetic drugs are the first choice for treating
severe hypotension or shock. The sympathomimetic
drugs are discussed in detail in Chapter 30. Sym-
pathomimetic drugs used to treat shock include the
following agents: adrenaline (
Adrenaline
,
EpiPen
),
dobutamine (
Dobutrex
), dopamine (generic), ephed-
rine (generic), isoprenaline (
Isuprel
), metaraminol
(generic), noradrenaline (
Levophed
) and phenylephrine
(
Neo-Synephrine
).
KEY POINTS
