C H A P T E R 4 3
Drugs affecting blood pressure
671
C
alcium channel blockers
Calcium channel blockers decrease blood pressure,
cardiac workload and myocardial oxygen consump-
tion. The effects of these drugs on cardiac workload
also make them very effective in the treatment of angina
(see Chapter 46). The calcium channel blockers avail-
able in immediate-release and sustained-release forms
that are used in treating hypertension are amlodipine
(
Norvasc
), felodipine (
Fendex ER, Plendil
ER
) and
isradipine (
DynaCirc SRO
[not available in Australia]).
Other calcium channel blockers are safe and effective
for this use only if they are given as sustained-release or
extended release preparations. These include diltiazem
(
Cardizem
,
Dilzem
), lercanidipine (
Lercan
,
Zanidip
,
Zircol
), nifedipine (
Adalat
,
Adefin
,
Nyefax
) and verapa-
mil (
Anpec
,
Isoptin
,
Cordilox SR
). See Contraindications
and cautions for important safety information regarding
use of controlled-release products.
Therapeutic actions and indications
Calcium channel blockers inhibit the movement of
calcium ions across the membranes of myocardial and
arterial muscle cells, altering the action potential and
blocking muscle cell contraction. This effect depresses
myocardial contractility, slows cardiac impulse forma-
tion in the conductive tissues and relaxes and dilates
arteries, causing a fall in blood pressure and a decrease
in venous return. See Table 43.2 for indications for each
of these drugs.
Pharmacokinetics
Calcium channel blockers are given orally and are gener-
ally well absorbed, metabolised in the liver and excreted
in the urine. These drugs cross the placenta and enter
breast milk (see Contraindications and cautions).
Contraindications and cautions
These drugs are contraindicated in the presence of
allergy to any of these drugs
to prevent hypersensitivity
reactions
; with heart block or sick sinus syndrome,
which could be exacerbated by the conduction-slowing
effects of these drugs
; and with renal or hepatic dysfunc-
tion,
which could alter the metabolism and excretion of
these drugs.
Although there are no well-defined studies
about effects during pregnancy, fetal toxicity has been
reported in animal studies; therefore, these drugs should
not be used during pregnancy unless the benefit to the
mother clearly outweighs any potential risk to the fetus
because of the potential for adverse effects on the fetus
or neonate. Because of the potential for serious adverse
effects on the baby
,
another method of feeding the
infant should be used if these drugs are required during
breastfeeding.
Adverse effects
The adverse effects associated with these drugs relate
to their effects on cardiac output and on smooth
muscle. CNS effects include dizziness, light-headedness,
headache and fatigue. GI problems include nausea and
hepatic injury related to direct toxic effects on hepatic
cells. Cardiovascular effects include hypotension, brady-
cardia, peripheral oedema and heart block. Skin flushing
and rash may also occur.
Clinically important drug–drug interactions
Drug–drug interactions vary with each of the calcium
channel blockers used to treat hypertension. A poten-
tially serious effect to note is an increase in serum levels
and toxicity of cyclosporin if taken with diltiazem.
Clinically important drug–food interactions
The calcium channel blockers are a class of drugs that
interact with grapefruit juice. When grapefruit juice
is present in the body, the concentrations of calcium
channel blockers increase, sometimes to toxic levels.
Advise people to avoid the use of grapefruit juice if
they are taking a calcium channel blocker. If a person
Evaluation
■
■
Monitor response to the drug (maintenance of
blood pressure within normal limits).
■
■
Monitor for adverse effects (hypotension, GI
distress, skin reactions, cough, headache, dizziness).
■
■
Evaluate the effectiveness of the teaching plan
(person can name drug, dosage, adverse effects
to watch for, measures to avoid them and the
importance of continued follow-up).
■
■
Monitor the effectiveness of comfort measures and
compliance with the regimen.
Safe medication administration
Several drugs that are used to treat hypertension cannot be
cut, crushed or chewed. This is very important information
to share with people on antihypertensive drugs. Sometimes
individuals cut tablets in half to facilitate swallowing or to get
twice the number of days for any given prescription. Most
drugs formulated for extended release or sustained release
are delivered in a matrix system that slowly dispenses the
drug into the system. If the coating of the matrix is cut, all
of the drug is released at once, leading to the release of too
much drug at one time and, consequently, toxic levels of the
drug when first taking it. Then the person receives no drug as
the day goes on. Some antihypertensives to be aware of are
diltiazem, nifedipine and verapamil.
