Antihistamines

THESE WELL-KNOWN ANTIALLERGY MEDICATIONS
may occasionally cause an increase in the pulse rate because
they have an atropine-like effect. In some individuals, the
heart rate may become rapid. These sensations, called
palpitations and caused by arrhythmia, subside over time
and usually cause no harm. Patients with angina, however,
should be cautious with the use of antihistamines, because
an increase in heart rate may trigger an attack of angina.
Blood pressure is usually not increased by antihistamine
use, but many remedies containing antihistamines contain
the decongestant phenylpropanolamine, which elevates
blood pressure. This should be avoided by patients with
hypertension and angina.

HISTAMINE ANTAGONISTS
These newer agents differ from the general group of
antihistamines in that they are more selective blockers of
specific histamine H1 receptors. They do not produce the
bothersome side effects usually noted with antihistamines,
such as drowsiness and dry mouth. Unlike antihistamines,
they are safe in patients with glaucoma or enlargement of
the prostate. Older histamine H1 antagonists include
astemizole and terfenadine. These agents are generally safe,
but abnormal heart rhythms, palpitations, transient loss
of consciousness, and cardiac arrest have been reported.
The electrocardiographic findings include an increase in
the QT interval. Agents that also increase the QT interval
should not be taken concurrently with these H1
antagonists.

These agents are broken down in the liver. Thus,
patients with liver disease or those using drugs that
interfere with liver enzymes, such as the antibiotic
erythromycin, may be predisposed to arrhythmias. Patients
taking agents with a similar chemical structure —
fluconazole, itraconazole, and metronidazole (Flagyl) —
may have a greater predisposition to developing bothersome,
and even serious, abnormal heart rhythms. Patients
with heart disease, those on diuretics or agents that reduce
the levels of potassium in the blood, should not take
astemizole or terfenadine. The antibiotics erythromycin,
azithromycin, and clarithromycin or similar types of
antibiotics should not be taken concurrently.
Newer histamine antagonists such as cetirizine and
loratadine have not been noted to cause serious arrhythmias,
but surveillance and caution are needed in patients
with heart disease or in those taking diuretics and other
substances that can lower blood potassium.

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