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Section IIICardiovascular drugs
Non-cardiac – these include chest discomfort, shortness of breath and facial flush-
ing. It should be used with caution in asthmatics as it may precipitate bron-
chospasm.
Drug interactions – its effects may be enhanced by dipyridamole (by blocking its
uptake) and antagonized by the methylxanthines especially aminophylline.Kinetics
Adenosine is given in incremental doses from 3 to 12 mg as an intravenous bolus,
preferably via a central cannula. It is rapidly de-aminated in the plasma and taken
up by red blood cells so that its half-life is less than 10 seconds.Verapamil
Verapamil is a competitive Ca^2 +channel antagonist.Presentation
Itis presented as film-coated and modified-release tablets and as a solution for
intravenous injection containing 2.5 mg.ml−^1.Uses
Verapamil is used to treat SVT, atrial fibrillation or flutter, which it may slow or convert
to a sinus rhythm. It is also used in the prophylaxis of angina and the treatment of
hypertension.Mechanism of action
Verapamil prevents the influx of Ca^2 +through voltage-sensitive slow (L) channels in
the SA and AV node, thereby reducing their automaticity. It has a much less marked
effect on the contractile tissue of the heart, but does reduce Ca^2 +influx during the
plateau phase 2. Antagonism of these Ca^2 +channels results in a reduced rate of
conduction through the AV node and coronary artery dilatation.Side effects
Cardiac – if used to treat SVT complicating Wolff–Parkinson–White (WPW)
syndrome, verapamil may precipitate VT due to increased conduction across the
accessory pathway. In patients with poor left ventricular function it may precip-
itate cardiac failure. When administered concurrently with agents that also slow
AVconduction (digoxin,β-blockers, halothane) it may precipitate serious brady-
cardia and AV block. It may increase the serum levels of digoxin. Grapefruit juice
has been reported to increase serum levels and should be avoided during verapamil
therapy.
Although its effects are relatively specific to cardiac tissue it may also
precipitate hypotension through vascular smooth muscle relaxation.