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9780521704632c14 CUFX213A/Peck 9780521618168 December 28, 2007 12:32
14 Anti-arrhythmicscardiac toxicity and should also be corrected. Where bradycardia is symptomatic
atropine or pacing is preferred to infusions of catecholamines, which may precipi-
tate further arrhythmias. Ventricular arrhythmias may be treated with lidocaine or
phenytoin.
Ifplasma levels rise above 20μg.l−^1 ,there are life-threatening arrhythmias or
hyperkalaemia becomes uncontrolled, digoxin-specific Fab are indicated. These are
IgGfragments. Digoxin is bound more avidly by Fab than by its receptor so that it
is effectively removed from its site of action. The inactive digoxin–Fab complex is
removed from the circulation by the kidneys. There is a danger of hypersensitivity or
anaphylaxis on re-exposure to digoxin-specific Fab.Adenosine
Adenosine is a naturally occurring purine nucleoside consisting of adenine (the
purine base) andd-ribose (the pentose sugar), which is present in all cells.Presentation
Adenosine is presented as a colourless solution in vials containing 3 mg.ml−^1 .It
should be stored at room temperature.Uses
Adenosine is used to differentiate between SVT, where the rate is at least transiently
slowed and VT, where the rate does not slow. Where SVT is due to re-entry circuits
that involve the AV node, adenosine may covert the rhythm to sinus. Atrial fibrillation
and flutter are not converted by adenosine to sinus rhythm as they are not generated
byre-entry circuits involving the AV node, although its use in this setting will slow
the ventricular response and aid ECG diagnosis.Mechanism of action
Adenosine has specific actions on the SA and AV node mediated by adenosine A 1
receptors that are not found elsewhere within the heart. These adenosine-sensitive
K+channels are opened, causing membrane hyperpolarization, and Gi-proteins
cause a reduction in cAMP. This results in a dramatic negative chronotropic effect
within the AV node.Side effects
Because of its short half-life its side effects are also short lived but for the patient
may be very distressing.
Cardiac – it may induce atrial fibrillation or flutter as it decreases the atrial refractory
period. It is contraindicated in those with second- or third-degree AV-block or with
sick sinus syndrome.