Atropine, adenosine and manifestations of digoxin toxicity
THE CONTEXT
These anti-arrhythmic drugs differ in their effects on cardiac conduction, clinical uses and precautions.
Atropine
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Atropine is a muscarinic receptor antagonist that increases sinus rate and sinoatrial and atrioventricular conduction.
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The source describes atropine as the treatment of choice for severe bradycardia or hypotension caused by vagal overactivity. It also lists initial management of symptomatic bradyarrhythmias complicating inferior myocardial infarction and cardiac arrest due to asystole.
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The stated dose is 0.6 mg intravenously, repeated if necessary to a maximum of 3 mg. Repeat dosing may be necessary because atropine disappears rapidly from the circulation after parenteral administration.
Adenosine
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Adenosine binds to A1 receptors in conducting tissue and produces transient atrioventricular block lasting a few seconds.
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It terminates supraventricular tachycardias when the atrioventricular node forms part of the re-entry circuit. It can also help establish the diagnosis of difficult arrhythmias, including atrial flutter with 2:1 atrioventricular block and broad-complex tachycardia.
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The listed responses to intravenous adenosine are termination of supraventricular tachycardia, transient atrioventricular block in atrial fibrillation, atrial flutter and atrial tachycardia, and no effect in ventricular tachycardia.
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The initial intravenous bolus is 3 mg over 2 seconds; escalating doses may be required if there is no response.
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Patients should be warned about short-lived, sometimes distressing flushing, breathlessness and chest pain. Adenosine can cause bronchospasm and should be avoided in patients with asthma.
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Dipyridamole greatly potentiates adenosine's effects, whereas theophylline and other xanthines inhibit them.
Digoxin toxicity
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Extracardiac manifestations include anorexia, nausea, vomiting, diarrhoea and altered colour vision termed xanthopsia.
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Cardiac manifestations include bradycardia, multiple ventricular ectopics, ventricular bigeminy with alternate ventricular ectopics, atrial tachycardia with variable block, ventricular tachycardia and ventricular fibrillation.