Medication Summary
In general, patients with second-degree atrioventricular (AV) block with symptomatic bradycardia may be medicated with intravenous isoproterenol or atropine acutely. However, these agents must be administered in a hospital setting with cardiac monitoring available. Patients with Mobitz I (Wenckebach) AV block secondary to increased vagal tone may respond to theophylline, glycopyrrolate, or scopolamine. Pacemaker therapy is required when medications fail to control symptoms.
Beta-adrenergic agonists
Class Summary
Isoproterenol has beta1-adrenergic and beta2-adrenergic receptor activity. It binds beta receptors of the heart, smooth muscle of bronchi, skeletal muscle, vasculature, and alimentary tract. Isoproterenol has positive inotropic and chronotropic actions.
Isoproterenol (Isuprel)
Isoproterenol is usually given as a continuous intravenous infusion for rate support. It is usually administered as a temporizing measure, initiated during the organization of temporary or permanent pacing system placement for symptomatic patients with bradycardia from heart block or sinus node disease.
Anticholinergics
Class Summary
The goal of anticholinergic therapy is to improve AV node conduction by reducing vagal tone via the muscarinic receptor blockade. This is effective only if the site of a block is within the AV node. Anticholinergic therapy is ineffective for patients with infranodal block.
Atropine
Atropine is administered to increase heart rate through vagolytic effects, causing an increase in cardiac output.
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A common pattern of second-degree atrioventricular (AV) block consists of gradual prolongation of the PR interval leading up to a nonconducted P wave; this pattern is known as Wenckebach AV block, or Mobitz I AV block. This rhythm strip is an example of classic Mobitz I, or Wenckebach, AV block, in which the PR interval prolongs by sequentially smaller increments, with consequent shortening of the RR intervals until the blocked beat occurs. However, classic Wenckebach block is present in only a minority of cases. Wenckebach block is most easily diagnosed by comparing the PR interval following the blocked beat with the PR interval preceding the blocked beat; if the PR interval shortens following the blocked beat, the block is most likely of the Wenckebach type.
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If there is no progressive prolongation of the PR interval and the PR interval fails to shorten following a blocked beat, non-Wenckebach AV block (or Mobitz II AV block) is said to be present. This block is usually located more distally in the His bundle or the His bundle branches, or both, and the escape rates are usually slower and less stable.
