Third degree AV block medical therapy

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Cafer Zorkun, M.D., Ph.D. [2]; Raviteja Guddeti, M.B.B.S. [3]

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Medical Therapy

A patient with inferior wall myocardial infarction and distal high grade complete heart block with a heart rate of more than 60 beats per minute is at immediate danger of asystole and requires an immediate placement of permanent pacemaker compared to a patient with inferior myocardial infarction and complete block at the level of AV node with a heart rate of 35-40 beats per minute.

Correction of reversible causes of the block such as ischemia, medications (beta-blockers, calcium channel blockers, antiarrhythmics, and digoxin), and vagotonic conditions should be considered. Treatment may also include medicines to control blood pressure and atrial fibrillation, as well as lifestyle and dietary changes to reduce risk factors associated with heart attack and stroke. A new third degree AV block is an emergency. Treatment in emergency situations are atropine and an external pacer.

Acute Pharmacotherapy

Atropine

Atropine can partially or completely restore conduction through the AV node when the cause for complete heart block is acute myocardial infarction (ischemia of the AV node). Atropine, in this setting, reverses the reduced conduction across the AV node (which is due to increased vagal tone). But caution is advised in such cases as the resulting vagolysis leads to unopposed sympathetic activity. This increased sympathetic activity causes ventricular irritability and can progress to fatal ventricular arrhythmias.

Use of atropine in situations where the block is at the level of His bundle can lead to increased atrial rate and a greater degree of heart block with reduced ventricular rate.

Atropine is unsuccessful in wide complex bradyarrhythmias (block below the AV node). It is also not helpful in a denervated heart, like in patients who have undergone a cardiac transplant procedure.

Isoproterenol

Isoproterenol may help accelerate a ventricular escape rhythm and restore conduction with distal level of block but the probability for efficacy is low. Active ischemic heart disease is an absolute contraindication for the use of isoproterenol.

DigiFab

DigiFab is an immunoglobulin fragment used in the treatment of digitalis overdose. It has specific high affinity for digoxin and digitoxin molecules and removes them from the tissues. The dose of DigiFab depends on the concentration of digoxin in the body.

Number of vials of DigiFab = (Digoxin concentration)×(Patient's weight)÷100

Transcutaneous Pacing

Transcutaneous pacing is the treatment of choice in symptomatic patients. Any patient with complete heart block associated with frequent pauses, inadequate ventricular escape rhythm and block below the AV node should be paced temporarily using a transcutaneous pacemaker to attain stability. Disadvantages of using a transcutaneous pacemaker are:

• It is not a reliable method and
• It is extremely uncomfortable for the patient.

If perfect capture is not obtained with a transcutaneous pacer, attempt should be made to pace the patient temporarily using transvenous pacing method. This method is employed in the emergency room for all patients with hemodynamic instability and in whom perfect capture cannot be obtained with a transcutaneous pacer.

Contraindicated medications

Third degree AV block(except in patients with a functioning artificial pacemaker) is considered an absolute contraindication to the use of the following medications:

• Adenosine
• Atenolol
• Betaxolol
• Bisoprolol
• Carteolol
• Diltiazem
• Disopyramide
• Dronedarone
• Fingolimod
• Metoprolol
• Mexiletine
• Nadolol
• Nebivolol
• Penbutolol
• Pindolol
• Procainamide
• Propranolol
• Sotalol
• Timolol
• Verapamil
• Labetalol