Selected Dysrhythmias in AMI

ATRIAL FIBRILLATION Atrial fibrillation associated with AMI most typically occurs in the first 24 hours and is usually transient. It more often occurs in patients with excess catecholamine release, hypokalemia, hypomagnesemia, hypoxia, chronic lung disease, and sinus node or left circumflex ischemia. Patients with supraventricular tachycardia, atrial fibrillation, and atrial flutter who have hemodynamic compromise should receive cardioversion with 100 J, then 200 to 300 J, then 360 J if lower energies fail. Patients without hemodynamic compromise, clinical left ventricular dysfunction, reactive airway disease, or heart block can be treated with beta-adrenergic antagonists, such as atenolol (2.5-5 mg over 2 min to a total of 10 mg) or metopropol (2.5-5 mg every 2-5 min to a total of 15 mg). Patients with contraindications to beta-adrenergic antagonists can be treated with digitalis (0.3 mg-0.5 mg initial bolus with a repeat in 4 h) or a calcium antagonist such as verapamil or dilitiazem. Digitalis will take longer to work but is still preferred by the ACC/AHA guidelines because of the potential negative inotropic effects of calcium antagonists, their lack of efficacy in reducing mortality during AMI, and potentially harmful effects in some subsets of AMI patients. 2 The etiology of the tachydysrhythmia should also be addressed. Heparin should be given since atrial fibrillation during AMI is associated with a 3-fold risk in systemic embolization.

BRADYDYSRHYTHMIAS The increased mortality in patients with heart block during AMI is related to more extensive myocardial damage and not the heart block itself. As a result, pacing has not been shown to reduce mortality in patients with AV block or intraventricular conduction delay. Nonetheless, pacing is still recommended to protect against sudden hypotension, acute ischemia, and precipitation of ventricular dysrhythmias in certain patients.

The risk of developing third degree heart block during AMI is increased in the setting of first degree AV block, both types of second degree AV block (more likely with Mobitz II), left anterior hemiblock, left posterior hemiblock, right bundle branch block, and left bundle branch block. The prognosis is related to the site of infarction, site of the block (intranodal vs. infranodal), the type of escape rhythm, and the hemodynamic response to the rhythm.

Atropine is recommended for sinus bradycardia when it results in hypotension, ischemia, or ventricular escape rhythms and for treatment of symptomatic AV block occurring at the AV nodal level (second degree type 1 or third degree AV block). Atropine will reverse decreases in heart rate, systemic vascular resistance, and blood pressure that are mediated by parasympathetic activity. It should be used cautiously in the setting of AMI because the parasympathetic tone is protective against infarct extension and ventricular fibrillation.

Temporary transcutaneous pacers are recommended for patients at moderate to high risk of progression to AV block, e.g: unresponsive symptomatic bradycardia, Mobitz-II, or higher AV blocks, new LBBB and bifascicular blocks, RBBB, or LBBB with 1 degree block. It can also be considered for some cases with stable bradycardia and new or indeterminate age RBBB. Transvenous pacing should be considered in patients who require permanent pacing and in patients with a very high likelihood (>30%) of requiring permanent pacing, e.g. asystole, unresponsive symptomatic bradycardia, Mobitz-II, 3 degree block, alternating BBB, or RBBB or LBBB with 1 degree block. It can also be considered in some cases of RBBB with left anterior or posterior hemiblocks, new or indeterminate age LBBB, for atrial or ventricular overdrive pacing in unresponsive VT, and unresponsive recurrent sinus pauses (>3 s). Patients with right ventricular infarction who are very dependent on atrial systole may require atrioventricular sequential pacing to maintain cardiac output.

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