Sinus Bradycardia

Almost 1/3 of ACS patients with acute MI will have sinus bradycardia. Those with inferior infarcts due to right coronary artery (RCA) occlusion can have a loss of blood flow to the sinus and AV node, resulting in slowed conduction. Bradycardia can also occur following RCA reperfusion. Atropine may be ineffective in these cases due to significant ischemia of the nodal tissue and should only be administered if the patient is symptomatic from the bradycardia. 

2nd- or 3rd-degree AV Block

Almost 1/5 of ACS patients with acute MI will have 2nd- or 3rd-degree heart block. About 40% will have the heart block upon presentation, while the remaining will develop it within 24 hours of the MI. The heart block is generally a result of ischemia to the tissue surrounding the conduction pathway. Other contributing factors may be hypoxia, electrolyte and acid-base disturbances, and the adverse effects of medications. 

The AV block alone is rarely life-threatening if managed quickly. However, AV block is more common in extensive or significant MI, and consequently, it is a marker of worse outcomes due to cardiac ventricular dysfunction. Thus, the prognosis is related to the location (anterior vs. inferior) and size of the infarct. Management of heart block depends on its site as well as any escape rhythms and ensuing perfusion abnormalities.

Using Atropine 

While atropine is indicated for symptomatic bradycardia, the clinician must keep the following cautions in mind:

• Atropine may negatively affect patients with ACS as the resulting increase in heart rate may lead to heightened oxygen demands and worsening ischemia or infarct. 
• Outside of the hospital and in emergency departments, the use of atropine in unstable bradycardia has similar outcomes in patients regardless of the diagnosis of acute MI. 
• Atropine is ineffective in patients with cardiac transplants as the vagus nerve is no longer innervating the heart. 
• Atropine is not effective for infranodal heart block (type II 2nd-degree block or 3rd-degree block with a wide QRS likely arising from acute MI). In these cases, atropine may worsen the AV blockage. 
• Amiodarone or lidocaine should not be given to patients with 2nd- or 3rd-degree block as this can worsen the block and terminate the ventricular escape rhythm, leading to asystole.