EMERGENCY MEDICAL SERVICES-ECG April Issue

This ECG demonstrates significant ST segment elevation in the inferior leads consistent with inferior wall myocardial infarction. This pattern accounts for nearly half of all myocardial infarctions (MIs) and results from right coronary artery (RCA) occlusion. Additionally, the ischemia in this case may extend into the posterior wall based on the ST depression in V1 and V2. This is common in RCA occlusion.  Posterior MIs were discussed in a previous Emergipress.

Additionally, this patient has a third degree AV block (also known as a complete heart block). In many cases, the RCA perfuses the AV node which conducts the electrical impulses from the heart’s pacemaker cells in the atria down into the ventricles. When the AV node is affected by ischemic damage, it is no longer able to conduct electrical impulses and acts as a “roadblock” within the heart’s conduction system. As a result, the heart can no longer contract in a coordinated fashion and instead, the atria and ventricles contract independently. In this case, ventricular contractions provide systemic blood circulation and the atrial contractions do not affect system perfusion.

On this ECG, third degree block is best observed on the V1 rhythm strip. Between the 2^nd and 3^rd QRS complexes. Note the space between the p-waves and marching this distance out across V1 reveals subtle, but equally spaced p-waves, some of which are buried within QRS complexes. Each p-wave is not followed by a QRS complex indicating that electrical impulses are not being transmitted from the atria or supraventricular region down into the ventricles.

Besides myocardial ischemia, other causes of third-degree AV block include progression of Mobitz type 1 or Mobitz type 2 often due to prior ischemic cardiac disease, AV nodal blocking drugs (i.e. calcium channel blocker, beta blockers), inflammatory conditions (i.e. myocarditis), or metabolic abnormalities (i.e. hyperkalemia).

Figure 1: Coronary arteries.

Source: https://commons.wikimedia.org/wiki/File:Blausen_0256_CoronaryArteries_02.png

Figure 2: Cardiac conduction system.

Source:https://commons.wikimedia.org/wiki/File:Conduction_system_en_(CardioNetworks_ECGpedia).png

This patient’s primary provider impression is Chest Pain – STEMI. Despite the lack of reported chest pain, left shoulder pain in this case is a chest pain equivalent. MIs can often present atypically in women. Complete heart block patients are at high risk for significant hemodynamic instability, symptomatic bradycardia, and sudden cardiac death, and require immediate intervention. In accordance with TP 1211, Cardiac Chest pain, Aspirin 325 mg chewable tablets PO should be given. One should hold Nitroglycerin in this case given the systolic blood pressure. Right ventricular dysfunction due to an inferior MI results in a reliance on adequate return of blood to the heart, known as preload, in order to sustain forward blood flow through the lungs and to the left heart and may be contributing to her hypotension. In such cases, a Normal Saline would be acceptable in increments of 250cc for poor perfusion (intermittent dizziness in this patient).  Because the patient is bradycardic, also treat in accordance with TP 1212, Cardiac Dysrhythmia – Bradycardia.  Atropine will not likely be effective as this is a complete heart block with AV dissociation.  Her heart rate is > 40 bpm at this time but may deteriorate.  Have pacer pads in place in preparation for transcutaneous pacing.

This patient presented with an atypical presentation of myocardial ischemia complicated by a third-degree AV heart block. The incidence of an atypical presentation is higher among older women and those with a history of diabetes. This patient requires transmission of the ECG to the STEMI Receiving Center and immediate transport for definitive management (i.e. percutaneous coronary intervention).