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CE ARTICLE ECG? Do you think the patient is having a STEMI? There are many clinical situations and circumstances in which a pace- maker implant is considered, but two factors drive the decision of permanent cardiac pacing more than others: the presence of a symptomatic brady- dysrhythmia and the location of the conduction dysfunction. The two most common indications for pacemaker implantation are sinus node dysfunc- tion and atrioventricular (AV) block. 1 Examples of sinus node dysfunction include symptomatic sinus bradycardia, chronic hear t rates below 40 bpm while awake in minimally symptom- atic patients, sinus node dysfunction in patients with unexplained syncope, and sinus node disease. 2 Symptomatic chronotropic incompetence is broadly defined as the inability of the heart to increase its rate with increased activity or demand. It is common in persons with cardiovascular disease. 3 Examples of AV block that may require the use of a permanent cardiac pacemaker include first-degree AV block (in cases where a very long PR interval effectively creates AV disassociation and hemodynamic instability), Mobitz I or Mobitz II second- degree AV block, and third-degree AV block. 2 In atrial pacing, the pacemaker wire is placed in the right atrium. The ECG will show a pacemaker spike followed by a P-wave, the morphology of which depends on the exact placement of the pacemaker wire. P-waves can be normal, small, inverted or biphasic. The P-wave is followed by a PR interval and QRS that is normal for that patient. In ventricular pacing, the pace- maker spike is followed by a wide QRS. As the patient in our scenario had a ventricular pacemaker, its specific ECG characteristics will be discussed below. With dual- chamber atrioventric- ular sequential pacing, pacing wires are plac ed in the atrium and the ventricle. The ECG pattern depends on two possible starting points. In the first case, if the patient's sinus node rate (hear t rate) is greater than the programmed rate of the pacer, an electrical impulse will originate from the AV node and no pacer spike will appear before the P-wave. Depending on the condition of the AV node, there may or may not be a pacer spike before the QRS: If the AV node is functioning properly and allows the atrial impulse through, a native QRS will appear. If the AV node is dysfunctional and does not allow the atrial impulse through, a pacer spike will appear and a wide QRS will be present. In the second case, if the patient's inherent sinus node rate is lower than the programmed rate of the pacer, then the pacer will fire, resulting in a pacer spike before the P-wave. If the AV node allows the impulse through, a normal QRS will appear. If the AV node is dysfunctional and does not allow the impulse through, the ventricular pacer will fire, and a pacer spike will appear before the paced QRS complex. In biventricular pacing, pacing wires are placed in the right atrium, right ventricle and coronar y sinus, which stimulates the left ventricle. The ECG characteristics of biventricular pacing are complex and beyond the scope of this article, and they may present with features characteristic of atrial pacing, ventricular pacing or both. DIFFERENTIATING The pacemaker lead of a ventricular pacemaker is typically placed in the apex of the right ventricle. As such, the ECG pat tern associated with a right ventricular paced rhythm typically has a left bundle branch block (LBBB) appearance, since the right ventricle is depolarized before the left ventricle. The pacemaker lead may sometimes be placed higher up in the right ventricle, resulting in an LBBB pattern with a vari- able axis. Typical ECG morphology for a right ventricular paced rhythm mirrors that of LBBB and includes: • Pacemaker spike preceding the QRS; • QRS duration greater than or equal to 120 ms in adults; • Broad notched or slurred R-wave in leads I, aVL, V5 and V6; • Absent Q-waves in leads I, V5 and V6; • ST and T-waves opposite in direc- tion to the QRS complex (discordance). Atrial activity, as evidenced by the presence of a P-wave, may or may not be present. Factors that influence whether a P-wave is present include the patient's underlying rhythm, the atrial rate and the occurrence of retrograde ventriculoatrial conduction of the pace - maker impulse through the atrioven- tricular node. 4 Worth noting is that newer-genera- tion pacemakers can have very small, almost imperceptible pacemaker spikes compared to older pacemakers, which had relatively large, easily discernible spikes. The amplitude of any pacemaker Left bundle branch block Pericarditis Left ventricular aneurysm Subarachnoid hemorrhage Left ventricular hypertrophy Brugada syndrome Paced rhythm Benign early repolarization Table 1: STEMI Imitators Figure 1: 12-lead ECG for the patient in Case 1. Note the very subtle, low-amplitude pacer spikes preceding each QRS comple EMSWORLD.com | DECEMBER 2014 91