ECG Challenge: A 30-year-old woman is seen in the emergency department with concerns about palpitation that developed while she was swimming. The palpitations had been present for 1 to 2 hours and were now associated with some lightheadedness. On presentation, her physical examination was normal except for a rapid heart rate. An ECG (A) was obtained. She was treated with intravenous metoprolol and 10 minutes later her heart rate slowed and an ECG was repeated (B).
ECG A shows a regular rhythm at a rate of 300 bpm. The QRS complex duration is normal (0.08 s). Therefore, this is a narrow complex supraventricular rhythm and at a rate of 300 bpm. The only cause for a supraventricular rhythm at this rate is atrial flutter with 1:1 atrioventricular conduction. The QRS complex morphology is normal, and there is a normal axis between 0° and +90° (positive QRS complex in leads I and aVF). There is increased QRS voltage with an S wave in lead V2 of 25 mm ([) and an R wave in lead V5 of 23 mm (]). Although the SV2+RV5=48 mm, which meets one of the criteria for left ventricular hypertrophy (ie, RV2+SV5≥35 mm), the voltage may be normal considering the young age. There are downsloping ST-segment depressions seen in leads II, III, and aVF (▲) and upsloping ST-segment depression in leads V4 through V6 (↑), as well. Although at a ventricular rate of 300 bpm this may be subendocardial ischemia, it is likely that the ST-segment changes are actually the result of superimposed atrial flutter waves. Also noted are beat-to-beat changes in QRS complex amplitude, particularly in leads V1 and V2 (+, v), which is termed QRS or electric alternans. There are also beat-to-beat changes in T-wave amplitude (*,↓), which is T-wave alternans. QRS and T-wave alternans may be seen with any rapid supraventricular tachyarrhythmia as a result of beat-to-beat changes in calcium fluxes into the myocardial cell. QRS alternans (also as a result of changes in calcium fluxes) may also be seen in a dilated cardiomyopathy, decompensated heart failure, or an acute myocardial infarction, and a large pericardial effusion or tamponade (attributable to the pendulum motion of the heart in the fluid-filled pericardial sac), as well.
After receiving metoprolol intravenously, there is an increase in atrioventricular nodal blockade and a slowing of the heart rate, which is regular at a rate of 150 bpm. The QRS complex duration, morphology, and axis are the same as seen in ECG 28A. The increased QRS complex amplitude in leads V2 ([) and V5 (]) is present. The QT/QTc intervals are normal (280/440 ms). There are regular atrial waveforms seen (+) at a rate of 300 bpm. The atrial waveforms are negative in leads II, III, and aVF with a continuously undulating morphology without an isoelectric baseline between each atrial waveform, consistent with atrial flutter, and there is 2:1 atrioventricular block. It can be seen that the apparent ST-segment depressions in leads II, III, and aVF are indeed the atrial flutter wave. There is no ST-segment elevation in leads V4 through V6, confirming the fact that the ST-segment depressions seen in ECG A were the atrial flutter waves.
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- © 2015 American Heart Association, Inc.