ECG Challenge: A 45-year-old woman with a history of asthma presents to the emergency department for an acute asthmatic episode. She is in marked respiratory distress and is started on oxygen, albuterol nebulizer, and intravenous steroids. Several minutes after therapy, she is noted to have a marked increase in her heart rate. An ECG is obtained (ECG A). After 15 minutes, her heart rate slows, and a second ECG is obtained (ECG B).
ECG A shows a rhythm that is regular with several long RR intervals (↔); therefore, the rhythm is regularly irregular. The rate is 210 bpm. The QRS complex duration is normal (0.08 second) and has a normal morphology. The axis is leftward, approximately −30° (positive QRS complex in lead I, negative in lead aVF, and isoelectric in lead II). Regular atrial waveforms are seen (+) at a rate of 240 bpm. The waveforms are negative in leads II and aVF. During the pauses (in which 2 sequential atrial waveforms are present), it can be seen that the atrial waves are continuously undulating (sawtooth), which is characteristic of atrial flutter, even though the atrial rate is slower than is typically seen with atrial flutter (ie, 260–320 bpm). There is primarily 1:1 conduction. However, the long RR intervals (↔) are the result of a nonconducted flutter wave (^), representing 2:1 atrioventricular block. However, before the nonconducted flutter wave, there is progressive lengthening of the flutter to the QRS complex interval (└┘). This is therefore a pattern of Mobitz type I or Wenckebach.
ECG B shows a regular rhythm at a rate of 105 bpm. The QRS complex duration, morphology, and axis are identical to what is seen in ECG A. The QT/QTc intervals are normal (320/415 milliseconds). Clear atrial waveforms are seen (^) at a rate of 210 bpm, similar to the rate seen in ECG A. The atrial waveforms are negative in leads II, III, and aVF, as well as leads V4 through V6. They are continuously undulating without an isoelectric baseline between them. This is characteristic for typical atrial flutter waves. There is now a continuous pattern of 2:1 atrioventricular block or atrioventricular conduction.
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- © 2015 American Heart Association, Inc.