ECG Challenge: An Unexpectedly Rapid Heart Rate in the Emergency Department

Philip J. Podrid, MD

Disclosures

October 07, 2019

The correct diagnosis is nonsustained atrial tachycardia, normal sinus rhythm (Figure 2).

Figure 2. Image courtesy of Dr Podrid.

Discussion

The initial part of the ECG shows a regular rhythm at a rate of 140 beats/min, with an abrupt slowing to rates ranging from 50 to 68 beats/min.

All of the QRS complexes are identical, with a normal duration (0.08 sec) and a normal axis between 0° and +90° (positive QRS complex in leads I and aVF). The morphology is normal except for a small R' in lead V1 (←), which represents a minor conduction delay to the right ventricle.

The QT/QTc intervals are normal (400/400 msec). The last four QRS complexes have P waves preceding them (+) with a stable PR interval (0.16 sec). The first QRS complex of the ECG has the same P wave (*) and PR interval.

The P wave is positive in leads I, II, and V4-V6. Hence, these are sinus complexes. The last four QRS complexes have a slight irregularity to their rates, representing sinus arrhythmia.

The episode of tachycardia has P waves after each QRS complex (^), and there is a long RP interval (0.24 sec) and short PR interval (0.20 sec). This is termed "long RP tachycardia," and etiologies for this include:

  • Sinus tachycardia;

  • Ectopic junctional tachycardia;

  • Atrial tachycardia;

  • Atrial flutter (with 2:1 AV block);

  • Atrioventricular reentrant tachycardia; and

  • Atypical atrioventricular nodal reentrant tachycardia (fast-slow).

Because the P waves are negative in leads II and aVF, this is not a sinus mechanism. There is no evidence for a second P wave, making atrial flutter unlikely. The atrial tachycardia abruptly terminates, and there is no P wave after the last QRS complex (↑); that is, it terminates with the absence of atrial activity. Atrial arrhythmia terminates when the atrial focus fails to generate an impulse. Therefore, this is nonsustained atrial tachycardia.

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