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