Introduction

ECG Challenge: A 45-year-old woman without any known heart disease is seen in an emergency room because of palpitations. She states that she has had palpitations for the past several years, but they are usually brief and self-terminating. She has not sought medical care. However, on this occasion the palpitations continued for several hours, at which point she sought medical care. ECG A was obtained on presentation. ECG B was obtained several minutes later after the palpitations resolved.

ECG A shows a regular rhythm at a rate of 140 beats per minute. The QRS complexes have a normal duration (0.08 s) and a normal morphology. There is a physiological leftward axis between 0° and –30° (positive QRS complex in leads I and II and negative in aVF). The QT/QTc intervals are normal (280/430 ms). There are no obvious P waves seen before any of the QRS complexes. However, there is evidence of atrial activity seen after the QRS complex (+). It is negative in leads II, III, aVF, and V4 through V6. There is a stable RP interval (0.20 ms; ]) and PR interval (0.28 s; [). Therefore, this is a short PR tachycardia. There are several causes for a short RP tachycardia:

Sinus tachycardia with a first-degree AV block (not the cause, because the P waves are negative in leads II and aVF). Ectopic junctional tachycardia with retrograde P wave Atrial tachycardia Atrial flutter with 2:1 AV block (not the cause, because a second flutter wave is not present) Typical atrioventricular nodal re-entrant tachycardia (slow-fast) but an unusual variant termed “slow-slow” (ie, the fast pathway conducts relatively slowly) Atrioventricular re-entrant tachycardia (as occurs with an accessory pathway)

Although the cause is not sinus tachycardia or atrial flutter, it is not possible to establish the cause from this ECG. However, ECG B is useful. There is initially a regular rhythm at a rate of 84 beats per minute. The QRS complex duration, morphology, and axis are identical to those seen in ECG A. There are P waves before each QRS complex (+) with a stable PR interval (0.20 s). The P waves are positive in leads I, II, aVF, and V6. Hence, this is a normal sinus rhythm. The seventh QRS complex (*) is premature, and it is preceded by a P wave (↓) that has a different morphology compared with the sinus P wave. This is a premature atrial complex. The 11th P wave (↑) is also premature, with a P wave morphology that is different from the sinus. It is also a premature atrial complex. It is associated with a longer PR interval (0.40 s). Immediately after this premature atrial complex is a narrow complex tachycardia, with a retrograde P wave (^) and a short PR interval (0.20 s). This arrhythmia is identical to the arrhythmia seen in ECG A. The arrhythmia was initiated by a premature atrial complex with a very long PR interval, and this is the mechanism for initiation of a typical atrioventricular nodal re-entrant tachycardia, which is attributed to dual AV nodal pathways. There is a fast pathway, which conducts rapidly but has a long refractory period and slower rate of recovery and a slow pathway that conducts slowly but has a short refractory period and recovers more quickly. These 2 pathways are linked proximally and distally in the AV node forming a circuit. If the premature atrial complex reaches the AV node early while the fast pathway is still refractory, it will be conducted antegradely to the ventricle via slow pathway, accounting for the very long PR interval. If the impulse reaches the distal part of the circuit when the fast pathway has recovered and is no longer refractory, the impulse will be conducted retrogradely to the atrium via this pathway. If the slow pathway has recovered, the impulse can also re-enter the slow pathway. If this process continues, a re-entrant arrhythmia is established, that is, a typical atrioventricular nodal re-entrant tachycardia (slow-fast). Most often this presents without any atrial activity seen (no RP tachycardia), because there is simultaneous atrial and ventricular activation. However, if the fast pathway conducts slowly, there will be a retrograde P wave seen with a short RP interval. This is termed “slow-slow atrioventricular nodal re-entrant tachycardia.” A slower conducting fast pathway may be seen with older individuals or in those receiving an antiarrhythmic agent.

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