ECG Challenge: A 25-year-old man who is active playing tennis, soccer, and basketball is seen in the emergency department with a several-hour history of palpitations. He states that he has had these symptoms in the past and they have resolved spontaneously. He has seen a cardiologist in the past, but no diagnosis has been made. As a result of the prolonged nature of the palpitations, he sought medical care. ECG A is obtained shortly after he arrives at the emergency department. After the patient is seen by the physician, there is a planned intervention, but before the intervention is performed, another ECG is obtained (ECG B).
ECG A shows a regular rhythm at a rate of 120 bpm. The QRS complex has a normal duration (0.08 second) and a normal morphology. The axis is normal between 0° and +90° (positive QRS complex in leads I and aVF). The QT/QTc intervals are normal (320/450 milliseconds). Distinct negative waveforms are seen before each QRS complex (+). Because the RP interval (└┘) is longer than the PR interval (┌┐), this is called a long-RP tachycardia. A number of causes are associated with this form of arrhythmia:
Sinus tachycardia. This is not the cause in this case because the P wave is negative in most of the leads, especially lead aVF, which is the only lead perpendicular to the atria. Hence, a negative P wave indicates atrial activation going from an inferior to a superior direction.
Ectopic junctional tachycardia with a retrograde P wave and a long ventriculoatrial conduction time (which is not typically the case).
Atrial flutter with 2:1 atrioventricular block, which is not the cause in this case because a second P wave is not seen.
Atrioventricular reentrant tachycardia, which is usually associated with a short-RP tachycardia.
Atypical atrioventricular nodal reentrant tachycardia, also known as fast-slow (ie, fast pathway antegradely to the ventricles and slow pathway retrogradely to the atria).
The initial part of ECG B shows the same narrow QRS complex tachycardia seen in ECG A. As before, a negative P wave (+) is seen before each QRS complex with a long RP (└┘) and short PR (┌┐)interval; that is, it is a long-RP tachycardia. However, the narrow complex tachycardia abruptly terminates, and there are 2 QRS complexes that have the same morphology as the tachycardia. There is a P wave (*) before these 2 QRS complexes, and they have a constant PR interval (0.14 second). These 2 sequential sinus complexes have a rate of 62 bpm. Note that the tachycardia terminates with a nonconducted P wave (^). Any arrhythmia originating in the atria (ie, atrial tachycardia or atrial flutter) terminates with the absence of atrial activity. Arrhythmias that require the atrioventricular node as part of the mechanism for the arrhythmia, that is, atrioventricular nodal reentrant tachycardia, atrioventricular reentrant tachycardia, or ectopic junctional tachycardia, terminate with a nonconducted P wave. In this situation, retrograde atrial activity has occurred before the cessation of the junctional focus and termination of the arrhythmia. As indicated, the very long RP interval is not usually seen with an ectopic junctional tachycardia. An atrioventricular reentrant tachycardia is the result of a pre-excitation pattern in which there is a circuit involving the atrioventricular node His-Purkinje system and an accessory pathway. Because the sinus QRS complexes have a normal PR interval and no delta wave, apre-excitation syndrome, due to either Wolff-Parkinson-White or Lown-Ganong-Levine, is not present. An atrioventricular reentrant tachycardia could be the result of a concealed bypass tract, which conducts only in a retrograde direction, However, such a bypass tract generally has rapid conduction, and hence the RP interval would likely be short. Thus, the most likely reason for this arrhythmia is an atypical atrioventricular nodal reentrant tachycardia (also known as fast-slow).
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