The ECG in the figure below was obtained from an otherwise healthy 13-year-old boy. He was alert and hemodynamically stable at the time this ECG was recorded. How should one interpret this tracing? Is there a clue on the tracing as to the mechanism of the arrhythmia?
The patient is hemodynamically stable. The rhythm is rapid and regular. The R-R interval is just over two large boxes in duration, which means that the ventricular rate is just under 150/min. The QRS complex is narrow (i.e., not more than half a large box in duration). Sinus P waves are absent. The red arrow in lead II points to a T wave, not a P wave. This description fits a regular supraventricular, or narrow-complex tachycardia (SVT), without sinus P waves.
The three principal diagnostic considerations for a regular SVT rhythm without sinus P waves are: 1) a reentry form of SVT, in which the reentry circuit is contained within the AV node; 2) atrial flutter; or 3) sinus tachycardia, in which sinus P waves are hidden within the preceding T wave. Atrial flutter is extremely uncommon in an otherwise healthy child. In addition, other than perhaps in lead I, there is no suggestion of a sawtooth flutter pattern on this tracing. The rhythm in the figure is also not sinus tachycardia because retrograde atrial activity is present, and therein lies the key to identifying the mechanism of this arrhythmia. AV nodal reentry forms of SVT (commonly referred to as AVNRT) are characterized by the presence of a regular, narrow-complex tachycardia at a rate between 130-240/min, without normal sinus P waves. Sometimes, retrograde P waves may be seen in some leads during tachycardia as a notch in the terminal portion of the QRS. This notch reflects conduction of the impulse back to the atria each time there is a QRS, which is why the notch appears as a negative deflection in the inferior leads. In contrast, retrograde atrial activity typically appears as a positive deflection in more superior leads, such as aVR or V1. Because the reentry circuit with AVNRT is contained within the AV node, retrograde conduction to the atria is usually quite fast. When seen, the notch most often will appear very close to the QRS complex. On the other hand, in patients with Wolff-Parkinson-White (WPW) syndrome, reentry forms of SVT typically involve retrograde conduction over an accessory pathway (AP) that is situated further away because the AP lies outside and at some distance from the AV node. The reentry circuit of such patients with WPW is therefore longer, and the RP interval (i.e., distance from the QRS complex to the retrograde P wave) is correspondingly greater.
Retrograde conduction during the tachycardia with a long RP interval clearly appears in the figure as a negative notch in the inferior leads (just before the red arrow in lead II) and as a positive notch at the very end of the T wave in leads aVR and V1. This finding should strongly suggest that this 13-year-old boy is predisposed to tachycardia episodes because of the presence of an “occult” accessory pathway. No delta wave appears on the ECG, because conduction to the atria over this AP is retrograde.
Please visit http://tinyurl.com/ACLS-14-PSVT for more on the mechanism of reentry SVT rhythms (See Section 14.3.7).
