ECG Quiz 4- Answer

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This is a narrow QRS tachycardia which is successfully terminated with carotid massage. This suggests that the sustenance of the tachycardia is dependant on the atrio-ventricular node. Supraventricular tachycardia’s dependant on AV node are atrio-ventricular nodal reentrant tachycardia (AVNRT) and accessory pathway mediated orthodromic AV reentrant tachycardia (AVRT). In AVNRT the reentrant circuit is around the AV node and therefore the RP interval is shorter than in patients with AVRT, as the retrograde limb can be anywhere along the atrio-ventricular annulus where the accessory pathway is and thus longer RP (usually ³ 90 ms).

In AVNRT, retrograde P wave may be buried in the QRS and may not be identifiable or is present in the terminal portion of the QRS complex, seen as pseudo S in inferior leads or pseudo R’ in lead V1. The atrial activation spreads from the AVN region towards both the atria and therefore the ST deflection leads avR & avL is concordant.

In AVRT, retrograde P wave is almost always identifiable within the ST-T wave complex. This often results in ST segment depression which is not due to ischemia but because of the retrograde P wave pulling down the ST segment. Also accessory pathways are often away from the septum which would result in discordant ST segment change in avR & avL.

When the tachycardia terminates there is a pause and often during a pause ventricular premature beats appear. These are always benign and are present only immediately after the tachycardia when there is a pause and no sooner the sinus rate picks up the VPB’s disappear.