Beginner ECG 8
- AV Nodal Reentry Tachycardia (AVNRT)
This is a narrow-complex QRS rhythm which is quite fast. Looking closely at the last part of the QRS complex in leads V1 and V2, P waves can be seen. This is considered a "short RP tachycardia" and is from AVNRT. Adenosine, carotid massage or vagal maneuvers can terminate this rhythm.
AVNRT occurs when a reentrant circuit is present within the AV node itself. In this situation, there are two separate conduction pathways within the AV node instead of just one (present in about 5% of the general population). This is sometimes termed “dual AV nodal physiology”. One pathway is slower and has a long refractory period while the other is faster and has a short refractory period. Normal conduction occurs through the faster pathway with the longer refractory period.
If a premature atrial contraction (PAC) or premature ventricular contraction (PVC) occurs at the right time, the normal conduction pathway will still be refractory, so the action potential will conduct through the slower AV nodal pathway with the shorter refractory period instead. After this action potential reaches the ventricles or atrium, it will conduct back through the normal AV nodal conduction pathway since it will no longer be refractory and a reentrant circuit will be created. See the ECG strip below which shows a PVC initiating AVNRT: