A 15 years old male presents to the cardiology ward due to palpitations and tachycardia. Symptoms appear once or twice a month and are present for almost half a year now. Palpitations and tachycardia mostly appear during rest. Tachycardia starts suddenly, almost as if someone "switched it on". It then persists for some minutes and spontaneously terminates abruptly (like someone "switched it off" again). During tachycardia the patient feels dizzy. However, no syncope in the history. No prior medications or diseases. No family history of sudden cardiac death or arrhythmia.
The ECG shows the following:
You can clearly see a very short PR interval (PQ in german literature) with the beginning of the R wave upstroke just at the end of the P wave (see leads II/III/aVF). Also, the initial portion of the R wave upstroke is slurred, forming what is called a delta wave (see inferior leads and V1-V5). This pre excitation of the ventricles (short PR & delta wave) represents the ECG pattern of an accessory pathway connecting the atria and the ventricles, allowing for integrate conduction.
The sinus impulse travels through the atria, reaching the AV node. Within the AV node the propagation of the electrical wavefront is slowed due to the decremental properties of the AV node. This is physiological, since the AV node functions as a gatekeeper of the ventricles, preventing extremely fast ventricular rates.
At the same time the sinus impulse reaches the accessory pathway (AP). The AP usually does not slow conduction and thus propagates the electrical wavefront to the ventricles sooner than via the AV node. This pre excitation results in shorter PR interval and, due to the early depolarization of parts of the ventricles ipsilateral to the AP, a slurred R wave upstroke (i.e. the delta wave).
This particular anatomy sets the stage for reentry tachycardia involving the atria and ventricles via the accessory pathway and AV node - his-purkinje system (atrioventricular reentry tachycardia - AVRT).
Typically, AVRT is initiated by atrial or ventricular premature beats. Typical history for reenrtry tachycardias involves sudden appearance and termination (ON/OFF phenomenon).
Vagal maneuvers (Valsalva, coolbag on chest...) often terminate tachycardia due to prolongation of the AV nodes refractory period. These maneuvers should be taught to patients as part of self-treatment.