Orthodromic tachycardia due to an accessory pathway (AVRT)

ECG 4A: Regular tachycardia, 180 bpm, narrow QRS; atrial activity difficult to visualize but seemingly present in the T wave with a 1:1 ratio;

ECG 4B

ECG 4B: Sino-carotid massage; termination of tachycardia; recovery of sinus rhythm with obvious pre-excitation (short PR + delta wave); repolarization disorders in conjunction with pre-excitation;

ECG 4C

ECG 4C: Electrocardiogram performed after ablation of a left posterior accessory pathway; disappearance of pre-excitation, normal PR and evidence of a physiological septal q wave in V5-V6; early repolarization;

Comments: This patient presented iterative episodes of reciprocating tachycardia due to a left posterior bundle of Kent (accessory pathway, Wolff-Parkinson-White syndrome). The recorded tracing corresponds to an orthodromic tachycardia: the tachycardia circuit is broad, with the impulse descending from the atrium to the ventricle through the bundle of His and ascending through the bundle of Kent.

This type of tachycardia, much more frequent than antidromic tachycardia where the circuit is reversed, displays certain electrocardiographic characteristics:

1. The tachycardia episodes are of sudden onset and termination with a 1:1 ratio between atria and ventricles; there are necessarily as many atrial as ventricular depolarisationsr; indeed, unlike intranodal reentrant tachycardia (AVNRT), the atria and ventricles are integral components of the circuit, there can therefore be no deficient atrial or ventricular activation;
2. The tachycardia generally varies between 150 and 250 bpm depending on the patient although remains relatively fixed from one episode to the other in the same patient;
3. In the vast majority of cases, these are narrow QRS tachycardias (ventricular activation linked exclusively to the descent through the bundle of His), even though a broadening of the QRS complexes is possible in the context of a pre-existing bundle branch block or onset of a tachy-dependent ventricular aberration; the presence of a QRS-amplitude alternans (alternation between 2 narrow QRS of different amplitude) is frequent but is a non-specific sign;
4. The P’ wave in tachycardia is located in the ST segment somewhat remote to the QRS and is therefore often difficult to identify (as on this tracing) because of the superposition with ventricular repolarization; the morphology of the P’ wave in tachycardia varies from one patient to another since depending on the atrial insertion zone of the accessory pathway which modifies the site of the primary atrial Tepolarization as well as the atrial activation vector;  the morphology of the P’ wave can sometimes help determine the localization of the bundle of Kent; indeed, a negative P’ wave in leads I and aVL point to a left lateral accessory pathway (atrial depolarization starting at the left lateral wall with atrial activation from left to right); retrograde conduction by an accessory pathway of septal topography results in an atrial depolarization sequence very similar to that observed during retrograde conduction originating in the atrioventricular node (intranodal reentry, antidromic tachycardia);
5.  The location of the P’ waves relative to the QRS complexes is variable and is dependent on the respective conduction velocities in the nodo-Hisian pathway (anterograde conduction), the ventricular myocardium, the accessory pathway (retrograde conduction) and the atrial myocardium; the P’R interval (traversing of the nodal filter) remains longer than the RP’ interval;
6. The initiation of an reciprocating tachycardia episode due to an accessory pathway is only possible if one of the pathways becomes blocked, the other pathway remaining permeable; this condition can sometimes be met by simple variations of the sinus rhythm; in the vast majority of cases, the episode begins as a result of an atrial extrasystole with a very precise timing in terms of refractory periods of the normal and accessory conduction pathways; a premature atrial extrasystole descends exclusively via the bundle of His (blocked by the bundle of Kent because in refractory period) with a non-pre-excited pattern (no delta wave); following ventricular activation, there is an atrial echo via the accessory bundle (the accessory pathway is no longer in refractory period and can thus conduct in retrograde manner) which is the initiation of a circular motion, the atrial activity being once again conducted to the ventricles through the normal pathways, the cycle thus repeating itself; a ventricular extrasystole can also act as a trigger, albeit more rarely (blocked in the nodo-Hisian pathway, invades the atrium via the accessory bundle and descends to the ventricles through the normal pathway);
7. The termination of the episode may be spontaneous (most often on a last P’ wave, although sometimes on a R wave, a ventricular extrasystole or a passage in atrial fibrillation) or induced by vagal stimulation, the interruption being generally preceded by a gradual prolongation of the last tachycardia RR cycles corresponding to a prolongation of the AH interval;

Take-home message: In orthodromic tachycardia, the impulse descends from the atrium to the ventricle through the bundle of His and ascends through the bundle of Kent; the tachycardia is regular, with sudden onset and termination, with a 1:1 A/V ratio, the P’R interval being longer than the RP’ interval;