Persistent (permanent) junctional reciprocating tachycardia (PJRT) is an unusual form of supraventricular tachycardia with a 1:1 AV relationship. PJRT classically occurs in children and is characterized by an incessant and sometimes even permanent narrow complex tachycardia – the tachycardia may be the patient’s predominant rhythm. PJRT does occur in adults but in about half these patients it is paroxysmal rather than incessant/permanent. Patients with persistent/permanent tachycardia tend to demonstrate a slower tachycardia than those in which the rhythm is paroxysmal. The ECG during periods of sinus rhythm is normal. PJRT can be a serious arrhythmia, particularly in children because of tachycardia-induced cardiomyopathy (TIC) - deterioration of ventricular contractile function caused by very prolonged periods in tachycardia. TIC can also occur in adults with the tachycardia and this is sometimes the sole presentation. Fortunately the LV dysfunction generally resolves following successful ablation of the tachycardia. The arrhythmia is sometimes an incidental finding with the patient essentially asymptomatic.
Fig 1 shows an ECG from a 29-year patient with PJRT. The tachycardia was an incidental finding on a routine ECG and the patient was asymptomatic. The tachycardia is described as a long RP tachycardia (see below). The p wave looks different to sinus rhythm – it is typically inverted in the inferior leads (II, III and aVF) - indicating atrial activation from “low to high” (the opposite direction to sinus rhythm). The p-R interval is typically relatively normal.
PJRT is a form of orthodromic AVRT and is caused by a concealed slowly conducting decremental accessory pathway. Anterograde conduction occurs over the AV node and His bundle. Retrograde conduction in orthodromic AVRT is always via an accessory pathway. But where PJRT differs to more commonly seen types of AVRT (such as the tachycardias associated with WPW) is that the conduction properties of this pathway are similar to the conduction properties of the AV node. Usually, the behaviour of accessory pathways is very different – displaying “all or none” conduction but in PJRT the pathway conduction is slow and decremental (conduction slows with increasingly premature stimuli).
Why is PJRT often incessant?
PJRT is a re-entrant rhythm. In a re-entry the cells in the circuit must have sufficient time to recover after each cycle if the rhythm is to continue. Changes in the speed of conduction in one part of the circuit may result in the impulse arriving at another portion of the circuit while it is still refractory – causing termination. In PJRT both limbs of the circuit are decremental - they will conduct more slowly if an impulse is more premature and this slowing protects other parts of the circuits that might otherwise be refractory. In the more usual form of AVRT only one limb – the AV node is decremental and changes in the circuit often result in termination. For re-entry rhythms to start an impulse must travel only one direction round the circuit. This is called unidirectional block. The pathway in PJRT does not conduct anterogradely from atrium to ventricle (it is concealed) and so anterograde unidirectional block is always present. In patients with incessant or permanent PJRT the AV node usually exhibits poor or absent retrograde conduction and so retrograde unidirectional block is also always present. Hence both atrial and ventricular beats can easily induce tachycardia.
Long RP Tachycardias
Tachycardias can be classified according to their appearance on the surface ECG. One such classification is narrow vs broad. A further classification of narrow complex tachycardia is into long RP vs short RP.
In long RP tachycardias the time interval between the R wave and the following p wave is longer than the interval between the p wave and the R wave that follows it.
Sinus tachycardia is (almost always) an example of a short RP tachycardia where the interval between p wave and subsequent R wave is shorter than between that R wave and the next p wave. In sinus tachycardia the R –P interval will change as a function of the sinus rate – it will be longer at slower rates and shorter at faster rates. In the case of sinus tachycardia and all other atrial tachycardias the p wave is not “retrograde” and the RP interval is not dependent on conduction through any kind of retrograde pathway. In a narrow complex tachycardia the p-R interval is determined by the rate of conduction through the AV node.
The reason for classifying tachycardias as long RP or short RP is to narrow down the differential diagnosis:
AVRT utilising a slowly conducting accessory pathway (AP) (e.g. PJRT)
Typical AVNRT (The p wave in typical AVNRT s not always visible however)
AVRT utilising a typical rapidly conducting AP
Atrial tachycardia is less likely but still possible - but it must be conducted to the ventricles with very significant AV conduction delay.
The R-p interval in PJRT is long because the retrograde limb of the circuit is over a slowly conducting pathway
The ECG in fig 2 shows a different long RP tachycardia. Note that the p wave (inverted in II, III and aVF) is further from the R wave to its left than to its right. The appearance and p wave morphology are very similar to the ECG of PJRT. However at EP study this tachycardia was shown to be atypical AVNRT – a tachycardia with a similar appearance to PJRT.
The pathway in PJRT is often located in the right postero-septal region – near the coronary sinus ostium. Consequently the earliest atrial activation during tachycardia is often on the proximal poles of the coronary sinus catheter. However PJRT pathways have been described in many other locations around both the tricuspid and mitral annulus. During the EP study PJRT must be distinguished from atrial tachycardia and atypical AVNRT. Differentiation from atypical AVNRT may be made more difficult because His synchronous V pacing may not advance the circuit due to decrementation in the accessory pathway. The pathway can be successfully ablated.
Fig 3 shows termination of PJRT with carotid sinus massage (CSM). CSM causes increased vagal tone that may cause termination of the tachycardia by blocking conduction either in the AV node or the pathway, which is also sensitive to autonomic tone. In this example the last inscription on the ECG is a retrograde p wave - indicating block in the anterograde limb of the circuit – the AV node. Termination of tachycardia with AV block strongly suggests that the rhythm is not atrial tachycardia because the atrial tachy would have to coincidentally terminate at exactly the same moment that AV block occurred – an unlikely scenario.
Fig 4 is from the same patient as fig 3 following cessation of CSM. The tachycardia resumed as soon as the autonomic tone normalised and the tachycardia was again incessant. Tachycardia re-initiation after a pause always requires in intervening sinus beat which is not the case with atrial tachycardia.