Zipes test:  AVJRT versus AVNRT

Ablation in the slow pathway area commonly results in junctional beats, and these are markers of effective ablation. However, these may be persistent following the end of ablation, or may be triggered by the use of isoprenaline. This can result in a tachycardia which is similar in appearance to the initial AVNRT, but is in fact an automatic junctional tachycardia (at 130-140bpm, for which no further treatment is required).

Two diagnostic tests can be used to differentiate the two tachycardias.

The first has to be described [Ref 1] uses atrial extra stimuli delivered during the tachycardia, scanned progressively earlier over a range of intervals, until His coincident, and then earlier.  (This has been explained previously in the Section “Arrhythmia Entrainment”)

If a His coincident atrial extra stimulus perturbs the timing of the following His signal, then the diagnosis is AVNRT as the junction could not have been affected (his coincident / refractory).

If the atrial extra results in advancement of the immediate His, and the tachycardia continues, the diagnosis is Junctional Tachycardia (in AVNRT the retrograde fast pathway would have been blocked causing termination).

The second test is called His-synchronous PVC (or Zipes test). During a tachycardia a synchronized ventricle extra stimuli will be delivered.  The timing of the extrastimulus that is delivered is critical, and should be synchronized with the tachycardia.  Single extra beats are introduced to capture the right ventricle, intended to occur at exactly the same time as the His signal seen on the His catheter.

By `scanning` earlier with successive 10ms decrements, the extra is delivered 20-30ms earlier than the observed His signal, then the earliest atrial signal following the PVC is monitored for any 'perturbation' in timing.

In AVNRT the ventricular extra stimulus should never alter the atrial activation time as the route up to the atrium is blocked by the His which has been depolarized, in contrast to AVRT where there is another route up to the atrium [Ref 2].

Differential atrial pacing after Flutter ablation

A very good example of differential pacing is performed after an atrial flutter ablation to demonstrate completeness of block of re-entrant pathway.  It involves pacing at different distances away from the ablation line along the Cavo-tricuspid isthmus (CTI). The physician will observe if the activation of the other side of the ablation line has occurred across the ablation or by going around the atrium (slow conduction versus block).

Pacing is required from 4 different sites on the tricuspid annulus, and conduction times compared to ensure that the impulse does not travel through the CTI.

Figure above shows a view of the Right atrium with diagnostic catheter at the lateral RA wall, a CS  and an ablation catheters. The Isthmus Ablation Line has been performed between the Inferior Vena Cava and the Tricuspid Annulus. 

The exact procedure may be found accessing here:

Para-Hissian pacing

This test can reveal the existence of a concealed pathway, but it does not show that the pathway is part of a circuit, nor responsible for palpitations or tachycardia.

The test is always performed in sinus rhythm [Ref 3].  The His catheter is used for pacing from the distal electrodes. Pacing at maximum output aims to capture the His bundle. This results in a surface ECG which appears similar to the normal QRS (that means relatively narrow).  The resultant atrial activation is observed (Stim-A time). The pacing output is then decreased until His bundle capture is lost, and the QRS becomes wider. The atrial activation is re-assessed.

Normal conduction results in a prolongation of the stim-A time with the transition from narrow complex capture to wide complex capture. Where there is no change in Stim-A time, a para‑hissian pathway is present. Ensure that direct atrial capture does not occur, as this is often seen when pacing from the His catheter.

It is important that when pacing the His bundle at high output, that the atrium is not simultaneously captured.

“Parahissian Button” appears in Pace & Burst Protocols when current is focused.   For performing a quick Parahisisian pacing: pressing the button boosts Stim Current to 20mA and releasing it Stim Current comes back again to its previous value.

Oesophageal pacing

Transesophageal atrial pacing is possible because of the proximity between the oesophagus and the posterior aspect of the atria.  This technique is non-invasive, therefore there is no need to bring the patient to the cathlab,  This method has been mainly tested in patients with a history of spontaneous supraventricular tachycardia (SVT).

Bipolar or unipolar recordings can be made, but obviously the strengh-duration curve of esophageal pacing is different than that of endocardial pacing.   Acording to some studies, lowest thresholds can be reached at pulse widths between 10 and 20 msec and 5-15mA. 

In order to minimize the patient discomfort, the studies have tried to use lower pacing thresholds using a bipolar electrode with wide interelectrode distance. [Ref 4]

In conclusion, using a rapid atrial pacing from the esophagus can be used to induce and terminate SVT for diagnostic or therapeutic purposes; and that esophageal pacing provides a convenient way to assess repeatedly the efficacy of long-term drug therapy and to screen patients for preexcitation syndromes. [Ref 5]

However, a few limitations to the technique have been found:

a) there is only one site of atrial pacing and recording,

b) there is no ventricular pacing and

c) sometimes atrial capture can be difficult to assess on the surface ECG during premature extrastimuli delivery.