Cure the patient with one impulse

• how to map and find accessory pathway
• what is the signal characteristic of the pathway
• how good and bad response to ablation looks like

Ablating accessory pathway (AP) is in opinion of many electrophysiologist the most rewording procedure you can experience… but only if you invest your time in careful mapping and find the right spot.

The goal is to find the connection between atrium and ventricle, i.e. accessory pathway or more precisely the atrial or ventricular exit of this conduction. Historically APs were mapped retrogradely i.e. through aorta into ventricle. However, as we are so used to transseptal approach most of APs are mapped form the atrial side.

• Mapping is the key to successful ablation. The approach to mapping depends on specific electrophysiological properties of the case but one think is always important: you want to ablate in a spot where the signal originate (is the earliest). You can map the earliest ventricle activation while conducting atrial pacing at the rate that assure maximal preexcitation. If the AP is conducting retrogradely you can do exactly the opposite and map earliest A while pacing from RVA. In both cases you are facing two problems: A) patient must tolerate well a high pacing rate and B) you need to know if the conduction is solely via AP. If AP is only partially responsible for conduction (in other terms AV-node is contributing to conduction) you see only fusion of activation from both AP and AV node. This means that the earliest activation will be somewhere between AP and AV node and not over AP directly. In order to be sure, you only have conduction via AP you need maximal preexcitation in case of V mapping or pure retrograde conduction for A mapping. Maximal preexcitation can be found by incremental atrial pacing: by decreasing CL of atrial stimulation the preexcitation is increasing up to a moment when a further decrease of CL causes no increase in preexcitation (further widening and “deformation” of QRS) or even a blocking of atrial impulse.  In practice it might be challenging while patients do not appreciate being paced at maximal heart rate for to long. Fortunately after establishing the critical CL in which only AP is conducting you can use a single beat at this CL for your measurement than move to another point and use single beat again (the combination of 2-3 beats at CL of 500ms followed by critical CL works for me the best). Another approach is mapping during AVRT. For that approach you need a stable and well tolerated AVRT. Since in AVRT AV node and AP are sequentially activated as two pathways of a reentry circle it you can be absolutely sure you are mapping only AP and not fusion.
• Signal characteristic is as always, a key to good mapping. Since we are looking for the connection between atrium and ventricle we want those two signals (i.e. A and V) to almost overlap. This makes the mapping a little bit tricky as it is sometimes hard to distinguish where one signal ends and the next begins.  In order to better understand the signal, you can use unipolar channel and look for QS morphology: catheter indicate the wave front direction of local activation and it shows a positive slope when the wave front is traveling towards the catheter and a negative slope then it is moving away from the catheter. Thus, at the origin of the signal only negative slope should be recorded (i.e. QS-morphology). Another signal characteristic can be sometimes spotet between both signals and it can be a direct signature of activation of the AP. Such sharp high-frequency signals are called pathway potentials.
• Optimal ablation setting depends on catheter specification and localization. In most cases 30-40W for 60s provides a sufficient lesion. If there is no response (termination, loss of preexcitation or retrograde conduction) within 10-20s after starting ablation it is probably a sign to burn elsewhere. If you decide to map and ablate during ongoing tachycardia it is advisable to keep the same heart rate (either by inhibited pacing or starting pacing with similar CL at the moment of termination) in order to prevent catheter from dislodgment caused by long diastole.