"Strange” respond to usual pacing maneuvers

• what are the first sign of presence of accessory pathway
• how to induce AVRT
• how to systematically assess the mechanism of tachycardia

In cases of left-sided accessory pathways (AP), pacing maneuvers can provide several diagnostic insights. There are many different approaches but it’s advisable to pick up one and master it through training and repetition.

1)    Look for eccentric activation:

- You can either start from the atrium or ventricle and look for activation in CS catheter indicating earliest activation region on the left side of the heart. This finding not necessarily has to be obvious right from the beginning as both AP and AV node might be conducting and you end up with fusion of those two conductions pathways. However, if you keep pushing and pace incrementally (faster) you will reach a point when one of the conductions routes will give up (reach ERP) and a sudden change in CS sequence will appear. In most of the cases it will be AV node to reach ERP first and you will end up with AP conduction.

2)    Observe preexcitation:

-       If you pacing form the atrium you should also get increasing preexcitation. You might observe His potential is slowly disappearing within ventricle signal as the AV node is conducting slower than the pathway.

-       In order to get preferential conduction via AP (meaning more preexcitation) you might try pacing atrium from the left side (CS 1/2).

-       In order to both get more preexcitation and eccentric activation you can block AV node with 12-18mg of adenosine (depending on patient weight). However, it should be your method of last resort.

3)    Measure decremental properties:

-       Delivering premature atrial or ventricular stimuli should reveal decremental conduction properties in the AV node. However, AP rarely have decremental properties so conduction of faster beats would not cause any delay in conduction when AP is in charge.

-       Remember that this test is most valid in the last beats before reaching ERP as it is most likely that they are conducted solely by AP.

4)    Inducing AVRT:

-       In general, an orthodromic AVRT (i.e. one using AV node for AV conduction) can be triggered by atrial stimulation while antidromic (Lesson 41) with ventricular pacing. Both rapid pacing and extra-stimulus should work but extra-stimulus method is more advisable as it is easier to follow.

-       You can use the protocol from the video: start with basic CL of 500ms followed by shorter and shorter extra-stimulus. You will observe increasing preexcitation which will suddenly disappear when AP reaches its ERP. This will initiate AVRT in next cycle as now AP is ready to conduct retrogradely and activate AN node in reentry mechanism.

-       During retrograde induction of orthodromic AVRT the blocked VA conduction via AV node will enable a AV conduction via AV node of the next atrial signal comming from AP conducting solely the ventricular extra-stimulus.

5)    Specific findings during tachycardia:

-       Typical orthodromic (i.e. narrow QRS) AVRT via left sides AP is characterized by a long RP interval as the distance from AP (A) to AV node (V) is quite big and the impulse need time. The best place to measure it is at His catheter where VA should be longer than 70ms or on HRA catheter where it should be longer than 110 ms.

-       Fast ventricular pacing during tachycardia (entrainment) should give PPI less than 110ms (in comparison to AVNRT) since ventricle is a part of the reentry.