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EP Procedures - Pacing Maneuvers

Understanding the role of Pacing

          Electrode pairs serve two purposes in an EP procedure. The first is to record and the second is to pace. Recording allows the practitioner to visualize the electrical activation at or around the specified electrodes while pacing provides a mechanism by which control may be exerted over the refractory periods of the tissue involved. Controlling the refractory period allows the physician to regulate where an electrical event will start and, by pacing at different rates, affect the duration of refractoriness of the cardiac tissue. Simply put, the faster we pace, the shorter the refractory period.

     This section demonstrates a variety of different pacing techniques. Each represents a specific tool the Electrophysiologist can use to diagnose cardiac arrhythmias. Understanding how each of these tools works will help expand your knowledge in the EP Lab.

Pacing to Evaluate Retrograde Conduction / Concentric and Eccentric Conduction

          Pacing in the ventricle can provide information on retrograde conduction through the AV Node or accessory bypass tracts.

          In this first image, ventricular pacing is being performed at a cycle length of 500ms. Note that the atrial activation sequence on the coronary sinus catheter is proximal to distal.

          This suggests that the electrical activation is conducting retrograde up through the AV node and activating the proximal CS channel before any of the other CS recording pairs. The last pair to activate is the distal CS. This pattern is considered normal and is referred to as concentric conduction.

          This image is from the same patient as the recording above. The ventricular pacing cycle length has been decreased to 400ms.

          Retrograde atrial activation on the CS catheter is now visualized first on CS channel 3-4. This suggests an accessory pathway near that electrode pair. Any abnormal patterns such as this one is considered to be eccentric conduction.

          Note that for the conduction pattern to change indicates that the AV node is now refractory to retrograde conduction. The only way for the electrical activation to reach the atrium is through the accessory bypass tract. Reducing the pacing cycle length further will allow the physician to determine the refractory period of retrograde conduction through the accessory bypass tract.

Anti-Tachycardia Pacing (ATP)

          Anti-Tachycardia Pacing, also known as Overdrive Pacing, allows the physician some degree of control over various arrhythmias. Using the principal that the fastest rate in the heart is the one that controls depolarization, ATP is performed by pacing faster than the arrhythmia cycle length, in this case 340ms, and then terminating the pacing train once it can be verified that the pacing stimuli are in control of depolarization or are capturing the myocardium.

          In this instance, a pacing train of 310ms is delivered to the high right atrium (labelled as Quad A). Capture is indicated by the disruption in cycle length visualized on the surface leads. Termination of pacing results in termination of the tachycardia.

Pace Entrainment


Pace Mapping

          One of the conventional mapping tools used in determining the origin of focal ectopics is a pacing maneuver referred to as pace mapping. This technique is performed by pacing from the RF catheter when it is believed to be at, or close to, the suspected location of breakout for the beat of interest.

          Pace mapping is performed by splitting the review screen into two windows. The first window is a recording of the surface 12 lead of the clinical VT. The second window shows a 12 lead of the paced morphology. Each lead of the two recordings is reviewed to determine if a 12 of 12 match is obtained. In the case, shown below, there is substantial difference between the two displays. We are not at the ideal location to target ablation energy. By moving the ablation catheter around and pacemapping at a number of different locations, a good 12 of 12 match is located as shown in the second set of images.  This degree of correlation between the paced recording on the right and the original morphology on the left indicates a prime target for successful ablation.

pace map 2

          In this image, the panel to the left is a recording of the clinical VT. The panel on the right is the pace map derived from pacing at the RF catheter. Note the differences listed below in each lead.

Lead I: Paced morphology (PM) is more negative.
Lead II: PM is not as positive.
Lead III: The peak of the QRS is predominately right in VT and left in PM.
AVR: PM is initially positive and biphasic.
AVL: PM shows small initial R wave and is more negative.
AVF: Similar to Lead III, the PM QRS is notched to the left.
V1: PM is initially positive and biphasic.
V2 - V6: R wave progression in the clinical VT show classic RVOT morphology. The paced morphology varies significantly in all the precordial leads.

pace map 1

          This image shows the same patient as above with the catheter at an a different location. The sweep speed has been increased in the right half of the screen.

          Evaluation of each of the 12 leads shows a high degree of correlation between the clinical VT and the paced rhythm. This would represent an ideal target for application of RF energy.

Advance the A


Catheter "Bump"

catheter bump

          During cases where the RF catheter is being maneuvered, it is possible to "bump" the endocardial surface and create a "false ectopic". While this is not a pacing maneuver, it has been included in this section as it often occurs during paced events.

          In the displayed leads, the morphology of the third beat is notably different that the other recorded beats. It also occurs significantly earlier and "out of sync" with the other beats in this run. An analysis of the timing between the onset of signal at the RF catheter and the onset of surface ECG show that the intracardiac signal initiates much ealier than the onset of surface for this beat only! This is a good indication of an ectopic caused by moving the RF catheter.




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