Techniques of Programmed Stimulation and Entrainment Technique

As a starting point for the discussion of programmed stimulation, it is helpful to briefly review the concept of reentry. Reentrant rhythms consist of a looping circuit of electrical activation, unlike automatic rhythms, which result from repetitive activation from one or several discrete sites. ^[16]

For reentry to be possible, certain anatomic and physiologic prerequisites must exist. Specifically, reentry requires the presence of tissues that differ in conduction velocity and refractoriness relative to one another; it also requires an area of functional or anatomical block.

Within this reentrant circuit, a propagating wavefront travels circumferentially around the area of block such that the leading edge (head) of the wavefront is “chasing” its trailing edge (tail). The portion of the circuit between the head and tail of the wavefront is termed the excitable gap. The tissue in the gap is either relatively refractory (partially excitable) or completely repolarized (fully excitable), allowing for continued propagation of the reentrant wavefront.

If the wavefront were to propagate so fast that the head met the tail (ie, arriving at tissue that was just recently depolarized and thus absolutely refractory), the wavefront would be extinguished and the arrhythmia terminated. Therefore, for reentrant circuit to sustain an arrhythmia, the distance from the head to the tail of the wavefront (wavelength) must be shorter than the distance around the circuit (path length). Also essential to sustaining reentry is an area of slow conduction within the circuit. It is the reduced conduction velocity of the propagating wavefront within this area of slowing (akin to speed bumps on a street) that allows for tissue downstream to recover from recent depolarization and once again become excitable.

Practical pearls

Stimulate and record from the same site/catheter. For example, when pacing from the right ventricular catheter during entrainment, the assessment of the post pacing interval should also be measured from the right ventricular catheter.

Use long drive trains, typically about 30 ms less than the tachycardia cycle length, but not faster. This increases the chances of having programmed stimuli fall on repolarized tissue and increases the chances of having a stable morphology.

Ensure that there is capture before proceeding to analyze any response.

Ensure the tachycardia was not terminated and reinitiated during the drive train.

Use the criteria for recognizing entrainment.

Do not assess the post-pacing interval when entrainment has not been confirmed.

A single extra stimulus (N) depolarizes myocardium around its point of origin and then travels toward the reentrant circuit, encountering (1) reentrant circuit tissue that is being actively depolarized and refractory (hence extinguishing the extra stimulus N) or (2) partially or completely excitable tissue (the gap). Within the gap, the extra stimulus N propagates both in the retrograde direction, colliding with the head of the preceding tachycardia wavefront (N - 1), and the anterograde direction, “pushing” against the tail of the propagating tachycardia and driving it around the reentrant circuit, after which it exits the circuit with resumption of the morphology and the rate of the original tachycardia.

A series of stimuli at a preprogrammed rate and sequence can be used to penetrate the reentrant circuit in a manner similar to that outlined above. As the series of stimuli are delivered, if they penetrate the excitable gap, they will propagate in the antegrade and retrograde directions, thereby extinguishing the previous wavefront and generating a new one at a rate determined by the pacing frequency and conductive properties of the tissue.

For example, consider the delivery of a three-beat drive train (N1, N2, N3) delivered at a pacing cycle length (PCL) of 350 ms in an effort to entrain a tachycardia with a fixed cycle length of 380 ms.

The first stimulus (N1) collides with and extinguishes the original tachycardia wavefront N - 1 in the retrograde direction while driving the tachycardia in the anterograde direction, accelerating it to 350 ms from 380 ms.

The following stimulus (N2) propagates in the retrograde direction, collides with and extinguishes the wavefront created by N1, and propagates in an anterograde direction around the circuit until it is extinguished by colliding with the retrograde propagation of stimulus N3 as this penetrates the gap.

Finally, the last stimulus in the drive train N3 completes its journey around the circuit, resulting in the next beat having the same morphology as the original tachycardia. The fact that stimuli N1 and N2 activated some portion of the myocardium in the retrograde or antidromic direction as well as in the antegrade or orthodromic direction leads to those paced complexes having a hybrid or "fused" appearance, looking in part like a purely paced beat and in part like the original tachycardia. The concept of fusion and the particulars of assessing fusion are beyond the scope of this article but are well described elsewhere. ^[17]

As noted above, during entrainment, the last stimulus of the drive train extinguishes the wavefront of the preceding stimulus in the retrograde direction, and, provided the pacing rate does not exceed the conduction properties of the tissue within the circuit, the stimulus propagates in the anterograde direction going completely around the circuit.

The response of an arrhythmia to entrainment is of critical importance in understanding its mechanism and guiding ablation. In performing this analysis, particular attention is paid to the post-pacing interval and to the sequence of atrial and ventricular events after pacing is discontinued.

The time between the electrogram that resulted in the last entrained beat (N3 in the example above) and the onset of next electrogram measured from the stimulation site is referred to as the post-pacing interval. When the difference between the post-pacing interval and the tachycardia cycle length is 30 ms or less, the pacing site is considered to be within the reentrant circuit. The ability to determine if a particular location is within the circuit is of paramount importance in deciding where to perform ablative therapy. A classic example of this is seen in the treatment of atrial flutter in which the operator confirms that they are “in the circuit” by entrainment before ablating the circuit.

The sequence of events that follows the cessation of entrainment are also key to understanding the nature of an arrhythmia. This sequence of events can be of particular use in distinguishing a focal atrial tachycardia from a reentrant supraventricular tachycardia. In atrial tachycardia, the atrial activation does not depend on ventricular activation; therefore, one expects to see an “A-A-V” response to entrainment. On the contrary, a reentrant supraventricular tachycardia displays an “A-V” response.