**5. The post pacing interval after ATP as discrimination tool**

The mechanism of the observed pause after an episode of ATP for true VT, in fact, would represent "entrainment" provided that the VT is advanced by the ATP and then continues unchanged post ATP. This pause then can thus be referred to as the post pacing interval (PPI). The difference between the PPI and the ambient tachycardia cycle length (TCL) has been established as indication of the proximity of the pacing source to the tachycardia circuit and is a fundamental electrophysiological concept [16,17] (figure 10).

In the case of AF or AT the tachycaria source is in a relatively distant chamber, namely the atria. The pause following ATP would represent retrogarde invasion of the infra nodal conducting system and concealed penetration of the AV node.

This illustration demonstrates the concept of the PPI. After delivery of ATP and the absolute PPI and the difference between the PPI and ambient TCL (PPI-TCL) is used to predict the source of the tachycardia (figure 11).

Episodes of failed ATP for detected tachycardias in a heterogenous cohort of 250 patients receiving dual chamber and biventricular ICDs were evaluated at our centre. Fifty one events (n=18 AT/AF and n=33 VT) were eventually compared after excluding episodes in which ATP terminated or altered the TCL ≥ 50ms ie. a significant pertubation of the un‐ derlying tachycardia.

The mean PPI after failed episodes of ATP for VT and AF/AT were 512±88ms vs 693±96ms (p<0.01). Thus a signficant difference was observed in the pause intervals for appropriately and inappropriately delivered ATP which is understandable given the different mechanisms accounting for the PPI in each context.

**Figure 11.** An episode of a 1:1 AT is inappropriately treated by the ICD. The A EGMs are dissociated from the V during

**Figure 10.** The pacing source is described as a distance X away from a macro-re rentrant circuit of tachycardia cycle

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length A +B + C. The PPI is therefore equal to the sum of 2 x X and the TCL (A+B+C).

ATP (arrows). There is an apparent long PPI with a pseudo VAAV response.

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**Figure 10.** The pacing source is described as a distance X away from a macro-re rentrant circuit of tachycardia cycle length A +B + C. The PPI is therefore equal to the sum of 2 x X and the TCL (A+B+C).

**5. The post pacing interval after ATP as discrimination tool**

is a fundamental electrophysiological concept [16,17] (figure 10).

The absence of an atrial EGM makes it difficult to be absolutely certain.

conducting system and concealed penetration of the AV node.

source of the tachycardia (figure 11).

accounting for the PPI in each context.

derlying tachycardia.

170 Cardiac Defibrillation

The mechanism of the observed pause after an episode of ATP for true VT, in fact, would represent "entrainment" provided that the VT is advanced by the ATP and then continues unchanged post ATP. This pause then can thus be referred to as the post pacing interval (PPI). The difference between the PPI and the ambient tachycardia cycle length (TCL) has been established as indication of the proximity of the pacing source to the tachycardia circuit and

**Figure 9.** A treated episode in a single chamber ICD detected as VT but may also be a regular SVT with rapid onset.

In the case of AF or AT the tachycaria source is in a relatively distant chamber, namely the atria. The pause following ATP would represent retrogarde invasion of the infra nodal

This illustration demonstrates the concept of the PPI. After delivery of ATP and the absolute PPI and the difference between the PPI and ambient TCL (PPI-TCL) is used to predict the

Episodes of failed ATP for detected tachycardias in a heterogenous cohort of 250 patients receiving dual chamber and biventricular ICDs were evaluated at our centre. Fifty one events (n=18 AT/AF and n=33 VT) were eventually compared after excluding episodes in which ATP terminated or altered the TCL ≥ 50ms ie. a significant pertubation of the un‐

The mean PPI after failed episodes of ATP for VT and AF/AT were 512±88ms vs 693±96ms (p<0.01). Thus a signficant difference was observed in the pause intervals for appropriately and inappropriately delivered ATP which is understandable given the different mechanisms

**Figure 11.** An episode of a 1:1 AT is inappropriately treated by the ICD. The A EGMs are dissociated from the V during ATP (arrows). There is an apparent long PPI with a pseudo VAAV response.

These were delivered either simulatenously with no AV delay (SAV) or with a Covenvergent AV delay (CAV) decrementing to 0ms which was thought to be less proarrhythmic (figure 13).

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**Figure 13.** The two methods by which ATP was delivered in the Dynamic Discrimination Download Study (DD).

or VT (figure 14).

The ATP was applied initially on detection and advanced discriminators in the form of PR logic (Medtronic, MN, USA) were only applied thereafter on the remaining rhythm disorder. If the tachycardia was not terminated by ATP, the chamber in which the first sequence of tachycardia was redetected post ATP, determined whether the rhythm was classified as SVT

**Figure 14.** In this example from the DD Study, ATP is applied in the CAV (convergent AV delay) format. The tachycar‐ dia resumes post ATP with the first event being an atrial sensed (AS) EGM. This then fulfills the criteria of being an SVT.

**Figure 12.** ROC curves for PPI and PPI-TCL and the cut-offs for each are indicated.

The same observation was made for the PPI-TCL difference for VT and AF/AT: 179±103ms vs 330±97ms (p<0.01), respectively.

The ROC identified cut off values of 615ms or greater for the PPI predicting AF/AT with a sensitivity of 77.8% (95% CI 58.6%-97.0%) and a specificity of 87.5% (95% CI 76.0%-99.0%) (figure 12).

A PPI-TCL ≥260ms also predicted AF/AT with a sensitivity of 72.2% (95% CI 51.5%-92.9%) and a specificty of 78.1% (95% CI 63.8% - 92.4%) (figure 12).

The use of the pause interval represents of form of active discrimination in that it relies on the response to ATP rather than a passive evaluation of EGMs which is the conventional method employed by device algorithms. This also represents a "downstream" evaluation after detection of the tachycardia by the device has already occurred.

Saba and colleagues in the Dynamic Discrimination Download Study (DD) presented a paradigm shift from the "diagnose before treating" to a "treat first and diagnose what is left" algorithm design in Medtronic dual chamber ICDs (MN, USA) [18,19]. Here, once a tachycardia was detected, ATP was applied with the delivery of 8 pulses of ATP in the atrium and ventricle.

These were delivered either simulatenously with no AV delay (SAV) or with a Covenvergent AV delay (CAV) decrementing to 0ms which was thought to be less proarrhythmic (figure 13).

**Figure 13.** The two methods by which ATP was delivered in the Dynamic Discrimination Download Study (DD).

The same observation was made for the PPI-TCL difference for VT and AF/AT: 179±103ms vs

The ROC identified cut off values of 615ms or greater for the PPI predicting AF/AT with a sensitivity of 77.8% (95% CI 58.6%-97.0%) and a specificity of 87.5% (95% CI 76.0%-99.0%)

A PPI-TCL ≥260ms also predicted AF/AT with a sensitivity of 72.2% (95% CI 51.5%-92.9%) and

The use of the pause interval represents of form of active discrimination in that it relies on the response to ATP rather than a passive evaluation of EGMs which is the conventional method employed by device algorithms. This also represents a "downstream" evaluation after

Saba and colleagues in the Dynamic Discrimination Download Study (DD) presented a paradigm shift from the "diagnose before treating" to a "treat first and diagnose what is left" algorithm design in Medtronic dual chamber ICDs (MN, USA) [18,19]. Here, once a tachycardia was detected, ATP was applied with the delivery of 8 pulses of ATP in the atrium and ventricle.

330±97ms (p<0.01), respectively.

a specificty of 78.1% (95% CI 63.8% - 92.4%) (figure 12).

detection of the tachycardia by the device has already occurred.

**Figure 12.** ROC curves for PPI and PPI-TCL and the cut-offs for each are indicated.

(figure 12).

172 Cardiac Defibrillation

The ATP was applied initially on detection and advanced discriminators in the form of PR logic (Medtronic, MN, USA) were only applied thereafter on the remaining rhythm disorder. If the tachycardia was not terminated by ATP, the chamber in which the first sequence of tachycardia was redetected post ATP, determined whether the rhythm was classified as SVT or VT (figure 14).

**Figure 14.** In this example from the DD Study, ATP is applied in the CAV (convergent AV delay) format. The tachycar‐ dia resumes post ATP with the first event being an atrial sensed (AS) EGM. This then fulfills the criteria of being an SVT.

The authors argued that if ATP terminated the tachycardia, then the time to effective therapy was shortened. If tachycardia was ongoing, then there was still no appreciable delay after the intial 8 pulses of ATP in making a definitive diagnosis. The DD algorithm terminated or correctly classified 1379/1381 SVT episodes with an overall specificity of 99.9% and 23/26 VT episodes with a sensitivity of 88.5%. There was no signficant difference in the effectiveness between the SAV and CAV ATP schemes (p>0.5). This upfront method of ATP delivery did not induce any atrial arrhythmias in the cohort studied but there was one episode of slow VT induced which spontaneously terminated.

**6.** 1:1 tachycardias are difficult to differentiate but VT with retrograde conduction is only

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observed in 10-30% of VT episodes.

\* Be aware of dual tachycardias in the A > V arm

**2.** VA dissociation suggests AT

a VVA response is suggestive of VT.

These points are summarised in the following flow chart:

response:

AVNRT

**Figure 15.** A broad classification of rhythm disorders according to the A and V electrogram relationship.

In 1:1 tachycardias, the atrial EGM in dual chamber devices, during the delivery of ventricular ATP and the EGM response post ATP with ongoing tachycardia can be evaluated for the atrial

**1.** VAAV response suggests AT, unless it is a pseudo VAAV which may occur in atypical

**3.** VAV response is compatible with AVNRT/AVRT. It does not completely rule out VT but

These points are summarised in the following flow chart:

The use of ATP as a means of pacing for SVT-VT discrimination does present some clinical challenges:


Some possible solutions to obviate these problems would be to:


## **6. General considerations when approaching device based tracings**


**6.** 1:1 tachycardias are difficult to differentiate but VT with retrograde conduction is only observed in 10-30% of VT episodes.

These points are summarised in the following flow chart:

The authors argued that if ATP terminated the tachycardia, then the time to effective therapy was shortened. If tachycardia was ongoing, then there was still no appreciable delay after the intial 8 pulses of ATP in making a definitive diagnosis. The DD algorithm terminated or correctly classified 1379/1381 SVT episodes with an overall specificity of 99.9% and 23/26 VT episodes with a sensitivity of 88.5%. There was no signficant difference in the effectiveness between the SAV and CAV ATP schemes (p>0.5). This upfront method of ATP delivery did not induce any atrial arrhythmias in the cohort studied but there was one episode of slow VT

The use of ATP as a means of pacing for SVT-VT discrimination does present some clinical

**1.** In the case of ventricular deliver of ATP, retrograde AV nodal conduction and advance‐ ment of the atrial EGM must be observed in order to interpret VAAV/VAV reponses.

**2.** ATP itself may induce premature atrial on ventricular complexes that truncate pause intervals and/or influence the assessment of the chamber of origin of tachycardia.

**4.** ATP may accelerate or decelerate the existing tachycardia or may induce further arrhyth‐

**3.** To automatically set variable blanking periods after the delivery of ATP in order not to

**1.** The chamber of onset of the tachcardia (if observed) in a dual chamber ICD has been

**2.** Although VT tends to be a stable rhythm, cycle length variations may occur during onset

**3.** SVTs may also present as regular tachycardias and AF may show pseudoregularisation

**4.** If A>V events are noted this usually defines an SVT (AT, AF or A Flutter) except in the

**5.** If V>A events are noted this suggests VT although AVNRT with intermittent retrograde

**1.** Deliver the ATP in multiple sequences in order to insure myocardial capture.

**6. General considerations when approaching device based tracings**

of the tachycardia or in the presence of anti-arrhythmic drugs.

block should be considered but this is fairly uncommon.

**3.** ATP must capture ventricular myocardium during the drive train.

Some possible solutions to obviate these problems would be to:

**2.** To deliver the ATP at high pacing output

implanted helps discriminate SVT from VT.

sense induced premature beats.

at rapid rates.

case of dual tachycadias.

induced which spontaneously terminated.

challenges:

174 Cardiac Defibrillation

mia.

\* Be aware of dual tachycardias in the A > V arm

**Figure 15.** A broad classification of rhythm disorders according to the A and V electrogram relationship.

In 1:1 tachycardias, the atrial EGM in dual chamber devices, during the delivery of ventricular ATP and the EGM response post ATP with ongoing tachycardia can be evaluated for the atrial response:


These points are summarised in the following flow chart:

[2] Klein RC, Raitt MH, Wilkoff BL et al. Analysis of implantable cardioverter defibrillator therapy in the Antiarrhythmics Versus Implantable Defibrillators (AVID) Trial. *J*

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[6] Wathen MS, DeGroot PJ, Sweeney MO et al. Prospective randomized multicenter trial of empirical antitachycardia pacing versus shocks for spontaneous rapid ventricular tachycardia in patients with implantable cardioverter-defibrillators: Pacing Fast Ventricular Tachycardia Reduces Shock Therapies (PainFREE Rx II) trial results.

[7] Schoels W, Swerdlow CD, Jung W et al.. Worldwide clinical experience with a new dual-chamber implantable cardioverter defibrillator system. *J Cardiovasc Electrophy‐*

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**Figure 16.** Arrhythmia classification based on atrial response post ATP.
