**1. Introduction**

The stored electrograms (EGMs) retrieved from the ICD provide a unique and useful source of information regarding the mechanism of the underlying arrhythmia.

Current ICD algorithms discriminate VT from SVT on the basis of **passive analysis** of detect‐ ed **rhythms** with positive predictive values of greater than 90% [1].

Despite this, the incidence of inappropriate therapies for SVT discrimination still remains high and varies from 16% to 31% as quoted in prior studies [2]. In many ways, the ICD bears a resemblance to a diagnostic electrophysiological study. The underlying cardiac rhythm is analysed and acted on often with the delivery of anti-tachycardia pacing (ATP) if the threshold is met. This therapeutic interaction by the ICD with the underlying arrhythmia can also be interpreted as a diagnostic manoeuvre similar to the pacing techniques employed in the electrophysiology laboratory before arriving at the diagnosis (figure 1). The success or failure of ATP to terminate the underlying rhythm may both have value.

#### **1.1. Anti-tachycardia pacing**

Anti-tachycardia pacing has been demonstrated to be a safe, effective and painless therapy in randomized controlled multicentre trials [3,4]. In the PAINFREE trial two sequences of ATP were delivered before a shock in the fast ventricular tachycardia (FVT) zone. A total of 446 FVT episodes with a mean cycle length of 301 ± 24 msec were documented in 52 patients. A total of 396 of these FVT episodes were terminated by ATP alone with an adjusted efficacy of 77% (95% CI 68% to 83%) [5]. Acceleration of the VT by ATP occurred in only 10 (4%) FVT episodes but these went on to delivery of a definitive shock aborting the episode (figure 2).

© 2013 Michael et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Figure 1.** The ICD bears a resemblance functionally to a diagnostic electrophysiology suite

Arrhythmia related syncope that may have been from the marginal delay during delivery of ATP occurred in just 4 patients (2%) and merely involved 9 device episodes.

The PAINFREE Rx II randomized ICD patients to 2 arms: standardised empirical ATP in the FVT zone (n= 313) before shock or directly to shock in the control group (n=321) [6]. Antitachycardia pacing was effective in 229 of 284 episodes in the ATP arm thus yielding an adjusted efficacy of 72%. The episode duration, incidence of arrhythmic syncope and acceler‐ ation of VT was similar in both arms.

In their evaluation of ATP as first line therapy, Schoels and coworkers evaluated 760 ventric‐ ular arrhythmia episodes in 128 patients. Five hundred were appropriately detected (82 patients) [7,8].

**2. Pacing to discriminate between atrial tachycardia and re-entrant SVT**

tivity for diagnosing AT figure 3 [9,10].

VT which then leads to a shock from the device terminating the episode.

pacing.

as discussed above [11].

Ventricular pacing and an evaluation of the atrial response after advancement of the A during retrograde conduction is a conventional manoeuvre of differentiating AT from a re-entrant SVT either AVNRT or AVRT. Knight et al. demonstrated that an A-A-V response after 1:1 VA conduction after ventricular pacing during ongoing tachycardia had a specificity and sensi‐

**Figure 2.** A. An episode of VT in the FVT zone terminated by a burst of ATP. B. The first burst of ATP in a detected VT episode fails to terminate the VT so a second burst at a shorter cycle length is delivered resulting in acceleration of the

Current Issues in ICD SVT-VT Discrimination: Pacing for SVT-VT Discrimination

http://dx.doi.org/10.5772/55047

163

An A-V response on cessation pacing, however, suggests either AVNRT or AVRT as the underlying mechanism (figure 4). This interpretation is based on condition that the A is advanced during V pacing and that the underlying tachycardia continues unperturbed post

Using this data, it therefore seems fairly intuitive to apply these atrial responses to the interpretation of device EGMs after ATP. If there is consequent conduction to the atrium in a 1:1 fashion with advancement of the A, then the return response after pacing maybe diagnostic

This concept was applied by Ridley and co-workers to the interpretation of ICD EGMs from dual chamber ICDs (Medtronic,MN, USA) [12]. The evaluation of responses, however, was

Their analysis however showed that with conventional ICD programming and detection there were 260 episodes that were inappropriately treated. Of these 224 (57 patients) were atrial tachycardia or atrial fibrillation (AT/AF) while the remaining 36 episodes (19 patients) were due to sinus tachycardia.

This suggests that conventional device detection algorithms are prone to misdiagnosis for supraventricular arrhythmias in a significant proportion of patients. In the case of devices programmed with ATP as first line therapy it would be painless and would not result in significant morbidity. This does not hold true if there was an inappropriate shock delivered.

Since pacing is a common way of differentiating arrhythmias in an electrophysiological study, the response to this form of pacing in the ICD, by deduction may therefore hold clues as to the mechanism of the underlying detected rhythm. This then has diagnostic potential for the device specialist evaluating stored EGMs in a clinic setting and possibly has the potential for further algorithm development.

**Figure 2.** A. An episode of VT in the FVT zone terminated by a burst of ATP. B. The first burst of ATP in a detected VT episode fails to terminate the VT so a second burst at a shorter cycle length is delivered resulting in acceleration of the VT which then leads to a shock from the device terminating the episode.

Arrhythmia related syncope that may have been from the marginal delay during delivery of

The PAINFREE Rx II randomized ICD patients to 2 arms: standardised empirical ATP in the FVT zone (n= 313) before shock or directly to shock in the control group (n=321) [6]. Antitachycardia pacing was effective in 229 of 284 episodes in the ATP arm thus yielding an adjusted efficacy of 72%. The episode duration, incidence of arrhythmic syncope and acceler‐

In their evaluation of ATP as first line therapy, Schoels and coworkers evaluated 760 ventric‐ ular arrhythmia episodes in 128 patients. Five hundred were appropriately detected (82

Their analysis however showed that with conventional ICD programming and detection there were 260 episodes that were inappropriately treated. Of these 224 (57 patients) were atrial tachycardia or atrial fibrillation (AT/AF) while the remaining 36 episodes (19 patients) were

This suggests that conventional device detection algorithms are prone to misdiagnosis for supraventricular arrhythmias in a significant proportion of patients. In the case of devices programmed with ATP as first line therapy it would be painless and would not result in significant morbidity. This does not hold true if there was an inappropriate shock delivered.

Since pacing is a common way of differentiating arrhythmias in an electrophysiological study, the response to this form of pacing in the ICD, by deduction may therefore hold clues as to the mechanism of the underlying detected rhythm. This then has diagnostic potential for the device specialist evaluating stored EGMs in a clinic setting and possibly has the potential for

ATP occurred in just 4 patients (2%) and merely involved 9 device episodes.

**Figure 1.** The ICD bears a resemblance functionally to a diagnostic electrophysiology suite

ation of VT was similar in both arms.

patients) [7,8].

162 Cardiac Defibrillation

due to sinus tachycardia.

further algorithm development.
