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

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

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 pacing.

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 as discussed above [11].

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

defining the rhythm as an AT (figure 5A). A type 2 response was due to variable VA conduction

**Figure 5.** A. Type 1 response: the ventricular EGMs are dissociated from the atrial events during ATP. This is consistent with a diagnosis of AT. B. Type 2 response: ATP results in a variable atrial response and therefore is inconclusive.

A type 3A response occurs if the post pacing phenomenon is a V-A-A-V which is essentially an A-A-V if the last paced V, is not taken into account, as was encountered in the Knight et al. study mentioned above (figure 5C). The overall sensitivity was 71.9% (95% CI 67.1-73.6) and a specificity of 95% (95% CI 83.5-99.1). A Type 3B or V-A-V response was felt to be less conclusive for a SVT and the authors felt this did not exclude VT. This study however was

A.

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

B.

and therefore leading to an inconclusive A response (figure 5B).

**Figure 3.** An A-A-V response after V pacing and 1:1 VA conduction with ongoing tachycardia suggests AT as the un‐ derlying mechanism.

based on the interval plot summary of episodes and not on the intracardiac signals. These were

categorized as a type 1 response if the ventricle (V) was dissociated from the atrium (A)

**Figure 4.** An A-V response on cessation of V pacing with 1:1 VA conduction suggests either AVNRT or AVRT.

defining the rhythm as an AT (figure 5A). A type 2 response was due to variable VA conduction and therefore leading to an inconclusive A response (figure 5B).

based on the interval plot summary of episodes and not on the intracardiac signals. These were

**Figure 3.** An A-A-V response after V pacing and 1:1 VA conduction with ongoing tachycardia suggests AT as the un‐

derlying mechanism.

164 Cardiac Defibrillation

categorized as a type 1 response if the ventricle (V) was dissociated from the atrium (A)

**Figure 4.** An A-V response on cessation of V pacing with 1:1 VA conduction suggests either AVNRT or AVRT.

**Figure 5.** A. Type 1 response: the ventricular EGMs are dissociated from the atrial events during ATP. This is consistent with a diagnosis of AT. B. Type 2 response: ATP results in a variable atrial response and therefore is inconclusive.

A type 3A response occurs if the post pacing phenomenon is a V-A-A-V which is essentially an A-A-V if the last paced V, is not taken into account, as was encountered in the Knight et al. study mentioned above (figure 5C). The overall sensitivity was 71.9% (95% CI 67.1-73.6) and a specificity of 95% (95% CI 83.5-99.1). A Type 3B or V-A-V response was felt to be less conclusive for a SVT and the authors felt this did not exclude VT. This study however was based on the the interval plot as opposed to the EGMs. In our opinion, by reviewing both near and farfield EGMs in conjunction with the scatterplot, a reasonable clinical deduction can be made with regards a V-A-V response to suggest either AVNRT/AVRT. A device-based algorithm might combine pacing response with EGM morphology to discriminate the Type 3B response.

**4. Limitations**

intervals.

blanking of the sensed event.

The following device related limitations need to be borne in mind:

**1.** Over/undersensing producing an incorrect A or V response on the interval plot.

**2.** Timing with automatic, decaying threshold sensing may not be accurate resulting a

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

**3.** The 10 ms resolution in Medtronic ICDs leading to an inherent error in the estimated

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A limitation in this study was the high exclusion rate of cases since 45.1% of data could not be reliably analysed for various reasons. In 74.5% of these cases the tachycardia was terminated by the ATP as well. This, in itself, does not imply that all these episodes were VT since SVTs may also terminate with ventricular pacing. The flow diagrams in the diagnostic approach discussed later in this chapter discuss how termination of tachycardia can be evaluated to obtain a rhythm diagnosis.

**Figure 6.** A type 3A response shows a VAAV pattern which is consistent with AT and a type 3B response where a VAV pattern is observed.
