**4. Limitations**

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

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

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

In order to evaluate the atrial response after V pacing, it is important to confirm that the atrium

Iso-arrhythmic dissociation of the ventricle at the atrial tachycardia rate may mimic 1:1 VA

The next common problem is to recognise the pseudo VAAV pattern. This could occur coincidentally post ATP delivered in the ventricle with VA dissociation during an episode of

A long VA interval during an SVT as is the case in atypical AVNRT or the so called "fast-slow"

conduction resulting in a misdiagnosis of a VAV interval on cessation of pacing.

**3. Pitfalls in interpreting the VAAV/VAV response**

variant can also yield a pseudo "VAAV" response (figure 7).

was indeed advanced during ongoing tachycardia.

3B response.

166 Cardiac Defibrillation

pattern is observed.

AT (figure 11).

obtain a rhythm diagnosis.

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


The tachycardias detected by the ICD therefore can be broadly classified as either 1:1 (A = V)

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

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

169

The 1:1 tachycardias are difficult for both device algorithms as well as the observer to resolve as either retrograde VA conduction during VT as opposed to a SVT (table 1). However, 1:1 VA conduction only occurs in about 10% of detected VTs and therefore has a low probability [13, 14].However, the consequences of misdiagnosis of VT is much greater than the misdiagnosis of SVT. If this is misclassified as an SVT, then the result may well be a withholding of appro‐

It seems rather intuitive to distinguish N:1 tachycardias as either AF, A flutter or AT if there are accompanying atrial EGMs on which to base this interpretation. Single chamber ICDs are frequently implanted if the indication is a primary prevention strategy or in the presence of persistent atrial tachycardias so that the atrial signals are lacking in these device EGMs. This then does not permit the use of algorithms that rely on A and V patterns of association in order to discriminate the rhythm. The result may well be inappropriate therapies in the form of ATP and/or shocks. It also makes it difficult for the device specialist when it comes to analysing the

In the example in figure 8, it is evident that the underlying arrhythmia is rapidly conducted AF. In the following clinical scenario, a single chamber ICD (figure 9), it is difficult to be certain that this is indeed an episode of VT. It may well be an organized atrial tachycardia or an episode of paroxysmal AF which is pseudo regularised and detected in tachycardia zone. Antitachycardia pacing is elicited as it is the first line therapy programmed in this detection zone of the device. It was deemed ineffective and therefore there was escalation to a shock. After each burst of ATP, a pause (arrows) is evident before resumption of the tachycardia. We postulated that the pause duration may help predict the chamber of origin of the tachycardia,

**Figure 8.** An interval plot showing uncontrolled atrial fibrillation detected in the VF zone of the ICD leading to ATP in

the form of a ramp and then progression to sequential inappropriate shocks.

tachycardias or N:1 tachycardias (ie. A > V).

priate device therapies which is best avoided.

tracings [11].

namely VT vs AT/AF [15].

**Figure 7.** a). This SVT was determined to be an atypical AVNRT using a fast-slow re-entrant substrate. This was inap‐ propriately detected and ATP was delivered by the device. On cessation of pacing a "VAAV" response is seen – or is it? (b). The same EGM is shown above. This time the arrows show each pacing spike delivered in the ventricle and the corresponding atrial signal. The last entrained atrial event (A) event is late and closer to the next ventricular sensed event(VS) because of retrograde conduction up the slow pathway. This in reality is a VAV sequence and not a VAAV response!


**Table 1.** Classification of Device Intracardiac EGMs

The tachycardias detected by the ICD therefore can be broadly classified as either 1:1 (A = V) tachycardias or N:1 tachycardias (ie. A > V).

(b)

**Figure 7.** a). This SVT was determined to be an atypical AVNRT using a fast-slow re-entrant substrate. This was inap‐ propriately detected and ATP was delivered by the device. On cessation of pacing a "VAAV" response is seen – or is it? (b). The same EGM is shown above. This time the arrows show each pacing spike delivered in the ventricle and the corresponding atrial signal. The last entrained atrial event (A) event is late and closer to the next ventricular sensed event(VS) because of retrograde conduction up the slow pathway. This in reality is a VAV sequence and not a VAAV

response!

168 Cardiac Defibrillation

**Table 1.** Classification of Device Intracardiac EGMs

The 1:1 tachycardias are difficult for both device algorithms as well as the observer to resolve as either retrograde VA conduction during VT as opposed to a SVT (table 1). However, 1:1 VA conduction only occurs in about 10% of detected VTs and therefore has a low probability [13, 14].However, the consequences of misdiagnosis of VT is much greater than the misdiagnosis of SVT. If this is misclassified as an SVT, then the result may well be a withholding of appro‐ priate device therapies which is best avoided.

It seems rather intuitive to distinguish N:1 tachycardias as either AF, A flutter or AT if there are accompanying atrial EGMs on which to base this interpretation. Single chamber ICDs are frequently implanted if the indication is a primary prevention strategy or in the presence of persistent atrial tachycardias so that the atrial signals are lacking in these device EGMs. This then does not permit the use of algorithms that rely on A and V patterns of association in order to discriminate the rhythm. The result may well be inappropriate therapies in the form of ATP and/or shocks. It also makes it difficult for the device specialist when it comes to analysing the tracings [11].

In the example in figure 8, it is evident that the underlying arrhythmia is rapidly conducted AF. In the following clinical scenario, a single chamber ICD (figure 9), it is difficult to be certain that this is indeed an episode of VT. It may well be an organized atrial tachycardia or an episode of paroxysmal AF which is pseudo regularised and detected in tachycardia zone. Antitachycardia pacing is elicited as it is the first line therapy programmed in this detection zone of the device. It was deemed ineffective and therefore there was escalation to a shock. After each burst of ATP, a pause (arrows) is evident before resumption of the tachycardia. We postulated that the pause duration may help predict the chamber of origin of the tachycardia, namely VT vs AT/AF [15].

**Figure 8.** An interval plot showing uncontrolled atrial fibrillation detected in the VF zone of the ICD leading to ATP in the form of a ramp and then progression to sequential inappropriate shocks.

**Figure 9.** A treated episode in a single chamber ICD detected as VT but may also be a regular SVT with rapid onset. The absence of an atrial EGM makes it difficult to be absolutely certain.
