**10. Morphology criteria**

Morphology and width of QRS complexes is a primary tool for physicians to differentiate SVT from VT (table 3 and 4). Accordingly, almost all manufacturers have developed QRS complex morphology algorithms. The device compare 8 or more voltage points on the EGM signals at different time points (figure 13). A point to point assessment of each complex to a defined standard complex (template) follows. Finally, decision is made based on a previous‐ ly programmed matching-ratio in percentage (figure 14). Currently, programming of this criterion is recommended in single chamber devices and in dual chamber devices in case of V<A and V=A. The algorithms use either EGM only or a combination of EGM and a can to coil lead (figure 15). In case of no intrinsic activation like third degree atrioventricular block or continuous biventricular pacing the automatic storing algorithm of QRS complex tem‐ plate may be problematic or even not possible. In the latter case QRS template should be stored manually during pacing inhibition. In case of HR dependent bundle branch block this algorithm may also fail to differentiate tachycardias.

**Figure 14.** Demonstration of sensitivity and specify of the Medtronic Wavelet™ algorithm to differentiate between

Tachycardia Discrimination Algorithms in ICDs

http://dx.doi.org/10.5772/52657

149

**Figure 15.** Demonstration of the Rhythm ID™ discrimination algorithm: discrimination between sinus rhythm and SVT with "normal" vector (green) to a potential VT vector (red). This algorithm is not based on EGM signal but on an inter‐

In the programming of single chamber ICD the three most important discrimination criteria are stability, morphology and sudden onset. However, it should be defined whether a single criterion or only matching of all three criteria should rule out a VT. Currently, if all three criteria are active, it is recommended to rule out VT in case of 2 out of 3 votes for SVT. If

**11. SVT/VT discrimination algorithms in single chamber devices**

nal ECG electrode from device can to RV shock coil and the vena cava shock coil.

SVT and VT by Klein et al [24]. As recommendation a value of 70% match is standard by Medtronic devices.

**Figure 13.** Demonstration of morphology score calculation: match or not match based on percentage template match threshold measurement by St. Jude Medical window detection algorithm. In this case of 6 matches from 8 the algorithm votes for SVT [75%).

therapy only within this specified time interval, when the time runs out, the device delivers VT therapy (SVT time out). Also in case of an episode identified as VT the device may switch to the VF therapy after a specified time interval (VT time out). Generally these coun‐ ters are not recommended because SVTs usually continue for longer time periods and such timer could force an ATP or shock delivery inappropriately. However, in individual cases (e.g. in patients with very low ejection fraction who could not tolerate higher HR for a lon‐

Morphology and width of QRS complexes is a primary tool for physicians to differentiate SVT from VT (table 3 and 4). Accordingly, almost all manufacturers have developed QRS complex morphology algorithms. The device compare 8 or more voltage points on the EGM signals at different time points (figure 13). A point to point assessment of each complex to a defined standard complex (template) follows. Finally, decision is made based on a previous‐ ly programmed matching-ratio in percentage (figure 14). Currently, programming of this criterion is recommended in single chamber devices and in dual chamber devices in case of V<A and V=A. The algorithms use either EGM only or a combination of EGM and a can to coil lead (figure 15). In case of no intrinsic activation like third degree atrioventricular block or continuous biventricular pacing the automatic storing algorithm of QRS complex tem‐ plate may be problematic or even not possible. In the latter case QRS template should be stored manually during pacing inhibition. In case of HR dependent bundle branch block this

**Figure 13.** Demonstration of morphology score calculation: match or not match based on percentage template match threshold measurement by St. Jude Medical window detection algorithm. In this case of 6 matches from 8 the

ger time) programming a time out intervals may be considered for safety reasons.

**10. Morphology criteria**

148 Cardiac Defibrillation

algorithm votes for SVT [75%).

algorithm may also fail to differentiate tachycardias.

**Figure 14.** Demonstration of sensitivity and specify of the Medtronic Wavelet™ algorithm to differentiate between SVT and VT by Klein et al [24]. As recommendation a value of 70% match is standard by Medtronic devices.

**Figure 15.** Demonstration of the Rhythm ID™ discrimination algorithm: discrimination between sinus rhythm and SVT with "normal" vector (green) to a potential VT vector (red). This algorithm is not based on EGM signal but on an inter‐ nal ECG electrode from device can to RV shock coil and the vena cava shock coil.

## **11. SVT/VT discrimination algorithms in single chamber devices**

In the programming of single chamber ICD the three most important discrimination criteria are stability, morphology and sudden onset. However, it should be defined whether a single criterion or only matching of all three criteria should rule out a VT. Currently, if all three criteria are active, it is recommended to rule out VT in case of 2 out of 3 votes for SVT. If only two criteria are active (standard), stability and morphology are recommended, and 1 out 2 votes is required for the diagnosis of SVT. Anyway, differentiation between SVT and VT in single as well as dual chamber ICD remains difficult and should be carefully checked by the physician at each follow-up visit (figure 16a-b).

**12. SVT/VT discrimination algorithms in dual chamber devices**

algorithm more orientated to CL stability called PARAD(+)™[9].

Detection and diagnose of arrhythmias in dual chamber devices is more complex, and the mechanism of decision making is harder to demonstrate. Discrimination algorithms are still based on the principles explained above. Significant addition is the comparison of atrial (A) and ventricular (V) frequency (figure 17). In case of V>A VT therapy is initiated directly. For cases with V=A or V<A further discrimination algorithms are used to differentiate between SVT and VT. For V<A programming of morphology and stability criteria is recommended. For V=A programming of morphology criterion may be sufficient. In addition for V<A episodes measurement of the A-V-A intervals may differentiate SVTs with 2:1 activation from VTs with VA dissociation during AF/Aflut. Measurement of A-V-A intervals by St. Jude Medical devi‐ ces (AV Detection Enhance™) is illustrated in figure 18a-b. This algorithm counts the last 12 AV intervals and calculates the difference between the second longest and second shortest AV interval; difference < 40ms suggests association between A and V and decides for SVT. A com‐ parable algorithm based on pattern recognition typical for Aflut or SVT with 1:1 AV-conduc‐ tion is used by Medtronic called PR-Logic™ and by Biotronik called SMART™. Sorin use an

Tachycardia Discrimination Algorithms in ICDs

http://dx.doi.org/10.5772/52657

151

**Figure 17.** The rate branch differentiates between V<A, V=A and V>A. For VA dissociation a VT will therapies by ICD directly. In case of V=A a sinus tachycardia or other 1:1 SVT should inhibited by ICD and in case of retrograde activa‐ tion of a VT the ICD should delivered therapy. For V<A and AF/AFl therapy should inhibited and V<A with VT and cur‐

rent AF/AFl therapy should delivered.

(b)

**Figure 16.** a: This episode of a dual chamber tachycardia detection (Atlas DR St. Jude Medical) with A>V during AF with a CL around 100ms demonstrates the hardly difficult decision-making in spite of all common discrimination algo‐ rithms. The ventricular EGM shows a fast stable tachycardia with a CL of around 300ms and sudden onset as well as nearly same EGM signal like intrinsic activation before. This tachycardia falls in VF zone and the morphology criteria founds a match to intrinsic activation of stored template. Although morphology votes for SVT, stability outvote for VT <40ms (sudden onset not active), but anyway the CL falls in VF zone in this case without active SVT discrimination algorithm the device detected this episode as VF and start charging shock therapy (\*). b: The same episode stopped spontaneously without any therapy and the shock is aborted by the device. The example underlined the importance of long detection intervals (TDI 18-24 not 12 like in this case), the need of competent discrimination algorithm also in short CL under 300ms and the difficult interpretation of an episode not only by the device but also by physicians in decision making SVT or VT.
