**7. Stability**

**6. Discrimination SVT/VT**

144 Cardiac Defibrillation

single or dual chamber ICDs.

Sinus Interval History (\*St. Jude

Discrimination of SVT and VT may present a challenge not only for devices using artificial intelligence and programmed algorithms but even for experienced physicians. Principally the following wide QRS complex arrhythmias should be considered: VT, SVT with bundle branch block, SVT with accessory pathway or pacing in left (via coronary sinus) or right ventricle. Table 3 shows the discrimination criteria of the 12 channel ECG for SVT/VT. No single criterion is sensitive and specific enough to provide sufficient discrimination value. Therefore, combination of several criteria should be implemented in the device algorithms to correctly diagnose arrhythmias. Table 4 shows the availability of discrimination criteria in

> **ECG criteria sensitivity specificity** QRS width high low VA dissociation low high capture beats low high northwest axis low high rS missed or long rS in V1 low medium concordance +/- low high

Brugada criteria LBBB/RBBB[22] medium high

**Table 3.** Sensitivity (in sense of frequency) and specificity of different ECG criteria for differentiation SVT and VT.

CL or heart rate + + + stability + + + Sudden onset + (Holter ECG) + + morphology + + + AV rate branch - - + QRS axis (\*Rhythm ID™ Boston) + +(\*) +(\*) AVA interval - - +

Medical) - +(\*) -

Capture beats (\*PR Logic™Medtronic) + - +(\*) RBBB and LBBB criterias + - -

**Table 4.** Variation of SVT and VT discrimination criteria in single and dual chamber ICDs (+ achievable; - not achievable).

**criteria ECG Single chamber Dual chamber**

Stability is the variability of tachycardia CL (figure 9). In general, VTs have a reasonably stable CL while numerous SVTs have beat-to-beat variability in their CL (AF, Sinus tachycardia, etc). However, some SVTs may have stable CL as well: circus movement tachycardia, atrioventricu‐ lar nodal re-entry tachycardia or atrial flutter (Aflut) – so this criterion has his limitations. Even AF may have a quite stable CL in the case of very high frequency. Anyway, several random‐ ized controlled studies {e.g. MADIT II [7]} proved a significant decrease (p=0.030) of inappro‐ priate ICD therapies (table 5). Therefore, programming of stability criterion is recommended in single chamber devices up to 260ms CL and indual chamber devices in the case V <A. The programmed value depends on the manufacturer, and lies in general for single and dual cham‐ ber devices around 40ms (-20 to + 20ms) [8]. Boston and St. Jude Medical use the last 12 consec‐ utive intervals and compare the second longest and the shortest interval to calculate the difference in ms (or percentage ratio). Medtronic and Biotronik use the last 3 and 4 consecu‐ tive intervals to calculate mean difference.

**Figure 9.** Notice variability of CL in ms (in red numbers) from beat to beat in normal sinus rhythm, monomorphic ven‐ tricular tachycardia and atrial fibrillation AF.
