**2. Cycle length / heart rate**

The cycle length (CL) or heart rate (HR) is a fundament in the detection of tachycardia in ICD patients. A sustained ventricular HR in adults >250 bpm or a CL <250 ms is very specif‐ ic for fast VT or VF (figure 1 red zone). However, artificial sensing of external impulses (noise), sensing caused by lead dysfunction (cluster) or inappropriate recognition of signals (T-wave-oversensing) present diagnostic challenges even in this HR zone. Below this HR al‐ most every type of cardiac arrhythmia should be considered as well for differential diagno‐ sis. The cycle length of both VTs and SVTs are influenced by most antiarrhythmic drugs. Therefore programming of the ICD should be accordingly adjusted.
