The graph shows a sinus rhythm followed by rapid ventricular pacing (8 stimuli at 150 beats/minute); the low-amplitude electrical shock delivered on the T wave cannot be visualized on the graph; induction of a probable rapid ventricular arrhythmia detected in the VF zone; highlighting of a few intervals falling outside the VF zone suggesting ventricular undersensing; detection of VF followed by electrical shock of 25 Joules terminating the arrhythmia.
This tracing allows detailing the functional operation of the VF counter in MedtronicTM devices. It is a probabilistic counter (fast X/Y intervals) which requires a minimum of 75% fast intervals (programming possibilities: 9/12, 12/16, 18/24, 24/32, 30/40, etc). A ventricular fibrillation is, by definition, an arrhythmia that is fast, disorganized, chaotic, with ventricular signals of low and/or variable amplitude. All of these characteristics increase the risk of undersensing with occasional signals with amplitude below the detection threshold (nominal value of 0.3 mV) and often a high variability in beat-to-beat amplitude, which is problematic given that, the device uses a sensitivity level which adapts to the amplitude of the preceding signal. This 75% ratio between fast and slow intervals was chosen so as to obtain an optimal balance between the proper detection of ventricular fibrillation (necessary tolerance for a certain number of long pseudo-intervals generated by undersensing) and the need to not fill the counters in the presence of an oversensing of T, P or R waves (frequently associated with a 50% ratio of short intervals.
