Discrimination in a single-chamber device
Patient implanted with a single-chamber ICD (Evera XT VR) for ischemic cardiomyopathy; this tracing illustrates the new guidelines which recommends a simple chamber discrimination based solely on morphology.
On the graph, there is a relatively irregular tachycardia detected in the VT zone and treated by a burst with termination of the arrhythmia.
- the EGM shows a tachycardia with a sudden onset;
- relatively unstable rhythm, the stability criterion being measured at 80 ms (80 ms difference between an interval and one of the previous 3 intervals); there is no resetting of the VT counter, the stability criterion not being programmed to ON;
- the VT counter is filled, a burst is delivered with termination of the arrhythmia;
- the morphology of the QRS complexes is objectively different from that of the tachycardia;
- in the text, it can be seen that the 8 analyzed complexes have a different morphology relative to the reference template with a lower percentage than the programmed threshold value (similarity at 70%);
- the sudden onset and stability parameters are programmed to OFF, the discrimination being therefore based solely on the Wavelet analysis.
The new guidelines for the optimal programming of implantable defibrillators have two key messages in terms of single-chamber discrimination:
- the discrimination parameter(s) must be programmed (except for patients with a complete AV block) up to high rates (limit of at least 230 bpm) in order to reduce the risk of inappropriate therapies (Class I indication). Indeed, rapid AF episodes with a rate greater than 200 beats per minute were observed in a certain number of patients; conversely, the probability that a tachycardia above 230 beats per minute is of supraventricular origin is extremely low with, in addition, a major risk if a discrimination error occurs for such a rapid arrhythmia compromising patient survival. It is important to remember that the application limit of Wavelet is not dependent on the programming of the detection zones (VT zone or VF zone) and can be applied partly in the VF zone. In contrast, the criteria for sudden onset and stability are limited to the VT zone or FVT via VT zone. The SVT limit must therefore be programmed at 230 beats/minute. When the median duration of the last 12 consecutive sensed intervals is less than this programmed limit, discrimination no longer applies;
- for a MedtronicTM single-chamber ICD, it is recommended to only program Wavelet to ON, with the Sudden onset and Stability criteria being programmed to OFF. When all three parameters are programmed to ON, all 3 must be verified for the arrhythmia to be diagnosed as VT and treated (sudden onset, stable rhythm with a different morphology compared to the reference). This type of programming would have led to a discrimination error in this patient; the ventricular arrhythmia being relatively irregular, the stability criterion would have inhibited the therapies. When the functioning of the morphology algorithm is optimal, as in this patient, it may therefore be advisable to base the discrimination solely on this parameter, the addition of the other two parameters potentially decreasing the sensitivity of the device.