Interpretation

1. The diagnosis proposed by the defibrillator is VT which lasted 1 minute and 34 seconds. This episode is in the monitoring zone and therefore, no treatment was initiated, since none is programmed
2. By analysing the tachogram, we observe that the spontaneous rate had slightly accelerated prior to the onset of the tachycardia, the rhythm being As-BiV.
3. From the EGM, the tachycardia is supraventricular in nature since beginning with an atrial event. Before stabilising, the PR interval lengthens rapidly, such that the P waves fall into the post-ventricular atrial blanking of the previous QRS complex, and are no longer sensed by the defibrillator. The tachycardia terminates on a premature atrial contraction with functional anterograde AV conduction block. The rhythm has returned to Slow, As-BiV
4. The first two ventricular tachycardia cycles, noted 515 and 484 ms are outside of the Slow VT zone, but accelerate the reference coupling interval, such that the acceleration criterion is not validated
5. For the device, the first cycle to enter the Slow VT zone is the ensuing cycle, noted 429 ms, excluded from both the stability and the association analysis as well as the next cycle noted 414 ms, as usual.
6. Thereafter, the next 6 cycles, in the Slow zone, stable and dissociated since the P waves are not visible, validate the diagnosis of VT, which is wrong !
7. The diagnosis does not change until spontaneous termination of this supraventricular tachycardia

Comments

1. This is an example of erroneous discrimination, resulting from the lack of detection of supraventricular tachycardia P waves which fall in the absolute refractory period of the defibrillator. This is the main fault of this defibrillator when the next P wave falls inside 75ms after the previous R wave! Indeed, the algorithm reveals the absence of AV association.
2. In this example, if a therapy had been programmed, it would have been triggered at the end of the programmed persistence of 30 cycles. This therapy would have been inappropriate and potentially dangerous.
3. However, the advantage of the system as it is, is a limited risk of poor discrimination of true ventricular tachycardias in case of ventricular-atrial crosstalk that can induce a false diagnosis of supraventricular tachycardia.
4. The solution lies elsewhere, in a morphology algorithm lacking from Microport defibrillators, which could compare the morphology of current tachycardia cycles with a reference model of the morphology of the conducted QRS complexes, and regularly updated. This type of algorithm is nevertheless wrong if, in case of supraventricular tachycardia, a conduction aberration appears (especially on the right branch), thus modifying the morphology of QRS complexes, albeit of supraventricular origin.

Message:

• In case of AV dissociation, the device does not look at acceleration
• The disappearance of the P waves created a false AV dissociation.