Defibrillation threshold and upper limit of vulnerability

• The arrhythmia episode in this patient is extremely worrying because it was only terminated by the fifth maximum output shock, the penultimate therapy available; in fact in Medtronic defibrillators, the maximum number of shocks for a single episode is limited to 6; the probability of a successful shock after 6 maximum unsuccessful attempts is low; conversely, it is preferable to limit the number of shocks if the therapies are inappropriate.
• This case illustrates one of the main problems encountered in patients implanted with a defibrillator: patients with a high defibrillation threshold are also those in whom an appropriate or inappropriate shock is most likely to induce a polymorphic ventricular arrhythmia that may compromise the patient’s survival (with a significant risk that subsequent shocks may be ineffective...).
• There is a direct relationship between the amount of energy required to induce ventricular fibrillation and the energy required to terminate it (concept of upper limit of vulnerability); in this patient, the first shock degrades a monomorphic, organised arrhythmia into a polymorphic, chaotic arrhythmia, suggesting the existence of a high upper limit of vulnerability; the shock did not capture a sufficient quantity of ventricular myocardium, but instead created sufficiently pro-arrhythmogenic myocardial heterogeneity to generate multiple reentry circuits; it is therefore not surprising that the subsequent delivery of the same amount of energy does not terminate the VF episode induced by the first shock and that several shocks are necessary (high defibrillation threshold).
• Therefore this tracing highlights 2 important points: 1) a shock from the device usually terminates episodes of ventricular arrhythmia and is the basic treatment of the defibrillator; however, a shock can sometimes be pro-arrhythmogenic and, as in this patient, transform an organised VT into a potentially lethal VF; 2) the defibrillation threshold is not a fixed value; in this case, several shocks at maximum energy were ineffective whereas the fifth shock of equal energy averted a dangerous situation.
• This patient’s safety was compromised with few reprogramming options available; a coil positioned in the coronary sinus was added; a shock delivered between two coils in contact with, or in the immediate vicinity of the myocardium is more likely to be effective by extending the electric field induced over a larger volume of myocardium.

What therapy is delivered by the device?

• The episode is detected in the VF zone; when the initial VF counter is filled, a burst during charging is delivered.

What does the «abandon» message mean?

• Charging of capacitors began as soon as the VF counter was filled; the burst was effective with termination of the arrhythmia; after 4 consecutive paced ventricular cycles, the criterion for confirming persistence of the arrhythmia is not met (criterion of 2/5 rapid cycles) and charging is interrupted (therapy abandoned).

Key messages