Upper vulnerability limit
Depending on the amplitude and timing of the shock delivered, the effect may be opposite: a high-amplitude shock (30 to 40 Joules) delivered in the neutral period (synchronized with the R wave) is likely to reduce a ventricular arrhythmia, whereas a low-amplitude shock (of the order of a Joule) delivered in the vulnerable ventricular period (peak of the T wave) may induce a polymorphic ventricular arrhythmia. There is a linear relationship between these 2 opposite effects, a hypothesis that has been validated in various animal models and human studies. In fact, there is a value directly related to the “defibrillation threshold” above which a shock does not induce arrhythmia. It is therefore possible to assess the defibrillation threshold indirectly by measuring the upper vulnerability value, which is the lowest value of energy delivered during the vulnerable period that does not induce arrhythmia.
The superior vulnerability value in clinical practice
First step: determine the coupling of electric shocks: a rapid pacing burst (between 120 and 150 beats/minute) is delivered, with measurement on a multi-lead electrocardiogram of the delay between pacing artifact and T-wave peak (vulnerable period).
Second step: determine the upper value of vulnerability: 3 to 4 high-amplitude shocks are delivered with different couplings (variations of 20 ms around the peak of the T wave); if no ventricular arrhythmia is induced, the amplitude delivered is progressively reduced (3 to 4 shocks of variable coupling for each amplitude) until ventricular arrhythmia is induced. The last value without induction corresponds to the upper limit of vulnerability. This probabilistic value is closely linked to the defibrillation threshold. In other words, if a 20 Joule shock delivered in the vulnerable period does not induce arrhythmia, it is likely that a shock of the same amplitude (20 Joules) delivered in the neutral period would reduce ventricular fibrillation.
Determining a safety margin: it is possible to deliver 3 to 4 shocks with different couplings at a given amplitude (e.g. 20 Joules), and if no arrhythmia is induced, this value is above the upper limit of vulnerability, suggesting a sufficient margin in terms of defibrillation for the device’s maximum capabilities.
Advantages and limitations
As explained above, the main advantage of this type of procedure is to confirm the existence of a safety margin without inducing ventricular arrhythmia, limiting the risks associated with cardiac arrest (non-reducible VF, cerebral and myocardial ischemia, electromechanical dissociation) observed during a traditional defibrillation threshold assessment procedure which begins with the induction of VF. What’s more, the reproducibility of upper limit of vulnerability assessment appears to be higher than that of defibrillation threshold assessment.
There are, however, certain limitations. The upper vulnerability value only provides an indirect assessment of the defibrillation threshold, and does not provide information on the quality of VF detection if no arrhythmia is induced. This test is therefore recommended only if detection in sinus rhythm is correct (R wave > 5 mV). What’s more, the “window” of vulnerability is narrow, and delivering a single shock without changing the coupling may lead to an overestimation of the upper limit of vulnerability if the shock has not been delivered in the most vulnerable period. It is therefore necessary to deliver 3 to 4 shocks for each amplitude, varying the coupling in 20 ms steps. This procedure reduces the risks associated with inducing a VF, but does not reduce the risks associated with shocks, 3 to 4 shocks being the minimum required.
