Depending on the amplitude of the shock delivered and of its timing in the cardiac cycle, the effect can be opposite: a strong shock (30 Joules) delivered during a neutral period (synchronized to the R-wave) is likely to terminate an arrhythmia, whereas a low-amplitude shock (1 Joule) delivered during the ventricular vulnerable period (peak of the T wave) is likely to trigger an arrhythmia. There is a direct and strong relationship between these opposite shock effects. This hypothesis has been validated in different animal and human studies. A value directly linked to the “defibrillation threshold” exists above which a shock does not induce an arrhythmia. Consequently, an alternate defibrillation test consists of measuring the upper limit of vulnerability, which is the lowest energy delivered during the ventricular vulnerable period that does not trigger VF.
Upper limit of vulnerability in practise
Determination of the shocking coupling interval: during pacing at high rate (400 to 500 ms pacing cycle lengths), measurement of the interval between the pacing spike and the peak of the T wave in multiple leads ECG.
Determination of the upper limit of vulnerability: deliver 3 to 4 shocks with different coupling (varying in 20 ms steps around the peak of the T wave) at high energy and if VF is not induced, progressively step down the energy until VF is induced; the last value that does not induce VF, defines the upper limit of vulnerability. This probabilistic value is closely correlated with the defibrillation threshold. For example, if a 20-J shock delivered during the vulnerable period does not induce an arrhythmia, the probability is high that a shock of same amplitude will terminate a ventricular arrhythmia.
Determination of a safety margin: deliver 3 to 4 shocks with different coupling at a given energy (usually between 15 and 20 Joules) and if VF is not induced, this shock strength is superior to the upper limit of vulnerability suggesting an adequate safety margin.
Advantages and limits
As explained before, the major advantage is the possibility to confirm a safety margin without inducing VF and circulatory arrest, limiting the risks associated with the conventional procedures of defibrillation threshold testing (intractable VF, cerebral and myocardial ischemia, electromechanical dissociation). The reproducibility of the upper limit of vulnerability (same field delivered during the same pattern of repolarization) is also probably higher than the defibrillation threshold (same field delivered during changing and variable patterns of repolarization).
However, the upper limit of vulnerability is an indirect measure and does not provide information with regard to the ability of the device to detect VF. This test should therefore only be performed when sensing during sinus rhythm is correct (> 5 mV) predicting a high probability of reliable sensing during VF. Moreover, the peak of the vulnerable zone is narrow and delivery of a single T-wave shock without changing the coupling interval may lead to the underestimation of the upper limit of vulnerability if the shock is not delivered during the most vulnerable period. Optimally, three to four shocks should be performed changing the coupling intervals (varying in 20 ms steps relative to the peak of the T wave). Therefore, vulnerability testing may reduce the risks related to VF but does not reduce or even increases the risks associated with the shocks (a minimum of 3 to 4 shocks being required).