8/3/2023 0 Comments Time out bostonThe programming of the shock vector depends on the number of high-voltage electrodes available. The shock waveform is necessarily biphasic and is not programmable with the latest Boston Science defibrillators. The second phase returns the membrane potential as close as possible to zero to prevent the re-induction of a tachycardia or of VF. The first phase of a biphasic shock is equivalent to that of a monophasic shock with a lesser critical mass. In a biphasic shock, the current is initially delivered in one direction, then, after a fixed amount of energy has been delivered, the current direction is reversed. Shock waveform : Initially monophasic, the shock waveform of state-of-the-art defibrillators is now biphasic, lowering the defibrillation threshold. Various characteristics of the shock waveform, shock vector, shock amplitude and number of shocks delivered determine the success of defibrillation and may or may not be programmable. The delivered energy can be increased by increasing either the capacitance or the voltage. The stored energy subsequently delivered by a defibrillator is expressed by the formula: A shock of excessive amplitude may injure the myocardium. Past a certain value, the risk of re-inducing an arrhythmia increases as well, thereby limiting the chances of therapeutic success. The probability of arrhythmia termination increases thereafter along an exponential curve as a function of the amplitude of the shock delivered, when synchronized with the R wave. The upper limit of vulnerability, a value correlated with the defibrillation threshold, is the lowest energy delivered in the ventricular vulnerable period that does not trigger VF. Weak energies, on the order of 1 J, delivered in the vulnerable period may induce an arrhythmia. The effects of an electrical shock vary as a function of the energy delivered. In practice, a Boston Science defibrillator systematically attempts to synchronize the shock to an R wave, including in the VF zone. In the VF zone, synchronization of the shock may not be possible because of instability of the ventricular electrograms. Cardioversion, which consists of delivering a low-energy shock synchronized to the upstroke of the R wave of an EGM, is distinguished from defibrillation, which consists of delivering a non-synchronized, high-energy shock. Defibrillation was historically conceived to terminate life-threatening ventricular tachyarrhythmias with an electrical shock.
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