The programming of this parameter depends on the number of shocking electrodes available. The defibrillation shock is delivered via a dedicated lead, which may be a single coil (one defibrillation electrode or coil placed inside the right ventricle), or a dual coil (one distal defibrillation electrode placed inside the right ventricle, and one more proximal defibrillation electrode placed in the superior vena cava) lead. Single coil shocks are delivered between the distal coil of the right ventricular lead and the coil, while dual coil shocks are delivered between 1) the distal coil, 2) the proximal coil and 3) the pulse generator. The anode is usually included between the 2 shocking electrodes and the cathode is the pulse generator. With a dual coil electrode, the shock vector can be changed by including or excluding (single coil shock) the proximal electrode in the superior vena cava. In presence of a high defibrillation threshold, the superior vena cava electrode can be excluded in case it is floating in the low right atrium, where some of the delivered energy is wasted.
The nominal polarity of Abbott ICD is anodal. In any given patient, the shock polarity is programmed either anodal or cathodal; the polarity cannot be changed during the delivery of a series of shocks. A high defibrillation threshold might be lowered by programming the right ventricular electrode as the anode in the first phase of a biphasic shock. Therefore, in presence of a high defibrillation threshold with an anodal polarity, the programming of a cathodal polarity is discouraged.
Tilt pulse duration
At a fixed tilt (nominally 65% for each of the 2 phases), the shock is interrupted when the residual capacitor voltage has reached a fixed percentage. The measured pulse duration is a function of the impedance, while the energy delivered is fixed. The tilt of both phases is the same. The optimal duration of the second phase depends on a) the duration of the first phase, b) the defibrillation impedance and c) a membrane time constant. In presence of a high defibrillation threshold, it is not advised to change the tilt (50% for each phase, for example). However, if the defibrillation threshold and impedance are both high, one can optimize the duration of both phases and reprogram a fixed pulse duration. A high impedance indicates an impediment in the transmission of current. Therefore, to deliver a same amount of energy, a long pulse duration is needed, incurring a risk of hyperpolarisation and loss of energy. Consequently, it is preferable to limit the pulse duration. Depending on the measured impedance, the ICD suggests a series of optimal durations in 3 colors: blue corresponds to a typical time constant, green to a more rapid constant, and yellow to a slower time constant.
In the VF zone, the strength of the first and subsequent shocks is usually programmed at the highest value the device is able to deliver. Programming of the defibrillation shock amplitude can be guided by the defibrillation threshold, defined as the least amount of energy that converts VF to sinus rhythm. In the VT zone, the first shock can be programmed either empirically between 5 and 10 J, sparing the battery and shortening the capacitor’s charge time, or at a higher amplitude with a view to optimize the likelihood of successful treatment of the arrhythmia.
Number of shocks
In the VF zone, the highest number of consecutive shocks is fixed at 6, limiting the risk of delivering an endless string of inappropriate shocks.
- the charge must have ended,
- the arrhythmia must have been re-confirmed by ≥6 short cycles, usually sensed during the charge unless the latter is very short,
- the event to which the shock is synchronized cannot be the event immediately following the end of the charge, explaining why the shock is often synchronized to the next cycle, and
- the mean and instantaneous sensed cycles to which the shock is synchronized cannot be sinus (no shock delivered after a long cycle).
Last generation defibrillators
Number of pulse per sequence
- the various arrhythmias recorded by the device and analyzed during the patient follow-up;
- the efficacy (termination of the episode) / adverse effect (acceleration of the tachyarrhythmia) ratio associated with an ATP sequence.