In a VT zone >140 bpm, shocks are often programmed in case of unsuccessful ATP. The first shock can be programmed as 1) a 5 to 10 J cardioversion or 2) a maximal output cardioversion to maximize the likelihood of success of a single shock.
The davantages of programming a low amplitude first shocks are: 1) a reduced consumption which may be important in terms of battery drain when the patient presents many episodes of VT requirring a shock 2) a shorter charge duration: which is much less determinant than in the VF zone in terms of risk of syncope since the shock is being delivered after sometimes more than 1 minute corresponding to the different ATP sequences 3) the detrimental impact of the electrical shocks on morbi-mortality of implanted patients has been demonstrated; therefore it may be recommanded to decrease the amplitude of the delivered therapies when possible.
In contrast, the advantages of programming a maximal output first shock are: 1) to increase the likelihood of VT termination at first attempt and to minimize the number of shocks delivered 2) to be above the upper limit of ventricular vulnerability in the atrias and in the ventricles and therefore to decrease the risk of transforming an organized VT to potentially lethal VF or to unduce atrial fibrillation 3) to increase the likelihood of converting atrial fibrillation, should the shock be inappropriate.
The question of the pain and discomfort associated with the amplitude of the shocks is debatable. It has been suggested that the first shock, in the VT zone, may be arbitrarily programmed at low strength to terminate the tachycardia, while limiting the pain it causes. However, it is clear that a second shock during a same episode is much more painful than the first one and therefore the priority of programming may be to increase at maximum the capacity of reducing the arrhythmias in a first attempt (with a single shock of maximal output).