The additional delay in setting up the ECG to provide accurate synchronisation has been the main reason for advocating this approach, although many current defibrillators allow accurate synchronisation via just the adhesive defibrillator pads.
The aim of this study was to investigate whether the timing of defibrillatory shocks in rapid VT-affected resuscitation outcome.
The timings of the shocks relative to the QRS complex were used to define whether each shock was acting as a 'synchronised' or 'unsynchronised' shock.
The study was a retrospective review of all diagnostic electrophysiological studies performed at Papworth Hospital. A total of 271 studies for ventricular arrhythmias were identified, with 144 studies resulting in stable monomorphic VT being induced.
Of these VT episodes, 40 stopped spontaneously, 61 cases were terminated with anti-tachycardia pacing, 1 required cardioversion for slow but incessant VT and 42 required defibrillation for severe haemodynamic compromise/cardiac arrest.
The electronic recordings of the defibrillation episodes were analysed to investigate the effects of shock timing on outcome.
Of the 42 patients who required defibrillation, 30 had shocks delivered within a 100 ms window of the peak of the QRS complex.
Of these, 28 patients converted to a perfusing rhythm and 2 patients deteriorated from VT to VF as a result of the defibrillation shock.
The remaining 12 patients received shocks outside this window, with 5 converting to a perfusing rhythm and 7 deteriorating to VF. Defibrillator shocks within the QRS complex had a success rate of 93% compared to a success rate of 42% for outside the QRS complex (p=0.0016 two-tailed Fishers' exact test, odds ratio=19.6, 95% limits=3.1-123.1). There was no significant effect of age or sex of the patient, the underlying heart disease, rate of VT or anti-arrhythmic medication on the outcome, although the number of patients was too small to definitively exclude this. Therefore, defibrillation shocks delivered shortly after the peak of the QRS complex in rapid VT do appear to offer significant advantages over defibrillation shocks at other parts of the cardiac cycle for very rapid ventricular tachycardia.