A Comparison of Biphasic Truncated Exponential and Pulsed Waveforms for Cardioversion of Atrial Fibrillation

Background:

External defibrillators are routinely used for treatment of lethal ventricular arrhythmias (defibrillation) as well as atrial and non-lethal ventricular arrhythmias (synchronized CVS). Commercially available defibrillators generate direct current (DC) shock with various waveforms using distinct technologies. These waveforms are suggested to have different efficacy and safety, aiming at a positive effect with lower cumulative energy, smaller number of shocks, and less post-shock complications and myocardial injury. Therefore, the efficacy and safety of different waveforms has been an object of continuous interest and clinical investigations over the last years. Although the superiority of biphasic over monophasic waveforms is well established, the relative efficacy and safety of the available biphasic waveforms is not clear.

Most defibrillator manufacturers use the biphasic truncated exponential (BTE) waveform. Almost all BTE defibrillators are embedding the impedance compensation technology, which adapts the pulse duration for proper delivery of the selected energy. However, various pulse commutation concepts produce differently shaped waveforms:

• rectilinear (impedance compensation with constant pulse duration);
• standard truncated exponential (impedance compensation with constant tilt);
• pulsed (impedance compensation with constant duty cycle and constant tilt). Clinical studies with patients presenting with shockable life-threatening arrhythmias are difficult to perform for ethical and practical reasons. Several studies have therefore included patients requiring elective CVS for atrial arrhythmias, who are able to give consent and also present a more controlled population. Elective CVS is a standard treatment for patients with atrial arrhythmias and is performed as a day case procedure under general anesthesia. The investigator's bibliographic research found 6 studies from which the dose-response curves of 4 rectilinear, 7 standard truncated exponential and 3 pulsed waveforms have been drawn. Although the major disparities between the studies (success accounted at different delays after CVS, rhythm inhomogeneity, different population sizes, different selected energy levels, etc.) the dose-response curves concentrate within 85-95% success rate and 300-500 joules (J) cumulative energy. More data on the efficacy of the pulsed waveform during CVS are needed.

Various BTE waveforms apply different potential gradients on the thorax that might produce various defibrillation effects. Apart from efficacy, the most important aspect is the patient safety, considering that larger potential gradients in the myocardium lasting longer could potentially induce an electroporation and then a fibrillation. A frequently observed effect of electroporation are the post-shock ST-segment deviations in the surface ECG, representing the potential difference between the normal tissue and sustained depolarized critical mass of myocardium closest to the associated origin of the electrical current. Although ST-segment changes are an easily ignored phenomenon, occurring acutely and resolving during the first few minutes post-shock, their presence in a short-term basis can identify electroporation by dangerously high potential gradients, while the sustained ST-changes in a long-term basis can identify cases with myocardial injury. The most reliable quantification of shock-induced myocardial injuries is based on measurements of the hsTnI biomarker after CVS. The safety of shocks contemplates any other complications after CVS, such as apnea, arrhythmias, etc. The variety of safety outcomes for different BTE waveforms is not clear.