There are a number of variables that influence the outcome of a cardioversion and/or defibrillation attempt. These can be grouped as patient characteristics such as body habitus, device characteristics including paddle size, waveform morphology and iatrogenic factors including administration of medications and ventilator support.
Electric shocks used in cardioversion and defibrillation are quantified by the amount of energy delivered. While this allows for the standardization of shocks delivered, it is important to understand that the determinant of an adequate shock is not the energy itself but the amount of electrical current that travels across the heart depolarizing the myocardium. The transmyocardial current generated is dependent directly on the energy level set and inversely related to the resistance/impedance offered by the circuit.
This resistance, termed as thoracic impedance, is determined by the electrode-to-skin interface, electrode pressure, body habitus and the phase of ventilation. Decreasing the interface between the skin and the paddle by placing more pressure on the paddles, applying adhesive or more conductive gel as well as delivering shocks during expiration decreases thoracic impedance and increases the effectiveness of cardioversion and defibrillation. Hairs should also be shaved off the chest if necessary to facilitate attachment of electrode pads to the skin.
Pad size is also an important determinant of transthoracic flow during delivery of shocks. A paddle or pad size with larger surface area has been associated with less thoracic resistance and less chances of myocardial injury.10 A standard adult electrode pad size usually measures about 8 to 12 cm and is commercially packaged and available for single use.
Observational studies have shown that persistent AF may be more easily converted using a hand-held paddle and the improved electrode-to-skin contact and reduced thoracic impedance are likely contributing to the higher success rate of cardioversion.11 However, there is no current data comparing the use of hand-held paddles and self-adhesive pad electrodes for other arrhythmias requiring cardioversion or defibrillation. Therefore, the decision to use which type of electrodes should base on equipment availability and the operator’s opinion regarding which electrodes are more likely to be effective in a particular patient.
There has been ongoing debate about the relative impact of the positioning of the electrodes on the outcome of the cardioversion attempt. An initial study from Germany demonstrated a statistically significant difference in the successful cardioversion of AF with anteroposterior positioning of the electrodes (96%) as compared to anterolateral position (78%).13 However subsequent studies have not demonstrated this benefit in a consistent manner.14 In the ICU it is often difficult to position patients for placement of posterior pads often resulting in delays in the delivery of shocks. Consequently we do not recommend a particular position for electrode placement over another.
A final point should be made about the use of antiarrhythmic drugs prior to attempted cardioversion. While evidence is limited, in patients who have been pretreated with amiodarone, ibutilide, propafenone or sotalol, the restoration of a sinus rhythm from AF required less electrical energy, fewer attempts and lower number of recurrences.15,16,17,18 Further studies however are required to determine if these findings are representative within the ICU population.