position. The need for an adequacy of anticoagulation should be considered in any patient undergoing cardioversion; this issue is explored in Chapter 6.
Just as a biphasic waveform markedly improved the ventricular defibrillation efficacy of implantable cardioverter defibrillators (ICD), biphasic cardioversion is more effective at restoring sinus rhythm than monophasic waveforms. In a randomized trial, Mittal and colleagues found that rectilinear biphasic shocks restored sinus rhythm in 94% of patients, compared to 79% of patients receiving damped sine-wave monophasic shocks (65). This increased efficacy was noted even though the maximum biphasic shock was 170 J, compared with a maximum monophasic shock of 360 J.
Internal cardioversion refers to a high-energy discharge between an intracardiac electrode in the right atrium (cathode) and a skin electrode (anode). Internal cardioversion allows greater energy delivery directly to the atrial myocardium. Initial trials of internal atrial defibrillation were performed by discharging high-energy shocks (up to 360 J) between a relatively small surface area electrode in the right atrium and a skinpatch electrode in patients refractory to external cardioversion (66,67). A randomized trial of 112 patients who had at least one prior failed cardioversion revealed a higher success rate with high-energy internal cardioversion than external cardioversion (62). However, the utility of high-energy internal cardioversion was limited by the relatively high rate of complete heart block, and the need for deep sedation to block the pain associated with the procedure.
Many subsequent studies have reported success with low-energy intracardic cardioversion (<10 J) using high-surface-area electrode catheters placed in the right atrial appendage and either the CS or left pulmonary artery (68-71). These catheter configurations allow the best current distribution between the two atria (68).
Patients who are refractory to high-energy external cardioversion can be defibrillated with a couple of J internally using these electrode configurations (68). Although internal defibrillation with a couple of joules is perceived to be painful by most patients (68,71), mild—rather than deep—sedation is all that is required. There is a wide range of variability in pain perception of internal shocks, with a general increase in pain awareness as energy increases (71). In general, patients with chronic AF or AF of longer duration require higher energies for cardioversion compared to patients with paroxysmal or short durations of AF (71). This probably reflects another adverse remodeling effect caused by long durations of AF.
The present techniques for low-energy internal cardioversion require placement of relatively stiff electrophysiology catheters with multiple electrodes and cannulation of either the CS or left pulmonary artery, thus restricting this technique to trained interventionalists. However, soft balloon-flotation pulmonary-artery catheters with defibrillation electrode elements on the distal and proximal (right atrial) portion of the catheter are under clinical evaluation at this time, and should allow more widespread application of internal cardioversion (72) (Fig. 4). In the initial experience with the ALERT catheter (EP Medical, Inc., Budd Lake, NJ), sinus rhythm could be restored in 25 of 27 patients with a mean energy of 6.7 ± 4.5 J. The catheter is also capable of right atrial and ventricular pacing immediately after cardioversion, either for overdrive arterial pacing for prevention of post-shock AF or ventricular pacing for bradycardia (72). Another application of internal cardioversion has been the development of temporary epicardial atrial electrodes applied to the left and right atrium at the time of cardiac
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