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Atrial fibrillation (AF) has a prevalence in the general population of
approximately 2%, with incidence increasing with advancing age1. This common arrhythmia is a poorly tolerated
disorder and it is becoming increasingly clear that medication often fails to prevent its recurrence or to restore
sinus rhythm. Conventional pharmacological therapy to prevent AF recurrence is only 50% effective at 6 months
and may also have life-threatening toxicity2-5. It is also known that high and irregular ventricular rate can lead
progressively to the development of tachycardiomyopathy with symptoms and signs of heart failure6. Beyond the
hemodynamic results, AF is one of the most usual causes of thromboembolic events7.
In this case the next step in eliminating the arrhythmia is electrical defibrillation, which was first demonstrated by
Lown in 19628. Using an electrical charge ranging from 100-360 Joules, the external cardioversion has a success
rate of 61-90%. Recent studies have shown that newer techniques of internal cardioversion are feasible and
effective methods for restoring sinus rhythm. Levy et al were the first to describe a method of high energy
transvenous atrial defibrillation in humans, delivering shocks between a proximal pole of a catheter in the right
atrium and a back plate and reported the relative advantages of internal defibrillation9. In patients who had had
AF for a mean of approximately 2 years, the arrhythmia was successfully converted to sinus rhythm in 91% of
patients randomly assigned to undergo internal cardioversion, compared to 67% of those assigned to external
cardioversion10. In addition to being acutely more efficacious, internal cardioversion offers a new option for
restoring sinus rhythm in patients for whom external cardioversion had failed11. It was however, already known
from experimental models of the 1980’s, that the restoration of AF is possible with low energy levels, if the
electrical field is closer to, or within, the atria12. Recent studies confirmed the efficacy and safety of low-energy
internal cardioversion in restoring sinus rhythm, with an acute success rate of 88% in patients with AF of
prolonged duration13.
In a multicenter trial including patients with paroxysmal, intermediate, chronic and induced AF the success rate
was between 75 and 94%14. The conversion voltage increased significantly with increasing left atrial size and
duration of the arrhythmia.
Critical factors related to the efficacy of internal defibrillation are the type of waveform, the electrode placement
and the time of the electrical intervention. Cooper et al systematically compared several electrode configurations
and shock waveforms in a sheep model of paroxysmal AF15. The lowest thresholds were obtained using
configurations embracing both atria. They showed that biphasic waveforms delivered between the right atrial
appendage and the coronary sinus with wavelength 3/3 ms were the most effective and cardioversion was
achieved with thresholds of 1.3±0.4 J. Later Cooper et al tested the efficacy of monophasic and biphasic
waveforms of several different durations in human studies16. They concluded that certain biphasic waveforms
were more effective than certain monophasic waveforms for internal atrial defibrillation in humans. Biphasic
waveforms with the first phase longer than the second phase appear to be more effective than biphasic
waveforms with both phases of the same duration as well as biphasic waveforms with the first phase duration
shorter than the second phase duration. The same author showed that in humans, the sequential biphasic
waveform, delivered over dual-current pathways, resulted in a markedly reduced (>50% reduction) atrial
defibrillation threshold compared with a single shock over a single-current pathway17. The decrease in voltage
and energy requirements for successful defibrillation afforded by biphasic waveforms may increase the efficacy
and lifetime of a device, and may also increase patients’ tolerance of therapy.
Apart from the type of waveform, the efficacy of the treatment depends on the placement of the electrodes.
Results from experimental studies revealed the optimal lead location to be a right-to-left shock vector with the
electrodes located in the anterior cardiac venous system18. According to the critical mass hypothesis, the most
effective electrode system would be one that is able to encompass the most fibrillating tissue between the
electrodes.
Several studies on humans have examined the optimal electrode position for the right atrial electrode and
results have shown the right atrial appendage or the lateral right atrium to be the most efficacious positions19.
Left-sided positioning had mainly concentrated on the coronary sinus as opposed to the left pulmonary artery,
which had significantly higher thresholds20.
Finally, efficacy depends significantly on the time that the electrical treatment is given. In animal studies early
timed internal atrial defibrillation shocks can reduce the atrial defibrillation threshold21. Many studies have
shown that long-lasting AF requires higher energy levels for successful cardioversion14. In contrast, when an
AF episode is defibrillated quite soon after its onset the level of energy required is lower, usually ranging
between one and three Joules22. It is also known that repeated internal cardioversion requires less energy than
primary internal cardioversion of chronic AF, given that the second episode is of shorter duration than the first23.
Recently, in our centre, we examined the efficacy of very rapid cardioversion of induced AF. It appears that
when the arrhythmia is cardioverted within the first thirty seconds the energy needed is significantly less. The
patient’s tolerance of low energy atrial defibrillation is a key point in this electrical treatment. While energy
determines the success of a defibrillation shock, it is the voltage which determines the pain perceived by the
patient24. The use of waveforms that deliver greater energy at lower peak voltages offers the possibility of
internal cardioversion with less sedation and greater patient tolerance.
Current research has focused on aspects that may reduce voltage and patient discomfort while maintaining
efficacy. Waveform rounding may be a useful method of reducing shock-related discomfort25. In another study,
compared with the high right atrial location, the inferomedial right atrial lead location had a higher defibrillation
threshold and the lowest defibrillation success rate, but this lead position produced more tolerable shocks26.
Concerning the chronic use of internal cardioversion for the treatment of AF, recent use of the Metrix device has
raised the issue of indications for its use, and has allowed conclusions to be drawn about the value of this
particular device.
Current indications are considered to be the recurring symptomatic, drug refractory AF without ischemia,
bradycardia, VT/VF and without congestive heart failure27. Wellens et al recently published the results for 51
patients who had received an implantable device for the treatment of AF. The device was safe and terminated
96% of the episodes. There was a 100% specificity of recognition of sinus rhythm and 92.3% sensitivity for
detection of AF28.
Internal defibrillation is a technique of proven efficacy, both for acute and chronic use. Recently, and over a
relatively short time, the parameters which affect efficacy of defibrillation – waveforms, electrode placement –
have become better understood.
Improvements in the efficacy, with lower voltages and energy levels, and/or a new approach to treatment, where
AF episodes are treated within the first seconds from their onset, could make the use of permanent implantable
atrial defibrillators more effective and more tolerable to the patient.
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