Friday, May 25, 2007
internal cardioversion en ablation
Internal Cardioversion/Atrial Defibrillators (Uit hetzelfde artikel zie hieronder)
Internal (catheter-based) cardioversion, using micro-joule internal shocks, is ineffective for AF.[43] Conversely, high-energy internal shocks, delivered between a conventional diagnostic electrophysiology catheter and an indifferent plate, were found by Levy et al. to be effective in cases resistant to external cardioversion.[44] However, the high energies required led to safety concerns.[45] Subsequent developments have lead to the use of biphasic shocks given between electrodes positioned in the right atrium and either the coronary sinus or the pulmonary artery. These efficient configurations allow cardioversion of short-lived AF with less than 5 J and are also generally effective in longstanding AF even if resistant to conventional cardioversion.[46-49] Two comprehensive reviews of internal cardioversion have been published recently.[50,51]
Alt et al.[52] compared the two modalities in 187 patients. They found higher success with internal cardioversion (internal -- 93% vs external -- 79%; P < 0.01) and also found that internal cardioversion could find success where external failed. Others[53] have also confirmed this in latter findings.
Internal cardioversion has found widespread use in the last decade for patients resistant to external shocks, although it is likely that this group will dwindle considerably in the era of biphasic defibrillators (see below). Nevertheless, the technique will probably retain a place for patients with very large thoracic cages, the obese, and those for whom general anesthesia poses a risk. Additionally, it is a very useful technique for AF occurring in the electrophysiology laboratory. The extraordinary efficiency of biphasic internal shocks led to the development of atrial cardioversion by ICDs and to that of a standalone atrial defibrillator. This technology found favor among certain groups[54]; however, as success rates with ablation have significantly improved the limitations of this device (with respect to patient tolerability and its limited effect on the underlying pathophysiology), have led to its withdrawal from the market.
Biphasic Versus Monophasic Shocks
Undoubtedly, the greatest advance in cardioversion efficacy has been the advent of biphasic shocks. A biphasic shock waveform is known to reduce the ventricular DFT.[55] The same was seen in early studies of atrial DFTs in sheep.[56] This effect appears to relate to the differing effects of the two phases of the shock.[57] The initial phase appears to hyperpolarize the myocardium, allowing some recovery of sodium channels. The second depolarizing phase is subsequently more effective at producing depolarization in sufficient myocardium to terminate the arrhythmia. In addition, biphasic shocks appear less likely to reinitiate fibrillation. There has been considerable interest therefore in the use of biphasic shocks for the cardioversion of AF and a number of these studies are summarized in Table 2 .
These studies differ in the energy delivery protocols, electrode configurations, and waveform characteristics but uniformly have found that biphasic shocks are more effective than monophasic. This advantage is present whatever the precise waveform; indeed two recent studies have directly compared biphasic shock waveforms and found no difference in efficacy.[72,73] Biphasic shocks have a higher success rate than monophasic, with lower energy, lower current, and less skin and muscle damage. Biphasic shocks also appear less sensitive to TTI, which in other studies is a major determinant of cardioversion success. Devices used for biphasic shocks compensate for this impedance but even with impedance-compensation monophasic shocks remain inferior.[68]
Adjuvant Anti-arrhythmic Medication
A number of studies have examined the effects of anti-arrhythmic medication on acute cardioversion success. These are summarized in Table 3 and can be divided into observational retrospective studies, randomized trials investigating specific anti-arrhythmic agents, and acute studies formally examining the effects of medication on the atrial DFT by reinitiating AF before a further test.
From these studies, it is evident that several anti-arrhythmic medications may lower the atrial DFT and improve the immediate likelihood of cardioversion; ibutilide has found particular favor particularly in the United States.[81] Certainly no agent has been found to reduce success. Although it is not clear whether anti-arrhythmic medication is routinely advisable as an adjunct to cardioversion, its use is recommended in the event of initial failure.[1] It has been proposed that the use of adjuvant medication in association with biphasic cardioversion may increase success rates sufficiently to make internal cardioversion obsolete except during electrophysiological procedures. Data are awaited in this area.
Internal (catheter-based) cardioversion, using micro-joule internal shocks, is ineffective for AF.[43] Conversely, high-energy internal shocks, delivered between a conventional diagnostic electrophysiology catheter and an indifferent plate, were found by Levy et al. to be effective in cases resistant to external cardioversion.[44] However, the high energies required led to safety concerns.[45] Subsequent developments have lead to the use of biphasic shocks given between electrodes positioned in the right atrium and either the coronary sinus or the pulmonary artery. These efficient configurations allow cardioversion of short-lived AF with less than 5 J and are also generally effective in longstanding AF even if resistant to conventional cardioversion.[46-49] Two comprehensive reviews of internal cardioversion have been published recently.[50,51]
Alt et al.[52] compared the two modalities in 187 patients. They found higher success with internal cardioversion (internal -- 93% vs external -- 79%; P < 0.01) and also found that internal cardioversion could find success where external failed. Others[53] have also confirmed this in latter findings.
Internal cardioversion has found widespread use in the last decade for patients resistant to external shocks, although it is likely that this group will dwindle considerably in the era of biphasic defibrillators (see below). Nevertheless, the technique will probably retain a place for patients with very large thoracic cages, the obese, and those for whom general anesthesia poses a risk. Additionally, it is a very useful technique for AF occurring in the electrophysiology laboratory. The extraordinary efficiency of biphasic internal shocks led to the development of atrial cardioversion by ICDs and to that of a standalone atrial defibrillator. This technology found favor among certain groups[54]; however, as success rates with ablation have significantly improved the limitations of this device (with respect to patient tolerability and its limited effect on the underlying pathophysiology), have led to its withdrawal from the market.
Biphasic Versus Monophasic Shocks
Undoubtedly, the greatest advance in cardioversion efficacy has been the advent of biphasic shocks. A biphasic shock waveform is known to reduce the ventricular DFT.[55] The same was seen in early studies of atrial DFTs in sheep.[56] This effect appears to relate to the differing effects of the two phases of the shock.[57] The initial phase appears to hyperpolarize the myocardium, allowing some recovery of sodium channels. The second depolarizing phase is subsequently more effective at producing depolarization in sufficient myocardium to terminate the arrhythmia. In addition, biphasic shocks appear less likely to reinitiate fibrillation. There has been considerable interest therefore in the use of biphasic shocks for the cardioversion of AF and a number of these studies are summarized in Table 2 .
These studies differ in the energy delivery protocols, electrode configurations, and waveform characteristics but uniformly have found that biphasic shocks are more effective than monophasic. This advantage is present whatever the precise waveform; indeed two recent studies have directly compared biphasic shock waveforms and found no difference in efficacy.[72,73] Biphasic shocks have a higher success rate than monophasic, with lower energy, lower current, and less skin and muscle damage. Biphasic shocks also appear less sensitive to TTI, which in other studies is a major determinant of cardioversion success. Devices used for biphasic shocks compensate for this impedance but even with impedance-compensation monophasic shocks remain inferior.[68]
Adjuvant Anti-arrhythmic Medication
A number of studies have examined the effects of anti-arrhythmic medication on acute cardioversion success. These are summarized in Table 3 and can be divided into observational retrospective studies, randomized trials investigating specific anti-arrhythmic agents, and acute studies formally examining the effects of medication on the atrial DFT by reinitiating AF before a further test.
From these studies, it is evident that several anti-arrhythmic medications may lower the atrial DFT and improve the immediate likelihood of cardioversion; ibutilide has found particular favor particularly in the United States.[81] Certainly no agent has been found to reduce success. Although it is not clear whether anti-arrhythmic medication is routinely advisable as an adjunct to cardioversion, its use is recommended in the event of initial failure.[1] It has been proposed that the use of adjuvant medication in association with biphasic cardioversion may increase success rates sufficiently to make internal cardioversion obsolete except during electrophysiological procedures. Data are awaited in this area.
# posted by roodbont @ 9:13 AM 
