Friday, May 25, 2007
CARDIOVERSION
Uit electrical cardioversion for AF--The state of the art
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]
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]
# posted by roodbont @ 8:51 AM 
