S-wave predominance of epicardial electrograms during atrial fibrillation in humans: indirect evidence for a role of the thin subepicardial layer.
OBJECTIVE
The purpose of this study was to characterize the morphology of fibrillation electrograms in patients in order to provide insight into the underlying electropathologic substrate of atrial fibrillation (AF).


BACKGROUND
Electrograms recorded during AF show a high degree of spatiotemporal variation.


METHODS
AF was induced by rapid atrial pacing in 25 patients undergoing cardiac surgery. A unipolar mapping array of 244 electrodes was positioned on the free wall of the right atrium to record multiple epicardial fibrillation electrograms. Local anisotropy in conduction and epicardial wavefront curvature during AF were determined by fitting the best quadratic surface on the activation times of rectangular areas of 3 x 3 electrodes.


RESULTS
During AF, unipolar epicardial electrograms revealed a clear predominance of S waves. The average RS difference during type I and II AF was -0.15 +/- 0.08 and -0.22 +/- 0.08. During type III AF, the predominance of S waves was less prominent (-0.07 +/- 0.05; P < .005). In all types of AF, the degree of anisotropy in conduction was remarkably low (anisotropy ratio: 1.24 +/- 0.09), and no clear directional effect on the relative amplitude of R and S waves was found. There was a weak relationship between local curvature of wavefronts and RS difference (r = 0.23; P < .01). Computer simulations showed that the negative RS difference could result from transmural activation in an epicardial to endocardial direction.


CONCLUSIONS
The clear predominance of S waves in epicardial fibrillation electrograms is not due to anisotropy and can only be partly explained by a high curvature of fibrillation waves. Predominant epicardial to endocardial activation seems to be important in producing rS electrograms on the epicardium. This finding provides indirect evidence that the thin epicardial layer of atrial myocardium plays an important role in propagation of fibrillation waves.
Houben
Richard P M
RP
Medtronic Bakken Research Center, Maastricht, The Netherlands.
de Groot
Natasja M S
NM
Smeets
Joep L R M
JL
Becker
Anton E
AE
Lindemans
Fred W
FW
Allessie
Maurits A
MA
D000328:Adult; D016880:Anisotropy; D001281:Atrial Fibrillation / Q000209:etiology / Q000503:physiopathology*; D002304:Cardiac Pacing, Artificial; D003198:Computer Simulation; D022062:Electrophysiologic Techniques, Cardiac; D005260:Female; D006325:Heart Atria / Q000473:pathology / Q000503:physiopathology; D006329:Heart Conduction System / Q000503:physiopathology*; D006801:Humans; D008297:Male; D008955:Models, Cardiovascular; D010496:Pericardium / Q000503:physiopathology*