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The illustration below shows, on the left, a phenomena electrophysiologists call continuous fractionated atrial electrograms or CFAE (pronounced by electrophysiologists as "cafe"). The CFAE (or high frequency tachycardia) recorded on the left-hand part of this slide was seen to occur from a pulmonary vein during an episode of atrial fibrillation. Various lines of evidence have show that CFAE type electrical activity in the atria is an important propagator, if not the propagator for atrial fibrillation. The illustration on the right side of the slide shows organized, albeit irregular, electrical activity in a pulmonary vein during an episode of atrial fibrillation. This activity represents passive activation of the vein and not propagation. Thus, the right hand part of the slide represents a pulmonary vein that is being paced by activity elsewhere and therefore being kept in atrial fibrillation by activity originating elsewhere. The high frequency tachycardia or CFAE on the left handpart of the illustration is the structure in the heart keeping the heart in atrial fibrillation and propagating the passive activation seen in other follower sites (as in the activity seen on the right). The CFAE activity is therefore the engine keeping the atria fibrillating. The organized activity on the right is simply following the high frequency activity originating elsewhere. In paroxysmal atrial fibrillation, most of both the left and right atria are being passively activated and driven by select regions of high frequency activity. Elimination of high CFAEs in the atria by selective catheter ablation of such acitivity has been shown to eliminate both paroxsysmal and chronic atrial fibrillation in the majority of patients with atrial fibrillation.
High frequency activity in the atria during atrial fibrillation tends to originate in and around the pulmonary veins (at least it does so for paroxysmal atrial fibrillation). The pulmonary veins are also the primary source for premature complexes which can trigger an episode of atrial fibrillation. Thus a catheter ablation procedure targeting the regions around the pulmonary vein entry points in the left atria has the potential to eliminate both the triggers and the propagators of atrial fibrillation. Did you notice how rapid the high frequency activity on the left-hand part of the slide is? The cells in the part of the heart generating this activity are beating at over 500 beats/minute. In order for cells in the heart to contract so rapidly and generate such rapid electrical signals, they must consume relatively large amounts of energy. In order for the cells in the body to generate energy they need to burn a fuel source which in the heart is glucose and free fatty acids present in the blood. When fuel is burnt in the body, oxygen is consumed (for the biochemists in the audience I will save the more detailed discussion on cardiac anaerobic metabolism for another time). Did you know that the region of the circulatory system with the highest known concentrations of oxygen are the pulmonary veins (which return freshly oxygenated blood from the lungs). Perhaps the need for large amounts of oxygen to allow for such high frequency electrical activity, explains the longstanding and irrefutable fact that the left atria is essential in the propagation of all types of atrial fibrillation. All known curative procedures for atrial fibrillation generally involve surgery to the left atria and its connecting veins. For the record, no one really knows why the left atria is essential in atrial fibrillation propagation but my personal theory is that high amounts of oxygen are needed to maintain such high frequency electrical activity (you probably won't read this elsewhere but it is a theory of mine which nicely explains the various phenomena about atrial fibrillation observed to date). |
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