Anatomical considerations dictate that the more proximal the level of ablation for the pulmonary veins, the greater the extent of disconnected myocardium, but this requires more extensive ablation because of increasing diameter and myocardial coverage proximally. Electrophysiologically definable sites of preferential inputs to the veins, however, enable disconnection to be achieved at the ostia without circumferential ablation in the majority.7
In effect therefore, these preferential inputs demarcate the electrophysiological equivalent of the left atrial–pulmonary vein junction and can be considered the electrophysiologically defined ostium.
Activation of the vein can best be appreciated with some form of circumferential mapping, as opposed to longitudinal mapping typified by a multielectrode catheter placed along the length of the vein. A thin (low profile, non-traumatic) preshaped circumferential multielectrode catheter allows continuous assessment of activation around the full circumference of the vein in addition to providing a flouroscopically visible marker for the vein with a verifiable three dimensional orientation. One or two bipole(s) or electrode(s) typically show earliest activation with later and sequential spread to the rest of the venous circumference; ablation proximal to this site will delay activation in this sector with the antipodal sector (usually) now becoming the earliest activated. Ablation of this secondarily manifest input eliminates all evidence of activation distally indicating the presence of interconnections beyond the site of ablation. Two sectors of early activation separated by one of later activation suggest two distinct breakthroughs; conversely multiple near simultaneous and contiguously activated sectors suggest multiple or coalescent (broad) inputs. Disappearance of activation in a part of the circumference after proximal ablation suggests the ablation of branched or fascicle like myocardial extensions. Integration of such activation patterns at multiple levels within the vein allows appreciation of complex branching and spiral loop like patterns. Because of the cul-de-sac nature of electrical activation in the pulmonary vein, the disappearance of all distal (circumferentially recordable) potentials is a clear and unarguable indicator of conduction block.
Distinguishing target pulmonary vein potentials from far field atrial potentials is important in order to avoid unnecessary ablation which could result in vein stenosis or avoidable collateral damage (for example, to the lung or the phrenic nerve). In the right sided pulmonary veins both right as well as left atrial potentials can be identified (by mapping both sides of the interatrial septum) while in the left pulmonary veins, the nearby appendage is the most common origin of non-pulmonary vein potentials: distal coronary sinus pacing by anticipating left atrial appendage activation can distinguish the two. Exit block is frequently seen during mapping of the pulmonary veins and may simply reflect a reduced safety factor; therefore entrance block of activation into the vein from the larger current source of the left atrium is likely to reflect complete bidirectional block with greater surety. In addition, the demonstration of pulmonary vein to left atrium block during sinus rhythm is hampered by the occurrence of threshold effects when pacing from the pulmonary veins; at low outputs pulmonary vein capture may or may not occur (depending upon the proximity to pulmonary vein muscle, which is difficult to ascertain in case of a discrete fascicle) while at higher outputs direct (electrotonic) capture of the adjacent left atrial appendage and sometimes posterior left atrium or right atrium can occur, even in the complete absence of pulmonary vein potentials.
Typically circumferential mapping allows electrophysiologically guided disconnection of the four pulmonary veins to be accomplished successfully in nearly 100% of patients and quite rapidly, sometimes within one hour. In this context, the potential role of circumferential ablation devices is debatable since the gain in efficacy (if any) would be limited. Ablative energy could be unnecessarily delivered at bystander sites with resulting collateral damage, and/or may not be sufficiently concentrated or focused to ablate a discrete fascicle.
This strategy of expeditious pulmonary vein disconnection without documented proof of arrhythmogenicity can only be justified by a sufficiently low risk of side effects—notably pulmonary vein stenosis. The use of limited radiofrequency power, minimising the circumferential extent of ablation and targeting the most proximal segment (usually the largest diameter) are all important in limiting the frequency of this difficult to treat complication to 1–2% of ablated pulmonary veins. Non-occlusive stenosis limited to a single pulmonary vein (typically draining about half of one lung) usually has no significant clinical consequences.