Atrial Fibrillation
Proper Threshold of Low Voltage and Reduced Conduction Velocity in Atrial Fibrillation
2022 Volume 86 Issue 2 Pages 200-201
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2022 Volume 86 Issue 2 Pages 200-201
Atrial fibrillation (AF) is a progressive, heterogeneous disease for which the underlying mechanisms of progression and perpetuation are not fully understood, and current treatments are inadequate. AF results from triggers and substrates that are responsible for the initiation and perpetuation respectively. Pulmonary veins (PVs) play a vital role in the pathogenesis of AF, and their isolation is associated with high rates of AF freedom in most affected patients. However, in some cases, AF recurs even though electrically durable PV isolation (PVI) has been achieved.1 It is difficult to identify the exact origin outside the PV that is initiating and sustaining AF because of the variability of temporal and spatial dispersion. Therefore, ablation in patients with AF for which PVI has been achieved has limited effectiveness. Regarding the recurrence of AF after PVI, substrate changes in the atrium other than the PV are important, and it is expected that adverse atrial remodeling will occur; however, the factors that are important are unknown.
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Complex fractionated atrial electrogram (CFAE)-guided ablation and linear ablation are often added after PVI as additional procedures, although their effectiveness is unproven.2,3 In addition, CFAE analysis is not constant, because it is difficult to evaluate objectively and quantitatively.4 Using an evaluation method during sinus rhythm, the existence of a low voltage area (LVA) has been reported as a factor predicting recurrence using a bipolar voltage map.5–7 The peak-to-peak value of the bipolar electrogram differs according to the spacing between the electrodes, the direction of conduction, and the degree of contact with the myocardium; therefore, it is not sufficient as an absolute evaluation.8 Furthermore, because the baseline of the bipolar levels also differs according to the site in the atrium, such as the posterior, anterior, roof or septum, it is difficult to evaluate with a single threshold. Some evaluation methods based on the conduction velocity (CV) using intracardiac electroanatomical mapping have been studied. Recently, the relationship between the total atrial conduction time using tissue Doppler imaging and AF recurrence after catheter ablation has been reported as a noninvasive evaluation.9 For tachyarrhythmias, conduction delay is electrophysiologically very important for their maintenance. Recently, substrate ablation has attracted attention regarding its effectiveness as an ablation strategy for ventricular tachycardia. The identification and elimination of the site with conduction delay with catheter ablation during sinus rhythm or atrial pacing are better than conventional electrophysiological analysis and treatment based on tachycardia induction.10 Therefore, it is extremely important to evaluate the conduction delay and functional conduction block, but it is difficult to evaluate it objectively in practice. The long conduction time of the atrium can be explained by the presence of multiple LVAs.11
In this issue of the Journal, Kurata et al12 describe dividing the left atrium into 6 areas to evaluate the electrophysiological degeneration of the entire left atrium, defining a proper threshold of low voltage for each area, and evaluating the voltage in the area. The number of LVAs was evaluated to determine how much of the potential was lowered in the entire atrium. To date, a LVA is often evaluated in terms of its existence and area ratio; therefore, it may be difficult to quantitatively predict the recurrence of AF, because it is averaged when looking at the entire atrium. It is difficult to set a proper threshold for low voltages. AF recurrence increased proportionally to the number of areas below the cutoff value for each LVA. The LVA could be defined more precisely because the cutoff value was set for each area instead of a single value for the entire atrium. This is not difficult, considering the tendency of the atrium to contract efficiently during sinus rhythm. The CV is non-uniform, maybe because the contraction of the entire atrium is adjusted to be efficient. If atrial remodeling occurs uniformly or is scattered throughout the atrium, it is important to set a proper threshold of bipolar voltage for each area instead of a single value for evaluation. In other words, knowing the number of areas where the bipolar voltage is below each threshold can bring us closer to a global left atrial assessment.
The CV is different for each area, and the proper threshold should be different, but it is not easy to accurately measure the CV. Previously, the CV and the time between 2 points on the atrium were calculated using an ablation catheter, but it was difficult to accurately determine the front of the activation line. Recently, high-density mapping systems have become available for a more accurate evaluation of the relationship between CV and bipolar voltage.13,14 The relationship between voltage and CV has been reported using unipolar, bipolar, and recently introduced omnipolar voltage mapping techniques. It has been reported that high CVs and large unipolar voltages are often present in LVAs identified using direction-independent bipolar voltages.15 In the future, various methods will be attempted to evaluate the substrate during atrial excitation.
If AF recurrence after PVI increases as atrial remodeling progresses throughout the atrium, the limits of intervention by catheter ablation also become apparent. Intervention is possible if the ablation is in a LVA within a very limited range, but if there are multiple LVAs, it is difficult to cauterize them all. It can be said that we are at the stage of considering interventions for cases that are expected to have a high risk of recurrence.
Because a highly objective and reproducible method of evaluation has not been developed, there is no standard for properly selecting the target to be treated and selecting the appropriate treatment for patients with AF. Traditional methods, such as CFAE ablation and linear ablation, may have offset their effectiveness by adding them to cases that do not require additional intervention other than PVI. To avoid this, an objective and reproducible evaluation method is required.
Remuneration for lectures: Johnson and Johnson, Medtronic, Toray
Affiliation with Endowed Department: Medtronic, Fukuda Denshi
