In the previous post, I have addressed the inefficiency of pulmonary vein isolation (PVI) in patients with persistent forms of atrial fibrillation (AF) and the need for alternative ablation targets. A natural choice is the AF driving mechanisms- anatomical or functional sources maintaining AF episodes. So, what are these mechanisms?
The older multiple wavelet theory holds multiple wavelets self-replicating across the atrial responsible for maintenance of AF (see Fig. 1c). According to this current of thought, fractionation of a larger wavefront (mother) gives birth to daughter wavelets through the atria. These daughter wavelets are self-perpetuating and their numbers are determined by many factors- the refractory period, conduction velocity as well as mass of atrial tissue. Increased atrial mass, shortened atrial refractory period, and delayed intra-atrial conduction increase the number of wavelets and promote sustained AF .
An interesting fact: this model was first observed by Garrey back in 1914! He cut fibrillating animal atrium tissue into smaller pieces and observed that the fibrillation was still going on in these smaller parts. This was against the idea of focal sources as only one piece could contain the source. He also reported the presence of a critical mass for perpetuation of AF as further tissue cutting caused the termination of AF .
The second theory (and certainly more interesting for my thesis work) is based on fibrillatory conduction in the atria maintained by localized AF drivers. These are fast and patient-specific localized sources with repetitive activity. Localized drivers are candidate ablation targets.
A Localized AF driver is a very broad term as it does not specify the underlying mechanism. There have been several studies reporting rotors, focal sources, breakthrough or a mixture of all these as driving mechanisms. If they could be anything, how will we detect them? Well… there are multiple signal processing procedures to detect these mechanisms, separately. For instance, phase singularities are reported to sometimes reflect rotor cores around which rotational activity is observed (note a very nice work from my group where phase singularities were observed to co-localize with conduction blocks! ). A natural question would be: Can we develop techniques which can detect a localized driver regardless of its mechanism? That would be very handy. The property renders this possible was already given: repetitiveness!
A local driver should have three properties: (i) High frequency, (ii) regularity and (iii) preferentiality. Therefore, we should observe a similar conduction pattern in the vicinity of the localized driver in repeating manner. Good news is that the repetitiveness is a fundamental property in dynamical systems and widely studied. In the next post, I will introduce recurrence plots for detection of atrial locations with repetitive activity.
I will be back soon!
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