Simultaneous monitoring of autonomic tone and ectopic activity could be used to prevent the occurrence of postoperative atrial fibrillation (POAF). Flavia Ravelli (Department of Physics, University of Trento, Povo-Trento, Italy) and the colleagues of the Cardiovascular Department of the Santa Chiara Hospital in Trento have examined triggering mechanisms of postoperative AF and have developed a framework for the combined assessment of autonomic tone and ectopic activity that characterises components of normal and abnormal atrial cycle length series. From this, they suggest a role for sympathetic activation in promoting ectopic activity and facilitating the induction of POAF.
Presenting the findings at the 2018 Heart Rhythm Congress (6–9 October, Birmingham, UK), Ravelli explained: “Our data show an increase in the number of premature beats and in the sympathovagal balance in the minutes preceding the occurrence of POAF. The increase in the number of premature beats is consistent with the increase in sympathetic activity which may indeed favour the initiation of AF by enhanced triggered activity. On the other side, sympathetic activity may non-uniformly shorten the refractory period by direct control or indirectly by rate adaptation. And so a higher occurrence of premature beats and tissue with higher response may increase the probability to trigger and support atrial fibrillation episodes.”
Despite its high prevalence, the pathophysiological mechanisms of postoperative AF remain incompletely understood. Facilitating factors are known to be acute peri-and postoperative responses to cardiac surgery, such as inflammation, autonomic tone alterations, and oxidative stress, as well as pre-existing elements, such as structural heart disease, and remodelling and ageing of the heart. These factors may provoke triggers to start the arrhythmia or may enhance substrates of arrhythmia to maintain atrial fibrillation.
Ravelli said: “Our aim was to characterise postoperative atrial fibrillation triggering factors, specifically to study the interplay between autonomic tone and ectopic activity. The study was performed by analysing long term epicardial recordings in post-operative patients.”
The study group consisted of nine patients, eight of whom were male, with sustained postoperative AF episodes lasting more than 30 seconds, and in a few patients more than 24 hours, which are preceded by three hours of continuous noise-free sinus rhythm recording. During this time, the atrial cycle length was estimated step-by-step using a custom made algorithm. She said: “The first and most important step of our algorithm is the binomial filtering procedure, which allows us to identify the abnormal beats, and also to reprise the abnormal beat values in the normal time series. This is very important, because we want also to make an analysis of autonomic tone in the normal atrial interval series.”
Ravelli pointed out that the unified framework developed by the investigators thoroughly characterises normal and abnormal atrial cycle length series components, allowing the simultaneous analysis of autonomic tone and ectopic activity. “Associated with an increase in the number of premature beats before the starting of AF, we could see a decrease in the atrial cycle lengths and an increase in the low frequency (LF) spectral component of the normal beat series, a decrease in the high frequency (HF), and an increase in the LF/HF ratio.” Characterisation of abnormal beats showed a concurrent increase in the number, and decrease in the coupling interval, of premature beats.
Ravelli added: “These patterns were consistent in the majority of patients. Furthermore, we found a correlation between the power of the low frequency band and the number of premature beats shortly before an atrial fibrillation episode.”
She outlined the two pathways leading to an increased AF vulnerability suggested by the findings. One, through autonomic modulation and increased sympathetic activity, leads to an enhanced triggered activity that feeds into an increase in premature beats and a decreased coupling interval. The second, concurrent, route is via a decreased effective refractory period (ERP), either by direct control or rate adaptation.