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This article has been accepted for publication in [Heart, 2019] following peer review, and the Version of Record can be accessed online at 
[http://dx.doi.org/10.1136/heartjnl-2018-314486]. Reuse of this manuscript version 
(excluding any databases, tables, diagrams, photographs and other images or illustrative material included where another copyright owner is identified) is permitted strictly pursuant to the terms of the Creative Commons Attribution-Non-Commercial 4.0 International (CC-BY-NC 4.0).
Except where otherwise noted, this item's license is described as BMJ Journals See here for full instructions. This article has been accepted for publication in [Heart, 2019] following peer review, and the Version of Record can be accessed online at [http://dx.doi.org/10.1136/heartjnl-2018-314486]. Reuse of this manuscript version (excluding any databases, tables, diagrams, photographs and other images or illustrative material included where another copyright owner is identified) is permitted strictly pursuant to the terms of the Creative Commons Attribution-Non-Commercial 4.0 International (CC-BY-NC 4.0).