Generally, a plot device is something added to a story that helps define part of the plot but it is often thought of as negative because it shows the author trying too hard to give away what he or she wants the reader to get from the story. In other words, the author may think you will not get the point of the work without the plot device.
Hawthorne uses the heart to demonstrate how a burden can cause mental and physical damage to someone. Arthur Dimmesdale suffers quiety because he does not own up to his role in the adultery with Hester Prynne. Though he is not punished by the town the way Hester is, she is able to live with herself where he is not. not only does he suffer mentally, he also suffers physically as demonstated by the physical issues with his heart.
This guilt and shame he feels could be the result of his own guilt or even be seen as punishment by God. as a minister, he is living a very hypocritical life.