Not really a question; but some info I found. It may be helpful because I know it's a question that's lingered in my mind for almost 2 years that i've tried to find answers for - exercise and arrhythmia's. I'm not sure how old this is but it was reprinted in 2006.
I stumbled on Myrvin H Ellestad's Cardiovascular and Pulmonary Responses to Exercise which explains alot about what happens when we exercise. The link is way too long to add, so if you google - myrvin h ellestad Cardiovascular and Pulmonary Responses to Exercise - and click on the top link which should be books.google.com isbn=01951592....you'll find parts of the book.
Page 12 shows about conditioned and deconditioned hearts...which I still have questions on; what exactly does that really mean? regular daily exercise, 1-3 times a week, being able to walk a certain distance, run etc etc etc?
Catecholamines (part of the sympathetic nervous system) seem to play a huge role in what goes on in our bodies when we exercise and why things go wrong...there's a long list of things that can make our hearts act strange.
Page 17 shows about heart rate - it says the HR is the result of a number of physical and emotional influences that are mediated by the autonomic nervous system (ANS). it also says both the Vagal and Sympathetic nerves are constantly stimulating the SA, which can cause rate increase/decreases. Page 17 is worth reading; it has alot of great info on what influences our HR's.
The 3rd paragraph on page 17 floored me - it starts off with A curious property of the heart is it's apparent age-related ceiling on rate. Then it goes on to talk about age, gender and more info such as at higher workloads the HR may be 40 beats more per minute in deconditioned patients. It seems HR's correlates with the oxygen consumption of the heart and the heart of a conditioned patient is 25% more efficient - what better reason to exercise on a regular basis. [someone kick me in gear to try again]
There's alot of information probably too much to discuss but it does go in depth about oxygen and heart workload etc - (page 33) I'm sure Hypoxia may play a big part in heart problems...such as sleep apnea; which goes undetected in many people and can cause arrhythmia's. I had no idea that body temperature played a part in our ability to adapt to exercise and recovery time is much longer with higher body temps; outside temps I can understand though being a major factor.
So really, if we have problems exercising it may not be our heart but other influences that are causing the problems.
here's some other info from the American Heart Association - http://circ.ahajournals.org/cgi/content/full/109/20/2374