Tweetif u use it properly, smart and not too often u could prevent that... i'm sure most of us that do couple of cycles a year for few years won't do as much damage to our hearts as someone who does it half their life, constantly...
every time ur on a cycle ur bloodpressure, heart rate etc goes through the roof... heart is a mucle like any other one... if u keep pushing it for too long and hard, it's gonna give in eventually... it never ever rests anyways, and adding even more stress on it will kill it over time...
beside the physical exhaustion, the increased HR and BP will also damage/burst/clog blood vessels (such as the fine cappilaries) that supply the heart with blood (food)...
body can replenish and fix most of that damage, but if u keep doing it over and over again, the body isn't able to keep up with the damages... parts of the heart start failing because they aren't getting any food delivered to them and they die off... (mild heart attacks are an example of it - a blood vessel gets blocked, and the part of the heart that it was supplying with blood dies off) u can have many little hert attacks (usually u can't even tell u had them, it might hurt a bit for few mins but then u feel fine afterwards) but once u reach the point where the damage done is too big, the heart will just stop completely... and fail...