system
Given the following post, respond as if you're commenting on Reddit:
human
What prevents a normal, healthy heart from going into cardiac arrest during very, very intense exercise?

During really high intensity anaerobic activity your heart rate can get to very high rates, close to 100 % of your theoretical maximum.  Assuming a normal, healthy heart what prevents the heart from going 'too fast' and going into an aberrant rhythm, and cardiac arrest?

I asked this question to an electro-physiologist and he basically said, you'd pass out, or be too fatigued, it really can't happen and that evolution would have sorted that issue out, if it ever existed, very rapidly.  This answer made sense to me.  I'm just curious as to what mechanisms exist to prevent it from occurring.  It seems as though the human body has contingencies in place for a lot of things and I assume there would be a few for this.
gpt
The heart adapts to higher rates. For instance heart rate is normally controlled by the sino-atrial node by a balance of sympathetic (+) and vagal (-) inputs. But the heart also gets sympathetic inputs to other parts of the muscle which alter the action potentials and conduction (e.g. shortening it), so that the fast rate doesn't cause problems. The atrio-ventricular node in a healthy heart won't conduct rates higher than 150-200. Arrest rhythms can be 300 beats per minute because they are generated in the ventricle itself or down an abnormal pathway. 

There is also probably regulation upstream of this, i.e. in the hormones and nerves. Some states and drugs can cause arrhythmias through abnormal stimulation so intense exercise must not cause as much.