I believe I have seen someone with a heart rate of 320... they didn't live long... We had to do CPR because their heart just quit.

In order to trigger a muscle cell to contract, an action potential has to be generated. As the electrical signal reaches each myocyte (the muscle cells in the heart) they are said to depolarise, meaning the cell goes from having a negative electrical balance across it's membrane, to having a positive electrical balance, resulting in an action potential. The electrical balance is reglated by various channels that allow certain charged atoms (ions) through. There are different types of channels that open and close at different times, but once an action potential has been generated, there is what is called an absolute refractory period, whereby the channels need to be reset before another AP can occur. This lasts about 0.2 seconds, so in theory you could have a heartbeat every 0.2 seconds, meaning a heartbeat of 300 beats per minute (5 per second x 60 seconds). However, following the ARP, there is a relative refractory period - some cells have reset and can produce another action potential, but the stimulus needs to be greater than before, as the original electrical potential across the membrane hasn't been fully restored. So, a 300 beats per minute heartrate is not sustainable, as not all the cells don't reset in time, and so can't keep up, and the stimulus needs to be progressively larger and larger each time. Furthermore, the cells would begin to get damaged, resulting in ions being released, disruption to the electrical conduction pathway and chaotic contraction sequences, leading to ventricular fibrillation (VF, the cells are all contracting out of sequence, preventing the heart from pumping blood). In this case, the defibrillator paddles are required to reset every cell to allow the heart to regain its natrual rhythm (hence when you see it on ER, the paddles don't restart the heart, they actually STOP it). Generally speaking, a person's maximum sustainable heart rate is roughly 220 - their age, so mine would be 220 - 24 = 196. Obviously, this differes from person to person, so no true maximum heart rate can be given.