What portion of cardiac cycle is shortened by rapid heart rate?
When the heart pumps (systole), it ejects blood forcefully and rapidly. The the muscles relax (diastole) and, as you know, it take longer to relax a muscle than to contract it. Additionally, blood is not pumped back to the heart, but returns passively. Thus, it is during diastole that the rate of return is most dependent on heart rate. A well-conditioned athlete can maintain a max heart rate of 200 – 220 bpm. Above that rate, there is not enough time for the ventricles to fill completely and so you start to fall behind complete circulation of the blood supply. Oxygen debt starts to build up and eventually you stop (if you are an athlete) or you faint. Occasionally, if you have atherosclerosis, not enough blood will flow to the heart or brain and you could have a myocardial infarction (heart attack) or a stroke or seizure.
