Heart rate node, where the faster pacemaker cells driving the self-generated rhythmic firing and responsible for the heart's autorhythmicity are located. For endurance athletes at the elite level, it is not unusual to have a resting heart rate between 33 and 50 bpm. The "maximum heart rate" (HR) is the highest heart rate an individual can achieve without severe problems through exercise stress, and generally decreases with age. Since HR varies by individual, the most accurate way of measuring any single person's HR is via a cardiac stress test. In this test, a person is subjected to controlled physiologic stress (generally by
Cardiac physiology heart with each contraction. To calculate ejection fraction, SV is divided by EDV. Despite the name, the ejection fraction is normally expressed as a percentage. Ejection fractions range from approximately 55–70 percent, with a mean of 58 percent. Many of the factors that regulate the heart rate also affect cardiac function by altering the stroke volume. While a number of variables are involved, stroke volume is dependent upon the difference between end diastolic volume and end systolic volume. The three primary factors involved are preload, afterload and contractility. Preload is another way of expressing EDV. Therefore, the greater the EDV,