Cardiovascular response to exercise
Heart rate increases steadily with exercise. The maximum predicted heart rate for an individual is 220-age. Stroke volume increases slightly due to sympathetic stimulation and increased contractility of the myocardium. Cardiac output being the product of heart rate and stroke volume, increases about six times at peak exercise. Blood flow to the exercising muscles increases markedly, to supply the increased nutritional demand and oxygen as well as to wash out the waste products of metabolism. The oxygen extraction from blood also increases with exercise upto three times the basal value. Thus total oxygen consumption can increase upto 18 times with peak exercise. The basal metabolic oxygen requirement is known as 1 MET or metabolic equivalent. The number of METs achieved will give the level of exercise and the physical capacity of the individual. In a standard Bruce protocol of treadmill exercise test a healthy, well trained individual may reach upto 18 METS. Such high levels of exercise cannot be maintained for a long period of time and is not recommended for a home treadmill exercise program.
The capacity to increase the heart rate with exercise is known as chronotropic reserve. If the heart is unable to increase it as per demand, it is known as chronotropic incompetence. This occurs in sick sinus syndrome as well as in therapy with beta blockers. Chronotropic reserve can also be reduced if the resting heart rate is already increased due to sympathetic overactivity as in heart failure. The facility to increase the contractility of the ventricular myocardium by sympathetic stimulation is known as inotropic reserve. This is also lost in heart failure.
Unlike the other organs, the heart extracts oxygen to the maximum possible extent even at rest so that coronary sinus has the least oxygen saturation. Hence increased oxygen delivery to the heart muscle during exercise is possible only by increasing the myocardial blood flow. This is known as myocardial flow reserve.