Both the rate at which your heart beats and the volume of blood your heart moves in a single beat determine how ef?ciently your heart pumps blood. Cardiologists calculate cardiac output to measure your heart’s
ef?ciency. Cardiac output is, quite simply, the amount of blood your heart pumps through your circulatory system in one minute. It is calcu- lated by multiplying how much blood the left ventricle squeezes out in a single contraction (stroke volume) by the number of times the heart contracts in a minute (heart rate).
Most typically, when your body needs more blood (for instance, when you are running up stairs) the heart increases its output by beat- ing faster. If your heart beats at a fast rate for very long, the muscle begins to tire and the resting phase of the heartbeat becomes too short for the chambers to ?ll adequately. If you are physically ?t, your heart muscle is stronger and can pump more blood with each contraction. That is, your stroke volume is higher, so your heart can deliver adequate blood to your body without tiring as quickly. A physically ?t person may actually have a low resting heart rate, because he or she has strength- ened the heart muscle so that it can pump more blood, delivering adequate oxygen to the body with fewer strokes. When a ?t person exercises, he or she may have the same heart rate as someone who is less ?t, but the ?t person is able to do more work, such as run longer with- out tiring.
A healthy resting heart rate is usually between 50 and 75 beats per minute. When you exercise, your heart rate may increase to as much as
165 beats or more. Age plays a role in determining your maximum heart rate; the maximum number of beats per minute can be very roughly predicted by the formula 220 minus your age. A number of other factors can cause your heart rate to increase, including stress, some medica- tions, caffeine, alcohol, and tobacco. When a healthy person sleeps, his or her heart rate may dip to as low as 40 beats per minute. As you age, your heart rate may decrease somewhat.
Stroke volume in most people is about 3 ounces. That means that the ventricles pump out about half the blood they contain. A good athlete may be able to increase his or her stroke volume by 5 percent or more. A diminishing stroke volume is one of the ?rst signs of a fail- ing heart.
A pregnant woman’s body demands more blood ?ow and oxygen for the developing placenta. Stroke volume increases early in preg- nancy, and later the heart rate increases to maintain a cardiac output 40 to 50 percent above normal. These changes reverse after the baby is delivered.