Biomed 101: Cardiac output basics and terminology

In human physiology, a volume of blood is expelled by either ventricle of the heart. It Is customarily expressed as minute volume, or liters of blood per minute, calculated as the product of stroke volume (output of either ventricle per heartbeat) and the number of beats per minute. Maintaining and regulating cardiac output, which is usually proportional to the tissues’ need for oxygen and other nutrients,is one of the circulatory system’s most intricate functions.

In the healthy human adult, resting (or basal) output is estimated to be slightly over five liters per minute. Normally, it decreases somewhat when a person changes from recumbent to upright position. It may be increased 50 to 100 percent by anxiety and excitement and as much as fivefold by exercise. Measurement of cardiac output makes possible an evaluation of respiratory exchange, i.e., the delivery of oxygen to the tissues.

The cardiac output represents the volume of blood that is delivered to the body, and is therefore an important factor in the determination of the effectiveness of the heart to deliver blood to the rest of the body, (i.e., determining heart failure, inadequate circulation, etc). By definition, two major factors contribute to the cardiac output: the heart rate, which is the number of times a given volume of blood is ejected per unit of time, and the stroke volume, which is effectively the volume of blood that the heart can fill with, which will be ejected upon contraction.

Increasing the heart rate effectively increases the cardiac output by increasing the number of cardiac volumes of blood released into the system. As long as the heart is given a long enough diastole (resting phase), the effective volume that the heart outputs will increase. The stroke volume can be increased by three different factors: the pre-load, the after-load and the myocardial contractility.

A high venous pressure (pre-load) can increase the stroke volume by causing the heart to fill more fully during diastole. The increased blood pressure forces more blood to flow into the heart, down a steep pressure gradient. Similarly, a decrease in the after-load (arterial blood pressure) can also increase cardiac output, by reducing the input resistance the heart must overcome in order to pump blood into the vasculature. If the arterial pressure is high, the heart must work harder, against a steep pressure gradient in order to pump more blood into the system. If the arterial pressure is lower, the heart need not work as hard, and more of the total heart volume actually ends up being ejected into the vasculature. The myocardial contractility affects the pumping efficiency of the heart. A higher contractility allows the heart to pump out more of its blood during systole, due to the increased ejection force of the cardiac muscles. By allowing the heart to pump out more of its volume into the vasculature (high cardiac efficiency), the cardiac output is increased. Assessment of the cardiac output is important in determining the work that the heart is actually performing with respect to the rest of the cardiovascular system. If the cardiac output is too low, then the body is not being properly supplied with blood (heart failure), which can and will lead to life threatening problems if left unchecked. Cardiac output is usually measured using the Fick Principle, named for German physiologist Adolf E. Fick, which relates the cardiac output of the patient to the oxygen consumption, or by thermodilution, in which cold saline is injected into the right atrium and changes in the temperature in the pulmonary artery are recorded. These methods allow...

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