Cardiac output (CO) is a measure of the amount of blood each ventricle pumps (the stroke volume) in one minute. Heart output is calculated by multiplying the stroke volume (SV) by the number of beats per minute (heart rate) (HR). Then the law becomes CO = SV x HR.[9] Cardiac output is closely related to body size and body surface area. This is called the cardiac index.
The average normal cardiac output is about 5.25 liters per minute, with an average heartbeat volume of 70 milliliters per minute, the normal range is 4 to 8 liters per minute.[9] The volume of the heartbeat is usually measured using echocardiography and can be affected by several factors such as the size of the heart, the physical and mental condition of the individual, sex, contractility, duration of contraction, and preload and postload.[9]
Preload refers to the filling pressure of the atria at the end of the diastole phase, when they are maximally filled. Another key factor is the filling time of the ventricles – when the ventricles contract faster, there is less time to fill and there will be less preload.[9] Preload can be affected by a person’s blood volume. The strength of each contraction of the heart is proportional to the preload, which is explained by the Frank-Starling mechanism, which indicates that the force of contraction is directly proportional to the initial length of the muscle fibre, meaning that the ventricles will contract more forcefully the more they expand during filling.[9] 32]
Afterload, or the amount of pressure the heart must generate to eject blood on systole, is affected by vascular resistance. It can be affected by narrowing of the heart valves or contraction or relaxation of peripheral blood vessels.[9]
The strength of heart muscle contractions controls the stroke volume, and can be affected positively or negatively by factors called inotropes.[33] These agents can be the result of changes within the body, given as drugs as part of treatment for a medical disorder, or as a form of life support, particularly in intensive care units. Inotropes that increase the force of contraction are called “positive” inotropes and include sympathetic factors such as adrenaline, noradrenaline, and dopamine.[34] Those that reduce the force of contraction of the heart are called “negative” inotropes and include calcium channel blockers.[33]
