What is cardiac output

Important terms related to the heart

Cardiac output

Cardiac output (HMV) is the amount of blood that the heart pumps into the bloodstream in one minute. It is also known as cardiac output (CO). The CO is a measure of how much the heart can pump. It is calculated from the heart rate (HR) and the stroke volume (SV): HR x SV = CO. At rest, the CO is about 4.5 to 5 liters per minute. If the heart has to pump faster, for example when people exert themselves physically, the CO can increase to up to 20 liters per minute, and for endurance athletes even to 35 liters.

The cardiologist determines the cardiac output most precisely with a right heart catheter examination. However, it can also be approximately calculated using other, non-invasive examinations such as magnetic resonance imaging (MRT) or echocardiography. In order to determine how well the patient's heart can pump, the cardiologist will determine the so-called ejection fraction in addition to the CO. The doctor can measure this easily and safely for the patient with an ultrasound examination of the heart (echocardiography).

When the heart can no longer pump properly, the CO is lower than in healthy people, for example with a weak heart or with heart valve disease. Even with an underactive thyroid, the CO can be lower than normal. CO is usually higher than usual when a person has a fever, anemia, or hyperthyroidism.

Heart rate: tachycardia & bradycardia

Heart rate (HR) is the number of heartbeats per minute. In healthy adults, the heart beats about 60 to 90 times a minute. In children and newborns, the heart beats much more frequently: in newborns about 120 times every minute. But that is nothing compared to the animal kingdom: Hummingbirds have a heart rate between 800 and 1,000 beats per minute! Elephants, however, only between 15 and 30.

How fast the heart beats depends not only on age, but also on the physical strain and the level of training: When exercised, the heart rate can increase to a maximum of 160-180 beats per minute. In endurance athletes, the heart beats much less often at rest than in untrained people, around 40 to 50 times per minute. How fast the heart beats is controlled by the sympathetic and parasympathetic nervous system as well as hormones, especially the stress hormones adrenaline and noradrenaline. When the heart beats faster than usual, this is called Tachycardia, a slower than normal heart rate means Bradycardia.

The doctor determines the heart rate by listening to it with a stethoscope or an electrocardiogram. The pulse measurement on the wrist gives a first clue.

Stroke volume

Stroke volume (SV) is the amount of blood that the heart pumps into the body when the heart beats. In healthy people, the SV at rest is about 70 milliliters (ml). With the help of the stroke volume, the cardiac output is calculated, i.e. the amount of blood that the heart pumps into the body in one minute.

Ejection fraction = ejection fraction

The ejection fraction (EF) is the amount of blood that the left ventricle pumps into the body in relation to the total amount of blood that is in the ventricle. In the relaxed state there is about 140 milliliters of blood in the left ventricle. When the heart beats, the heart pumps 80 milliliters of blood into the main artery, so the ejection fraction is 57%. For healthy people, the EF is over 50 to 60%, young people have an EF of 65-70%. In people with severe cardiac insufficiency, the EF even sinks below 25%; a heart that is too weak can then not supply the body with sufficient blood.

The doctor can determine the EF using a variety of examination techniques. The simplest and least dangerous is the ultrasound examination of the heart (echocardiography).

Critical heart weight

The healthy heart of an adult weighs around 300 grams. In certain diseases, for example high blood pressure or heart valve defects, the heart enlarges in order to be able to supply the body with sufficient blood. At the beginning, the heart muscle cells enlarge (hypertrophy). If the heart weight exceeds 500 grams, the coronary arteries can no longer supply the heart with sufficient oxygen. The result is an enlargement of the heart, known in technical jargon as "dilatation". The heart can no longer pump sufficient blood into the body, and cardiac insufficiency develops.

In people who exercise regularly and intensely, the heart also enlarges. In this way, the heart muscle can easily do the extra work during physical exertion. However, even in endurance athletes, the heart does not weigh more than about 500 grams.