29. Cardiac output. Measurement, normal standards and physiological variations
Important terms
The cardiac output (CO) is the amount of blood the heart pumps out every minute. It’s normally 5 litres per minute.
The end-systolic volume (ESV) is the amount of blood which is left inside the left ventricle after systole.
The end-diastolic volume (EDV) is the amount of blood which is left inside the left ventricle after diastole, right before systole begins.
The stroke volume (SV) is the amount of blood the heart ejects with each cardiac cycle. The stroke volume is the end-diastolic volume minus the end-systolic volume, or:
SV = EDV – ESV
The normal value for the stroke volume is 70 mL, meaning that 70 mL of blood is ejected with each cardiac cycle.
The ejection fraction (EF) refers to how much of the end-diastolic volume was ejected during the cardiac cycle. In other words, how many percent of the blood which was in the left ventricle was ejected. The normal value for the EF is 60 – 70%. It is calculated like this:
EF = SV / EDV
The heart rate (HR) is the number of systoles per minute. It varies considerably between people, but an average adult has a heart rate of approx. 70 bpm.
The mean arterial pressure (MAP) is the average blood pressure in the arteries during a single cardiac cycle. The normal value for MAP is around 90 mmHg. MAP can be calculated like this:
The total peripheral resistance (TPR) is the resistance the heart must overcome to push blood through the circulatory system. This resistance is mostly determined by the diameter of the arterioles of the body.
Cardiac output
The cardiac output depends on two things: how often the heart pumps, and how much blood the heart ejects with each pump. As such, we can use the following formula to calculate the cardiac output:
Stroke volume x heart rate = cardiac output or SV x HR = CO
The normal value of the cardiac output is 5 litres per minute. This corresponds approximately with the normal values for the stroke volume and the heart rate. 70 mL x 70 beats per minute = 4900 mL per minute.
The cardiac output can also be calculated like this:
CO = MAP / TPR
The normal value of CO is 5 L/min, but this is in rest. When the body is in activity the different organs, especially the muscles, require more oxygen and therefore more blood. The cardiac output must therefore increase. Indeed, during exercise the cardiac output may increase up to 3 – 4 x!
The body increases cardiac output when necessary by increasing the heart rate and the contractility of the heart, and by vasodilating the systemic circulation so that the total peripheral resistance decreases. The heart then has less resistance to pump against.
The capacity the body has to increase the cardiac output during exercise decreases with age.
Fick principle
The Fick principle is a method of measuring cardiac output. According to the Fick principle, cardiac output can be calculated if we can measure the O2 consumption of the body, the O2 concentration in the pulmonary vein and the O2 concentration in the pulmonary artery. The blood in the pulmonary vein has equal O2 concentration as blood in the systemic arterial circulation, and the blood in the pulmonary artery has equal O2 concentration as blood in the systemic venous circulation.
As an example, the normal [O2]pulmonary vein is 0,20 mL O2/mL blood, the normal [O2]pulmonary artery is 0,15 mL O2/mL blood and the O2 consumption is 250 mL O2/mL. This gives a cardiac output of 5000 mL/min.
Venous return
The venous return refers to how much blood flows back to the heart per minute, more specifically back into the right atrium. The venous return and the cardiac output should always be equal, so that the same amount of blood flows out of the heart as back into it. The body constantly regulates both the cardiac output and the venous return so that they are equal. There may be transient differences between the two, but these differences cause adjustments in the regulation of the circulation so that they become equal again.
The mechanism which ensures the venous return and cardiac output are equal is called the Frank-Starling mechanism/law. It’s described in topic 32.
Preload
The preload refers to how much the muscle fibres in the right atrium are stretched at the end of diastole. When the muscle fibres are very stretched, we say that the muscle experiences increased strain.
It’s directly related to the venous return and the end-diastolic volume. If more blood flows into the heart the right atrium will be stretched more, so the preload increases.
Afterload
The afterload refers to how much the muscle fibres in the left ventricle are stretched at the end of systole. When the muscle fibres are very stretched there is increased muscle strain. You can think of the afterload as the “resistance” or “load” the heart must eject blood against.
The afterload is directly related to the resistance the left ventricle must eject blood against. The higher this resistance, the harder it is for the left ventricle to eject blood. The resistance in question is the resistance inside the aorta, the aortic pressure.
If the pressure inside the aorta increases the heart must work harder to pump blood.