Preload and Afterload of Heart

It is true that the basic function of heart is to pump blood and supply oxygenated blood to body tissues, but in reality, the heart is a complex body organ and so many factors determine how much blood is pumped through the body. Preload and afterload are two determinants of how much blood the heart pumps in a minute (cardiac output).

What is cardiac output?

The volume of blood pumped by the heart per minute is its cardiac output. It is measured in mL blood/min. Cardiac output refers to the function of stroke volume and heart rate. The heart rate refers

to the number of times your heart beats in a minute. The stroke volume refers to how much blood get pumped out of heart with each beat. Cardiac output will increase if stroke volume or heart rate increases.

A resting heart rate for an average person is 70bpm, whereas a resting stroke volume is 70 mL per beat. Using the above formula, an average person will have a cardiac output of 4900 mL per minute. Now, if you consider the total volume of blood in the circulatory system, which is around 5000mL, it will become clear that the heart of an average person pumps the entire volume of blood in the circulatory system after every minute.

What is preload?

Preload, also known as the left ventricular end-diastolic pressure (LVEDP), is the amount of ventricular stretch at the end of diastole. Think of it as the heart loading up for the next big squeeze of the ventricles during systole. Some people remember this by using an analogy of a balloon – blow air into the balloon and it stretches; the more air you blow in, the greater the stretch.

What is afterload?

Afterload, also known as the systemic vascular resistance (SVR), is the amount of resistance the heart must overcome to open the aortic valve and push the blood volume out into the systemic circulation. If you think about the balloon analogy, afterload is represented by the knot at the end of the balloon. To get the air out, the balloon must work against that knot.

What is contractility of heart?

It measures the performance of cardiac pump, the degree to which muscle fibers may shorten when they are activated by certain stimulus independent of preload and afterload. If everything else remains constant and contractility increases, it will make the ventricle to eject more blood, which in turn will increase stroke volume. It implies that if preload and afterload stay unchanged, raising contractility will directly increase stroke volume and vice versa.

High and low of preload and afterload

Cardiac output will get affected with the highs and lows of preload and afterload. Hypotension, shock and tamponade are some states when preload will be low. Sometimes, preload is low due to volume issue. This can be resolved by adding volume through blood and fluids. However, some states like brady-arrhythmias and heart failure will create excessive volume and lead to an increase in preload.

Similarly, afterload, which basically refers to vascular resistant within the lungs and aorta, may have highs and lows as well. Afterload is usually high during SNS stimulation, aortic stenosis and HTN. If that’s the case, you may benefit from vasodilators. Afterload will be low due to hypotension or sepsis. If that's the case, vasopressors may benefit the patient.

HSN,RN,BSN