Afterload is defined as the force against which the contraction of the ventricle occurs. It can also be considered as the end load against which the blood is ejected when the heart contracts. It thus results in ventricular resistance, particularly of the arteries and by the physical characteristics like the viscosity of blood and mass of the blood. 

It is an essential determinant concerning myocardial energy consumption because it depicts the resistance against which the ventricle of the heart should pump. It also indicates the efforts that the ventricles must put forward concerning pushing and forcing the blood from the heart towards the systemic circulation.

The factors that mainly affect and increase the myocardial afterload are systemic hypertension, pulmonary hypertension, aortic stenosis, pulmonary stenosis, and a high level of peripheral resistance.

Factors Affecting Afterload

Many factors increase or decrease the afterload. Thus, some factors which affect the myocardial afterload are as follows:

1. Systolic hypertension: Systolic hypertension is defined as elevated blood pressure. It is the main type of hypertension in the elderly population. The left ventricle works harder to eject the blood from the heart to the aorta. Thus, this factor increases the afterload of the left ventricle. This is because the aortic valve doesn’t open until the ventricular pressure is higher than increased pressure in the aorta.
2. Pulmonary hypertension: Pulmonary hypertension refers to an increase in blood pressure on the right side of the heart, which leads to lungs. It occurs when the pressure in the blood vessel that carries blood from the heart to the lungs is higher than normal. It causes an increase in afterload in the right portion of the heart.
3. Aortic stenosis: The condition in which left ventricular pressure exceeds aortic pressure during the ejection of blood from the left ventricle is called aortic stenosis. The left ventricle needs to overcome the pressure difference caused by the stenotic and calcified valves of the aorta. It is, thus, responsible for the increase in the afterload. This leads the ventricle to work harder to eject blood into the systemic circulation.
4. Pulmonary stenosis: Pulmonary stenosis is the condition in which the pulmonary valve which connects the right ventricle to the pulmonary artery doesn’t grow. This leads to a pressure gradient causing the ventricles to apply more force leading to afterload.
5. Aortic insufficiency: Aortic insufficiency is defined as heart disease, which occurs when the aortic valve of the heart doesn’t work properly and results in the flow of blood in the wrong direction. This causes the amount of blood that moves forward to return to the heart leading to an increase in systolic blood pressure and a decrease in diastolic blood pressure, thus, causing an increase in afterload.
6. Mitral insufficiency: Mitral insufficiency is also referred to as mitral regurgitation. It is the condition of heart disease in which the mitral valve is unable to close properly during the ejection of blood. This leads to the backflow of blood, causing the introduction of an extra pathway for the flow of blood through the mitral valve. This results in less working of the left ventricle to eject blood, causing a decrease in afterload.

How Does Afterload Affect Cardiac Output?

Cardiac output is defined as the product of heart rate and stroke volume, where stroke volume means the amount of blood ejecting from the left ventricle with each heartbeat, and heart rate means the number of times the heartbeat per minute. Thus, in simple words, cardiac output is the amount of blood pumped by one ventricle in one minute.

Afterload is particularly affected for the left ventricle by the aortic pressure and the right ventricle by the pulmonary artery pressure. The pressure inside the ventricles needs to be greater than the pulmonary and the systemic pressure to open the pulmonary and aortic valves, respectively. 

Thus, if ventricles will apply more pressure to eject blood, then the afterload will increase, but it will result in the low amount of blood coming out of the ventricles per minute, thus, overall decreasing the cardiac output.

Calculation Of Afterload

Afterload can be quantitatively calculated with the help of the Young-Laplace equation. It can be calculated by determining the tension in the walls of the ventricles. The equation is as follows-

(EDP X EDR) ÷ 2h

Where EDP stands for end-diastolic pressure inside the left ventricle and EDR stands for end-diastolic radius, particularly at the position of the midpoint of the left ventricle, and h refers to the mean thickness of the ventricular wall.

Conclusion

Afterload is considered as the pressure experienced by the walls of the ventricle to eject the blood from the heart to systemic circulation. The afterload increases with the increase in the pulmonary and aortic pressure. Factors like systolic and pulmonary hypertension, aortic and mitral insufficiency, etc., affect the afterload, which either increases or decreases the afterload. It is also responsible for affecting cardiac output. The amount of blood leaving the ventricles decreases as the pressure experienced by the walls of the ventricle to eject the blood increases. We are also able to quantitatively calculate the afterload by using the Young Laplace equation