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Nadine has taught nursing for 12 years and has a PhD in Nursing research
Have you ever seen a movie where a car has plunged into a lake with people in the car? What happens when the some of the people try to push open the door? They cannot get it open because the pressure that is exerted on the car door by the water in the lake is too great.
This is similar to what happens with cardiac afterload. The definition of cardiac afterload is the resistance that the ventricle of the heart has to overcome to eject the blood from the ventricle chamber during systole. The resistance comes from the blood in the vessels and the constriction of the vessel walls.
Systole occurs during contraction of the heart muscles. The blood flows from the atria to the ventricles. Diastole is the period of relaxation. Together they make up the cardiac cycle. One full cycle is what happens with each heartbeat.
To really understand afterload you must first know how the heart functions. The heart is a muscle and just like other muscles it has fibers that expand and contract. This process of expanding and contracting fill and empty the blood in the chambers of the heart.
There are 4 chambers in the heart divided into 2 atria and 2 ventricles. The atria are pathways to the ventricles for the blood. In other words the blood cannot get to the ventricles without first going to the atria. At a specified point in time, the ventricles contract sending the blood out into the body. The blood returns to the atria when it comes back from the body.
How does the blood move from one chamber in the heart to the next? There are 4 valves that open and close to allow the movement of blood through the chambers. Two of the valves are classified as arterioventricle (AV) and the other two are semilunar (SL).
What makes the valves open and close? It is the pressures inside and outside of the chambers. These pressures increase and decrease during systole and diastole.
In the first part of diastole the AV valves open and blood rushes into the ventricle from the atria. When the ventricle contracts the AV valves close and they remain closed until the pressure inside the ventricle chambers reach a certain level. Once that level is reached, the AV valves open again and the blood leaves the ventricles but the ventricles remain contracted so the cycle is still in systole.
What is happening with the SL valves during systole? The SL valves are forced open when the ventricles contract, allowing the blood to move into the aorta and the pulmonary artery. Once the ventricles relax the pressure in the arteries (outside of the heart) push on the SL valves. The SL valves close - this begins diastole again.
Now that you understand how the heart functions let's move back to the discussion on cardiac afterload. It is one of the 4 heart functions that make up cardiac output (the amount of blood ejected from the heart per minute). The other functions are preload (the pressure of the blood on the heart muscle fibers), contractility (the force of the contraction), and heart rate (the number of beats per minute).
The lower the afterload the more blood the heart will eject with each contraction this is known as stroke volume. So you can think of this as an inverse relationship - reduced afterload = greater contraction (increased stroke volume). The opposite is also true: increased afterload = reduced contraction (reduced stroke volume).
Afterload rises with age and hypertension (increased blood pressure). With age the arteries stiffen due to degeneration; therefore, less contraction of the ventricle occurs and less blood is ejected from the ventricle. Hypertension has a slightly different effect - especially on the left ventricle. Because of the increased pressure on the left ventricle from hypertension over time it grows known as hypertrophy. The enlarged ventricle cannot contract as well and so cardiac output is reduced.
Increased afterload also occurs when the arteries constrict. So what do you think happens? If you said stroke volume is reduced you were correct. When stroke volume is decreased less blood is ejected from the heart with each contraction.
The diagram above shows how an increase in afterload lowers the stroke volume and increases left ventricle end-diastolic pressure (LVEDP). This is represented as the change in points A to C. Likewise, a decrease in afterload increases stroke volume and lowers LVEDP. This is represented as the change of points A to B.
Afterload reduction agents support patients with cardiogenic shock (usually occurs after a heart attack), severe mitral and aortic valve regurgitation (leaking of blood backwards once the valve is closed), and aortic stenosis (stiffening) by reducing the volume in the left ventricle thereby increasing the left ventricle contraction. This improves coronary artery perfusion and cardiac output. The medications used for afterload reduction include:
1. Sodium nitroprusside which dilates the blood vessels. When the blood vessels are dilated, afterload decreases.
2. Nitroglycerin which also dilates the blood vessels. It's mainly in the veins, so it has the same effect as sodium nitroprusside.
3. Hydralazine is an antihypertensive medication: it lowers blood pressure. When blood pressure is lowered, the ventricle can have a greater contraction and the heart can eject more blood.
4. Ace inhibitors that lower blood pressure and have the same effect as hydralazine.
The cardiovascular system is complex. However, when broken down into the processes involved in the cardiac cycle it can become more understandable. Cardiac afterload, or the resistance the ventricle has to overcome in order to pump out blood, is one part of the process that influences how blood flows through the heart. Specifically, the resistance comes from the blood vessels and the contraction of the blood vessel walls.
Remember that cardiac output is the amount of blood ejected by the heart in one minute. Afterload is one of four processes that make up cardiac output. The other three processes are preload (the pressure of the blood on the heart muscle fibers), contractility (the force of the contraction), and heart rate (the number of beats per minute).
To have more cardiac output you have to have lower afterload. Stroke volume is the actual contraction of the muscle fibers that eject the blood from the heart. There is an inverse relationship between afterload and stroke volume. Factors that affect afterload include age (stiffness = less contraction), increased blood pressure, or hypertension, (enlarged ventricle = less contraction), and constriction of the arteries.
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Back To CourseGED Science: Tutoring Solution
34 chapters | 724 lessons