Preload/Afterload

[noele1213]

Hi there... Is there anyone out there who has had A&P2 and can take a few minutes to explain preload and afterload to me. I have read it too, too many times in many different ways and I'm still not able to envision it (which is what I have to do in order to retain or understand something) So if you can explain with an analogy, all the better. I need a visual... LOL Thanks guys.

[Race Mom, ASN, RN]

I'm sure you need just one more thing to read on this!!! This is not out of my brain, that's why it may make sense!

Preload is what comes to the heart before contraction. The heart muscle is much like a rubber band. The more you stretch it, the better it will contract. This stretch is accomplished in the heart through filling of the ventricles with blood. Therefore, preload is related to the amount of blood in the ventricle before contraction. If a person is overhydrated, preload will increase. If a person is dehydrated, preload will decrease.

Afterload is what comes after ventricular contraction or the resistance against which the heart must pump blood. Afterload is determined by two conditions; the blood volume ejected from the ventricle and the compliance of the vascular space into which the blood is ejected. Think of afterload as a hose nozzle. If the hose nozzle is wide open, afterload is decreased due to decreased compliance. If the hose nozzle is almost closed, afterload will increase because the water has so much resistance to push against. Now, if you increase or decrease the amount of water that comes from the nozzle, there will be a further effect on the afterload. Think of afterload as blood pressure. Increased blood pressure is increased afterload, while decreased blood pressure is decreased afterload.

Hmmmmmmmm.

Woogy

[Fun2, BSN, RN]

First check out this allnurses.com thread:

OR......

I'm not sure about this, but maybe this way will help:

The clearance of the pump - imagine a pump which is basically a hollow rubber ball with tubes coming from it (an inlet and an outlet). If you squeeze the ball slowly the air comes out gradually and without much force, but if you give it a good hard squeeze a lot of air comes out quickly. The same can be done with the heart, the body can increase the amount of blood it pumps out with each squeeze (contraction). The technical term for the amount of blood squeezed out with each contraction is stroke volume (SV). This variable is linked with the stretch of the heart prior to contraction (preload), the force of the contraction (contractility) and the pressure that must be obtained to actually get the blood out of the heart (afterload).

Preload - If the muscle walls of the heart are stretched prior to a stroke then they will squeeze harder on the stroke. To get a greater stretch more blood must be in the heart. This observation is called the 'Frank-Starling law of the heart'.

Contractility - The contractility of the heart is influenced by inotropic agents that make it pump either stronger or weaker. Positive inotropic agents such as Calcium ions and adrenaline make it pump stronger, negative inotropic agents such as potassium ions make the stroke weaker.

Afterload - In order to get blood out the heart the back- pressure in the blood vessels must be overcome. The harder it is to get blood out, the less blood will actually leave the heart. You can imagine if the blood vessels are clogged or narrow that this will make things harder.

[NurseB_, BSN, RN]

Great thread! I'm currently studying this concept and I think I understand it a whole lot better now!

[akulahawkRN, ADN, RN, EMT-P]

Preload is affected by venous blood pressure, rate of return, and the "atrial kick". Blood will normally passively fill the ventricles. The atria will add a little bit to essentially complete the filling, thus increasing the volume in the ventricle, stretching it a bit, and thus by the Frank-Starling law, increase cardiac contractility. If you decrease venous pressure or the venous return, you're going to drop the preload. This decreases the stretching, and thus decreases the contractility.

In other words, drop the preload and you ease the heart's work because it's not contracting as hard. Drop the afterload and you drop the workload on the heart because it doesn't have to push as hard to move blood around.

[P-rosie]

Thanks for your explanations! I am taking A&P 2 during the summer and it is getting more and more confusing. Anyone has anything to suggest about how to retain the material and pass this class during summer?