Cardiac Preload and Afterload

hoolahan, ASN, RN · Feb 23, 2003

Think of it like a garden hose.

Preload is about volume. If the watre pressure is low, the water output will be a trickle, not enough to water and sustain your pretty garden.

If the volume is too much, it will back up your plumbing system (Right-sided heart failure engorged liver, systemic edema, etc, or left-sided faiolure pulmonary edema.)

Afterload is about pressure or resistance. If there is a link or narrowing in your garden hose, the volume will back up AND the output will drop.

I can't really make a good analogy about the hose and low afterload, just less resistance, may end up w a relative hypovolemia ie use water glass and med cup analogy.

Fill a med cup (30cc) with water. Med cup is your normal vascular system, filled w normal blood volume. Pt becomes septic, results in massive vasodilation, which is like taking that 30cc of water, and pouring it into an 8-oz glass, it is a relative hypolvolemia, so most blood volume will be shunted to major organs, and periphery and kidneys will be sacrificed.

Just think in this way Preload = Volume

Afterload = Pressure/Resistance

orca1 · Feb 23, 2003

Hoolahan....

THANK YOU!!! Finally a response I can understand :roll :idea:

It is time to look over the notes and apply everything.

Lisa

caroljon55 · Apr 28, 2007

iam new here LPN to RN program very helpful explaination. thanks

gt4everpn, BSN, RN · Apr 28, 2007

1. Pre-load is the filling pressure of the heart, the pressure the heart has when it is relaxing. (Volume- In)

2. After-load is the pressure of the contracting heart. (Resistance-push- Out)

ghmccart · May 13, 2007

preload=volume/stretch measured in CVP/PCWP

afterload=resistance measured in SVR

jaquelynne · May 13, 2007

Gross analogy, but works for me....

Thing about flushing a toilet......

you flush, and then flush again right away......nothing happens right? THis is because the tank doesn't have time to fill....PRELOAD is decreased in the tank.

What if the toilet is plugged up? When you flush, it backs up....this is too much afterload.

jmca26 · May 22, 2007

Hi,

I am a student nurse and I need help from a professional

I want to know why hypertension or high blood pressure is common in patient with diabetes mellitus how is this happen?

thanks!

hikingcole · Mar 27, 2009

funny thing, i am taking my cardiac classes right now. i asked the professor about preload and afterload today and she simply replied that i need to read the material over again. i do hate it when they won’t take a minute or two to explain anything and very glad that most of our professors are not like that.

i just want to see if i understand preload and afterload now that i have read the replies to the question. is it that preload is the blood in the heart and the pressure caused by this and the afterload is the blood in the arteries when it leaves the heart and the pressure exerted by such?

i am very interested in the heart and wish our cardiac professor knew more about it and was more interested in answering our questions. she has admitted not understanding much of the content in the powerpoints she has posted. there is much she simply does not cover and tells us we will have to learn it on our own. we shall see what happens.

i’m so glad someone else had posted about this. i have yet to figure out how to post on this site. i joined, got the confirmation e-mail but every time i click on the link, internet explorer closes. i finally gave up.

hikingcole

Sarcolemma · Mar 27, 2009

in student terms:

preload = pressure inside the heart during diastole (resting phase)

-starling curve. to a point, the more volume sitting in a chamber, the better a heart works

afterload = pressure the heart has to overcome to squirt blood out of the ventricle

-anything that causes resistance. from a malfunctioning valve to tight arteries to standing on your head.

hikingcole · Mar 27, 2009

Thanks so much for the explanation. It's making more sense all the time. I have a week before my next exam on cardiac but have two exams and clinical during that week so I want to KNOW this and not just "get it" long enough to pass the exam. I was actually thinking about it when I woke up this morning. I think I know this one now.

Gale

joeyzstj, LPN · Apr 11, 2009

Think of preload in relation to a rubber band. If you remember back to Anatomy and Physiology class, there is an optimal point in which you can stretch a muscle fiber and it will have the most effiency. I belive its 120% of the orginal length of the muscle fiber. Sooooo..........think of a rubber band. You stretch it a little and let go and it barely moves. You pull it back farther and farther it really starts to snap back hard. You pull it back unti you cant pull it anymore and it snaps the strongest. This is the same concept as preload............you are Preloading the cardiac muscle sarcomere (muscle fibers) by giving volume we stretch the heart far enough that we get a good snap from the heart. Afterload is the resistance that the heart pumps against when it ejects the blood. Its actually the measurement of Left ventricular wall tension during systole. SO, we preload the heart with a certain volume and then we eject it. Now, afterload will be high if we have to eject that volume through something the size of a coffee stirer. Afterload will be low if we eject the volume through a vessel the size of a car tire. On a extra note, CHF is similar to us stretching that rubber band sooooo far that it loses its optimal stretch. We stretch it out so far that it is beyond that 120% point and is too lose and streched out to recoil well. That is CHF in essence.