Cardiac Preload and Afterload

emily_mom · Feb 23, 2003

What exactly are you looking for? This is what I know....

Preload - it's the pressure at the end of diastole (filling of ventricles)...aka left ventricular end diastolic pressure. It's primarily a reflection of volume and is influenced by venous return. Increased preload (to a point) = increased cardiac output

Afterload - (in aorta) resistance to ejection during systole (the force opposing ventricular ejection). The ventricle must overcome aortic pressure in order to eject blood. Increased afterload = increased workload and decreased cardiac output and slower contraction.

Basically, preload is how much blood are you giving your heart to pump. Along with this is the Frank-Starling Law (see below).

With afterload, the left vent pressure has to be more than the aorta so the blood can get out. There is no benefit to increased afterload.

Contractility goes hand in hand with this also. It's the degree of myocardial shortening. It depends on ATP and other factors including hypoxemia. Moderate hypoxemia (O2 = 50-80) = increased contractility. Severe hypoxemia (O2 =

Frank-Starling Law- the bigger the stretch (to a point), the stronger the contraction.

Stretch of the myocardium is increased by increased filling.
Increase or decrease in the volume of blood = an increase or decrease in the force of contraction (like a rubber band flying)
This can max out...your actin and myosin can disengage and your force of contraction will drop to zero (like a rubber band breaking)

Hope this helps!!!

baseline · Feb 23, 2003

Well...no need for me to post now! Good job Em's M!

RN always, BSN, RN · Feb 23, 2003

WOW! You Go Girl! I wish I had those brains!

orca1 · Feb 23, 2003

Oh my gosh!!! I wish I could think like that. I am printing your response as I write and will be memorizing it for tomorrow.

Thanks

Lisa

RNIAM, BSN, RN · Feb 23, 2003

Hi .I just wanted to say thanks for the info.I am testing on this info on Tuesday and you make alot of sense.

emily_mom · Feb 23, 2003

It's easier to remember with arrows, but I couldn't do that with this BB.

You're welcome! Glad to help (nice to know I retained something!!). Let me know if you have any other problems. I can try!

One thing I forgot to mention.

The L side of the heart is usually bigger due to the intense amount of work needed to pump that blood out against aortic pressure. The R side is smaller b/c the pressure is smaller as it really wants to be oxygenated by the lungs.

Good luck on your tests all!

Kristy

hoolahan, ASN, RN · Feb 24, 2003

I don't this you would need to know this for your test, it's one of those "nice to know, but not need to know for a student, but....

Just a note to add on afterload. Resistance can occur at any point in the heart, not just the aorta, ex mitral valve stenosis can cause inc afterload to the left atrium, pulmonary Hypertension can create inc afterload to the right side of the heart, etc... Pulm HTN would be treated much differently than ordinary HTN, that is why I threw that in.

emily_mom · Feb 24, 2003

Pulm HTN will cause the R side of the heart to enlarge, as it's doing way more work than it did before.

StudentRN · Feb 26, 2003

Thanks Kristy!!!!! I to have this on a Midterm Monday!!!! Almost 1/2 way thru my second and hardest semester!!!

orca1 · Feb 26, 2003

Hi Everyone!!!

I want to THANK YOU all of you for your help. Unfortunately there was only 1 question on the test regarding preload/afterload. Anyhow, I hope everyone who has to still take their cardiac/neuro test does better than myself and my classmates. Apparently we all did not do well. I passed but once again here is an "A/B" student now getting "C"s. Believe me, I am happy I passes just in shock....I am wondering where my brain went??? :confused:

Thanks again

Lisa

tarty/sweet, BSN, RN · Sep 17, 2008

Thanks as a new grad and a new member to site I am soooo grateful to have access to understandable knowledge. It is daunting trying to recall and apply at the bedside what I probably crammed during school.