why is preload and afterload important?

Your question directly refers to Starling's Law - pre-load is the amount of blood entering the ventricles during systole in order to be pumped to the body with the next contraction. The greater the stretch the more force the heart will use to pump the blood a direct increase in stroke volume.

After load is the resistance that the blood must be forced though during contraction. vasodilation would lower the afterload and allow more blood to be pumped by the heart and vasoconstriction would do the increase the afterload and more blood would remain in the heart. Thus an increase in afterload would decrease stroke volume and a decrease would increase stroke volume.

As far as measuring these pressures -- this is something that is usually done in ICUs and CCUs, but there are hemodynamic monitors and calculations that provides accurate information.

A good example of why it would be important for a nurse - consider someone having (or suspected to be having) an active MI. Wouldn't it make sense that the harder the heart has to work the more potential for damage there would be. Think about why you would give NTG - a vasodilator, it decreases the afterload (the resistance that the heart has to pump against) that allows more blood enter systemic circulation and thus more O2 to get to the body as well as the myocardial muscle itself.

Thank you so much 203bravo!!! That answered all my questions! In fact Frank-Sterling's law is in my book I guess I wasn't understanding. I'm basically writing your reply in my notes :) :)

Since preload is the stretch of heart muscle, its increases ultimately has to do with volume. Increase stroke volume could lead to pulmonary congestion and edema; to lower preload, you'll see diuretics being used to shed the excess water. As a nurse, then you have to watch the patient's diet, I&Os, daily weights etc. also auscultate lung sounds to make sure fluid isn't back up there.

To decrease afterload you use meds that dilate the vessels (NTG, ACE inhibitors etc.), which lowers workload. usually inotropes like dobutamine is used to boost heart's contractility. assess BP, pulses, and ekg etc?

This is sort of treatment for people with CHF. If you know what each drugs is suppose to do then you'll know signs when something isn't working. Maybe that's the point. It's a simple example but I actually worked with a patient with CHF so the individual had all these meds and then some.

Thank you Umbdude that helped also! I could feel it struggling to come together in my brain haha.

One thing though please. I got really confused about what I read on this site CV Physiology: Afterload

It says "When afterload increases, there is an increase in end-systolic volume and a decrease in stroke volume." But I thought the stronger the contraction (from the greater the afterload) means blood volume and stroke volume increases?

One thing though please. I got really confused about what I read on this site CV Physiology: Afterload

It says "When afterload increases, there is an increase in end-systolic volume and a decrease in stroke volume." But I thought the stronger the contraction (from the greater the afterload) means blood volume and stroke volume increases?

Remember that afterload is the pressure that the heart must pump against to get blood out.. the greater that pressure the harder the heart must work... with significant pressure less blood is able to be pumped (decreased stroke volume) with each contraction and more blood is left in the ventricles and in the short term while this will increase preload (the stretch of the ventricles filling) it will trigger a stronger contraction to push the excess out (increased stroke volume)... but left untreated over time the ventricles will become less susceptible to this trigger and the contractions will not be as strong resulting in less blood being pumped (decreased stroke volume) and in cases such as cardiomegaly this can become permanent.