Preload and Afterload...does anyone have any good definitions?

Bruno Matos · Nov 9, 2001

Preload consists in the volume of ventricular filling at the end of diastole (pre-systole). The ventricular preload controls the strenght of the subsequent cardiac contraction.

Afterload is the amount of strenghts that the ventricule have to beat to eject the blood. It's directly determinated by diastolic pressure.

CVP (central venous pressure) - it's the pressure in the right atrium, to show how the right side of the heart it's working (elevated in case of right cardiac insufficiency).

I hope this answer can help you.

(sorry the ortographyc mistakes, I'm from Portugal...)

Bruno M. :rolleyes:

PhantomRN · Nov 10, 2001

firsr consider that when we speak of the workings of the heart we are mostly referring to the left vent as it is the most important part. it gives all the pumping power to purfuse the body.

preload is the amount of blood returned to the heart that is in the ventricles, right before the heart squeezes to give you the systolic part of the blood pressure.

you can measure preload by the pap (pulmonary artery pressure).

range 20-30 (systolic) / 10-20 (diastolic)

with a mean of 10-15.

the systolic pressure in the pap is a reflection of right vent pressures.

where as the diastolic pressure in the pap is a reflection of the left vent of the heart.

the mean is used to calculate the pvr, which we will get to.

afterlaod.......ok now we have all this blood in the vents ready to be expelled..........bang it is expelled...we are now into afterload. because afterload is the resistance that the blood will meet when it tries to leave the heart.

so the blood expelled from the left vent is hitting the aorta and pulmonary artery (going to the lungs). we will look at how much resistance is met when it hits the aorta. a lot of resistance ie. the vessel is vasoconstricted we get a higher systolic blood pressure.

there are of course other factors that can inhibit the blood leaving the heart, but you get the idea.

so the blood tries to leave the heart and goes out via the pulmonary artery and the aorta, so here is where we will measure afterload. we use pvr (pulmonary vascular reistance) for the pulmonary and svr (systemic vascular resistance) to measure the aorta side.

like i said above we use the mean of the pap to calculate the pvr. well, to calculate the svr we use the cvp.

the cvp is a reflection of the right side of the heart, the right atrium and the right vent.

PhantomRN · Nov 10, 2001

To help you get a better understanding of CVP I should state that we use the CVP to figure the systemic vascular resistance. BY this we take the Cardiac output and measure it against the input IE the CVP.

PhantomRN · Nov 13, 2001

I come to this site looking for information to get and give such as what I gave above. :cool:

prn nurse · Jan 3, 2002

Thanks a lot for your clear and comprehensible explanation. I had given up on anyone replying and forgotten I posted...then , this morning I rolled outa bed early to let the critters in and decided to see whats on all-nurses, and to my surprise there was my old post with your reply.

It was just like receiving an unexpected present! Your explanation makes it easy to understand and remember...I'm going to print it..thanks again.