Systemic vascular resistance

Nursing Students General Students

Published

Can anyone explain what SVR is (in easy to understand terms) and what the major mechanism is that increases SVR??? I appreciate any information. Thanks :)

Up Vote Up Vote ×
Can anyone explain what SVR is (in easy to understand terms) and what the major mechanism is that increases SVR??? I appreciate any information. Thanks :)

Lets say I give you a hand pump, like the one you would use to pump up a bike tire. Then I give you a 100 foot 1 inch diameter hose and say, "Now go ahead and pump a bunch of water through this hose." You say, "OK" and get to work. You pump and can feel the resisance as you push down the handle of the pump.

Then I say, "Here's a different hose that is the same length, but 1/4 the diameter (much thinner). It's only a quarter inch. Now start pumping that one."

You will find it MUCH harder to pump the same amount of water through that smaller hose. You will have to work a heck of a lot harder to get the same amount of water moved. And the water moving through the hose will be moving faster in order to move the same volume of fluid.

The increased pressure you felt was because the smaller diameter hose had dramatically increased the systemic resistance - the resistance you had to overcome by pushing the pump harder. The "Systemic Hose Resistance" increased as the hose diameter decreased.

Let's say I then said, "Hey, now try pumping maple syrup instead of water." Do you think you would have to work harder or less hard to pump thicker fluid through those hoses?

In the body, you have a pump, the heart and you have "hoses" - blood vessels. Many of those blood vessels, arterioles, have smooth muscle to allow them to change their diameter. They can constrict, which is like putting a thinner hose on your bike pump while still needing to move the same amount of fluid. This changing diameter causes the resistance the heart has to overcome to pump blood to change - this resistance is the Systemic Vascular Resistance. The heart has to work harder when the Systemic Vascular Resistance increases.

There are many factors that cause blood vessels to constrict or dilate (look them up), but it is the constriction and dilation that mainly affects SVR. The viscosity (or "thickness") of the blood can also affect SVR.

As opposed to the hose example, where you would always want the pump to not have to work hard, you often need that SVR to increase - say to keep your kidneys perfused or keep the brain's supply of 02 coming, so the tradeoff of having the heart work harder is a good one.

Good luck!

Up Vote Up Vote ×

Wow, you're amazing!! This was perfect...I get it now! Thanks a bunch.

Up Vote Up Vote ×
Wow, you're amazing!! This was perfect...I get it now! Thanks a bunch.

You're welcome. That was just a start, but you should be able to build on it.

I guess I kinda do miss my study groups this summer!

Up Vote Up Vote ×
Specializes in ER, progressive care.

I also want to add - that when it comes to SVR and PVR, they deal with afterload. Think of afterload and arteries, and what do arteries do? They constrict or dilate. Vasoconstriction will cause an increase in SVR because of the small diameter and the heart must work harder to pump the blood through a smaller space. Vasodilation will cause a decrease in SVR.

Up Vote Up Vote ×
Specializes in Nursing Education.

I love the hose explanation! It's a very clear way of illustrating it. Not much to add, except how disease processes can fit into it...

When you have plaques or clots, that narrows the "hose" and makes it harder to pump blood through.

There are also many things that can cause damage to our blood vessels, which causes scar tissue and (over time) thickening and hardening of arteries (arteriosclerosis). If the vessels become hard instead of elastic, that will increase vascular resistance. To mix metaphors, it's like the difference between blowing up a regular balloon (pretty easy), or one of those balloons that you use to make balloon animals (I blow and blow, but all I get is lightheaded and a red face). When there's less elasticity, it's also much harder to get blood flowing.

Up Vote Up Vote ×

That helps me too. Let me ask this. If you have an increase in SVR, that means that you have more resistance for a variety of reasons-plaque, high BP, etc. On the other hand if you want to decrease SVR, then you may be using drugs like beta blockers, ACE inhibitors, or other drugs that are going to vasodilate (so the blood vessel has less resistance)...am I on the right track?

Up Vote Up Vote ×

Oops sorry, didn't real Melosaur's post, which answers my question.

Up Vote Up Vote ×
Specializes in ER, progressive care.
That helps me too. Let me ask this. If you have an increase in SVR, that means that you have more resistance for a variety of reasons-plaque, high BP, etc. On the other hand if you want to decrease SVR, then you may be using drugs like beta blockers, ACE inhibitors, or other drugs that are going to vasodilate (so the blood vessel has less resistance)...am I on the right track?

And don't forget about nitro! It decreases preload more than afterload but it will also decrease your afterload. Calcium channel blockers are another drug class that will decrease your afterload. Hydralazine is another drug.

Up Vote Up Vote ×

SO helpful, thank you!!!!:woot:

Up Vote Up Vote ×

I know that the comment about the hose has been posted for 6 years, but I am just reading it! If you could explain SVR in laymen's term as well, I would greatly appreciate it!!! That was an AWESOME way to explain PVR and I totally get it whereas when it was being explained in class, I had no clue of what the instructor was talking about!! Sad, but its true! Thanks in advance for the explanation.

Up Vote Up Vote ×

This may have been posted in 2010, but the post is still helping lost students like myself. Thanks so much!

Up Vote Up Vote ×
+ Add a Comment