How do Chest Compressions Work?

Are You Really Pumping the Heart?

When can you stop CPR?. Gary Ombler/Getty Images


How do Chest Compressions Work?

First, there was internal cardiac massage -- basically the doc cut open your chest, reached in and squeezed -- but eventually came this new fangled thing called external cardiac massage or closed cardiac massage. These days all the kids are calling them chest compressions, which is a better term since the heart is too deep in the chest to really be getting a massage at all.

So, you might ask, if the heart isn't getting its spa time, then how in the world do chest compressions make the blood go 'round?


Veins should get most of the credit.

To be fair, all the blood vessels are helping guide blood flow through the chest cavity during CPR, but there's one unique feature of veins that helps move the whole thing along. Without the help of veins, I don't think this CPR thing would work all that well.

Blood Vessels 101

The pipes that carry blood around our bodies are roughly categorized into one of three types: arteries, veins and capillaries. Capillaries are the tiniest of blood vessels, so much so that they only allow red blood cells through in single file. There are so many capillaries in each square inch of muscles and other tissues that it's impossible to see them with the naked eye. Indeed, if you look at muscle and other soft tissue, it appears to be soaked in blood rather than having defined blood vessels running through it.

I always think of tissues as sponges and capillaries as the channels in the sponge that help it soak up fluid. We'll come back to the sponge idea in a minute.

Arteries are the vessels that carry blood from the heart to the tissues. These are high pressure pipes with thick walls that have the ability to expand or constrict in order to help control the flow.

When we're new, young and healthy, these are clear and open. Over time as we age, we get some crust (called plaque) that forms on the inside. The plaque forms a little bit in everyone, but in folks that get plaque really bad -- especially in the arteries that carry blood to the heart muscle -- they run the risk of blocking blood flow, which can lead to heart attacks and strokes.

Veins collect blood from the tissues and ferry it back to the heart. These guys deal with much lower pressures than arteries do and have thinner walls. They are not as capable of expanding and constricting. To keep the blood moving with such little pressure, however, veins have one important thing that arteries don't: valves.

The valves that are found in veins only allow blood to flow in one direction. You can see how this works if you can see the veins in the back of your hand. Try this: let your hand dangle by your side for 10 or 15 seconds. Shake it out a little. Let the blood settle. Now make a fist and find a vein on the back of your hand.

Run a finger from your other hand up your vein toward your heart. The vein flattens out but doesn't disappear. Now run your finger the other way along your vein, it stays flat until you lift your finger. When you push the blood out, the vein flattens and as long as you keep your finger on the vein to stop new blood from entering upstream, it will stay flat. The blood that's downstream can't come back because of the valves.

You might even be able to find a valve on your hand; look for a spot where the vein isn't flat while your finger is on the vein.

Blood Flow by Compression

Now that we understand how the blood vessels work, let's see how that translates into blood flow when we compress. As I said, body tissues and muscles act like sponges. When you squeeze a soaked sponge, fluid oozes out of it. In the case of body tissues, squeezing forces blood out. Blood getting squeezed out of the tissues can go into the veins or the arteries. Blood that goes into the veins can't come back because of the valves. After a few compressions, there's enough pressure to start moving blood through the veins and back to the heart.

The heart is included in this one-way valve business. Each of the chambers of the heart has a valve. Once blood leaves a heart chamber, it's not allowed back in until it goes all the way around the body and back. You might not be massaging the heart directly during compressions, but you are definitely squishing the chambers closed.

Sucking and Squeezing

Squishing blood out of the tissues isn't the only way that blood is encouraged to flow when we're doing chest compressions. As important as it is to push on the chest, it's also important to allow the chest to recoil (return to its original shape). Just like in a sponge, when you release the squished tissues, they suck up fluid. Plus, since the chambers of the heart are roughly in the middle of the chest and they have those nifty one-way valves in them, they suck up blood as well during recoil.

There is a lot of evidence that the sucking action of recoil between chest compressions is just as important during CPR as pushing is. One of the theories of why Hands-Only CPR is so successful has to do with the fact that nobody is blowing into the chest and reducing its ability to suck blood in. Besides, for patients who collapse from sudden cardiac arrest, there's plenty of oxygen left in the bloodstream so mouth to mouth isn't really necessary.


Andreka P, Frenneaux MP. "Haemodynamics of cardiac arrest and resuscitation." Curr Opin Crit Care. 2006 Jun;12(3):198-203. Review. PubMed PMID: 16672776.

Lurie KG, et al. "Comparison of a 10-breaths-per-minute versus a 2-breaths-per-minute strategy during cardiopulmonary resuscitation in a porcine model of cardiac arrest." Respir Care. 2008 Jul;53(7):862-70.

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