Why does air in the bloodstream trigger a heart attack or whatever it does?

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Why does air in the bloodstream trigger a heart attack or whatever it does?

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The blood system uses tiny capillary blood vessels to deliver the blood all the way to the cells. A lot of these are thin enough that normal cells can not even squeeze through them. If you get an air bubble in your blood stream then this air bubble might end up in one of these small capillary vessels and get stuck. If that happens then no blood can pass and the cells will not get as much nutrition.

How serious this is depends on where it is. The body does have a lot of redundancy and can handle quite a few different things going wrong. However if this happens in the heart or the brain it can cause quite a bit of damage.

Doctor here:
What you’re describing is known as an air embolus – an embolus being anything (air, fat, blood clots) travelling through vessels to the point where they become too small and the anything gets stuck.

The effect of an air embolus will depend on what kind of vessel it enters, as this will dictate where it gets stuck.
If you introduce air through a vein (i.e. vessel carrying de-oxygenated blood towards the heart), it will (usually…) go to the heart. If it’s small enough the heart will pump it out to the lungs, where it will then sit in blood vessels and block blood getting to the lung tissues downstream. The effect of this is you will have a patch of lung that isn’t doing it’s job and is dying – this is known as a pulmonary embolism/PE. How severe this is depends on how big the bubble/embolus is, as a bigger embolus will prevent a larger area of lung from working properly. This is painful, and can make you very breathless and need oxygen. Very big emboli can be fatal as they stop enough lung from working and you can’t compensate for it. That being said an air embolus that big is very rate and it tends to be emboli made from blood clots that cause PEs this severe.

If the air bubble is too big to make it out the heart, it will get lodged in either the right atrium or right ventricle. This will stop the heart from being able to send enough blood to the lungs to be re-oxygenated through several mechanisms. In turn this means no blood comes back to the heart to be sent around the body, and cardiac output (the amount of blood pumped by the heart to the body) drops. A large enough drop in cardiac output can cause the heart rythmn to fail, and result in no meaningful flow of blood around the body (cardiac arrest) which is fatal.

Alternatively you could get air entering an artery, but this would be quite an achievement: arteries are under higher pressure than the atmosphere. So if you open an artery the blood rushes out, – the air doesnt rush in.
As a result arterial emboli are rare – but they are possible! Some people have a “patent foramen ovale”. This essentially means there is a hole between the right (venous) side of your heart into the left (arterial) side of your heart that failed to close when you were born. It is possible for emboli such as air bubbles to enter a vein, travel to the heart and pass through the hole and get pumped out down an artery to the rest of the body. If it hits the brain – it blocks off blood to brain tissue causing a stroke. If it goes to the coronary arteries (blood supply to the heart) it will cause a heart attack. It can get to other places, but these are the most common. Both strokes and heart attacks are fatal.
If it doesnt get pumped down an artery the embolus can sit in the left side of the heart and cause decreased cardiac output leading to the same problems as outlined above.

The amount of air needed to actually kill someone has estimated to need to be quite high (300-500ml given over 3-5 seconds) according to one paper I read. Non air emboli however are common and do cause a lot of problems… such as most heart attacks Avoid them if you can.