Dear Cecil: What's the deal with air in your bloodstream being lethal? Is this true? I have to assume it is; presumably it's the reason for tapping on a syringe to get the air out of it. But if so, how does it stop your heart? How much is too much? Is it 100 percent lethal? Please give us all the gruesome details. I love gruesome details. Scott Black

Cecil replies:

Sure thing, Scott — maybe this will tide you over till the next car wreck. Air embolism, as the MDs call air in the bloodstream, can definitely kill you. The mechanism of death or injury depends on the size of the air embolus (the bubble) and where it lodges in the body. One way is akin to vapor lock, an automotive problem in the beaters of my youth. If vapor developed in the fuel line, the engine died. If an air bubble gets into a blood vessel, so might you.

I’ll explain that shortly, but first we need to understand how air gets into the blood in the first place. We’ve discussed some bizarre routes here in the past — for example, by blowing into the vagina of a pregnant woman during oral sex. More common is air entering accidentally via injection or IV tube, or when blood vessels are cut during surgery. Another possibility arises during ascent after scuba diving, where an increase in air volume in the lungs pushes tiny bubbles of air into the bloodstream that expand as you rise.

Here we need to distinguish between little bubbles and big ones, because they do damage in different ways. Small bubbles can block capillaries in vital organs, most urgently the brain, causing anything from pain and inflammation to neurological damage and paralysis. A small bubble impedes blood flow the same way a solid obstruction would — the bubble’s surface tension relative to its size is too great for the force of blood to break it up or shove it along. Bad? Yes. Fatal? Probably not, although see below.

A big bubble, on the other hand, gets us into the vapor lock scenario. Your heart, like the fuel pump in an old car (cars with modern fuel injection work differently), is a simple mechanical device. In ordinary operation, its contracting chambers squeeze the blood out and force it through the circulatory system. All is well. Now imagine a massive air embolus shows up and your heart starts squeezing on that. There’s nothing to get any purchase on; the air just compresses. Blood flow stops, and eventually so does your heart.

It also makes a difference where the air bubbles enter. Emboli from injections or IVs are typically confined to veins, but if a bubble ends up in your arteries (which can happen if you have the double misfortune of air in your veins plus a fairly common congenital heart defect), then the bubble can block your coronary arteries or the blood supply to your brain. The former type of blockage, at least, can mean death.

How much air is needed to kill you? That’s debatable and doubtless varies, but generally speaking, a lot. One journal article I saw boldly declared that 300 milliliters can be lethal — three-tenths of a liter! You’d need a bicycle pump to inject that. But much less will do the trick; it’s said serious damage can result from as little as 20 milliliters, which still isn’t a small amount. In 1949 New Hampshire physician Hermann Sander ended the life of a terminal cancer patient by injecting her with 40 milliliters of air — four syringes of 10 milliliters each. (He called it a mercy killing when arrested but on the stand improbably claimed that he thought the patient was already dead; at any rate he was acquitted.) But people have survived much larger amounts. French doctors reported in 2006 on an 82-year old man scheduled for a CAT scan who was supposed to get 90 milliliters of contrast solution but instead got 90 milliliters of empty syringe. Prompt treatment with pure oxygen saved him.

Despite the uncertainties, air embolism has served as a reasonably dependable method of execution. After public outcry stopped Nazi gassing of mental patients in 1941, psychiatric institutions were ordered to continue so-called mercy killings by less conspicuous means. A program described as “wild euthanasia” began at the Meseritz-Obrawalde hospital in 1942, with doctors selecting the victims and nurses doing the deed. While most of the murders were carried out with overdoses of sedatives, some patients were injected with air, which usually killed them within minutes. Though thousands of patients died, at trial years after the war 14 nurses claimed they were just following orders and were acquitted. Decades later Germany saw another rash of murders-by-embolism when a nurse confessed to injecting an estimated 60 to 130 milliliters of air into the veins of 15 seriously ill elderly patients. All died. So maybe not 100 percent lethal. But lethal enough.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.