An
embolism caused by air entering the
bloodstream. It eventually travels to the heart, where it fills the
right atrium with frothy bubbles, which form an
air lock in the
heart.
The heart can't pump blood around the body, because the usual compressing action that causes this to happen just squeezes the bubbles a bit, rather than forcing blood out of the atrium. If nothing is done, death usually occurs.
Air embolism is quite often deliberately inflicted during syringe attacks by people who are unaware of its serious effects.