A medical procedure to drain fluid from the pericardium - the sac in which the heart sits. In the emergency situation this is likely to be blood causing cardiac tamponade (sac full of fluid, heart can't fill).
The procedure essentially involves sticking a large needle through the chest, into the pericardium and sucking out the fluid. The usual technique is to have the patient lying at 45 degrees, and to place the tip of the needle just under the xiphisternum (the little nub you can feel at the bottom of your breastbone). Push through, aiming for the lower tip of the left scapula.
You do not want to go too far.
This is why the procedure is usually performed under echo guidance.
The Seldinger method may be used (thread wire through needle, thread tube over wire) to leave a pericardial drain in place.
In expert hands, pericardiocentesis is safe and effective and can be a life-saver.