Pericardial effusion is a condition in which there is an abnormal accumulation of fluid in the pericardium, the sac that surrounds the heart. If this accumulation of fluid is great enough, it can impair the function of the heart, and this is called cardiac tamponade. If aortic dissection causes a pericardial effusion, it is usually due to blood leaking from the wall of the aorta into the pericardium. This can rapidly progress to cause cardiac tamponade. Prompt treatment can relieve this.