A difference in blood pressure between the arms can be a sign of aortic dissection, although it is only present in about 15% of patients with an aortic dissection. So, if there is no difference in blood pressure between the arms this does not allow an aortic dissection to be ruled out. Aortic dissection is a serious condition that involves a tear in the inner layer of the aorta, the large blood vessel that carries blood from the heart to the rest of the body. This tear can cause blood to flow between the layers of the aortic wall, leading to the formation of a false lumen, or channel.
In some cases, the difference in pressure between the false lumen and the true lumen can affect blood flow to the arms. If the pressure in the false lumen is higher than the pressure in the true lumen, it can cause the blood vessels in the arms to become compressed, reducing blood flow and leading to a lower blood pressure in the affected arm.