Causes of First Degree Heart Block. Many highly conditioned athletes with an increased vagal tone have a first degree heart block. Vagal tone is an intrinsic inhibition of heart beats by the vagal nerve that is responsible for slowing down the heart. Other causes of first degree heart block are: Infectious disease – such as Lyme disease, which is associated with carditis (a heart complication of the disease) and usually found in children; viruses can also cause myocarditis or inflammation of the heart muscles.