Bradycardia
Problems with the heart's electrical system.  There are two atria (upper chambers) in the heart: the right atrium and the left atrium.  For the heart to beat normally, special parts of the right atrium have to send electricity through the heart to make it squeeze out blood.  The electricity has to start in a certain place and pass through a certain route in the heart.  If this does not happen correctly, the heart will not beat correctly.