The main diseases that can slow the heart rate are atrioventricular block or sick sinus node syndrome, etc. First-degree AV block may have a relatively small effect on the pulse rate. However, second- and third-degree AV block can seriously affect the pulse rate, as in severe third-degree AV block, the pulse rate may be mainly in the form of ventricular escape, or junctional escape, and the heart rate generally fluctuates between 30-40 beats per minute. Severe sinus bradycardia or a high degree of AV block may occur in sick sinus node syndrome due to malfunction of the sinus node itself, and the pulse rate may be below 45 beats/min. Other degenerative changes in the sinus node may also predispose to a slowed pulse rate, and the diagnosis of a slowed pulse rate is primarily made by performing an ambulatory electrocardiogram.