A Sinus Rhythm is the pattern of electrical activity that is seen on an electrocardiogram (ECG or EKG) when the heart is beating normally. Normal sinus rhythm has waves in the cycle that are named P, QRS and T. These waves correspond to events in the cardiac cycle. The ECG is the most important and definitive noninvasive diagnostic test for cardiac arrhythmias. An arrhythmia is characterized on an ECG as a deviation from the normal sinus rhythm in rate, regularity or as a different pattern of activation of the cardiac muscle.