Normal sinus rhythm (NSR) is the rhythm that originates from the sinus node and describes the characteristic rhythm of the healthy human heart. The rate in NSR is generally regular but will vary depending on autonomic inputs into the sinus node.
A sinus rhythm is any cardiac rhythm in which depolarisation of the cardiac muscle begins at the sinus node. It is characterised by the presence of correctly oriented P waves on the electrocardiogram. Sinus rhythm is necessary, but not sufficient, for normal electrical activity within the heart.