What does the term 'normal sinus rhythm (NSR)' refer to?

Normal sinus rhythm (NSR) refers to a heart rhythm that originates from the natural pacemaker of the heart, known as the sinoatrial (SA) node. It signifies a regular rhythm where the electrical impulses are generated at a consistent and normal pace, typically between 60 to 100 beats per minute. This rhythm is characterized by a sequential activation of the heart chambers, allowing for efficient and effective pumping of blood.

In NSR, the electrical activity in the heart is orderly, leading to predictable heartbeats that maintain proper timing and coordination among the atria and ventricles. This orderly function is crucial for normal heart function and ensures adequate blood flow throughout the body. A clear indicator of NSR is the presence of consistent P waves preceding each QRS complex on an electrocardiogram (ECG), which reflects the atrial depolarization before ventricular contraction.

Other options describe heart rhythms that deviate from this normal pattern, such as a fast heart rate (tachycardia) or irregular rhythms (like atrial fibrillation), which do not originate from the SA node and thus do not represent a normal sinus rhythm.