Final answer:
The normal sinus rhythm is a regular heartbeat rate of 60-100 beats per minute, established by the heart's natural pacemaker, the sinoatrial node.
Step-by-step explanation:
The correct option is A:
Normal sinus rhythm is the normal, rhythmical beating of the heart established by the sinoatrial node, the heart's natural pacemaker. It is established by the heart's pacemaker cells in the sinoatrial node. Electrical signals from the pacemaker cells travel to the atria and cause them to contract, and then to the atrioventricular node and from there to the ventricles, causing them to contract as well.
It indicates that the electrical signals governing the heart's contractions are functioning correctly. Sinus rhythm is considered normal if the heart rate is between 60 to 100 beats per minute. Tachycardia refers to a resting heart rate over 100 beats per minute, and bradycardia indicates a heart rate below 60 beats per minute in adults. Symptoms like dizziness or fainting can accompany these conditions and should be assessed by a healthcare provider. In athletes, lower rates may be seen due to conditioning but are not typically harmful unless symptoms suggest inadequate blood flow.