The heart is like a pump that constantly provides oxygen and nutrients to all organs of the body and carries away metabolic waste and toxins. It can be said that human life is built on the basis of the normal beating of the heart. People have long been aware of this in their observations, so they generally know to judge whether a person is alive or dead based on whether there is still a heartbeat. So, what kind of heartbeat is considered normal? What is an abnormal heartbeat? Structurally, the heart is divided into four parts, the upper part is the left and right atria, and the lower part corresponds to the left and right ventricles, and the normal human heartbeat is controlled by a piece of tissue called the sinus node, which is located in the right atrium. So how is it controlled? In life, if you are accidentally stimulated by electric current or static electricity, a person’s hand will suddenly contract, which is the effect of electric current on the muscles. There are special cells in the sinus node that discharge regularly like fireflies, and the heart’s muscles contract regularly according to its instructions, producing a pounding, or commonly known as a heartbeat. Of course, this current is very weak and will not be felt. Since a normal heartbeat follows the instructions of the sinus node in a regular manner, the medical term for a normal heartbeat is sinus rhythm. In other words, a normal, regular heartbeat is a sinus rhythm. Sinus rhythm is constantly changing. Normally, the majority of people in sinus rhythm should have a heart rate – that is, the number of beats per minute – between 60 and 100 beats per minute, but this is not absolute. Some people are born slightly slow and others may be slightly fast. Moreover, each person must also be variable throughout the day, otherwise it is rather abnormal. For example, in a normal adult, if a 24-hour ambulatory ECG is done, then it can often be as slow as 37-40 beats per minute at night when sleeping, and it may be as fast as about 120-130 beats per minute during the day. This is related to a person’s physical activity, emotional changes, and changes in body position. People who exercise regularly generally have a slow heart rate, while those who lack exercise, lead an irregular life and suffer from insomnia will have a faster heart rate. Deep inhalation, lying down and other actions will slow down the heart rate, while stress, drinking tea or coffee, drinking alcohol, walking or going upstairs may make the heart rate speed up again in different degrees. Due to the complexity of the human body, the same person does not respond to the same stimulus in exactly the same way at different times. Therefore, the heartbeat may not feel uncomfortable when the person is in good health, but when the person is not well rested or fatigued, the same action may cause a feeling of panic. If the heart rate is more than 100 beats/min, we call it sinus tachycardia, normal people have some times of sinus tachycardia every day. If an adult’s slowest heart rate falls below 50 beats/min at night while sleeping, it may also be abnormal. If your heart rate is always over 100 beats/minute or slower than 60 beats/minute, there is no need to panic too much. If the heartbeat is not beating according to the instructions given by the sinus node, the situation is relatively more complicated and we call it an arrhythmia, which means that there is a lesion in the heart that will discharge abnormally and the doctor must capture the ECG at the time of the offense in order to determine where this arrhythmia originates from in the heart. Depending on the location of the lesion, it can be broadly classified as atrial, atrioventricular junctional, or ventricular. In general, atrial and atrioventricular junctional arrhythmias are less dangerous and severe, while ventricular arrhythmias are relatively more severe.