Sinus rhythm is the term used to describe the normofrequency and regular heartbeat in humans. This rhythm is formed in the sinus node.
What is sinus rhythm?
Sinus rhythm is the term used to describe the normofrequency and regular heartbeat in humans. Sinus rhythm is the normal heart rhythm. The number of heartbeats per minute is called the heart rate or heart beat rate. In humans, heart rate depends on exercise, age, and physical condition. While sinus rhythm results in about 120 heartbeats per minute in a newborn, a person in his 70s has a rate of about 70 beats per minute. The physiological range of heartbeat frequency, and thus of sinus rhythm, in a healthy person at rest is 50 to 100 beats per minute. Sinus rhythm is formed in the sinus node in the right atrium of the heart. The heart consists of two chambers and two atria. Blood enters the right atrium from the systemic circulation and flows from there into the right ventricle. The right ventricle ejects the blood into the pulmonary circulation. After oxygenation, it flows into the left atrium and from there into the left ventricle. The sinus node is located in the right atrium in the superior vena cava area. This area of the orifice of the superior vena cava into the right atrium is called the sinus venarum cavarum. The term node is misleading. The sinus node is not a visible or palpable node. Rather, the sinus node can be detected electrically. In addition, there is a subtle difference in tissue from the neighboring cells. The sinus node lies close to the epicardium. The location and size of the sinus node varies greatly depending on the individual. For example, the node can be between 10 and 20 millimeters long and between 2 and 3 millimeters wide. The sinus node is supplied with blood by a branch of the coronary arteries. In addition, there is a collateral supply with other vascular branches. This ensures that if the coronary artery (part of the coronary arteries) is blocked, the blood supply can be maintained. Compared to the cells of the working myocardium, sinusoidal cells have fewer mitochondria and myofibrils. Therefore, they are less susceptible to oxygen deprivation.
Function and task
Histologically, the sinus node consists of several specialized cardiac myocytes. These, unlike the other muscle and nerve cells, have the capacity for spontaneous depolarization. During depolarization, the membrane potential at the cell membrane decreases. In the unexcited state, a resting potential is present. During spontaneous depolarization, the voltage-gated sodium ion channels of the sinusoidal cells open and an action potential is triggered. This occurs between 50 and 100 times per minute in healthy humans. Because of the enlargement of the heart, the sinus rhythm in endurance athletes is often less than 40 excitations per minute. The excitation that originates in the sinus node travels to the atria via the working muscles of the heart. Via so-called internodal bundles, the electrical excitation is conducted to the AV node. The AV node is located in Koch’s triangle in the right atrium. Like the sinus node, it consists of specialized heart muscle cells. The AV node continues into the His bundle. The His bundle is also part of the conduction system. It lies below the AV node toward the apex of the heart and continues into the tawara bundle. At the cardiac apex, the two Tawara legs divide into the Purkinje fibers. These represent the final conduction pathway of the excitation conduction system and are in direct contact with the cardiac fibers of the working muscles. The excitation conduction system is responsible for the contraction of the individual cardiac muscle cells and thus for the contraction of the entire cardiac muscle. The excitation propagates downward from the sinus node. As a result, the upper part of the heart contracts minimally sooner than the lower part. This is necessary for proper blood ejection. To ensure that the cardiac output is always adjusted to the respective requirements, the sinus node is connected to the sympathetic and parasympathetic nervous systems. The sympathetic nervous system exerts a positive chronotropic effect at the sinus node. This means that the sinus rhythm is increased. The parasympathetic nervous system, on the other hand, has a negative chronotropic effect, meaning that the sinus rhythm is decreased.
Diseases and ailments
Above a frequency of 100 per minute, a so-called sinus tachycardia is present. In most cases, this goes unnoticed.In children, adolescents, in stressful situations, such sinus tachycardia is physiological. However, there are also numerous underlying diseases that are associated with sinus tachycardia. These include hyperthyroidism, for example. The increased metabolic output also causes the heart to beat faster. Sinus tachycardia is also found in circulatory shock, heart failure, fever, anemia, and withdrawal from intoxicants. Pheochromocytoma is also associated with increased sinus rhythm. Various medications can also increase sinus rhythm. Sinus bradycardia, or slowed sinus rhythm, is physiologic during sleep and in athletes. Causes of pathological sinus bradycardia, on the other hand, include tissue damage in the sinus node, medication use, and increased vagal tone. The tissue of the sinus node can be damaged by hypoxia in coronary artery disease (CAD). Infections leading to myocarditis can also damage the sinus node. The same is true for autoimmunologic processes. Other causes of sinus bradycardia include hypothyroidism (hypothyroidism), hypothermia (hypothermia), poisoning, increased intracranial pressure, and bradycardic (pulse-lowering) medications. A functional defect of the sinus node can also lead to sick sinus syndrome. Sick sinus syndrome is a term used to describe a variety of arrhythmias, all of which originate in the sinus node. The main symptoms of sick sinus syndrome are palpitations and a slow pulse.
