The sinoatrial node is the electrical pacemaker of the heart, responsible for the generation of excitation or heart rate. A pacemaker cell can discharge itself, so the heart rhythm is dictated by it. A malfunction of the sinus node slows the heartbeat, in which case a pacemaker can take over.
What is the sinus node?
The sinoatrial node (SA node, Keith-Flack node, or nodus sinuatrialis) is located in the right atrium and is responsible for sinus rhythm. It is also known as the stimulation center of the heart. It transmits electrical excitation through depolarization, which determines the heart rhythm. The SA node is located in a spindle shape on the epicardium (outer layer of the heart wall), although the size of the node often varies considerably (width 2 to 3mm, length 10 to 20mm). It is composed of cardiac muscle cells that can spontaneously depolarize, producing electrical excitation. Three fiber bundles branch off from the sinus node toward the atrioventricular node:
- Bachmann-James bundle (anterior internodal bundle).
- Wenckebach bundle (middle internodal bundle).
- Thorel bundle (posterior internodal bundle).
Anatomy and structure
The heart pumps independently and is not dependent on nerve excitation. This is due to the fact that there are so-called pacemakers here. These cells discharge spontaneously, with the main pacemaker being the sinus node. It is located at the outermost layer of the heart muscle, where the superior vena cava joins the right atrium. It is a node that is not palpable and is supplied with blood from the right coronary artery. In healthy individuals, it reaches a rate of approximately 70 beats/minute. However, this number depends on age, training condition and various individual factors. During physical exertion, the frequency increases to 120 beats, often even up to 200 beats. At night, the frequency is then only 50 beats per minute.
Function and tasks
The sinus node is also called the autonomic pacemaker, which forms the excitation of the heart. To do this, sodium ions flow into the cells and calcium channels open, leading to the excitation of the SA node. When a certain threshold is reached, the cell discharges completely (depolarization). Subsequently, the voltage is equalized, the particles are concentrated again by the sodium–potassium pump, and the initial position is restored (repolarization). The electrical curve that results is called an action potential. The excitation of the sinus node then continues to the atrioventricular node, which is located between the ventricles and the atria. The atrioventricular node relays the signals from the sinus node to the so-called His bundle, which moves toward the ventricular septum. There, the excitation conduction divides into a left and right ventricular bundle, respectively. The ventricular bundles then branch at the apex of the heart, and the terminal branches are called Purkinje fibers.
Diseases and disorders
The sinus node can be affected by various disorders, which are grouped under the term “sick sinus syndrome.” These include frequency changes of various kinds: if the frequency is too slow, it is called bradycardia; if it is too fast, it is called tachycardia. Another variant is sinus arrest. In this case, the sinus node fails completely and an acute cardiac arrest occurs. Normally, the atrioventricular node steps in and takes over the function of the sinus node, although it operates at a somewhat lower frequency. This is sufficient, however, so that sinus arrest is life-threatening only in rare cases. In addition, phases of increased excitation can alternate with phases in which the number of beats is reduced. The rapid phases are then referred to as atrial fibrillation or atrial flutter. Sinus node syndrome occurs more frequently in patients suffering from coronary artery disease or high blood pressure, as a result of which the heart muscle is not supplied with sufficient oxygen.Depending on the beat frequency, a wide variety of symptoms develop: If the heart rate is less than 50 per minute, those affected suffer from dizziness or fainting spells; if the heart rhythm is permanently slowed, there is shortness of breath, reduced performance or water retention in the legs and lungs. Patients also complain of frequent urination at night and the inability to lie flat in bed. Hyperactivity is manifested by breathing difficulties, chest tightness, and palpitations. Chest pain, which can also radiate to the left arm or neck, can be very threatening. If the heart rate does not increase during physical exertion, this is called chronotropic incompetence. If the electrical impulses of the SA node are no longer transmitted to the ventricle, an AV block occurs, and three different forms can be distinguished here:
- First-degree AV block: Here, delayed conduction of impulses occurs. However, this form is usually not in need of treatment.
- Second-degree AV block: the transmission of signals fails from time to time. If heart disease is present, treatment should be considered.
- Third-degree AV block: conduction is completely interrupted and typical symptoms of bradycardia occur.
A disturbance of the excitation conduction the doctor diagnoses with the help of an ECG. Possibly also a long-term ECG is required, thereby we wear the device for a day on the body. Therapies are the diseases with the help of drugs or by inserting a pacemaker.
