Cardiac rhythm is the complete repetitive sequence of heartbeats, including electrical excitation and heart muscle contractions. In people with healthy cardiovascular systems, the atria contract first, pumping blood into the ventricles, which then contract, pushing their blood into the great systemic circulation and into the pulmonary circulation. Normally, the complete heartbeat sequences move in a frequency band of 60 to 80 Hz without physical stress.
What is the heart rhythm?
Cardiac rhythm is the complete repetitive sequence of heartbeat including electrical excitation and heart muscle contractions. The heart has four cavities, two atria (ventricles) and two ventricles (atria). In order to fulfill its task of constantly supplying the body tissues with oxygenated blood, the atria and the ventricles alternately contract and relax in a specific sequence, in a specific rhythm. The “correct” sequence of a complete beat cycle is regulated electrically. The heart has, so to speak, its own pacemaker, the so-called sinus node, which is located in the right atrium near the junction of the superior vena cava. The sinus node is the primary excitation center and sets the pace. Due to the electrical impulse it emits, the atria contract while the ventricles relax (diastole) and take over blood from the atria in their cavities when the leaflet valves are open. The electrical impulse originating from the sinus node is then picked up by the atrioventricular (AV) node, the secondary pacemaker, which transmits it to the two chambers in a complex conduction system. The two chambers then contract (systole) and squeeze their blood into the large systemic circulation and pulmonary circulation, respectively.
Function and purpose
The main task and function of the heart rhythm is to adapt the sequence of beats between the atria and ventricles to the respective demand during different body loads. This ensures a sustained optimal oxygen supply to the body tissues. At the same time, the heart rhythm adapts to the performance capacity of the heart muscles (myocardium) in order to keep them healthy and prevent damage due to long-term overload. The sinus node in the right atrium near the confluence of the superior vena cava is primarily responsible for maintaining and adjusting the optimal beating sequence and beating frequency. It is composed of a network of nerves and generates the initial electrical stimulus, which is distributed to the smooth muscle cells of the atria and causes them to contract. The contraction stimulus, and thus the contraction itself, proceeds from top to bottom, pumping blood through the open leaflet valves into the ventricles. Subsequently, the AV node focuses the electrical impulse and is responsible for transmitting and distributing the electrical beating impulse to the ventricular muscles through the septa. Here, contraction stimulus and thus contraction proceed from the bottom to the top because the outputs of the ventricles are each located at the top, near the septa to the atria. The contraction sequences in the atria and the ventricles is somewhat comparable to the swallowing reflex, which ensures a certain contraction sequence of the esophagus so that food is transported from the pharynx to the stomach in an orderly fashion. Although the resulting beating sequence, the heart rhythm, is largely autonomous, it must also be subject to regulation by the autonomic nervous system, above all in order to be able to adapt the beating frequency, the beating force and the blood pressure to the momentary requirement. The sympathetic nervous system can therefore influence the sinus node, atria, AV node and the ventricles and drive the heart to peak performance via the neurotransmitters norepinephrine and epinephrine, which have a stimulating effect. The counterpart is the vagus nerve, which, as part of the parasympathetic nervous system, influences the sinus node, atria and AV node, but not the ventricles. The vagus nerve can release the neurotransmitter acetylcholine, which has a calming effect on heart rhythm and blood pressure. In extreme cases, it can even lead to circulatory collapse.
Diseases and ailments
The complex interaction of the herzeigen excitation centers with the physical requirements of the heart and the influence of the autonomic nervous system can be disturbed and lead to typical symptoms and complaints. In addition to a relatively rare abnormally high heart rate (tachycardia), which does not occur due to increased physical demands, and an abnormally low heart rate (bradycardia), arrhythmia, a cardiac arrhythmia, can develop. It involves a disturbance in the sequence of the normal heart rhythm and is caused by a dysfunction of the electrical excitation formation or conduction in the heart. By far the most common form of arrhythmia is atrial fibrillation, which is associated with disordered and rapid contractions of the atria at a frequency usually exceeding 140 Hz. Unlike ventricular fibrillation, atrial fibrillation is not immediately life-threatening, but it can be associated with noticeable and unpleasant loss of performance. If the sinus node fails as the primary pacemaker, the AV node steps in as the secondary pacemaker and clock generator. However, the heart rate is 40 – 60 beats per minute, which is lower than the frequency of the sinus node. This ensures that the sinus node normally “overrides” the AV node as a pacemaker and that two independent contraction stimuli do not coexist. If the AV node also fails as a pacemaker, the myocardial cells of the ventricles can depolarize (excite) themselves at a low frequency of 20-40 Hz, so that an otherwise imminent danger of death is initially overcome. Arrythmia caused by so-called ventricular fibrillation at a frequency above 300 Hz causes a reduction in blood flow volume that tends toward zero, creating an immediately life-threatening situation.
