Keith-Flack Node

Keith-Flack Node (Keith-Flack), otherwise the sinus node, is part of that system in the hearts of warm-blooded animals which is called the specific muscular conducting system or the atrioventricular bundle (see). The Keith-Flack node was discovered by the Englishmen Keith and Flack in 1906 during their investigation of the muscular connection between the venae cavae and the atria. This node is a formation of repob-like shape, 1-2 cm long and 3-5 mm wide, located in the sulcus terminalis. This last groove begins at the angle between the right auricle and the superior vena cava and extends outward and downward to the inferior vena cava, forming the boundary between both venae cavae and the proper atrial musculature. According to Koch, the Keith-Flack node can be divided into 4 parts. 1. The head portion of the node, which represents a thickening of the trunk and lies in the angle between the auricle and cava sup. almost under the pericardium. 2. The upper branches, originating from the head portion, directed medially upward and sometimes ascending along the v. cava sup. 3. The trunk portion of the node, which begins in the middle of the groove and ends before reaching the angle between the right auricle and the superior vena cava. 4. The lower branches, originating from the lower part of the trunk and reaching the v. cava inf.; they can be traced for a distance of 30 mm (see figure).-In the center of the node is the artery supplying it-ramus cristae terminalis-according to Spalteholz, ramus v. cavae super. - according to Gross. This artery originates either from branches of the left coronary artery (ramus atrialis sinist. anter. or ramus atrialis sin. inter.), or from branches of the right coronary artery (ramus atrialis dext. anter.), or from the aorta or bronchial artery (Geraudel). Histologically, the Keith-Flack node consists of poor in fibrils, rich in sarcoplasm multinuclear embryonic muscle fibers located in a coarse-fibrous connective tissue mesh. These fibers in hoofed animals as well as in humans contain a large amount of glycogen. In addition to muscle fibers, the node is richly supplied with nerve elements, especially ganglion cells, which correspond to the Remak node of cold-blooded animals. A sharp boundary between the node and the rest of the atrial musculature cannot be drawn, as there is no sheath separating them. When studying the transition of the node fibers to the ordinary atrial musculature, groups of fibers can be traced going in different directions. Monckeberg distinguishes the following 5 groups of fibers. 1. Fibers that emerge from the posterior-upper part of the node, slightly curve around the upper part and go upward parallel to the axis of the v. cava sup. 2. Fibers emerging from the anterior-upper part and passing under the epicardium along the upper edge of the auricle. 3. Fibers also emerging from the anterior-upper part of the node, but belonging to the circular musculature of the venae cavae and passing subendocardially or intramuscularly at the boundary between the atrium and auricle. 4. Fibers originating from the tail of the node. These fibers represent a continuation of the node, are directed obliquely downward and reach the posterior surface of the atria, and sometimes also the septum. 5. Fibers emerging from the middle part of the node and directed downward.At some distance from the node, it is difficult to distinguish these fibers from ordinary atrial fibers. Physiologically, the Keith-Flack node possesses the greatest automatic capability of the entire specific system. In the normal heart, this node is the site of origin of the cardiac impulse. This is proven by the fact that isolated heating of the node leads to acceleration of the cardiac rhythm, while cooling leads to slowing of the rhythm, which is not characteristic of other parts of the heart. Wybauw, and then Lewis, when locally recording action currents from various parts of the heart, found that the initial point of origin of the electronegative wave is the head portion of the Keith-Flack node. This finally confirmed that the node is the pacemaker of the cardiac rhythm (Pacemaker, Schrittmacher).-The influence of extracardiac (right and left vagus and sympathetic) nerves on the Keith-Flack node is not the same. The right vagus and sympathetic nerves act predominantly on the Keith-Flack node, while the left ones act predominantly on the atrioventricular node. Irritation of the right vagus nerve causes cardiac arrest, while that of the left causes atrioventricular block (Rothberger, Winterberg) (see Heart block). Irritation of the right sympathetic nerve causes a significant acceleration of the rhythm and mainly enhances atrial contractions; irritation of the left sympathetic nerve does not accelerate the cardiac rhythm as strongly, but mainly enhances ventricular contractions and in 30-40% of cases causes a transition to atrioventricular rhythm.-The question of the effect on cardiac rhythm of the exclusion of the Keith-Flack node has been subjected to detailed experimental investigation; however, it cannot yet be considered clarified. A number of authors obtained upon destruction or extirpation of the node a transition to atrioventricular automaticity. The same transition was obtained by Rothberger and Scherf upon ligation of the arteries supplying the Keith-Flack node. All these authors consider that the atrioventricular node possesses the greatest automatic capability after the Keith-Flack node and is the center of the second order. However, there are experimental data indicating that the exclusion of the Keith-Flack node does not cause the appearance of atrioventricular automaticity. The source of impulse generation in this case becomes other accumulations of embryonic tissue in the atria, possessing automatic capability and subordinating the atrioventricular node.-It should be noted that in experimental atrioventricular rhythm obtained by complete removal of the Keith-Flack node, irritation of both vagus nerves and the right sympathetic nerve causes the source of impulse generation to shift upward to the upper parts of the atria and the appearance of a positive atrial P wave and a normal or almost normal atrioventricular interval on the electrocardiogram.