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Copyrighted Material 1 1 Anatomy of the Conduction System Pedro Oliveira ­Introduction In dogs, it lies less than 1 mm beneath the epicardium and occupies almost the entire thickness of the atrial wall 1 The heart possesses a specialised conduction system that from epicardium to endocardium. The total size of the is responsible for generating and transmitting electrical canine node was described as being approximately 3 stimuli to the whole heart in a specific and ordered fashion. 5 mm with an oblong shape, although with significant 1,2 It is composed of the sinoatrial node (SA), internodal and variation observed amongst individuals. Other reports inter‐atrial pathways, atrioventricular junction, bundle suggest a more extensive location of up to 3–4 cm 3,4 branches and Purkinje fibres (Figure 1.1). The SA contains between both venae cavae. specialised ‘pacemaker’ cells that have the ability to sponta­ In cats, according to one study including five male neously depolarise, generating electrical impulses. The and five female domestic shorthair cats, the SA node remainder of the conduction system is composed mainly of was located 0.06–0.11 mm beneath the epicardium 5 cells organised in bundles that allow conduction of the with an almost triangular shape. In males the reported electrical stimuli. These structures are present in the walls size was 2.78 × 2.80 × 0.54 mm, and in females it 5 of the heart and are interwoven with the myocardial tissue was 2.75 × 2.64 × 0.45 mm. A different study involving 6 itself. It is not possible to distinguish them from the rest of 12 cats produced different results. A reconstruction the myocardium (working myocardium) with the naked of the SA node based on histological and electrophy­ eye, only with certain stains under the microscope. siological data was performed in five of these cats, The anatomy of these structures is presented in this revealing an ellipsoid shape with a total area of 2 chapter. To avoid confusion, the use of human anatomical 10.5 ± 0.76 mm , a maximum length of 7.4 ± 0.74 mm, a terminology is avoided, and terminology commonly used maximal width of 2.2 ± 0.10 mm and a thickness of 6 in veterinary medicine for quadruped patients is ­preferred. 0.41 ± 0.060 mm. Given the different orientation of the heart within the chest of dogs and cats in comparison to humans, the fol­ Histology lowing terms are used: cranial instead of anterior; caudal Histologically, the SA is composed of specialised mus­ instead of posterior; dorsal instead of superior; and ventral 7 instead of inferior. However, since some terms are so cle fibres arranged in a network. Many small bundles widespread in veterinary literatureCOPYRIGHTED (e.g. left anterior or are MATERIALpresent with irregular courses interspersed with posterior fascicles), it is difficult to avoid the use of such connective tissue accompanied by capillary vessels terms even though they are not entirely appropriate. and nerve cells. The surface of the SA is covered by epicardium, and the remaining areas are surrounded by atrial muscle. Each nodal fibre shows a smooth ­Sinoatrial Node transition to ordinary atrial muscle fibres at the periphery of the SA. Anatomy Three different types of cells are present: normal working myocardial cells, transitional cells and P (pacemaker) cells. The SA, also known as the sinus node, is found in the The P cells are responsible for the ability of the SA to wall of the right atrium at its junction with the cranial spontaneously generate electrical stimuli. They repre­ vena cava in the upper portion of the terminal groove sent approximately 50% of the cells in the SA and are (sulcus terminalis) (Figure 1.1). also present in other areas of the conduction system Guide to Canine and Feline Electrocardiography, First Edition. Ruth Willis, Pedro Oliveira and Antonia Mavropoulou. © 2018 John Wiley & Sons Ltd. Published 2018 by John Wiley & Sons Ltd. Companion website: www.wiley.com/go/willis/electrocardiography Chapter No.: 1 Title Name: Willis 0003457848.INDD 0003457848.INDDComp. 1 by: R. RAMESH Date: 01 Jun 2018 Time: 08:59:54 PM Stage: Printer WorkFlow:CSW Page Number: 1 6/1/2018 8:59:54 PM 2 Guide to Canine and Feline Electrocardiography Sinoatrial Exit Pathways The existence of discrete exit sites has been described at the cranial and caudal ends of the canine sinus node.8 Ablation of these sites resulted in sinoatrial block, sug­ gesting that the SA was not anatomically continuous with the atrial myocardium.8,9 It was suggested that ves­ sels and connective tissue around the SA tissue were responsible for anatomical and physiological blocks on both sides of the node with the exception of the exit sites.9 These findings, together with the reports of a length of up to 3–4 cm,3,4 provide a plausible explanation for the occurrence of ‘wandering pacemaker’ in this spe­ cies (see chapter 5). Internodal Pathways The presence of preferential pathways that connect the SA and AVN has been the subject of debate for the past century, and there is still disagreement about their exist­ ence and significance. In the dog, there is anatomical and electrophysiological evidence to support the presence of three distinct pathways in the right atrium, with cells that possess characteristics similar to those of Purkinje cells.10 However, these pathways are not insulated from neighbouring atrial muscle and are not composed of Figure 1.1 Cardiac conduction system. The sinoatrial node, also ­specialised conduction cells only. This raises questions known as the sinus node, is found in the wall of the right atrium at about their role, and some argue that they should not be its junction with the cranial vena cava in the upper portion of the termed bundles for this reason.11 Nonetheless, surgical terminal groove (sulcus terminalis). It is connected to the resection of these pathways was shown to result in a atrioventricular node (AVN) located on the floor of the right atrium junctional rhythm in dogs, supporting their role as inter­ via the anterior (AIP), middle (MIP) and posterior (PIP) internodal 10 pathways. Connections also exist between the right and left atria, nodal pathways. of which Bachmann’s bundle and the inferior interatrial pathway The anterior internodal pathway originates in the (not illustrated) are the most important. Bachmann’s bundle runs sinus node and courses through the cranial aspect of the in the upper portion of the atrial septum towards the left auricle. cranial vena cava, at which point it bifurcates into The inferior interatrial pathway is composed of fibres that are continuous with the right atrial myocardium at the level of the Bachman’s bundle (see the ‘Inter‐atrial pathways’ sec­ ostium of the coronary sinus (CSO) and with the left atrial tion) and a branch that courses ventrally through the cra­ myocardium from which they separate approximately 20–30 mm nial inter‐atrial septum to join the cranial aspect of the from the CSO. The AVN is continued by the Bundle of His that AVN (Figure 1.1). penetrates the fibrous skeleton of the heart and ramifies into the The middle internodal pathway originates in the sinus right (RBB) and left (LBB) bundle branches. The LBB is composed of anterior (AF) and posterior ramifications (not illustrated). The node and travels downwards cranially to the fossa ovalis bundle branch subdivisions give rise to numerous small branches towards the AVN (Figure 1.1). that spread all over the subendocardium of both ventricles, The posterior internodal pathway originates in the forming the Purkinje network that connects the conduction sinus node, then it courses along the crista terminalis system to the working myocardium. and downwards through the caudal aspect of the inter‐ atrial septum, past the coronary sinus (CS) ostium and (e.g. atrioventricular node [AVN]) in fewer numbers. joining the caudal aspect of the AVN (Figure 1.1). They are organised in small groups of approximately five cells surrounded by connective tissue that function Inter‐atrial Pathways as a unit. The transitional cells are also present in other parts of At least four distinct inter‐atrial electrical connections the conduction system (e.g. internodal tracts and the have been identified in dogs. atrioventricular junction) and seem to provide a link Bachman’s bundle, or the inter‐atrial band, originates between the specialised cells and the normal working close to the SA and traverses the upper portion of the myocardium. inter‐atrial septum towards the left auricle (Figure 1.1). 0003457848.INDD 2 6/1/2018 8:59:55 PM Anatomy of the Conduction System 3 It is composed of normal atrial muscle and of specialised coordinated fashion. The only point of electrical connec­ conducting fibres capable of rapid conduction, similar to tion is provided by a specialised conduction structure the Purkinje fibres in the ventricles.12 that traverses the central portion of the fibrous skeleton Another connection is present ventrally via striated (the central fibrous body), commonly described as the muscle fibres identical to atrial myocardium that sur­ atrioventricular node or junction (Figures 1.1 and 1.2). It round the CS.13 These fibres are continuous with the is located approximately 1 mm beneath the epicardium right atrial myocardium at the level of the CS ostium on the floor of the right atrium in an area known as the and with the left atrial myocardium from which they triangle of Koch (Figure 1.2).11,18 The CS ostium limits separate approximately 20–30 mm from the CS ostium.14 the base of this triangle, and the apex is formed by the A tract of atrial muscle that terminates blindly within the junction between the fibrous tendon (tendon of Todaro) ligament or vein of Marshall (a remnant of the left cranial and the septal leaflet of the tricuspid valve (Figure 1.2).
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