Electro-morphology of the heart: atrial arrhythmias

Siew Yen Ho
Professor of Cardiac Morphology, Head of Cardiac Morphology Unit, Imperial College London
Royal Brompton and Harefield NHS Trust
London, UK

The cardiac conduction system in congenitally malformed hearts can be located differently from structurally normal hearts. The abnormal locations impact upon surgical techniques, e.g. placement of patches for closing defects or resection of musculature. Increasingly, there is also a need for cardiology interventionists to be aware of the conduction tissues. Although there are guides to the abnormal locations, each case should be considered individually.
　Beginning with the sinus node, the variations are related to the arrangement of the atrial appendages (situs). Juxtaposition of both atrial appendages on the left side can also displace the sinus node.
　In terms of the atrioventricular conduction system, there is normal or near-normal distribution in hearts with biventricular atrioventricular connection and right hand ventricular topology but special care should be taken in those with perimembranous ventricular septal defects or atrioventricular septal defects where the bundle or node can be in the immediate vicinity of the defect’s margin. A particular abnormality to note is when there is straddling of the tricuspid valve because in these cases the atrioventricular conduction bundle penetrates not at the apex of Koch’s triangle but at the point where the inlet portion of the ventricular septum rises up to meet the ‘annulus’ of the tricuspid valve. The same deviation of the septum in hearts with atrioventricular septal defect will also shift the site of the penetrating atrioventricular conduction bundle. On the other hand, hearts with biventricular atrioventricular connections but left-hand ventricular topology will have anterior location of the atrioventricular node and bundle, with the anterior rim of the septal defect at risk. This is a well known association in hearts with congenitally corrected transposition in the setting of usual atrial arrangement. In some cases, both an anterior and a posterior node may be present and the atrioventricular bundle is like a sling connecting to both nodes.
　The majority of hearts with univentricular atioventricular connections have abnormal, anterior location of the atrioventricular node and bundle. An example is absence of the right atrioventricular connection (so-called classical tricuspid atresia) in which the atrioventricular node is located in the muscular floor of the right atrium and the atrioventricular bundle emerges onto the right-inferior margin of the ventricular septal defect (patient’s right) when viewed from the rudimentary right ventricle. A similar course of the atrioventricular conduction system is present in hearts with double inlet left ventricle except that the node is related to the right-anterior margin of the atrioventricular junction.
　Dilation of the right atrium is a common finding in patients with the common form of atrial flutter. Compared to normal patients, the inferior isthmus is also enlarged. This area which is targeted for ablation of common atrial flutter has zones of different morphologic characteristics. Although the anatomic substrate for arrhythmia is still unclear, the stretched fibres in the atrial wall and composition of the wall itself probably have a role in invoking the arrhythmia or in reducing the threshold for normal conduction. Increasingly, incisional tachycardia is recognised in patients who had previous surgery for congenital heart defect. The substrate is related to a surgical scar(s), and ablation of the narrow isthmus bounded by scars or other anatomical barriers has been shown to be effective in interrupting the flutter circuit.
　Atrial fibrillation is common amongst the elderly and also in patients with atrial dilatation with congenital heart defects. Ectopic foci in the muscular walls of pulmonary veins and autonomic neural inputs have been implicated in recent years. Structurally, the muscular sleeves do not show histological specialisation. However, the variations in myofibre orientations interspersed with areas of fibrosis may well contribute to slow and non-uniform conduction or micro-reentry.
　Ventricular arrhythmias occurring longterm after surgical repair is often macroreentry, with an isthmus of myocardium between two barriers being the critical isthmus. For example, in Fallot’s tetralogy, the isthmus may be at the pulmonary infundibulum between the ventriculotomy scar and the pulmonary valve.
　In summary, electro-morphology of the heart has many variants and the pathomorphologic substrates of arrhythmias need further investigations with clinico-morphological correlations.