Assessing the criteria for definition of perimembranous ventricular septal defects in light of the search for consensus.

BACKGROUND: Discussions continue as to whether ventricular septal defects are best categorized according to their right ventricular geography or their borders. This is especially true when considering the perimembranous defect. Our aim, therefore, was to establish the phenotypic feature of the perimembranous defect, and to establish the ease of distinguishing its geographical variants. METHODS AND RESULTS: We assessed unrepaired isolated perimembranous ventricular defects from six historic archives, subcategorizing them using the ICD-11 coding system. We identified 365 defects, of which 94 (26%) were deemed to open centrally, 168 (46%) to open to the outlet, and 84 (23%) to the inlet of the right ventricle, with 19 (5%) being confluent. In all hearts, the unifying phenotypic feature was fibrous continuity between the leaflets of the mitral and tricuspid valves. This was often directly between the valves, but in all instances incorporated continuity through the atrioventricular portion of the membranous septum. In contrast, we observed fibrous continuity between the leaflets of the tricuspid and aortic valves in only 298 (82%) of the specimens. When found, discontinuity most commonly was seen in the outlet and central defects. There were no discrepancies between evaluators in distinguishing the borders, but there was occasional disagreement in determining the right ventricular geography of the defect. CONCLUSIONS: The unifying feature of perimembranous defects, rather than being aortic-to-tricuspid valvar fibrous continuity, is fibrous continuity between the leaflets of the atrioventricular valves. While right ventricular geography is important in classification, it is the borders which are more objectively defined.

Sequential segmental analysis of the crocodilian heart.

Differences between hearts of crocodilians and those of mammals and birds are only partly understood because there is no standardised approach and terminology for describing cardiac structure. Whereas most reptiles have an undivided ventricle, crocodilians have a fully septated ventricle. Their hearts, therefore, are more readily comparable with the hearts of mammals and birds. Here, we describe the heart of a crocodile (Crocodylus noliticus). We use the versatile sequential segmental approach to analysis, juxtaposing several key views of the crocodilian heart to the comparable views of human hearts. In crocodiles, the atrial and ventricular septums are complete but, unlike in placental mammals, the atrial septum is without an oval fossa. The myocardial component of the crocodilian ventricular septum dominates, but the membranous septum likely makes up a greater proportion than in any mammal. In the crocodile, the aortic trunk takes its origin from the left ventricle and is not wedged between the atrioventricular junctions. Consequently, there is a common atrioventricular junction, albeit with separate right and left atrioventricular valvar orifices. As in mammals, nonetheless, the crocodilian left atrioventricular valvar orifice is cranial to the right atrioventricular valvar orifice. By applying a method of analysis and terminology usually restricted to the human heart, we build from the considerable existing literature to show neglected and overlooked shared features, such as the offset between the left and right atrioventricular valvar orifices. Such commonalities are surprising given the substantial evolutionary divergence of the archosaur and synapsid lineages, and likely reflect evolutionarily shared morphogenetic programmes.