Atrial septal defects (ASDs) represent a diverse group of differing anatomic lesions that all result in intracardiac shunting. The common features of all ASD types that should be systematically evaluated and reported for all ASD types are listed in Table 5.1. These include the type of ASD (primum or secundum) or other atrial communication (venosus or unroofed coronary sinus); the presence and direction of Doppler flow through the defect; and associated findings such as anomalous pulmonary vein drainage, the presence and size of a Eustachian valve or a Chiari network, the size and shape of the defect or defects, the location in the septum, the presence or absence of multiple fenestrations and the size of the ASD at end-systole and end-diastole. Ostium secundum ASD is the most common defect encountered and most commonly occurs as a deficiency in septum primum. Secundum ASDs can vary considerably in their size, shape and configuration, as has been described previously. A small ASD is typically described as less than 5 mm in the maximal measured ASD diameter. With favourable anatomic features, ostium secundum ASDs can be amenable to percutaneous transcatheter closure. Secundum ASDs have a variable amount of surrounding tissue that borders the defect, and these ‘rims’ of surrounding tissue are named for the corresponding surrounding adjacent anatomic structures. By convention, there are six anatomically named rims of surrounding tissue. These rims should be assessed carefully, using echocardiography in all patients and, in particular, before consideration of percutaneous closure. A rim length of 5 mm or more is considered a favourable characteristic for percutaneous transcatheter closure of a secundum ASD. An ASD rim length of less than 5 mm is described as ‘deficient’ and could present challenges for transcatheter closure.¹ Secundum ASD rims can be defined as follows:

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