5.7 Atrial septal defect
Atrial septal defects (ASDs, communications between the RA and LA) are rare in horses. They can occur at a number of points in the atrial septum. Defects of the septum can take the form of:
· a sinus venosus ASD associated with a hole at the top of the septum at the junction of the LA and the pulmonary veins; · a secundum defect in the mid-portion of the septum; · a primum defect at the base of the septum (associated with an endocardial cushion defect); · a patent foramen ovale (persistence of the normal fetal structure which allows blood to shunt from the right to the LA, see section 5.1).
Small ASDs may go unnoticed because they may not result in a cardiac murmur or any clinical abnormalities. Large ASDs occurring in the absence of other defects are very rare. Theoretically these defects would have a similar effect to a VSD except that there would be volume overload of the RA in addition to the other chambers. However, ASDs are usually associated with other cardiac defects, and may be essential for life in some defects such as tricuspid atresia (see section 5.8).
5.7.1 Haemodynamic effects
5.7.2 Clinical signs