Unbalanced complete atrioventricular septal defect (AVSD) with right dominance
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The term “unbalance” has been used for decades in atrioventricular septal defect (AVSD) to describe a lack of symmetry between left and right sides of the heart. Even though we all think to understand its meaning, the concept it alludes to is quite vague. What does then “unbalance” truly mean? We still strive to find a precise definition.
Perhaps, the main issue is that we are trying to sort out a heart with AVSD as if it only had two pieces (left and right) that could be similar in size or not. Actually, unbalance can strike different levels independently. Therefore, the affected anatomic level should always be specified (i.e., unbalance at the atrioventricular valve [AVV] level). Being “balanced” at one level does not necessarily mean that this condition will be fulfilled in the rest. Besides, finding a certain degree of asymmetry at one level does not imply that the same degree will be found in the others.
Unbalance is related to a lack of symmetry in: (a) size or (b) distribution of blood flow between the pulmonary and systemic circulations. The first might also be described as “anatomic unbalance” and reflects the actual absence of symmetry in size (diameter, length, and volume) between the right and left structures. On the other hand, asymmetry in blood flow distribution can be conceived as a “physiologic unbalance.” Certain anatomic features such as the sizes of the atrial septal defect and ventricular septal defect (VSD) or malalignment of the atrial septum and/or ventricular septum have a higher impact on distribution of blood flow between the systemic and pulmonary circulations than on the presence or absence of symmetry in size between right and left structures (although they might contribute to the development of anatomic asymmetry).