Management of the left ventricular outflow tract in the setting of deficient atrioventricular septation

It has long been recognized that one of the problems confronted by the surgeon, when repairing hearts with deficient atrioventricular septation, now usually described as atrioventricular septal defects, is to ensure postoperatively an unobstructed channel from the left ventricle to the aorta [1]. It has similarly long been recognized that the potential problem reflects the so-called ‘unwedging’ of the aortic root relative to the common atrioventricular junction [2, 3]. In many ways, the evolution of surgical repair for these defects parallels the amazing advances made in paediatric cardiac surgery in general over the last half century. The review provided by Kirklin and Barratt-Boyes in their ground-breaking textbook serves to encapsulate the understanding of the situation towards the end of the 20th century. It is salutary to remember that paediatric cardiac surgery, in itself, is a phenomenon of the second half of the 20th century. It is no exaggeration to state that Kirklin and Barratt-Boyes themselves were responsible for many of the advances that revolutionized the happenings in the operating room. I was privileged to be working throughout this period, and I was fortunated to be able to count Kirklin as one of my mentors. In the textbook, these giants point to the fact that Jesse Edwards, the consummate pathologist of the 20th century, and Lodewyk Van Mierop, an aspiring cardiac surgeon who became a paediatric cardiologist and an outstanding morphologist, emphasized the similarity between the so-called ‘ostium primum’ defect, still considered incorrectly by many to be an atrial septal defect, and the allegedly complete defect, characterized by the presence of a common valvar orifice within the common atrioventricular junction. As Kirklin and Barratt-Boyes indicate, my own involvement with the lesion began with the series of investigations headed by Gian Piero Piccoli [2, 3], with one of these studies focused on the morphological details of the left ventricular outflow tract [4]. Kirklin and Barratt-Boyes also cite the work published by Anton Becker and myself, in which we first promoted the benefits of describing the lesion as an atrioventricular septal defect [5]. In that review, having stripped away the leaflets of the potentially common atrioventricular valve, we endorsed the fact that there was no way of distinguishing the so-called ‘partial’ and ‘complete’ variants. It is, nonetheless, the relationship between the superior bridging leaflet and the crest of the muscular ventricular septum that underscores the difference between the potential for obstruction in the 2 sub-sets. This is because, as is well demonstrated by Chandiramani et al. [6] from the paediatric cardiac surgical centre in Glasgow, it is the attachment of the superior bridging leaflet to the ventricular septal crest that elongates the outflow tract beneath the virtual basal ring of the aortic root. This is a point that we emphasized in our own textbook on surgical cardiac anatomy. This book is now in its 4th edition, and appropriately named for Benson Wilcox, who joined me in producing the first edition. This association is itself then of further significance, since it was Wilcox, whilst working at Chapel Hill in North Carolina, who noted that the surgeon could effectively transform the so-called ‘complete’ defect into the ‘partial’ variant simply by attaching the superior leaflet to the septal crest when there was the potential for ventricular shunting beneath it [7]. The repair is now rightly known as the Australian repair, since Graham Nunn deserves all the credit for popularizing the approach [8]. As the Glasgow group point out, however, the technique is not without its own dangers. This is because, by using the superior leaflet to close the interventricular communication, the surgeon automatically elongates the subaortic area. And as they also show, it is this very feature that compromises the outflow tract in the group of atrioventricular septal defects as a whole. Much depends, therefore, on how the surgeon remodels the attachment of the superior bridging leaflet. There is a major difference between folding over the leaflet, as opposed simply to depressing its ventricular surface to the septal crest [9]. The key point made by the Glasgow group is that, although there is always the potential for outflow tract obstruction, the outflow tract itself and the aortic root are not themselves intrinsically narrow. Their dimensions are sufficient to permit unobstructed flow in the ‘native’ situation. This begs the question as to whether it is, indeed, the surgical reconstruction that exacerbates the subsequent occurrence, in a minority of cases, of an obstructed outflow tract. There is no question, nonetheless, that any potential for obstruction is further exaggerated by the presence of additional lesions, such as a hypertrophied antero-lateral muscle bundle or formation of tissue tags from the fibrous structures forming the boundaries of the outflow tract. So as to demonstrate the propensity for obstruction, the group has performed elegant measurements based on their echocardiographic findings. The studies, however, were only carried out postoperatively. The measurements made unequivocally confirm the differences between the sub-sets to be found within the overall group of atrioventricular septal defects. Of equal importance is that those making the measurements were blinded to the morphology and the type of surgical repair. It was also appropriate for them to compare the ‘ostium primum’ sub-set with other patients having defects within the oval fossa and those with ventricular shunting to be compared to those having ventricular septal defects. In the hands of the Glasgow surgeons, it then transpired that those undergoing the Australian repair postoperatively had smaller aortic roots and outflow tracts. Despite these findings, nonetheless, only a small proportion of the patients in any of the groups were found to have overt postoperative obstruction. It is difficult, therefore, to argue with their conclusions, namely that the topic, despite now having been studied at length for half a century, remains in need of still further evaluation. Their thoughtful investigation serves to point the direction. Conflict of interest: none declared.