Single ventricle means that one ventricular sinus, body, or inflow tract is present; in other words, one ventricular sinus, body, or inflow tract is absent.
In single LV (morphologically left ventricle), the right ventricular sinus is absent (component 2, Fig. 17.1A ).
In single RV (morphologically right ventricle), the left ventricular sinus is absent (component 2, see Fig. 17.1B ).
In single LV, the most common form of single ventricle occurring in about 74% of cases, , there is an infundibular outlet chamber (component 3, see Fig. 17.1A ).
The infundibular outlet chamber which is characteristic of single LV is shown in Figs. 17.2 , 17.3 , and 17.4 .
In single RV with absence of the left ventricular sinus (about 26% of cases) even though the infundibulum is present, it does not look like an infundibular outlet chamber . Why not? With single LV, the infundibulum looks like an outlet chamber because the infundibulum has an inlet constriction or narrowing that is produced by displacement of the ventricular septum. When the right ventricular sinus is absent, the ventricular septum is displaced in that direction—toward the location of the absent right ventricular sinus. This is similar to the shift of the mediastinum in the direction of a small or absent lung.
This “right ventriculad” shift of the ventricular septum brings the crest of the muscular ventricular septum abnormally close beneath the inferior rim of the infundibular septum, creating the appearance of an inlet narrowing leading into the infundibulum. The inlet narrowing makes the subarterial infundibulum look like a chamber—a diagnostically helpful appearance. An outlet chamber appearance strongly suggests a single LV with an infundibular outlet chamber. By contrast, with single RV, because of an absent left ventricular sinus, the ventricular septum moves away from beneath the infundibular septum. The ventricular septal remnant moves “left ventriculad,” posteriorly and to the left with a ventricular D-loop, or posteriorly and to the right with a ventricular L-loop. Consequently, there is no inlet constriction leading into the infundibular or outlet part of the heart: no inlet constriction and no outlet chamber appearance.
The premorphologic definition of single or common ventricle used to be as follows. Single or common ventricle is present if both atrioventricular (AV) valves or a common AV valve open into the same ventricular chamber. ,
Lambert reported a case of single ventricle with a rudimentary outlet chamber in 1951 in which the tricuspid valve opened predominantly into the outlet chamber. In other words, we knew in the early 1960s that single ventricle with an infundibular outlet chamber did not necessarily have double-inlet single ventricle. In other words, we knew that the premorphologic definition of single ventricle was not always accurate.
When I speak of the premorphologic definition of single or common ventricle I mean that before 1964 the morphologic anatomy of single or common ventricle was not understood. That is why Dr. Jeremy Swan, my boss in the Cardiac Catheterization Laboratory at the Mayo Clinic in Rochester, Minnesota, gave me this as a research project in 1960 to 1961. I was his fellow in the Catheterization Laboratory for 1 year, a high privilege. Before this time, I had been an Assistant Resident in Pathology in 1956 to 1957 at Boston Children’s Hospital, between my junior and senior years as a Resident in Pediatrics. During my year of training in Pathology, I had fortunately learned of the work of Dr. Maurice Lev in Chicago in which he introduced the morphologic anatomic approach to the diagnosis of the cardiac chambers by means of the morphologic anatomy of their septal surfaces, not in terms of their relative positions, such as right-sided or left-sided.
I expanded Lev’s approach by including the morphologic anatomy of the septal surfaces and of the free wall surfaces. Both were specific, different, and diagnostically very helpful. Then it became obvious that using the AV valves to diagnose the presence or absence of single ventricle was a violation of logic, in the following sense. Using one variable (the AV valves) to diagnose the status of another, different variable (the presence or absence of single ventricle) was a violation of logic in the sense that each variable should be diagnosed primarily in terms of itself, not in terms of some other, different variable. The principle that each variable should be diagnosed primarily in terms of itself, if possible, not in terms of some other different variable, is important and has widespread applicability. In addition to cases like Dr. Edward Lambert’s, , later we would also learn that the type of ventricular loop (D- or L-) should be diagnosed specifically in terms of itself, not in terms of whether the AV alignments and connections are concordant or discordant :
When the atria are in situs solitus and the AV alignments (“connections”) are concordant, L-loop ventricles can be present, not the expected D-loop ventricles.
When the atria are in situs solitus and the AV alignments are discordant, D-loop ventricles can be present, not the expected L-loop ventricles.
In other words, to avoid diagnostic errors, the ventricular situs must be diagnosed specifically per se, not in terms of a different variable.
Before morphologic anatomic understanding, single ventricle used to be called common ventricle, based on the impression that single ventricle is a huge ventricular septal defect (VSD), that is, absence of the ventricular septum. Does common ventricle occur? The answer is yes, but it is rare ( Fig. 17.5A–B ). In our last large study of single ventricle, we found no cases of common ventricle.
Single Left Ventricle With Infundibular Outlet Chamber
Six different anatomic types of single LV with infundibular outlet chamber were found ( Table 17.1 ). Single LV with an infundibular outlet chamber and a segmental situs set of solitus atria, ventricular D-loop, and solitus normally related great arteries were found in only 1 case (3% of this series as a whole).
Our Findings in 31 Postmortem Cases of Single Ventricle ( Table 17.1 )