Further morphologic studies on tetralogy of Fallot, with particular emphasis on the prevalence and structure of the membranous flap.

We have investigated 158 hearts from patients with tetralogy Fallot so as to probe the precise anatomy of so-called membranous flaps and their relationship to the ventricular septal defect, the tricuspid valve, and the atrioventricular conduction axis. A membranous flap was detected in 61 hearts (38.6%) of the 158, all being found when the ventricular septal defect was perimembranous. Two types of flap were discovered. A remnant of the interventricular membranous septum (or a real flap) was seen in 32 and reduplication of the leaflets of the tricuspid valve (pseudoflap) was present in 32. The pseudoflaps either were connected to the anteroseptal commissure of the tricuspid valve or were attached to the medial papillary muscle complex. The medial papillary muscle was abnormally attached more frequently in those hearts with flaps (31%, 52%) than in the ones lacking a flap (5%) (p less than 0.01). An anomalous arrangement of the leaflet of the tricuspid valve was significantly more prevalent in those hearts with a pseudoflap (41%) than in those with a real flap (14%) and in those without a flap (9%). There was a tendency for real flaps to be smaller (p less than 0.05) and thinner than pseudoflaps. A few flaps were considered too tiny to warrant placement of sutures within their structure during surgical repair. During our investigation, we noted also the morphologic arrangement of the rest of the hearts.