How constant anatomically is the tendon of Todaro as a marker for the triangle of Koch?

Introduction: Although well recognized by anatomists as a border of the tri angle of Koch demarcating the location of the AV node, the tendon of Todaro is not visible in the operating room or in the catheterization laboratory. Instead, clinicians use as surrogate a projected line between the eustachi an valve and the central fibrous body. The constancy of the tendon of Todar o within this border remains to be determined. Materials and Results: We reexamined serial histologic sections from 25 adu lts and 50 infants and gross dissections in four normal hearts. The tendon of Todaro was identified in all cases and traced to the central fibrous bod y in all but one case. It tended to be thicker in the hearts of infants cas es (0.2 to 0.8 mm vs 0.1 to 0.6 mm), The tendon and the hinge-line of the s eptal leaflet of the tricuspid valve were consistent as landmarks for locat ion of the compact AV node in all the cases studied by histology, Gross dis sections traced the tendon to the free edge of the eustachian valve. Conclusion: The tendon of Todaro is present in hearts obtained from both ad ults and infants. It, or its surrogate, is a reliable border for the triang le of Koch and serves as a landmark to location of the atrial components of the AV conduction axis.