AGE‐RELATED CHANGES IN THE STRUCTURE OF THE CAVOTRICUSPID ISTHMUS IN AN EAST AFRICAN POPULATION
Authors: Kipkorir Vincent , Olabu Beda , Ongeti Kevin , Ndung'u Bernard , Cheruiyot Isaac , Misiani Musa , Munguti Jeremiah 
Published: May 2022

The cavotricuspid isthmus (CTI) is a concave, quadrilateral region of the right atrium between the ostium of the inferior vena cava (IVC) and the tricuspid valve (TV). Three levels of the isthmus have been described from supero‐medial to inferolateral aspects as para‐septal, central and inferolateral isthmus. Antero‐posteriorly, the CTI is also described in three sectors; posterior, middle and anterior sectors, with variable morphologies. The CTI is a common site for atrial flutter (AFL) macro‐reentrant circuits and its histological structure, dimensions and proximity to the right coronary artery (RCA) and minor coronary vein (MCV) are the key determinants of both the occurrences as well as success of management of these AFL. Interestingly, both the prevalence as well as post‐ablation complications of AFL are higher in elderly populations, suggesting possible age‐related histological and morphometric changes in the CTI. Despite this possibility, age‐related histomorphological changes of the CTI are yet to be described. Thus, this study’s objective was to determine the age‐related changes in the structure of the CTI by testing the hypothesis that there are age‐related changes in the structure of the CTI.

40 morphologically normal hearts with intact CTI were obtained from post‐mortem heart specimens at Chiromo Funeral Parlor and Kenyatta National Hospital Mortuary following ethical approval by the Kenyatta National Hospital ‐ University of Nairobi Ethical Review Commission. Subjects were classified according to 4 age groups; <20, 21‐35, 36‐50, >50. This study used vertical inter‐caval incisions on the posterior aspect of the right atrium. The gross appearance of the CTI sectors was determined as either trabeculated/membranous/recessed. Additionally, light microscopy and histology were used to identify myocardial architecture. Means and standard deviations of linear measurements were calculated and correlated for heart weight. Gross morphology and histo‐architecture were presented in photo‐macrographs and photomicrographs. Results demonstrate that myocardial thickness and proximity of the RCA to the endocardium increased among the 0‐20 to 21‐35 age groups, but reduced in the 36‐50 and >50. The distance of the MCV from the endocardium increased with age but dropped in the >50 age group. Moreover, an increase in age is correlated with an increase in sub‐Eustachian recess prevalence, prevalence of a trabeculated posterior sector, a combination of trabeculae bridges, ridges and intra‐trabeculae recesses within the middle sector. These findings suggest that thinning of the myocardium as well as the increase in heterogeneity of trabeculations, associated with an increase in age, are the structural basis behind the high prevalence of AFL among the elderly. Furthermore, the close proximity of minor coronary veins to the endocardium and the high prevalence of sub‐Eustachian recess may account for the difficulties during ablation and recurrence of AFL post‐ablation associated with advancement in age.