This major criterion for the diagnosis of ARVC/D is that of fibrofatty replacement of myocardium on endomyocardial biopsy. Fatty tissue may be a normal component of the right ventricular free wall [20-23]. The definition of "fibrofatty replacement"was not provided in the original Task Force criteria. The major problem with the pathological evaluation from my-ocardial biopsy relates to the fact that, in ARVC/D, the pathology may be spotty, and biopsies may not be ob tained from the affected region. Importantly, the most common site for right ventricular biopsy is the septum and the septum is seldom involved in ARVC/D. Fortunately, the risk of perforation due to biopsy of the free wall of the right ventricle is small when performed according to strict guidelines, including avoiding biopsy of the thin wall dysfunctional regions [24-25]. Biopsies are obtained from adjacent regions.
Angelini et al.  proposed that diagnostic values for ARVC/D in right ventricular endomyocardial biopsies are the presence of <45% myocytes, >40% fibrous tissue, and >3% fatty tissue. The normal amount of myocytes is 80%-85%. These measurements are based on analysis of endomyocardial biopsies from 30 patients with ARVC/D, 29 patients with dilated car-diomyopathy, and 30 controls. The following criteria have been utilized to evaluate right ventricular biopsy samples in the ARVC/D North American Multidisci-plinary Study of Right Ventricular Dysplasia. If there are <45% myocytes due to replacement by either fibrous tissue or fatty tissue, the biopsy is consistent with ARVC/D. If the residual myocardium is between 45%-70%, findings are classified as indeterminate; finally if the residual myocardium exceeds 70%, the biopsy is considered negative. Sensitivity and specificity of these criteria are undergoing evaluation in the above registry.
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