fibrous skeleton

**Structure/components:** - Annuli fibrosi (fibrous rings) around the aortic, pulmonic, mitral, and tricuspid valves - Right and left fibrous trigones (the right trigone is the thickest, formed by union of the aortic, mitral, and tricuspid annuli; the central fibrous body) - Tendon of Conus (connects the pulmonary and aortic annuli) - Membranous part of the interventricular septum - Composed of dense irregular connective tissue (primarily type I collagen), with some elastin. Cartilage or bone (os cordis in some animals) may be present with age.

**Functions:** 1. Structural support/anchor: attachment point for valve leaflets (cusps) and for atrial and ventricular myocardium, which keeps the valve orifices patent and prevents over-dilation. 2. Electrical insulation: it is non-conductive, so it separates atrial from ventricular myocardium. The AV node and bundle of His, which pierces the right fibrous trigone/central fibrous body, form the only normal electrical connection between atria and ventricles. This creates the physiologic AV nodal delay, which allows ventricular filling. 3. Provides a rigid base for coordinated valve opening and closure, and for myocardial contraction.

**Clinical significance/high-yield associations:** - Accessory pathways: bypass of the fibrous skeleton by an accessory pathway (bundle of Kent) causes Wolff-Parkinson-White syndrome: short PR interval, delta wave (slurred QRS upstroke), risk of orthodromic/antidromic AVRT, and AF with rapid ventricular conduction. Avoid AV nodal blockers (e.g., verapamil, digoxin, beta-blockers) in pre-excited AF; use procainamide or cardioversion if unstable. Definitive treatment is catheter ablation. - Infective endocarditis: aortic root (annular) abscess extends into the fibrous skeleton and the conduction system, causing a new prolonged PR interval or heart block. Suspect it with a new conduction abnormality in a patient with endocarditis (especially S. aureus, aortic valve). - Valvular disease: annular dilation (e.g., from LV dilation in dilated cardiomyopathy or heart failure) causes functional mitral or tricuspid regurgitation. Mitral annular calcification occurs in the elderly and in patients with ESRD, and can cause conduction disease or mitral regurgitation or stenosis. - Surgery: annuloplasty rings repair annular dilation; the conduction bundle near the membranous septum is at risk during aortic valve surgery/TAVR (heart block, need for pacemaker). - Lack of conduction between the atria and ventricles also explains why atrial fibrillation or flutter does not transmit 1:1 to the ventricles under normal conditions (the AV node filters impulses). - Embryology: the fibrous skeleton develops from endocardial cushions; endocardial cushion defects (seen in Down syndrome) cause ASD (ostium primum), VSD (membranous), and AV septal defects.