The anatomic substrate of mitral annular contraction

Area of Science:

• Cardiac Anatomy
• Cardiovascular Physiology

Background:

• The mitral annulus exhibits sphincter-like contraction, reducing its area by ~25% without intrinsic contractile elements.
• The anatomical basis for this annular contraction remains unclear.
• Understanding annular contraction is crucial for elucidating the pathogenesis of mitral regurgitation.

Purpose of the Study:

• To delineate the anatomical basis of mitral annular contraction.
• To identify the muscle bundles responsible for the annulus's sphincter-like function.

Main Methods:

• Gross dissection of over 100 bovine, ovine, and human hearts.
• Histological examination of 5 ovine hearts.
• Tracing the origins, course, and insertion points of atrial and ventricular muscle bundles related to the mitral annulus.

Main Results:

• Circumferentially-oriented left atrial fibers from Bachman's bundle insert into the right fibrous trigone, flanking the annulus internally.
• Externally, the annulus connects to superficial, obliquely-oriented left ventricular inlet fibers.
• These atrial and ventricular muscle fibers exert extrinsic forces on the annulus.

Conclusions:

• The mitral annulus's position between left atrial and ventricular musculature subjects it to extrinsic contractile forces, causing sphincter-like narrowing.
• Left atrial fibers contract, pulling the inner annulus inward during late diastole.
• Left ventricular inlet fibers exert torsional forces, causing inward annular translation during systole, with implications for mitral regurgitation.