The Dynamic Anterior Mitral Annulus

Background The anterior mitral annulus is considered a fixed structure. Recent data suggest otherwise. This study tested the hypothesis that the size of the anterior annulus varies with hemodynamic loading and ventricular contractility. Methods Sonomicrometry array localization measured annular area, total annular circumference, anterior circumference, and posterior circumference in 6 sheep before and after neosynephrine increased systolic blood pressure by at least 150% during atrial pacing at 120 beats/min. In 6 additional animals the same dimensions were measured during atrial pacing (at 120 and 150 beats/min) and during isoproteronol infusions to increase heart rate to 120 and 150 beats/min. Results Neosynephrine increased systolic total annular circumference from 99.7 ± 5.5 mm to 106.9 ± 9.6 mm. Anterior circumference increased from 40.8 ± 4.0 mm to 45.3 ± 5.7 mm whereas posterior circumference only increased from 59.0 ± 5.5 mm to 61.6 ± 7.0 mm. Low isoproteronol infusion decreased systolic total annular circumference from 107.5 ± 8.3 mm to 101.9 ± 10.6 mm. Most of this change occurred in the posterior circumference. Higher infusions of isoproteronol decreased total annular circumference from 106.8 ± 8.3 mm to 98.3 ± 9.7 mm. At this higher inotropic state the decrease in annular size was similar in the anterior and posterior annulus. Conclusions In sheep, the anterior annulus is a dynamic structure that varies in size in response to changes in hemodynamic loading and ventricular contractility.