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 min to 106.9 +/- 9.6 mm. Anterior circumference increased from 40.8 +/- 4.0 min to 45.3 +/- 5.7 min whereas posterior circumference only increased from 59.0 +/- 5.5 min to 61.6 +/- 7.0 mm. Low isoproteronol infusion decreased systolic total annular circumference from 107.5 +/- 8.3 min 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 min 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. (C) 2004 by The Society of Thoracic Surgeons.