The role of atrial contraction in mitral valve closure

Ovine mitral valve closure is associated with presystolic mitral annular reduction coincident with atrial contraction, which is abolished with ventricular pacing. Whether lack of properly timed atrial contraction influences mitral valve closure or competence, however, is not known.

Eight sheep underwent myocardial marker implantation on the left ventricle, mitral annulus (MA), and mitral leaflets. After 7-10 days, the animals were studied with biplane videofluoroscopy at baseline and during ventricular or atrioventricular (AV) sequential pacing. Valve closure was timed from end-diastole (ED) and defined as minimum distance between two leaflet edge markers. ED was defined as peak of ECG R wave, end-systole as peak negative left ventricular (LV) dP/dt, and end-isovolumic contraction (EIVC) as 83.5 ms after ED. Septal-lateral (S-L) annular diameter was defined as distance between two markers at the middle of the anterior and posterior annulus. Regurgitant volume (RV) was calculated as relative volume change between ED and EIVC.

V-pacing was associated with delayed leaflet closure (65 +/- 5 versus 29 +/- 10 ms, p = 0.008); moreover, RV (4.1 +/- 0.5 versus 1.4 +/- 0.5 ml, p = 0.02), end-diastolic S-L diameter (2.87 +/- 0.10 versus 2.67 +/- 0.09 cm, p = 0.0005), and MA area (8.12 +/- 0.37 versus 7.26 +/- 0.31 cm2, p = 0.009) all increased. RV and leaflet and annular dynamics during AV-pacing were similar to baseline.

V-pacing increased S-L MA diameter by only 8 +/- 1%, but this change was associated with delayed leaflet coaptation and a 16 +/- 1% regurgitant fraction. These findings provide direct evidence that a properly timed atrial contraction is functionally important for effective mitral leaflet closure.