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Presystolic mitral annular septal–lateral shortening is independent from left atrial and left ventricular contraction during acute volume depletion

^a Department of Cardiothoracic Surgery, Falk Cardiovascular Research Center, Stanford University School of Medicine, Stanford, CA 94305-5247, United States
^b Department of Statistics, Stanford University, Stanford, CA, United States
^c Laboratory of Cardiovascular Physiology and Biophysics, Research Institute of the Palo Alto Medical Foundation, Palo Alto, CA, United States

Received 31 August 2008; received in revised form 2 March 2009; accepted 10 March 2009.

^_* Corresponding author. Tel.: +1 650 725 3826; fax: +1 650 725 3846. (Email: dcm{at}stanford.edu).

Background: The mitral annulus (MA) is a dynamic structure that joins the left atrium (LA) and left ventricle (LV), but it is unknown whether MA motion is coupled to the LA or the LV or neither of the two. Since a well orchestrated coordination of LA, MA and LV septal–lateral (S–L) dynamics is essential for efficient valve closure, we assessed their functional coupling in an experimental ovine model. To assess the coupling under a wide range of physiological conditions, data were acquired in normal and acutely volume depleted hearts. Methods: In 10 sheep, radiopaque markers were placed in LA, MA and LV base (LV_base). Twelve weeks postoperatively, 4-D marker coordinates were obtained by stereo videofluoroscopy (60 frames/s) before (CTRL) and during acute inferior vena caval occlusion (VCO). Septal–lateral dimensions were calculated as distances between corresponding marker pairs in the LA, MA and LV_base 5 frames before end-diastole (ED – 84 ms) and at end-diastole. Dynamics during late diastole are described as changes from ED – 84 ms versus end-diastole. To study the functional coupling between LA, MA and LV_base we calculated slopes during late diastole from simple linear regressions on an animal-by-animal basis. Results: During late diastole in CTRL, the LA and MA both shortened along the S–L dimension (32.9 ± 6.6 mm vs 31.0 ± 5.5 mm, p = 0.026 and 27.3 ± 3.7 mm vs 24.6 ± 4.1 mm, p = 0.005, respectively) whereas the LV_base lengthened (56.2 ± 9.3 mm vs 57.3 ± 9.3 mm, p = 0.012). VCO abolished septal–lateral dynamics of LA and LV_base during late diastole (27.8±4.3 mm vs 27.4 ± 3.9 mm, p = 0.155 and 49.4 ± 7.7 mm vs 49.5 ± 7.5 mm, p = 0. 752, respectively) while the MA still shortened (19.0 ± 2.9 vs 18.0 ± 2.8, p = 0.042). Under CTRL conditions LA dynamics were linearly dependent from MA dynamics (average coefficient 0.57, p = 0.001), suggesting that LA and MA are functionally coupled. With acute volume depletion, MA dynamics were linearly independent from both, LA and LV (average coefficient 0.28, p = 0.159 and 0.58, p = 0.192, respectively). Conclusion: Whereas MA and LA dynamics are coupled during late diastole in hearts with normal LV volumes, presystolic mitral annular septal–lateral shortening is independent from LA and LV dynamics with acute volume depletion. A better understanding of mitral annular dynamics and their functional coupling may help improve mitral valve repair strategies.

Key Words: Mitral annular dynamics • Acute volume depletion • Radiopaque markers • Ovine model