Transient stiffening of mitral valve leaflets in the beating heart

Krishnamurthy G, Itoh A, Swanson JC, Miller DC, Ingels NB Jr. Transient stiffening of mitral valve leaflets in the beating heart. Am J Physiol Heart Circ Physiol 298: H2221-H2225, 2010. First published April 16, 2010; doi:10.1152/ajpheart.00215.2010.-Anterior mitral leaflet stiffness during isovolumic contraction (IVC) is much greater than that during isovolumic relaxation (IVR). We have hypothesized that this stiffening is due to transient early systolic force development in the slip of cardiac myocytes in the annular third of the anterior leaflet. Because the atrium is excited before IVC and leaflet myocytes contract for <= 250 ms, this hypothesis predicts that IVC leaflet stiffness will drop to near-IVR values in the latter half of ventricular systole. We tested this prediction using radiopaque markers and inverse finite element analysis of 30 beats in 10 ovine hearts. For each beat, circumferential (E-c) and radial (E-r) stiffness was determined during IVC (Delta t(1)), end IVC to midsystole (Delta t(2)), midsystole to IVR onset (Delta t(3)), and IVR (Delta t(4)). Group mean stiffness (E-c +/- SD; E-r +/- SD; in N/mm(2)) during Delta t(1) (44 +/- 16; 15 +/- 4) was 1.6-1.7 times that during Delta t(4) (28 +/- 11; 9 +/- 3); Delta t(2) stiffness (39 +/- 15; 14 +/- 4) was 1.3-1.5 times that of Delta t(4), but Delta t(3) stiffness (32 +/- 12; 11 +/- 3) was only 1.1-1.2 times that of Delta t(4). The stiffness drop during Delta t(3) supports the hypothesis that anterior leaflet stiffening during IVC arises primarily from transient force development in leaflet cardiac myocytes, with stiffness reduced as this leaflet muscle relaxes in the latter half of ventricular systole.