Extracellular matrix remodeling in wound healing of critical size defects in the mitral valve leaflet.

The details of valvular leaflet healing following valvuloplasty and leaflet perforation from endocarditis are poorly understood. In this study, the synthesis and turnover of valvular extracellular matrix due to healing of a critical sized wound was investigated. Twenty-nine sheep were randomized to either CTRL (n = 11) or HOLE (n = 18), in which a 2.8-4.8 mm diameter hole was punched in the posterior mitral leaflet. After 12 weeks, posterior leaflets were harvested and histologically stained to localize extracellular matrix components. Immunohistochemistry was also performed to assess matrix components and markers of matrix turnover. A semi-quantitative grading scale was used to quantify differences between HOLE and CTRL. After 12 weeks, the hole diameter was reduced by 71.3 ± 1.4 % (p < 0.001). Areas of remodeling surrounding the hole contained more activated cells, greater expression of proteoglycans, and markers of matrix turnover (prolyl 4-hydroxylase, metalloproteases, and lysyl oxidase, each p ≤ 0.025), along with fibrin accumulation. Two distinct remodeling regions were evident surrounding the hole, one directly bordering the hole rich in versican and hyaluronan and a second adjacent region with abundant collagen and elastic fiber turnover. The remodeling also caused reduced delineation between valve layers (p = 0.002), more diffuse staining of matrix components and markers of matrix turnover (p < 0.001), and disruption of the collagenous fibrosa. In conclusion, acute valve injury elicited distinct, heterogeneous alterations in valvular matrix composition and structure, resulting in partial wound closure. Because these changes could also affect leaflet mechanics and valve function, it will be important to determine their impact on healing wounds.

How do annuloplasty rings affect mitral annular strains in the normal beating ovine heart?

BACKGROUND: We hypothesized that annuloplasty ring implantation alters mitral annular strains in a normal beating ovine heart preparation. METHODS AND RESULTS: Sheep had 16 radiopaque markers sewn equally spaced around the mitral annulus. Edwards Cosgrove partial flexible band (COS; n=12), St Jude complete rigid saddle-shaped annuloplasty ring (RSA; n=10), Carpentier-Edwards Physio (PHY; n=11), Edwards IMR ETlogix (ETL; n=11), and GeoForm (GEO; n=12) annuloplasty rings were implanted in a releasable fashion. Four-dimensional marker coordinates were obtained using biplane videofluoroscopy with the ring inserted (ring) and after ring release (control). From marker coordinates, a functional spatio-temporal representation of each annulus was generated through a best fit using 16 piecewise cubic Hermitian splines. Absolute total mitral annular ring strains were calculated from the relative change in length of the tangent vector to the annular curve as strains occurring from control to ring state at end-systole. In addition, average Green-Lagrange strains occurring from control to ring state at end-systole along the annulus were calculated. Absolute total mitral annular ring strains were smallest for COS and greatest for ETL. Strains for RSA, PHY, and GEO were similar. Except for COS in the septal mitral annular segment, all rings induced compressive strains along the entire annulus, with greatest values occurring at the lateral mitral annular segment. CONCLUSIONS: In healthy, beating ovine hearts, annuloplasty rings (COS, RSA, PHY, ETL, and GEO) induce compressive strains that are predominate in the lateral annular region, smallest for flexible partial bands (COS) and greatest for an asymmetrical rigid ring type with intrinsic septal-lateral downsizing (ETL). However, the ring type with the most drastic intrinsic septal-lateral downsizing (GEO) introduced strains similar to physiologically shaped rings (RSA and PHY), indicating that ring effects on annular strain profiles cannot be estimated from the degree of septal-lateral downsizing.

Rigid, complete annuloplasty rings increase anterior mitral leaflet strains in the normal beating ovine heart.

BACKGROUND: Annuloplasty ring or band implantation during surgical mitral valve repair perturbs mitral annular dimensions, dynamics, and shape, which have been associated with changes in anterior mitral leaflet (AML) strain patterns and suboptimal long-term repair durability. We hypothesized that rigid rings with nonphysiological three-dimensional shapes, but not saddle-shaped rigid rings or flexible bands, increase AML strains. METHODS AND RESULTS: Sheep had 23 radiopaque markers inserted: 7 along the anterior mitral annulus and 16 equally spaced on the AML. True-sized Cosgrove-Edwards flexible, partial band (n=12), rigid, complete St Jude Medical rigid saddle-shaped (n=12), Carpentier-Edwards Physio (n=12), Edwards IMR ETlogix (n=11), and Edwards GeoForm (n=12) annuloplasty rings were implanted in a releasable fashion. Under acute open-chest conditions, 4-dimensional marker coordinates were obtained using biplane videofluoroscopy along with hemodynamic parameters with the ring inserted and after release. Marker coordinates were triangulated, and the largest maximum principal AML strains were determined during isovolumetric relaxation. No relevant changes in hemodynamics occurred. Compared with the respective control state, strains increased significantly with rigid saddle-shaped annuloplasty ring, Carpentier-Edwards Physio, Edwards IMR ETlogix, and Edwards GeoForm (0.14 ± 0.05 versus 0.16 ± 0.05, P=0.024, 0.15 ± 0.03 versus 0.18 ± 0.04, P=0.020, 0.11 ± 0.05 versus 0.14 ± 0.05, P=0.042, and 0.13 ± 0.05 versus 0.16 ± 0.05, P=0.009), but not with Cosgrove-Edwards band (0.15 ± 0.05 versus 0.15 ± 0.04, P=0.973). CONCLUSIONS: Regardless of three-dimensional shape, rigid, complete annuloplasty rings, but not a flexible, partial band, increased AML strains in the normal beating ovine heart. Clinical studies are needed to determine whether annuloplasty rings affect AML strains in patients, and, if so, whether ring-induced perturbations in leaflet strain states are linked to repair failure.

Contribution of myocardium overlying the anterolateral papillary muscle to left ventricular deformation.

Previous studies of transmural left ventricular (LV) strains suggested that the myocardium overlying the papillary muscle displays decreased deformation relative to the anterior LV free wall or significant regional heterogeneity. These comparisons, however, were made using different hearts. We sought to extend these studies by examining three equatorial LV regions in the same heart during the same heartbeat. Therefore, deformation was analyzed from transmural beadsets placed in the equatorial LV myocardium overlying the anterolateral papillary muscle (PAP), as well as adjacent equatorial LV regions located more anteriorly (ANT) and laterally (LAT). We found that the magnitudes of LAT normal longitudinal and radial strains, as well as major principal strains, were less than ANT, while those of PAP were intermediate. Subepicardial and midwall myofiber angles of LAT, PAP, and ANT were not significantly different, but PAP subendocardial myofiber angles were significantly higher (more longitudinal as opposed to circumferential orientation). Subepicardial and midwall myofiber strains of ANT, PAP, and LAT were not significantly different, but PAP subendocardial myofiber strains were less. Transmural gradients in circumferential and radial normal strains, and major principal strains, were observed in each region. The two main findings of this study were as follows: 1) PAP strains are largely consistent with adjacent LV equatorial free wall regions, and 2) there is a gradient of strains across the anterolateral equatorial left ventricle despite similarities in myofiber angles and strains. These findings point to graduated equatorial LV heterogeneity and suggest that regional differences in myofiber coupling may constitute the basis for such heterogeneity.

Myocardial strains from 3D displacement encoded magnetic resonance imaging.

BACKGROUND: The ability to measure and quantify myocardial motion and deformation provides a useful tool to assist in the diagnosis, prognosis and management of heart disease. The recent development of magnetic resonance imaging methods, such as harmonic phase analysis of tagging and displacement encoding with stimulated echoes (DENSE), make detailed non-invasive 3D kinematic analyses of human myocardium possible in the clinic and for research purposes. A robust analysis method is required, however. METHODS: We propose to estimate strain using a polynomial function which produces local models of the displacement field obtained with DENSE. Given a specific polynomial order, the model is obtained as the least squares fit of the acquired displacement field. These local models are subsequently used to produce estimates of the full strain tensor. RESULTS: The proposed method is evaluated on a numerical phantom as well as in vivo on a healthy human heart. The evaluation showed that the proposed method produced accurate results and showed low sensitivity to noise in the numerical phantom. The method was also demonstrated in vivo by assessment of the full strain tensor and to resolve transmural strain variations. CONCLUSIONS: Strain estimation within a 3D myocardial volume based on polynomial functions yields accurate and robust results when validated on an analytical model. The polynomial field is capable of resolving the measured material positions from the in vivo data, and the obtained in vivo strains values agree with previously reported myocardial strains in normal human hearts.

In Vitro Mitral Valve Model with Unrestricted Ventricular Access: Using Vacuum to Close the Valve and Enable Static Trans-Mitral Pressure.

Current in vitro models of the left heart establish the pressure difference required to close the mitral valve by sealing and pressurizing the ventricular side of the valve, limiting important access to the subvalvular apparatus. This paper describes and evaluates a system that establishes physiological pressure differences across the valve using vacuum on the atrial side. The subvalvular apparatus is open to atmospheric pressure and accessible by tools and sensors, establishing a novel technique for experimentation on atrioventricular valves. Porcine mitral valves were excised and closed by vacuum within the atrial chamber. Images were used to document and analyze closure of the leaflets. Papillary muscle force and regurgitant flow rate were measured to be 4.07 N at 120 mmHg and approximately 12.1 ml/s respectively, both of which are within clinically relevant ranges. The relative ease of these measurements demonstrates the usefulness of improved ventricular access at peak pressure/force closure.

Anterior mitral leaflet curvature during the cardiac cycle in the normal ovine heart.

BACKGROUND: The dynamic changes of anterior mitral leaflet (AML) curvature are of primary importance for optimal left ventricular filling and emptying but are incompletely characterized. METHODS AND RESULTS: Sixteen radiopaque markers were sutured to the AML in 11 sheep, and 4-dimensional marker coordinates were acquired with biplane videofluoroscopy. A surface subdivision algorithm was applied to compute the curvature across the AML at midsystole and at maximal valve opening. Septal-lateral (SL) and commissure-commissure (CC) curvature profiles were calculated along the SL AML meridian (M(SL))and CC AML meridian (M(CC)), respectively, with positive curvature being concave toward the left atrium. At midsystole, the M(SL) was concave near the mitral annulus, turned from concave to convex across the belly, and was convex along the free edge. At maximal valve opening, the M(SL) was flat near the annulus, turned from slightly concave to convex across the belly, and flattened toward the free edge. In contrast, the M(CC) was concave near both commissures and convex at the belly at midsystole but convex near both commissures and concave at the belly at maximal valve opening. CONCLUSIONS: While the SL curvature of the AML along the M(SL) is similar across the belly region at midsystole and early diastole, the CC curvature of the AML along the M(CC) flips, with the belly being convex to the left atrium at midsystole and concave at maximal valve opening. These curvature orientations suggest optimal left ventricular inflow and outflow shapes of the AML and should be preserved during catheter or surgical interventions.

Electromechanical coupling between the atria and mitral valve.

Anterior leaflet (AL) stiffening during isovolumic contraction (IVC) may aid mitral valve closure. We tested the hypothesis that AL stiffening requires atrial depolarization. Ten sheep had radioopaque-marker arrays implanted in the left ventricle, mitral annulus, AL, and papillary muscle tips. Four-dimensional marker coordinates (x, y, z, and t) were obtained from biplane videofluoroscopy at baseline (control, CTRL) and during basal interventricular-septal pacing (no atrial contraction, NAC; 110-117 beats/min) to generate ventricular depolarization not preceded by atrial depolarization. Circumferential and radial stiffness values, reflecting force generation in three leaflet regions (annular, belly, and free-edge), were obtained from finite-element analysis of AL displacements in response to transleaflet pressure changes during both IVC and isovolumic relaxation (IVR). In CTRL, IVC circumferential and radial stiffness was 46 ± 6% greater than IVR stiffness in all regions (P < 0.001). In NAC, AL annular IVC stiffness decreased by 25% (P = 0.004) in the circumferential and 31% (P = 0.005) in the radial directions relative to CTRL, without affecting edge stiffness. Thus AL annular stiffening during IVC was abolished when atrial depolarization did not precede ventricular systole, in support of the hypothesis. The likely mechanism underlying AL annular stiffening during IVC is contraction of cardiac muscle that extends into the leaflet and requires atrial excitation. The AL edge has no cardiac muscle, and thus IVC AL edge stiffness was not affected by loss of atrial depolarization. These findings suggest one reason why heart block, atrial dysrhythmias, or ventricular pacing may be accompanied by mitral regurgitation or may worsen regurgitation when already present.

The presence of two local myocardial sheet populations confirmed by diffusion tensor MRI and histological validation.

PURPOSE: To establish the correspondence between the two histologically observable and diffusion tensor MRI (DTMRI) measurements of myolaminae orientation for the first time and show that single myolaminar orientations observed in local histology may result from histological artifact. MATERIALS AND METHODS: DTMRI was performed on six sheep left ventricles (LV), then corresponding direct histological transmural measurements were made within the anterobasal and lateral-equatorial LV. Secondary and tertiary eigenvectors of the diffusion tensor were compared with each of the two locally observable sheet orientations from histology. Diffusion tensor invariants were calculated to compare differences in microstructural diffusive properties between histological locations with one observable sheet population and two observable sheet populations. RESULTS: Mean difference ± 1SD between DTMRI and histology measured sheet angles was 8° ± 27°. Diffusion tensor invariants showed no significant differences between histological locations with one observable sheet population and locations with two observable sheet populations. CONCLUSION: DTMRI measurements of myolaminae orientations derived from the secondary and tertiary eigenvectors correspond to each of the two local myolaminae orientations observed in histology. Two local sheet populations may exist throughout LV myocardium, and one local sheet population observed in histology may be a result of preparation artifact.

Significant changes in mitral valve leaflet matrix composition and turnover with tachycardia-induced cardiomyopathy.

BACKGROUND: Dilated cardiomyopathy (DCM) involves significant remodeling of the left ventricular-mitral valve (MV) complex, but little is known regarding the remodeling of the mitral leaflets. The aim of this study was to assess changes in matrix composition and turnover in MV leaflets with DCM. METHODS AND RESULTS: Radiopaque markers were implanted in 24 sheep to delineate the MV; 10 sheep underwent tachycardia-induced cardiomyopathy (TIC), whereas 14 sheep remained as controls. Biplane videofluoroscopy was performed before and after TIC. Immunohistochemistry was performed on leaflet cross-sections taken from the septal, lateral, anterior, and posterior commissures attachment segments. Staining intensity was quantified within each attachment segment and leaflet region (basal, mid-leaflet, and free edge). Mitral regurgitation increased from 0.2+/-0.4 before TIC to 2.2+/-0.9 after TIC (P<0.0002). TIC leaflets demonstrated significant remodeling compared to controls, including greater cell density and loss of leaflet layered structure (all P<0.05). Collagen and elastic fiber turnover was greater in TIC, as was the myofibroblast phenotype (all P<0.05). Compositional differences between TIC and control leaflets were heterogeneous by annular segment and leaflet region, and related to regional changes in leaflet segment length with TIC. CONCLUSIONS: This study shows that the MV leaflets are significantly remodeled in DCM with changes in leaflet composition, structure, and valve cell phenotype. Understanding how alterations in leaflet mechanics, such as those induced by DCM, drive cell-mediated remodeling of the extracellular matrix will be important in developing future treatment strategies.

Transient stiffening of mitral valve leaflets in the beating heart.

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 < or = 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 (Deltat(1)), end IVC to midsystole (Deltat(2)), midsystole to IVR onset (Deltat(3)), and IVR (Deltat(4)). Group mean stiffness (E(c) + or - SD; E(r) + or - SD; in N/mm(2)) during Deltat(1) (44 + or - 16; 15 + or - 4) was 1.6-1.7 times that during Deltat(4) (28 + or - 11; 9 + or - 3); Deltat(2) stiffness (39 + or - 15; 14 + or - 4) was 1.3-1.5 times that of Deltat(4), but Deltat(3) stiffness (32 + or - 12; 11 + or - 3) was only 1.1-1.2 times that of Deltat(4). The stiffness drop during Deltat(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.

Determinants of evolution and progression of acute ovine ischemic mitral regurgitation.

BACKGROUND AND AIM OF THE STUDY: The optimal treatment of moderate ischemic mitral regurgitation (IMR) remains contested. Thus, radiopaque markers were implanted on valvular structures to investigate the geometric and hemodynamic variables associated with the evolution and progression of acute ovine IMR. METHODS: Eight adult sheep underwent implantation of five radiopaque markers on the edge of the posterior mitral leaflet (PML), and five on the edge of the anterior mitral leaflet (AML). Eight additional markers were sewn around the mitral annulus (MA). The animals were studied immediately after surgery, using biplane videofluoroscopy and transesophageal echocardiography. Data were acquired at Baseline and at two time points (IMR1 and IMR2) during acute snare occlusion of the proximal left circumflex coronary artery and progressive IMR. The orthogonal distance of each leaflet edge marker to the least-squares annular plane, mitral annular area (MAA), and septal-lateral diameter (SL) were calculated at end-systole. The leaflet tenting area (TA) was calculated at valve center (CENT) and near the anterior (ACOM) and posterior (PCOM) commissures. RESULTS: The degree of MR was 0.6 +/- 0.4, 1.8 +/- 0.7, and 2.8 +/- 0.7 for Baseline, IMR1, and IMR2, respectively (p < 0.005). IMR1 was associated with annular dilatation and leaflet restriction near the valve center, and prolapse near the PCOM versus Baseline. Although both left ventricular pressure (LVP) and left ventricular dP/dt decreased significantly from IMR1 to IMR 2, there were no differences in leaflet or annular geometry. CONCLUSION: The initiation of moderate IMR was associated with significant alterations in annular and leaflet geometry, but only a small decrease in LV systolic function, was needed for IMR progression. These data suggest that the surgical repair and optimization of LV function may be important in combination to treat moderate IMR, as only small hemodynamic deterioration and perturbations in valvular geometry are necessary for significant IMR progression.

The effects of mitral regurgitation alone are sufficient for leaflet remodeling.

BACKGROUND: Although chronic mitral regurgitation results in adverse left ventricular remodeling, its effect on the mitral valve leaflets per se is unknown. In a chronic ovine model, we tested whether isolated mitral regurgitation alone was sufficient to remodel the anterior mitral leaflet. METHODS AND RESULTS: Twenty-nine sheep were randomized to either control (CTRL, n=11) or experimental (HOLE, n=18) groups. In HOLE, a 2.8- to 4.8-mm diameter hole was punched in the middle scallop of the posterior mitral leaflet to create "pure" mitral regurgitation. At 12 weeks, the anterior mitral leaflet was analyzed immunohistochemically to assess markers of collagen and elastin synthesis as well as matrix metalloproteinases and proteoglycans. A semiquantitative grading scale for characteristics such as intensity and delineation of stain between layers was used to quantify differences between HOLE and CTRL specimens across the heterogeneous leaflet structure. At 12 weeks, mitral regurgitation grade was greater in HOLE versus CTRL (3.0+/-0.8 versus 0.4+/-0.4, P<0.001). In HOLE anterior mitral leaflet, saffron-staining collagen (Movat) decreased, consistent with an increase in matrix metalloproteases throughout the leaflet. Type III collagen expression was increased in the midleaflet and free edge and expression of prolyl-4-hydroxylase (indicating collagen synthesis) was increased in the spongiosa layer. The proteoglycan decorin, also involved in collagen fibrillogenesis, was increased compared with CTRL (all P

Alterations in transmural myocardial strain: an early marker of left ventricular dysfunction in mitral regurgitation?

BACKGROUND: In asymptomatic patients with severe isolated mitral regurgitation (MR), identifying the onset of early left ventricular (LV) dysfunction can guide the timing of surgical intervention. We hypothesized that changes in LV transmural myocardial strain represent an early marker of LV dysfunction in an ovine chronic MR model. METHODS AND RESULTS: Sheep were randomized to control (CTRL, n=8) or experimental (EXP, n=12) groups. In EXP, a 3.5- or 4.8-mm hole was created in the posterior mitral leaflet to generate "pure" MR. Transmural beadsets were inserted into the lateral and anterior LV wall to radiographically measure 3-dimensional transmural strains during systole and diastolic filling, at 1 and 12 weeks postoperatively. MR grade was higher in EXP than CTRL at 1 and 12 weeks (3.0 [2-4] versus 0.5 [0-2]; 3.0 [1-4] versus 0.5 [0-1], respectively, both P<0.001). At 12 weeks, LV mass index was greater in EXP than CTRL (201+/-18 versus 173+/-17 g/m(2); P<0.01). LVEDVI increased in EXP from 1 to 12 weeks (P=0.015). Between the 1 and 12 week values, the change in BNP (-4.5+/-4.4 versus -3.0+/-3.6 pmol/L), PRSW (9+/-13 versus 23+/-18 mm Hg), tau (-3+/-11 versus -4+/-7 ms), and systolic strains was similar between EXP and CTRL. The changes in longitudinal diastolic filling strains between 1 and 12 weeks, however, were greater in EXP versus CTRL in the subendocardium (lateral: -0.08+/-0.05 versus 0.02+/-0.14; anterior: -0.10+/-0.05 versus -0.02+/-0.07, both P<0.01). CONCLUSIONS: Twelve weeks of ovine "pure" MR caused LV remodeling with early changes in LV function detected by alterations in transmural myocardial strain, but not by changes in BNP, PRSW, or tau.

Heterogeneity of left ventricular wall thickening mechanisms.

BACKGROUND: Myocardial fibers are grouped into lamina (or sheets) 3 to 4 cells thick. Fiber shortening produces systolic left ventricular (LV) wall thickening primarily by laminar extension, thickening, and shear, but the regional variability and transmural distribution of these 3 mechanisms are incompletely understood. METHODS AND RESULTS: Nine sheep had transmural radiopaque markers inserted into the anterior basal and lateral equatorial LV. Four-dimensional marker dynamics were studied with biplane videofluoroscopy to measure circumferential, longitudinal, and radial systolic strains in the epicardium, midwall, and endocardium. Fiber and sheet angles from quantitative histology allowed transformation of these strains into transmural contributions of sheet extension, thickening, and shear to systolic wall thickening. At all depths, systolic wall thickening in the anterior basal region was 1.6 to 1.9 times that in the lateral equatorial region. Interestingly, however, systolic fiber shortening was identical at each transmural depth in these regions. Endocardial anterior basal sheet thickening was >2 times greater than in the lateral equatorial region (epicardium, 0.16+/-0.15 versus 0.03+/-0.06; endocardium, 0.45+/-0.40 versus 0.17+/-0.09). Midwall sheet extension was >2 times that in the lateral wall (0.22+/-0.12 versus 0.09+/-0.06). Epicardial and midwall sheet shears in the anterior wall were approximately 2 times higher than in the lateral wall (epicardium, 0.14+/-0.07 versus 0.05+/-0.03; midwall, 0.21+/-0.12 versus 0.12+/-0.06). CONCLUSIONS: These data demonstrate fundamentally different regional contributions of laminar mechanisms for amplifying fiber shortening to systolic wall thickening. Systolic fiber shortening was identical at each transmural depth in both the anterior and lateral LV sites. However, systolic wall thickening of the anterior site was much greater than that of the lateral site. Fiber shortening drives systolic wall thickening, but sheet dynamics and orientations are of great importance to systolic wall thickening. LV wall thickening and its clinical implications pivot on different wall thickening mechanisms in various LV regions. Attempts to implant healthy contractile cells into diseased hearts or to surgically manipulate LV geometry need to take into account not only cardiomyocyte contraction but also transmural LV intercellular architecture and geometry.

Characterization of mitral valve anterior leaflet perfusion patterns.

BACKGROUND AND AIM OF THE STUDY: Although previous histologic studies have demonstrated the presence of blood vessels in the anterior mitral leaflet (AML) and second-order chordae (SC), little is known of the pattern of leaflet perfusion. Hence, the pattern and source of AML perfusion was investigated in an ovine model. METHODS: Fluorescein angiograms were obtained in 17 ovine hearts immediately after heparinization and cardioplegic arrest, using non-selective left coronary artery (LCA) and selective left anterior descending (LAD), proximal, mid- and distal left circumflex (LCx) perfusion. Serial photographs using a flash/filter system to optimize fluorescence were obtained through a left atriotomy. RESULTS: The proximal half of the AML was seen to be richly vascularized. A loop of vessels was consistently observed in the mitral annulus and AML; these vessels ran along the annulus, extended to the sites of SC insertion, and created anastomoses between these insertions. The SC contributed to the AML perfusion and the anastomotic loop. Selective perfusion of the LAD or proximal LCx artery (ligated before the first obtuse marginal artery) did not perfuse the AML (n = 6). Perfusion of the mid- and distal LCx (n = 7) consistently supplied the AML via SC insertion sites and annular branches. CONCLUSION: The ovine AML is perfused by vessels that run through the SC and annulus simultaneously, and then create a communicating arcade in the leaflet. These vessels originate from the mid- and distal portions of the LCx. A loss of perfusion as a result of microvascular disease could have adverse implications. Derangements in the extensive vascular component of the mitral valve could be an important contributing factor to valve disease.

Regional mitral leaflet opening during acute ischemic mitral regurgitation.

BACKGROUND AND AIM OF THE STUDY: Diastolic mitral valve (MV) opening characteristics during ischemic mitral regurgitation (IMR) are poorly characterized. The diastolic MV opening dynamics were quantified along the entire valvular coaptation line in an ovine model of acute IMR. METHODS: Ten radiopaque markers were sutured in pairs on the anterior (A1-E1) and corresponding posterior (A2-E2) leaflet edges from the anterior (A1/A2) to the posterior (E1/E2) commissure in 11 adult sheep. Immediately after surgery, 4-D marker coordinates were obtained before and during occlusion of the proximal left circumflex coronary artery. Distances between marker pairs were calculated throughout the cardiac cycle every 16.7 ms. Leaflet opening was defined as the time after end-systole (ES) when the first derivative of the distance between marker pairs was greater than a threshold value of 3 cm/s. Valve opening velocity was defined as the maximum slope of marker pair tracings. RESULTS: Hemodynamics were consistent with acute ischemia, as reflected by increased MR grade (0.5 +/- 0.3 versus 2.3 +/- 0.7, p < 0.05), decreased contractility (dP/dt(max): 1,948 +/- 598 versus 1,119 +/- 293 mmHg/s, p < 0.05), and slower left ventricular relaxation rate (dP/dt(min): -1,079 +/- 188 versus -538 +/- 147 mmHg/s, p < 0.05). During ischemia, valve opening occurred earlier (A1/A2: 112 +/- 28 versus 83 +/- 43 ms, B1/B2: 105 +/- 32 versus 68 +/- 35 ms, C1/C2: 126 +/- 25 versus 74 +/- 37 ms, D1/D2: 114 +/- 28 versus 71 +/- 34 ms, E1/E2: 125 +/- 29 versus 105 +/- 33 ms; all p < 0.05) and was slower (A1/A2: 16.8 +/- 9.6 versus 14.2 +/- 9.4 cm/s, B1/B2: 40.4 +/- 9.9 versus 32.2 +/- 10.0 cm/s, C1/C2: 59.0 +/- 14.9 versus 50.4 +/- 18.1 cm/s, D1/D2: 34.4 +/- 10.4 versus 25.5 +/- 10.9 cm/s; all p < 0.05), except at the posterior edge (E1/E2: 13.3 +/- 8.7 versus 10.6 +/- 7.2 cm/s). The sequence of regional mitral leaflet separation along the line of coaptation did not change with ischemia. CONCLUSION: Acute posterolateral left ventricular ischemia causes earlier leaflet opening, probably due to a MR-related elevation in left-atrial pressure; reduces leaflet opening velocity, potentially reflecting an impaired left ventricular relaxation rate; and does not perturb the homogeneous temporal pattern of regional valve opening along the line of coaptation. Future studies will confirm whether these findings are apparent in patients with chronic IMR, and may help to refine the current strategies used to treat IMR.

Characterization of 3-dimensional papillary muscle displacement in in vivo ovine models of ischemic/functional mitral regurgitation.

OBJECTIVE: Papillary muscle (PM) displacement contributes to ischemic/functional mitral regurgitation (IMR/FMR). The displaced PMs pull the mitral leaflets into the left ventricle (ie, toward the apex) thus hampering leaflet coaptation. Intuitively apical leaflet tethering results from apical PM displacement. The 3-dimensional directions of PM displacement are, however, incompletely characterized. METHODS: Data from in vivo ovine models of IMR (6-8 weeks of posterolateral infarction, n = 12) and FMR (9-21 days of rapid left ventricular pacing, n = 11) were analyzed. All sheep had radiopaque markers implanted on the anterior and posterior PM (PPM) tips, around the mitral annulus, and on the left ventricular apex. To explore 3-dimensional PM displacement directions, differences in marker coordinates were calculated at end-systole before and during IMR/FMR using a right-handed coordinate system centered on the mitral annular "saddle horn" with the y-axis passing through the apical marker. RESULTS: No apical PM displacement was observed during either IMR or FMR. The anterior PM displaced laterally during FMR. Posterolateral PPM displacement was observed during IMR and FMR. CONCLUSIONS: Experimental in vivo ovine models suggest posterolateral PPM displacement as a predominant pathomechanism leading to apical leaflet tethering during IMR/FMR.

Do annuloplasty rings designed to treat ischemic/functional mitral regurgitation alter left-ventricular dimensions in the acutely ischemic ovine heart?

OBJECTIVE: To quantify the effects of annuloplasty rings designed to treat ischemic/functional mitral regurgitation on left ventricular septal-lateral (S-L) and commissure-commissure (C-C) dimensions. METHODS: Radiopaque markers were placed as opposing pairs on the S-L and C-C aspects of the mitral annulus and the basal, equatorial, and apical level of the left ventricle (LV) in 30 sheep. Ten true-sized Carpentier-Edwards Physio (PHY), Edwards IMR ETlogix (ETL), and GeoForm (GEO; all from Edwards Lifesciences, Irvine, Calif) annuloplasty rings were inserted in a releasable fashion. After 90 seconds of left circumflex artery occlusion with the ring implanted (RING), 4-dimensional marker coordinates were obtained using biplane videofluoroscopy. After ring release, another data set was acquired after another 90 seconds of left circumflex artery occlusion (NO RING). S-L and C-C diameters were computed as the distances between the respective marker pairs at end-diastole. Percent change in diameters was calculated between RING versus NO RING as 100 × (diameter in centimeters [RING] - diameter in centimeters [NO RING])/diameter in centimeters [NO RING]). RESULTS: Compared with NO RING, all ring types (PHY, ETL, and GEO) reduced mitral annular S-L dimensions by -20.7 ± 5.6%, -26.8 ± 3.9%, and -34.5 ± 3.8%, respectively. GEO reduced the S-L dimensions of the LV at the basal level only by -2.3 ± 2.4%, whereas all other S-L dimensions of the LV remained unchanged with all 3 rings implanted. PHY, ETL, and GEO reduced mitral annular C-C dimensions by -17.5 ± 4.8%, -19.6 ± 2.5, and -8.3 ± 4.9%, respectively, but none of the rings altered the C-C dimensions of the LV. CONCLUSIONS: Despite radical reduction of mitral annular size, disease-specific ischemic/functional mitral regurgitation annuloplasty rings do not induce relevant changes of left ventricular dimensions in the acutely ischemic ovine heart.

Effect of chronotropy and inotropy on stitch tension in the edge-to-edge mitral repair.

BACKGROUND: Our prior studies suggest that mitral annular septal-lateral (SL) diameter is the chief determinant of "Alfieri stitch" tension, but hemodynamic parameters may also play a role. We approximated the central edge of the mitral leaflets with a miniature force transducer to measure tension (T) at the leaflet approximation point during inotropic and chronotropic stimulation. METHODS AND RESULTS: Eight sheep were studied under open-chest conditions immediately after surgical placement of a miniature force transducer to approximate the leaflets and implantation of radiopaque markers on the LV and mitral annulus (MA). Chronotropic stimulation was induced with atrial pacing at 130 minutes(-1) (n=5) whereas inotropic state was increased with i.v. CaCl2 bolus (n=8). Hemodynamic data, stitch tension, and 3-D marker coordinates were obtained throughout the cardiac cycle before and during each intervention. Peak stitch tension (T(MAX)) under all conditions was observed in diastole and temporally correlated with peak annular SL (SL(MAX)) size. Atrial pacing did not change peak transducer tension or annular size. Calcium infusion also did not alter peak transducer tension (0.29+/-0.11 versus 0.32+/-0.10 N; P=NS) and only slightly reduced SL dimension (29.9+/-3.3 versus 29.3+/-3.5 mm; P<0.05). CONCLUSION: Isolated increase in heart rate or inotropic state did not alter peak stitch tension whereas enhanced contractile state decreased SL diameter minimally. These data, combined with those from our previous study, suggest that geometric (SL diameter) rather than hemodynamic parameters are the main determinants of "Alfieri stitch" tension. This implies that any interventional or surgical edge-to-edge repair performed without concomitant annular reduction to limit the SL dimension could expose the leaflet junction to forces which could limit repair durability.