Three-dimensional dynamic geometry of the normal canine mitral annulus and papillary muscles.

BACKGROUND: Despite an incomplete knowledge of the geometry and dynamics of the mitral annulus (MA), papillary muscle (PM), and the chordae tendineac, chordal-sparing MVR is popular. METHODS AND RESULTS: The systolic reduction in three-dimensional distance between each PM tip and eight MA sites (DT-A) was measured in nine normal closed-chest dogs by use of surgically implanted radiopaque markers. Three loci (tip, junction, and base) on each PM were also projected onto the MA plane at end diastole and end systole to assess PM dynamics. The anterior PM tip showed significant shortening of DT-A toward the opposite side of the MA or the midanterior MA region (P < .005 or P < .05, respectively, versus same MA side [MANOVA]); conversely, the posterior PM tip DT-A shortened toward the opposite side of the MA near the anterior commissure or the area between the anterior commissure and midposterior MA (P < .005 versus same MA side). Annular projection revealed three-dimensional motion (relative to the MA) of the anterior PM tip, junction, and base toward the right trigone, while posterior PM motion was oriented toward the opposite side of the MA. CONCLUSIONS: Both PMs in normal canine hearts demonstrated systolic relative motion in a direction compatible with the "oblique" chordal configuration, ie, from the anterior PM to the anterior MA near the right trigone and from the posterior PM to the opposite side of the posterior MA. These observations warrant further investigation of three-dimensional PM-MA dynamics with various methods of chorda preservation during MVR to assess their impact on left ventricular systolic and diastolic function.

Three-dimensional regional dynamics of the normal mitral anulus during left ventricular ejection.

The mitral anulus is a dynamic structure that undergoes alterations in size and shape throughout the cardiac cycle, contracting during systole. Numerous reports have shown this systolic orifice reduction to be due chiefly to posterior annular contraction, whereas the anterior perimeter was unchanged. Segmental motion of the mitral anulus from true in vivo three-dimensional data, however, has not been described. We used radiopaque markers and simultaneous biplane videofluoroscopy to measure the lengths of mitral anular segments in seven closed-chest, sedated dogs. Eight markers were placed equidistant from each other around the mitral anulus, As viewed from the left atrium, segment 1 began at the posteromedial commissure, and the remaining segments were numbered sequentially clockwise around the anulus (that is, the posterior mitral anulus encompassed segments 1 to 4 and the anterior anulus encompassed segments 5 to 8). Marker image coordinates obtained from two orthogonal views 7 to 12 days after implantation were merged to construct three-dimensional marker coordinates at end-diastole and end-systole. From end-diastole to end-systole, mean annular area decreased by 11% +/- 8% (5.5 +/- 0.9 cm2 to 4.9 +/- 0.8 cm2, p = 0.005) and perimeter by 5% +/- 4% (8.8 +/- 0.7 cm to 8.3 +/- 0.7 cm, p < 0.01). Mitral annular segmental percent systolic shortening (negative values indicate lengthening) were as follows (mean +/- standard deviation): segment 1, 7% +/- 9%; segment 2, 8% +/- 10%; segment 3, 16% +/- 6%; segment 4, 10% +/- 7%; segment 5, -4% +/- 5%, segment 6, -7% +/-7%; segment 7, 3% +/- 2%; and segment 8, 6% +/- 5%. With the exception of segment 1, all posterior (2 to 4) and two anterior (7 and 8) mitral annular segments contracted significantly (p < or = vs zero, paired t test). Two anterior annular segments (5 and 6, regions overlapping aortic-mitral continuity), however, unexpectedly lengthened during left ventricular systole. We conclude that the anterior mitral anulus may be a much more dynamic component of the mitral apparatus that previously thought. Such heterogeneous dynamic annular motion should be taken into account when various mitral valve reparative techniques are being designed.

Dynamics of normal and ischemic canine papillary muscles.

This investigation was designed to elucidate the dynamics of the left ventricular (LV) papillary muscles. Miniature tantalum myocardial markers were placed on the tip and base of each papillary muscle in six dogs. Markers were also implanted into the LV myocardium to define two orthogonal equatorial diameters and the long-axis dimension. Two weeks later, after recovery from thoracotomy, markers were visualized by biplane fluoroscopy, and video images were recorded during control conditions, after autonomic blockade, after inotropic stimulation with calcium, after methoxamine infusion (to increase afterload), and after blood volume augmentation (to increase preload). Two days later, radiographic recordings were made before and after occlusion of the left circumflex coronary artery. Computer-aided analysis of the video recordings was used to determine three-dimensional coordinates of the markers. It was found that before circumflex coronary occlusion, the dynamics of both papillary muscles closely mimicked the dynamics of the LV as a whole. The papillary muscles shortened during ejection and lengthened during diastole. Their lengths changed minimally during the isovolumic periods, and this behavior was not altered by any of the interventions except coronary occlusion. During circumflex coronary artery occlusion, the ischemic posterior papillary muscle lengthened during isovolumic contraction and most of ejection and shortened only when LV pressure began to fall. Hence, we believe that previous studies demonstrating papillary muscle lengthening during isovolumic contraction and shortening during isovolumic relaxation may have been confounded by coexistent myocardial ischemia or stunning.

Annuloplasty with flexible or rigid ring does not alter left ventricular systolic performance, energetics, or ventricular-arterial coupling in conscious, closed-chest dogs.

Eighteen dogs were randomly chosen to undergo mitral annuloplasty with either a Carpentier-Edwards rigid ring (n = 6 in each group) or a Duran-Medtronic flexible ring or to undergo a sham procedure with an operation, but no ring. Tantalum markers were inserted to measure left ventricular volume and geometry. After 1 and 6 weeks, biplane videofluoroscopic images were obtained during steady-state conditions and during vena caval occlusion. Global and regional systolic function was assessed with load-insensitive indexes. Comparison of all three groups and both times (1 and 6 weeks) showed no significant differences among the three groups in global or regional (basal, equatorial, and apical) left ventricular systolic performance. Furthermore, neither type of annuloplasty ring significantly affected left ventricular pump efficiency, ventricular-arterial coupling ratio, or systolic circumferential contraction and rotation of the basal left ventricular sites.

Mitral valve replacement in dilated canine hearts with chronic mitral regurgitation. Importance of the mitral subvalvular apparatus.

The importance of the mitral subvalvular apparatus in terms of left ventricular (LV) mechanics and energetic efficiency in the chronically dilated canine heart was assessed in nine dogs with surgically induced mitral regurgitation. Miniature radiopaque tantalum markers were implanted into the myocardium to measure LV volume. Biplane cinefluoroscopic images obtained 1 week and 3 months after creation of mitral regurgitation confirmed the presence of LV dilatation. Mitral valve replacement with preservation of all chordae tendineae was then performed. LV systolic function and derived energetics were then assessed during transient caval occlusion both before and after chordal division by using exteriorized snares. Global LV systolic mechanics, as assessed by the slopes of the end-systolic pressure volume (Ees) and end-systolic stress volume (Ms) relations, fell by 46% (11.7 +/- 2.8 versus 6.3 +/- 1.4 mm Hg/ml, p less than 0.001) and 33% (17.8 +/- 4.0 versus 12.0 +/- 5.1 kdyne/cm5, p = 0.0001), respectively, when the chordae were divided. Chordal severing also increased systolic LV wall stress or LV afterload. In terms of calculated myocardial energetics, the slopes of the stroke work-end-diastolic volume and pressure volume area-end-diastolic volume relations declined significantly by 20% (85 +/- 14 versus 68 +/- 16 mm Hg) and 11% (116 +/- 20 versus 104 +/- 20 mm Hg) after cutting the chordae, thereby indicating reduced external stroke work and mechanical energy generated at any given level of preload. Moreover, the efficiency of energy transfer from pressure volume area to external stroke work fell by 19% (p less than 0.001). Since effective systemic arterial elastance (Ea) did not change, the Ea/Ees ratio (index of ventriculoarterial [V-A] coupling) increased from 0.93 +/- 0.27 to 1.67 +/- 0.62 (p = 0.006). Therefore, chordal division in dilated dog hearts due to chronic mitral regurgitation resulted not only in deterioration of systolic LV mechanics but also deleterious changes in calculated LV energetics and efficiency due to exacerbated mismatch in V-A coupling between the left ventricle and the systemic arterial bed, unfavorable loading conditions, and exhaustion of preload reserve. These observations in the low-pressure, volume-overloaded heart due to chronic mitral regurgitation underscore the importance of the mitral subvalvular apparatus for optimal LV systolic performance and energetic efficiency.

Regional epicardial and endocardial two-dimensional finite deformations in canine left ventricle.

We evaluated subepicardial and subendocardial two-dimensional finite deformations in the left ventricular (LV) anterior, lateral, and posterior regions in the closed-chest, conscious dog heart. Eight dogs underwent placement of 22 radiopaque markers in the LV myocardium. Sets of three markers were implanted in the anterior, lateral, and posterior subepicardium and subendocardium at the mid-ventricular level; reference markers were placed at apical and basal sites. Eight hours later, biplane videofluoroscopy was performed. Finite deformations for each subepicardial and subendocardial region were analyzed during three consecutive beats at end expiration. Circumferential shortening occurred in all layers and regions; similarly, longitudinal shortening occurred in all layers except that of the posterior endocardium. Values of principal strain were -0.19 +/- 0.08 (SD) and -0.10 +/- 0.03 for the anterior subendocardium and subepicardium, -0.20 +/- 0.07 and -0.10 +/- 0.02 for the lateral subendocardium and subepicardium, and -0.13 +/- 0.02 and -0.10 +/- 0.03 for the posterior subendocardium and subepicardium respectively (P less than 0.05 subendocardium vs. subepicardium). Second principal strain tended to be near zero or positive (from -0.01 +/- 0.05 to 0.04 +/- 0.05) in all regions. The end-systolic direction of principal strain was -29 +/- 32 degrees and -34 +/- 29 degrees in the anterior subepicardium and subendocardium, -47 +/- 10 degrees and -30 +/- 37 degrees in the lateral subepicardium and subendocardium, and -4 +/- 29 degrees and +7 +/- 23 degrees in the posterior subepicardium and subendocardium. Anterior and lateral directions of principal strain were similar in the subepicardial and subendocardial layers and oriented along the epicardial fiber axis, but the posterior direction tended to be circumferentially oriented.(ABSTRACT TRUNCATED AT 250 WORDS)

Automatic tracking and digitization of multiple radiopaque myocardial markers.

An 80386 PC-based system was designed to track automatically multiple, miniature radiopaque markers implanted in the heart wall. This system eliminated the need for tedious, time-consuming manual digitization of marker coordinates. Use of a MATROX MVP-AT/NP image processing board incorporated advanced image processing and graphics features into the low-cost PC environment. Digital image enhancement and segmentation techniques (such as limiting analysis to predefined windows of interest, spatial band-pass and matched filtering, contrast stretching and clipping, linear adaptive prediction, intensity histogram analysis, adaptive binary thresholding, region growing, expanding region of analysis, and feature extraction) were incorporated into a user-friendly integrated marker processing software environment. Improved speed, accuracy, and reproducibility of the marker digitizing process were realized. These basic techniques have broad applications to other image processing needs in biomedical research.

Alterations in left ventricular diastolic twist mechanics during acute human cardiac allograft rejection.

BACKGROUND: Contraction of obliquely oriented left ventricular (LV) fibers results in a twisting motion of the left ventricle. The purpose of this study was to assess the effects of acute human cardiac allograft rejection on LV twist pattern and the twist-volume relation. METHODS AND RESULTS: Tantalum markers were implanted into the LV midwall in 15 transplant recipients to measure time-varying, three-dimensional chamber twist using computer-assisted analysis of biplane cinefluoroscopic images. Twist was defined as the mean longitudinal gradient of circumferential rotation about the LV long axis. When plotted against normalized percent ejection fraction (%EF), the resulting twist-normalized %EF relation could be divided into three phases. In systole, LV twist was linearly related to ejection of blood. In contrast, diastolic untwist was characterized by early rapid recoil with little change in LV volume, followed by more gradual untwisting when the bulk of diastolic filling occurred. During 10 acute rejection episodes in 10 patients, maximum twist, peak systolic twist rate, and the slope of the systolic twist-normalized %EF relation did not change. In contrast, the slope of the early (first 15% of filling) diastolic twist-normalized %EF relation (M(early-dia)) decreased significantly (-0.194 +/- 0.062 [prerejection] versus -0.103 +/- 0.054 rad/cm [rejection], p = 0.0003), resulting in a prolonged tau 1/2 (time required to untwist by 50% [20 +/- 5% versus 28 +/- 5% of diastole], p = 0.0003) and decrease in percent untwisting at 15% diastolic LV filling (62 +/- 11% versus 36 +/- 13%, p = 0.0003). Therefore, a greater proportion of LV untwisting occurred later in diastole during rejection, as reflected by an increase in the slope (M(mid-dia)) of the middle to late (from 15 to 90% filling) diastolic twist-normalized %EF relation (-0.018 +/- 0.009 versus -0.030 +/- 0.010 rad/cm, p = 0.0015). Peak rate of untwist was not affected. With resolution of rejection, M(early-dia) and percent untwist during early diastole returned to baseline levels (p = NS versus baseline). There was also a trend for M(mid-dia) to return toward prerejection values (p = NS versus baseline), but this change did not reach statistical significance compared with rejection values. CONCLUSION: Acute cardiac allograft rejection is associated with altered diastolic twist mechanics in the absence of any demonstratable systolic abnormalities. During rejection, myocardial edema and other factors may result in intrinsic changes of the elastic properties of the myocardium, thereby leading to modification of recoil forces responsible for the early, rapid unwinding of the deformed ventricle.

Importance of mitral subvalvular apparatus in terms of cardiac energetics and systolic mechanics in the ejecting canine heart.

To assess the importance of the intact mitral subvalvular apparatus for left ventricular (LV) energetics, data from nine open-chest ejecting canine hearts were analyzed using piezoelectric crystals to measure LV volume. After mitral valve replacement with preservation of all chordae tendineae, baseline LV function was assessed during transient caval occlusion: A quadratic fit of the LV end-systolic pressure-volume data was used to determine the curvilinear end-systolic pressure-volume relationship (ESPVR). All chordae were then divided with exteriorized snares. Reassessment revealed deterioration of global LV pump function: (a) the coefficient of nonlinearity, decreased (less negative) by 90% (P = 0.06); (b) slope of the curvilinear ESPVR at the volume axis intercept, decreased by 75% (P = 0.01); and V100, end-systolic volume at 100 mmHg end-systolic pressure, increased by 42% (P less than 0.02). Similarly, preload recruitable stroke work fell significantly (-14%) and Vw1,000 (end-diastolic volume [EDV] at stroke work [SW] of 1,000 mmHg.ml) rose by 17% (P less than 0.04). With respect to LV energetics, the total mechanical energy generated by the ventricle decreased, as indicated by a decline in the slope of the pressure volume area (PVA)-EDV relationship (120 +/- 13 [mean +/- SD] vs. 105 +/- 13 mmHg, P less than 0.001). Additionally, comparison of LV SW and PVA from single beats with matched EDV showed that the efficiency of converting mechanical energy to external work (SW/PVA) declined by 14% (0.65 +/- 0.13 vs. 0.56 +/- 0.08, P less than 0.03) after chordal division. While effective systemic arterial elastance, Ea, also fell significantly (P = 0.03) after the chordae were severed, the Ea/Ees ratio (Ees = slope of the linear ESPVR) increased by 124% (0.91 +/- 0.53 vs. 2.04 +/- 0.87, P = 0.001) due to a proportionally greater decline in Ees. This indicates a mismatch in ventriculo-arterial interaction, deviating from that required for maximal external output (viz., Ea/Ees = 1). These adverse effects of chordal division may be related to the observed changes in LV geometry (i.e., eccentricity). We conclude that the intact mitral subvalvular apparatus is important in optimizing LV energetics and ventriculo-vascular coupling in addition to the enhancement of LV systolic performance.

Global and regional left ventricular systolic performance in the in situ ejecting canine heart. Importance of the mitral apparatus.

The importance of the intact mitral apparatus in left ventricular (LV) systolic performance has been indirectly suggested by clinical studies of chordal-preserving mitral valve replacement (MVR) or reconstruction. The importance of the intact mitral apparatus has been clearly demonstrated in isovolumic experimental preparations but has not been demonstrated unequivocally in ejecting hearts. Therefore, this question was assessed independently of load in an in situ, open-chest ejecting canine heart preparation (n = 9). Three orthogonal LV dimensions were measured by sonomicrometry. During MVR with complete chordal preservation, snares were placed around the anterior and posterior papillary muscles. After the hearts were weaned from cardiopulmonary bypass, LV function was assessed by caval occlusion to alter LV preload abruptly. The slopes of the end-systolic--pressure-volume (end-systolic elastance, Ees) and stroke-work--end-diastolic volume (preload-recruitable stroke work, PRSW) relations were used to measure global LV systolic function; similarly, the slopes of the end-systolic--pressure-dimension (regional end-systolic elastance, rEes) and stroke-work--end-diastolic dimension changes in regional LV systolic performance. All chordae were then divided by pulling the snares. Immediate reassessment revealed deterioration of global LV function: Ees declined by 72% (14.1 +/- 11.2 mm Hg/ml [mean +/- SD] vs. 3.9 +/- 3.5 mm Hg/ml, p less than 0.001), and PRSW declined by 39% (129 +/- 37 vs. 79 +/- 29 mm Hg, p = 0.0001). Regional LV function was also adversely affected but somewhat selectively: rEes decreased significantly in all three LV dimensions (p less than or equal to 0.03), but rPRSW decreased significantly (-21%) only in the anteroposterior minor LV axis (89 +/- 19 vs. 70 +/- 15 mm Hg, p = 0.005) and in the septal-lateral axis (-19%, p = NS). These data demonstrate the importance of the intact mitral apparatus on LV systolic performance in ejecting hearts, particularly in the LV regions subtended by the papillary muscles.

Physiologic role of the mitral apparatus in left ventricular regional mechanics, contraction synergy, and global systolic performance.

In animal models, severing the chordae tendineae of the mitral valve reduces the maximum global left ventricular elastance (Emax,g), a load-independent measure of left ventricular systolic performance; moreover, chamber geometry is altered with systolic bulging in the region of the papillary muscle insertions. This suggests that forces transmitted by the mitral apparatus increase the regional volume elastance (Emax,r) of segments subtending the insertions of the papillary muscles, and these regions contribute substantially to overall left ventricular systolic function (Emax,g). To test this hypothesis, we developed a method to evaluate changes in the magnitude and uniformity of Emax,r as quantitated by the slopes (E'max,i) of regional left ventricular isovolumetric pressure-dimension relations. Such measurements were obtained before and after all chordal attachments of the mitral valve were surgically divided in seven open-chest swine preparations. Significant declines in E'max,i were limited to the region of the posteromedial papillary muscle insertion. Although the mean E'max,i of all ventricular regions (E'max,ave) was unchanged, regional left ventricular elastances were less uniform after the mitral chordae tendineae were severed, which indicated a less synergistic contraction, and Emax,g fell by 21% from 7.1 +/- 2.0 to 5.6 +/- 1.2 mm Hg/ml (p less than 0.05). These data demonstrate that the mitral apparatus contributes importantly to the magnitude and uniformity of regional left ventricular elastances and suggest that such alterations in regional mechanics underlie the deterioration in global left ventricular systolic performance (Emax,g) after excision of the mitral apparatus.