Suzanne Sullivan

Design of a New Surgical Approach for Ventricular Remodeling to Relieve Ischemic Mitral Regurgitation Insights From 3-Dimensional Echocardiography

BACKGROUND Mechanistic insights from 3D echocardiography (echo) can guide therapy. In particular, ischemic mitral regurgitation (MR) is difficult to repair, often persisting despite annular reduction. We hypothesized that (1) in a chronic infarct model of progressive MR, regurgitation parallels 3D changes in the geometry of mitral leaflet attachments, causing increased leaflet tethering and restricting closure; therefore, (2) MR can be reduced by restoring tethering geometry toward normal, using a new ventricular remodeling approach based on 3D echo findings. METHODS AND RESULTS We studied 10 sheep by 3D echo just after circumflex marginal ligation and 8 weeks later. MR, at first absent, became moderate as the left ventricle (LV) dilated and the papillary muscles shifted posteriorly and mediolaterally, increasing the leaflet tethering distance from papillary muscle tips to the anterior mitral annulus (P<0.0001). To counteract these shifts, the LV was remodeled by plication of the infarct region to reduce myocardial bulging, without muscle excision or cardiopulmonary bypass. Immediately and up to 2 months after plication, MR was reduced to trace-to-mild as tethering distance was decreased (P<0.0001). LV ejection fraction, global LV end-systolic volume, and mitral annular area were relatively unchanged. By multiple regression, the only independent predictor of MR was tethering distance (r(2)=0.81). CONCLUSIONS Ischemic MR in this model relates strongly to changes in 3D mitral leaflet attachment geometry. These insights from quantitative 3D echo allowed us to design an effective LV remodeling approach to reduce MR by relieving tethering.

Reverse Ventricular Remodeling Reduces Ischemic Mitral Regurgitation Echo-Guided Device Application in the Beating Heart

Background—In ischemic mitral regurgitation (MR), mitral leaflet closure is restricted by ventricular remodeling with displacement of the papillary muscles (PMs). Therapy is uncertain because ring annuloplasty does not alleviate PM displacement. We tested the hypothesis that echo-guided PM repositioning using an external device can reduce MR without compromising left ventricular (LV) function. Methods and Results—We studied 10 sheep with ischemic MR produced by circumflex ligation with inferior infarction, 6 acutely and 4 eight weeks after myocardial infarction (MI). A Dacron patch containing an inflatable balloon was placed over the PMs and adjusted under echo guidance to reverse LV remodeling and reposition the infarcted PM. 3D echo assessed mitral valve geometric changes. In 7 sheep, sonomicrometry and Millar catheters assessed changes in end-systolic and end-diastolic pressure-volume relationships, and microspheres were injected to assess coronary flow. Moderate MR after MI resolved with patch application alone (n=3) or echo-guided balloon inflation, which repositioned the infarcted PM, decreasing the PM tethering distance from 31.1±2.5 mm after MI to 26.8±1.8 with patch (P <0.01; baseline=25.5±1.5). LV contractility was unchanged (end-systolic slope=3.4±1.6 mm Hg/mL with patch versus 2.8±1.6 after MI). Although there was a nonsignificant trend for a mild increase in stiffness constant (0.07±0.05 mL−1 versus 0.05±0.03 after MI, P =0.06), LV end-diastolic pressure was unchanged as MR resolved. Coronary flow to noninfarcted regions was not reduced. Conclusions—An external device that repositions the PMs can reduce ischemic MR without compromising LV function. This relatively simple technique can be applied under echo guidance in the beating heart.

Active Adaptation of the Tethered Mitral Valve Insights Into a Compensatory Mechanism for Functional Mitral Regurgitation

Background— In patients with left ventricular infarction or dilatation, leaflet tethering by displaced papillary muscles frequently induces mitral regurgitation, which doubles mortality. Little is known about the biological potential of the mitral valve (MV) to compensate for ventricular remodeling. We tested the hypothesis that MV leaflet surface area increases over time with mechanical stretch created by papillary muscle displacement through cell activation, not passive stretching. Methods and Results— Under cardiopulmonary bypass, the papillary muscle tips in 6 adult sheep were retracted apically short of producing mitral regurgitation to replicate tethering without confounding myocardial infarction or turbulence. Diastolic leaflet area was quantified by 3-dimensional echocardiography over 61±6 days compared with 6 unstretched sheep MVs. Total diastolic leaflet area increased by 2.4±1.3 cm2 (17±10%) from 14.3±1.9 to 16.7±1.9 cm2 (P=0.006) with stretch with no change in the unstretched valves despite sham open heart surgery. Stretched MVs were 2.8 times thicker than normal (1.18±0.14 versus 0.42±0.14 mm; P<0.0001) at 60 days with an increased spongiosa layer. Endothelial cells (CD31+) coexpressing &agr;-smooth muscle actin were significantly more common by fluorescent cell sorting in tethered versus normal leaflets (41±19% versus 9±5%; P=0.02), indicating endothelial-mesenchymal transdifferentiation. &agr;-Smooth muscle actin-positive cells appeared in the atrial endothelium, penetrating into the interstitium, with increased collagen deposition. Thickened chordae showed endothelial and subendothelial &agr;-smooth muscle actin. Endothelial-mesenchymal transdifferentiation capacity also was demonstrated in cultured MV endothelial cells. Conclusions— Mechanical stresses imposed by papillary muscle tethering increase MV leaflet area and thickness, with cellular changes suggesting reactivated embryonic developmental pathways. Understanding such actively adaptive mechanisms can potentially provide therapeutic opportunities to augment MV area and reduce ischemic mitral regurgitation.

Mitral Leaflet Adaptation to Ventricular Remodeling Prospective Changes in a Model of Ischemic Mitral Regurgitation

Background— Ischemic mitral regurgitation is caused by systolic traction on the mitral leaflets related to ventricular distortion. Little is known about how chronic tethering affects leaflet area, in part because it cannot be measured repeatedly in situ. Recently, a new method for 3D echocardiographic measurement of mitral leaflet area was developed and validated in vivo against sheep valves, later excised. Clinical studies (n=80) showed that mitral leaflet area increased by >30% in patients with inferior myocardial infarction and dilated cardiomyopathy versus normal; greater adaptation independently predicted less mitral regurgitation. This study explored whether mitral valve area changes over time within the same heart with ischemic mitral regurgitation. Methods and Results— Twelve sheep were studied at baseline and 3 months after inferior myocardial infarction by 3D echocardiography; 6 were untreated and 6 were treated initially with an epicardial patch to limit left ventricular dilation and mitral regurgitation. Untreated sheep developed left ventricular dilation at 3 months, with global dysfunction (mean±SD ejection fraction, 24±10% versus 44±10% with patching, P=0.02) and moderate mitral regurgitation (vena contracta, 5.0±1.0 versus 0.8±1.0 mm, P<0.0002). In untreated sheep, total diastolic leaflet area increased from 13.1±1.3 to 18.1±2.5 cm2 (P=0.0001). In patched sheep, leaflet area at 3 months was not significantly different from baseline sheep values (13.0±1.1 versus baseline, 12.1±1.8 cm2, P=0.31). Conclusions— Mitral valve area, independent of systolic stretch, increases over time as the left ventricular remodels after inferior myocardial infarction. This increase, however, fails to compensate adequately for tethering to prevent mitral regurgitation. Understanding the mechanism of valve adaptation can potentially suggest new biological and surgical therapeutic targets.

New Efficient Catheter-Based System for Myocardial Gene Delivery

Background—Manipulating gene expression in the failing heart has therapeutic promise, but until now efficient and homogeneous cardiac gene delivery has required an open-chest approach. This study examines the hypothesis that vector delivery promoted by echo contrast microbubbles will be maximized by injection of the vectors into the aortic root with brief balloon occlusion above the sinuses, while at the same time prolonging diastole and vasodilating with acetylcholine (ACh) to maximize coronary exposure. Methods and Results—After incubation with albumin-coated perfluorocarbon microbubbles, an adenovirus encoding a reporter gene was infused into the aortic root of rats. To maximize delivery, the aortic root was transiently occluded with a balloon catheter during a brief ACh-induced asystole. Ultrasound was used to image the delivery and disrupt the microbubbles. Aortic occlusion with concomitant ACh increased myocardial gene expression for virus + microbubbles by >2.5-fold, from 925±165 to 2358±376 relative units (RU;P <0.01). This delivery system also produced substantial expression with vector alone (1473±549 RU). All uptakes were significant compared with 433±332 RU without virus. Conclusions—An adenoviral delivery system combining echo contrast with a catheter-based technique to maximize coronary perfusion increases gene delivery compared with echo contrast alone. This novel method permits efficient percutaneous gene delivery in closed-chest animals.

Mitral Regurgitation Augments Post-Myocardial Infarction Remodeling: Failure of Hypertrophic Compensation

OBJECTIVES We examined whether mitral regurgitation (MR) augments post-myocardial infarction (MI) remodeling. BACKGROUND MR doubles mortality after MI, but its additive contribution to left ventricular (LV) remodeling is debated and has not been addressed in a controlled fashion. METHODS Apical MIs were created in 12 sheep, and 6 had an LV-to-left atrial shunt implanted, consistently producing regurgitant fractions of approximately 30%. The groups were compared at baseline, 1, and 3 months. RESULTS Left ventricular end-systolic volume progressively increased by 190% with MR versus 90% without MR (p < 0.02). Pre-load-recruitable stroke work declined by 82 +/- 13% versus 25 +/- 16% (p < 0.01) with MR, with decreased remote-zone sarcoplasmic reticulum Ca(2+)-ATPase levels (0.56 +/- 0.03 vs. 0.76 +/- 0.02, p < 0.001), and decreased isolated myocyte contractility. In remote zones, pro-hypertrophic Akt and gp130 were upregulated in both groups at 1 month, but significantly lower and below baseline in the MR group at 3 months. Pro-apoptotic caspase 3 remained high in both groups. Matrix metalloproteinase (MMP)-13 and membrane-type MMP-1 were increased in remote zones of MR versus infarct-only animals at 1 month, then fell below baseline. The MMP tissue inhibitors rose from baseline to 3 months in all animals, rising higher in the MI + MR-group border zone. CONCLUSIONS In this controlled model, moderate MR worsens post-MI remodeling, with reduced contractility. Pro-hypertrophic pathways are initially upregulated but subsequently fall below infarct-only levels and baseline; with sustained caspase 3 elevation, transformation to a failure phenotype occurs. Extracellular matrix turnover increases in MR animals. Therefore, MR can precipitate an earlier onset of dilated heart failure.

Gene Delivery of Sarcoplasmic Reticulum Calcium ATPase Inhibits Ventricular Remodeling in Ischemic Mitral Regurgitation

Background—Mitral regurgitation (MR) doubles mortality after myocardial infarction (MI). We have demonstrated that MR worsens remodeling after MI and that early correction reverses remodeling. Sarcoplasmic reticulum Ca+2-ATPase (SERCA2a) is downregulated in this process. We hypothesized that upregulating SERCA2a might inhibit remodeling in a surgical model of apical MI (no intrinsic MR) with independent MR-type flow. Methods and Results—In 12 sheep, percutaneous gene delivery was performed by using a validated protocol to perfuse both the left anterior descending and circumflex coronary arteries with occlusion of venous drainage. We administered adeno-associated virus 6 (AAV6) carrying SERCA2a under a Cytomegalovirus promoter control in 6 sheep and a reporter gene in 6 controls. After 2 weeks, a standardized apical MI was created, and a shunt was implanted between the left ventricle and left atrium, producing regurgitant fractions of ≈30%. Animals were compared at baseline and 1 and 3 months by 3D echocardiography, Millar hemodynamics, and biopsies. The SERCA2a group had a well-maintained preload-recruitable stroke work at 3 months (decrease by 8±10% vs 42±12% with reporter gene controls; P<0.001). Left ventricular dP/dt followed the same pattern (no change vs 55% decrease; P<0.001). Left ventricular end-systolic volume was lower with SERCA2a (82.6±9.6 vs 99.4±9.7 mL; P=0.03); left ventricular end-diastolic volume, reflecting volume overload, was not significantly different (127.8±6.2 vs 134.3±9.4 mL). SERCA2a sheep showed a 15% rise in antiapoptotic pAkt versus a 30% reduction with the reporter gene (P<0.001). Prohypertrophic activated STAT3 was also 41% higher with SERCA2a than in controls (P<0.001). Proapoptotic activated caspase-3 rose >5-fold during 1 month in both SERCA2a and control animals (P=NS) and decreased by 19% at 3 months, remaining elevated in both groups. Conclusions—In this controlled model, upregulating SERCA2a induced better function and lesser remodeling, with improved contractility, smaller volume, and activation of prohypertrophic/antiapoptotic pathways. Although caspase-3 remained activated in both groups, SERCA2a sheep had increased molecular antiremodeling “tone.” We therefore conclude that upregulating SERCA2a inhibits MR-induced post-MI remodeling in this model and thus may constitute a useful approach to reduce the vicious circle of remodeling in ischemic MR.

Early Repair of Moderate Ischemic Mitral Regurgitation Reverses Left Ventricular Remodeling A Functional and Molecular Study

Background— Mitral regurgitation (MR) doubles postmyocardial infarction (MI) mortality. We have shown that moderate MR augments remodeling in an apical MI model (no intrinsic MR) with independent left ventricle-to-left atrial MR-type flow. We hypothesized that repairing moderate MR 1 month after MI reverses this remodeling. Methods and Results— Anteroapical MIs were created in 18 sheep, and a left ventricle-to-left atrial shunt implanted in 12 (regurgitant fraction, 30%). Six sheep had the shunt closed at 1 month (repair group). Sheep were compared at baseline, and at 1 and 3 months. Sheep in the MI+MR (unrepaired) and repaired groups remodeled during the first month (120% increased left ventricular end-systolic volume [ESV; P<0.01]), but shunt closure reversed remodeling at 3 months, with end-diastolic volume (EDV) and ESV 135% and 128% of baseline versus 220% and 280% without repair (P<0.001). At 3 months, dP/dt and preload-recruitable stroke work were relatively maintained in the repaired and MI-only groups versus nearly 50% decreases without repair. Prohypertrophic gp130 and antiapoptotic pAkt increased followed by exhaustion below baseline without repair, but remained elevated at 3 months with repair or MI only. With repair, matrix metalloproteinase-2 decreased to ≤50% that without repair in remote and border zones at 3 months, and the matrix metalloproteinase inhibitor TIMP-4 increased dramatically. Conclusions— Early repair of moderate MR in the setting of apical MI substantially reverses the otherwise progressive remodeling process, with reduced left ventricular volumes, relatively maintained contractility, persistently activated intracellular signals promoting hypertrophy and opposing apoptosis, and reduced matrix proteolytic activity. These findings are of interest for the current controversy regarding potential benefits of repair of MR after MI.

Chordal Cutting Does Not Adversely Affect Left Ventricle Contractile Function

Background— Severing a limited number of second-order chordae to the anterior leaflet can improve ischemic mitral regurgitation (MR). Some concerns have been raised regarding possible influence on regional and global left ventricle (LV) function. We evaluated changes in cardiac function in 5 normal sheep with cutting of pre-instrumented chords in the beating heart to maintain constant load. Methods and Results— Under cardiopulmonary bypass, wires were placed around the 2 central basal chordae and brought outside the heart, which was restarted. Hemodynamic and imaging data were collected before and after chordal cutting by radiofrequency ablation using those wires. Segmental contractility was assessed invasively using sonomicrometers and noninvasively using Doppler tissue velocity and strain rate (with strain rate viewed as less load-dependent than ejection fraction) at 6 sites: base, mid-ventricle, and apex along the anteroseptal and posterolateral walls. We found no changes from before to after chordal cutting in LV end-diastolic volume (47.2±3.3 after cutting versus 48.4±4.6 mL before cutting, P=0.66), end-systolic volume (21.5±1.2 versus 22.3±2.8 mL, P=0.68), ejection fraction (54.2±1.8 versus 54.2±2.7%, P=0.96), systolic ventricular elastance (7.28±1.68 versus 7.66±2.11 mm Hg/mL, P=0.64), preload-recruitable stroke work (46.6±7.7 versus 50.2±10.7 mm Hg, P=0.76), and LVdP/dt (1480±238 versus 1392±250 mm Hg/s, P=0.45). Doppler tissue velocities and longitudinal strain rates surrounding the papillary muscles were unchanged, as were sonomicrometer longitudinal and mediolateral absolute strains. No wall motion abnormalities were visible around the papillary muscles, and no MR developed. Conclusion— We find no evidence for acutely decreased global or segmental LV contractility with chordal cutting. This absence of adverse effects is consistent with long-term clinical experience with cutting these chords in valve repair.

Persistent Reduction of Ischemic Mitral Regurgitation by Papillary Muscle Repositioning Structural Stabilization of the Papillary Muscle–Ventricular Wall Complex

Background— Recurrent ischemic mitral regurgitation (IMR) is frequent despite initial reduction by annuloplasty because continued LV remodeling increases tethering to the infarcted papillary muscle (PM). We have previously shown that PM repositioning by an external patch device can acutely reduce IMR. In this study, we tested the hypothesis that IMR reduction persists despite possible continued LV remodeling. Methods and Results— In 7 sheep, we used a chronic ischemic posterior infarct model that produces LV dilatation and MR over 10 weeks. An epicardial patch device was adjusted under echo guidance to reduce MR, with follow-up over a further 8 weeks and evaluation by 3D echo and sonomicrometry. In all 7 sheep, moderate IMR resolved with acute patch application and PM repositioning (6.5±1.8 mm to 0.6±1.3 mm proximal jet width, P<0.001) without decrease in LVEF (43±3% to 44±8%). Eight weeks after PM repositioning, MR was not significantly greater (0.6±1.3 mm versus 1.0±1.0 mm, P=NS) despite an increase in LV volumes in 3 animals (2 had increases of 50±15%). On average, LV volumes did not change significantly (ESV: 46±8 mL versus 49±15 mL; P=NS and EDV: 85±16 mL versus 89±30 mL; P=NS). LVEF was unchanged from acute to chronic patch (44±8% versus 43±8%). Contractility as end-systolic elastance did not decrease from the chronic MI to the acute and chronic patch stages, nor were there any significant changes in dP/dt, LV stiffness constant, or time constant of LV relaxation (Tau). Conclusion— PM repositioning is persistently effective in reducing moderate chronic IMR, even when LV volume increases. This may reflect structural stabilization by an external patch device of the papillary muscle-LV wall complex that controls mitral valve tethering.