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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Circ Cardiovasc Imaging. Author manuscript; available in PMC 2010 March 1.
Published in final edited form as:
PMCID: PMC2760031
NIHMSID: NIHMS126600

Reduced systolic torsion in chronic “pure” mitral regurgitation

Daniel B. Ennis, Ph.D.,1 Tom C. Nguyen, M.D.,1 Akinobu Itoh, M.D.,1 Wolfgang Bothe, M.D.,1 David H. Liang, M.D.,2 Neil B. Ingels, Ph.D.,1,3 and D. Craig Miller, M.D.1

Abstract

Background

Global left ventricular (LV) torsion declines with chronic ischemic mitral regurgitation (CIMR), which may accelerate the LV remodeling spiral toward global cardiomyopathy; however, it has not been definitively established whether this torsional decline is due to the infarct, the MR, or their combined effect. We tested the hypothesis that chronic “pure” MR alone reduces global LV torsion.

Methods and Results

Chronic “pure” MR was created in thirteen sheep by surgically punching a 3.5–4.8mm hole (HOLE) in the mitral valve posterior leaflet. Nine control (CNTL) sheep were operated upon concurrently. At one (WK-01) and twelve weeks (WK-12) postoperatively, the 4-D motion of implanted radiopaque markers was used to calculate global LV torsion. MR-grade in HOLE was greater than CNTL at WK-01 and WK-12 (2.5±1.1 vs. 0.6±0.5, P<0.001 at WK-12). HOLE LV mass index (LVMI) was larger at WK-12 compared to CNTL (195±14 vs. 170±17 g/m2, P<0.01), indicating LV remodeling. Global LV systolic torsion decreased in HOLE from WK-01 to WK-12 (4.1°±2.8° vs. 1.7°±1.7°, P<0.01), but did not change in CNTL (5.5°±1.8° vs. 4.2°±2.7°, P=N.S.). Global LV torsion was lower in HOLE relative to CNTL at WK-12 (P<0.05), but not at WK-01 (P=N.S.).

Conclusions

Twelve weeks of chronic “pure” MR resulting in mild global LV remodeling is associated with significantly increased LVMI and reduced global LV systolic torsion, but no other significant changes in hemodynamics. MR alone is a major component of torsional deterioration in “pure” MR and may be an important factor in CIMR.

Keywords: mechanics, mitral valve, myocardium, ventricles, torsion

Introduction

Chronic ischemic mitral regurgitation (CIMR) remains one of the most challenging, life-threatening clinical problems in cardiac surgery, affects a large number of patients, leads to congestive heart failure which limits life expectancy and functional capacity, and has major adverse implications on U.S. health care costs1.

In CIMR, the mitral valve leaks, yet the leaflets appear normal. The mitral regurgitation (MR) associated with CIMR is the result of left ventricular (LV) geometric and microstructural2 remodeling which leads to alterations in the geometry of the mitral valve, annulus, and sub-valvular apparatus. These alterations include annular dilation, which reduces leaflet coaptation, and papillary muscle displacement, which leads to leaflet tethering.

Uncorrected CIMR is associated with decreased LV torsion3. LV torsion in the normal heart reflects a balance between epicardial and endocardial function and helps produce a nearly homogeneous transmural distribution of myocardial stress and fiber strain during ejection4. An impairment of LV torsion may increase transmural differences in stress and foment deleterious LV remodeling or reduce the elastic energy stored in the myocardium, thereby impairing filling5. In this way decreases in LV torsion as seen in CIMR may contribute to a vicious cycle wherein “MR begets MR”6. It has not been definitively established, however, whether the reduced LV torsion in CIMR is a direct result of the infarct, the MR, or a combination of the two.

With respect to the treatment options for patients with CIMR, sorting out the impact of MR from that of ischemia on ventricular function may have important clinical consequences. For example, the surgical treatment options for patients with CIMR include coronary artery bypass grafting (CABG) and/or mitral valve annuloplasty or replacement. Currently, the most common technique to restore valve competence is the placement of an undersized annuloplasty ring. There exists, however, an ongoing debate about whether or not mitral valve annuloplasty at the time of CABG improves outcomes over and above CABG alone79.

Hypothetically, if the reduction in LV torsion could be attributed to ischemia alone, then CABG might be sufficient to either restore or stop the deterioration of LV torsion. If, however, MR alone contributes to impaired LV torsion, mitral valve repair may be important for minimizing the deterioration in LV torsion.

The goal of this study was to sort out the impact of MR alone on left ventricular torsion in an ovine model of chronic “pure” MR. We tested the hypothesis that chronic “pure” MR alone reduces global left ventricular torsion.

Methods

Animal Welfare

All animal procedures were approved by our Institutional Animal Care and Use Committee and followed guidelines set forth by the National Institutes of Health. Results from some of the animals used in this report were described previously10. The authors had full access to and take full responsibility for the integrity of the data. All authors have read and agree to the manuscript as written.

Adult male sheep (Dorsett-hybrid) were pre-medicated with an injection of ketamine, anesthetized, intubated, mechanically ventilated, and sedation was maintained with inhalational isoflurane. Blood pressure, body temperature, blood gases (CO2 and O2), and ionic concentrations were monitored throughout the study.

Surgery

A left lateral thoracotomy was performed under surgically sterile conditions to expose the heart for the implantation of miniature radiopaque markers. Thirteen left ventricular markers were placed subepicardially to silhouette the LV chamber. One marker was placed at the apex and four each were placed in the septal, anterior, lateral, and posterior walls at apical, mid-equatorial, and basal levels (Figure 1). These markers were used to generate the results reported herein. To investigate changes in regional function additional beads were implanted transmurally in the antero-basal and lateral-equatorial LV wall10.

Figure 1
Diagram of the anatomical location of all implanted markers. Markers at the apical and basal levels (#1, 2, 4, 5, 7, 8, 10, 11, 13) were used to calculate left ventricular global torsion. Lower right inset is a photograph of the hole in the posterior ...

The animal was then placed on cardiopulmonary bypass, the heart arrested, and the left atrium opened to provide access to the mitral valve annulus, leaflets, and subvalvular apparatus. To investigate changes in annular and leaflet function, markers were sewn to the mitral valve annulus, leaflets, and on the papillary muscle tips.

Animals were randomized into a control group (CNTL, N=8) and a chronic “pure” mitral regurgitation (PMR) group (HOLE, N=12) while on bypass. In HOLE animals, a 3.5–4.8mm hole was created in the posterior leaflet using an aortic hole punch to generate “pure” mitral regurgitation. The animals were then weaned off bypass. A Konigsberg micromanometer pressure transducer was placed in the LV chamber through the apex and exteriorized. The chest was closed in layers and the animal was recovered and returned to the holding facility. Throughout the whole study period the animals were followed for clinical signs of heart failure (tachypnea, lethargy, and anorexia). Body weight and transthoracic echocardiographic (TTE) MR-grade were also monitored.

Data Collection – Catheterization Laboratory

At WK-01 and WK-12 all animals underwent examination in the catheterization laboratory. As before, the animals were premedicated with ketamine, anesthetized, intubated, mechanically ventilated, and sedation was maintained with inhalational isoflurane. LV pressure (LVP) was recorded using the implanted Konigsberg pressure transducer and aortic pressure (AoP) was recorded using a micromanometer catheter (Millar Instruments) introduced via the left carotid artery. Pressure and electrocardiogram data were recorded at 240Hz and synchronized with the recording of the x-ray images. MR was graded qualitatively (0–4+) by an expert echocardiographer (D.L.) on the basis of transesophageal echocardiography color doppler regurgitant jet extent and width11.

Marker motion was recorded during normal sinus rhythm with controlled apnea12 using biplane cine fluoroscopy at 60 frames/second. After WK-01 data collection the animals were recovered and returned to animal care facility. After the WK-12 study the animals were euthanized with a bolus infusion of potassium chloride (80mEq).

Data Analysis

The animals selected for this study were required to have all LV markers in place, regardless of the successful placement of the other markers. During three consecutive heartbeats, each markers’ image coordinates were digitized using custom computer aided detection software13 and merged to reconstruct 3D marker motion14.

The 3D left ventricular marker coordinates were used to calculate the volume of space-filling tetrahedra that approximate the total left ventricular volume during the cardiac cycle. The change in left ventricular volume calculated from sub-epicardial markers is an accurate measurement of the change in LV chamber volume15, despite the inclusion of the LV wall volume. Sphericity index (SI) was calculated as the ratio of the length of the LV long axis and the LV diameter16 (SI>1 is an elongated ellipsoidal heart, SI=1 is spherical). Sheep weight, W (kg), was used to approximate the body surface area (BSA) using a formulae (BSA=0.087·W0.67) provided by Fischer et al 17. End diastolic (ED), end isovolumic contraction (IVC), end systolic (ES), and end isovolumic relaxation (IVR) cardiac cycle time points were determined semi-automatically from plots of LV pressure-volume loops. The BSA was then used to calculate indexed values of the LV end-diastolic volume (LVEDVi), LV end-systolic volume (LVESVi), and LV mass (LVMi). Peak systolic (LVPmax) and LV end-diastolic pressure (LVEDP) were extracted from the LV pressure curves.

Calculation of Gobal LV Torsion

Global left ventricular torsion was calculated from the apical LV markers (#2, 5, 8, 11) and the basal LV markers (#4, 7, 10, 13, Figure 1). An internal cylindrical coordinate (R,Θ,Z) for each marker was calculated using the LV long-axis defined as a vector pointing from the centroid of the basal markers to the centroid of the apical markers. Ventricular torsion within each wall (anterior, lateral, posterior, and septal) was then calculated as the difference between the basal Θ coordinate and the apical Θ coordinate for all time points. Global ventricular torsion was calculated as the average torsion of the four wall regions, averaged across three consecutive heartbeats. Positive torsion values indicate counter-clockwise rotation of the apex relative to the base about the LV long-axis when viewed from apex-to-base. Negative torsion values indicate clockwise rotation when viewed from apex-to-base relative to ED. The global ventricular torsion during the cardiac cycle was then decomposed into torsion during IVC, ejection, IVR, filling, and systolic torsion from ED to ES. Significant regional differences (e.g. anterior vs. lateral wall torsion) were not evident (data not shown). Note, that we have defined torsion for all phases of the cardiac cycle though some authors prefer that this term be reserved only for ejection.

Changes in ventricular torsion as a function of LV volume during ejection were also quantified. A least squares estimate of the slope of the torsion-volume curve during ejection for each group was computed.

Statistical Analysis

The results are presented as mean±1SD. To determine if there were significant differences between the CNTL and HOLE groups at both WK-01 and WK-12 two-way repeated measures ANOVA was performed (SigmaStat 3.5, Systat Software). A Holm-Sidak multiple comparisons procedure was used. The threshold for statistically significance was P<0.05. Statistical differences both “between groups” (CNTL vs HOLE at WK-01 or CNTL vs HOLE at WK-12) and “within groups” (HOLE WK-01 vs HOLE WK-12 or CNTL WK-01 vs CNTL WK-12) were considered.

Results

Hemodynamics

Hemodynamic data are shown in Table 1. MR grade was significantly elevated in HOLE relative to CNTL at both WK-01 (2.7±0.7 vs. 0.6±0.5, P<0.01) and WK-12 (2.5±1.1 vs. 0.6±0.5, P<0.01). There were no significant differences within CNTL or within HOLE between WK-01 and WK-12. Therefore, MR grade was elevated throughout the experiment in the HOLE group and was typically moderate-severe to severe, whereas the CNTL group had trace MR on average throughout the experiment. Aortic cross-clamp times for the HOLE and CNTL groups were significantly different (34±3 and 29±3 minutes, P<0.01). Creation MR in the HOLE group accounts for the time difference.

Table 1
Hemodynamics for control (CNTL) and chronic “pure” mitral regurgitation (HOLE) animals at one (WK-01) and twelve (WK-12) weeks post-op.

LV mass index at WK-12 (the only time point available) was significantly increased in HOLE compared to CNTL (195±14 vs. 170±17g, P<0.01). There were, however, no significant differences between groups or over time for heart rate, body weight, LV end diastolic volume index, LV end systolic volume index, peak systolic LV pressure (LVPmax), nor sphericity index. LVEDP, however, was decreased from WK-01 to WK-12 in both CNTL and HOLE.

Global Left Ventricular Torsion – No Week-01 Differences

A summary of the phases of global left ventricular torsion is found in Table 2 and graphically depicted in Figure 2. There were no significant differences between CNTL and HOLE groups at WK-01, indicating that one week of “pure” mitral regurgitation did not result in a significant alteration in global left ventricular torsion during any phase of the cardiac cycle.

Figure 2
LV torsion for the control (CNTL) and chronic “pure” MR (HOLE) group at one (WK-01) and twelve (WK-12) weeks post-op during five phases of the cardiac cycle. Notably, global LV torsion during ejection, filling, and systole (ED to ES) all ...
Table 2
LV torsion for the control (CNTL) and chronic “pure” MR (HOLE) group at one (WK-01) and twelve (WK-12) weeks post-op during each phase of the cardiac cycle.

Global Left Ventricular Torsion – Similar Week-12 Changes

Twelve weeks after surgery there were significant differences within the CNTL group (WK-01 vs. WK-12) for torsion during IVC and IVR. LV cocking during IVC was restored from WK-01 to WK-12 in the CNTL group (0.6°±0.9 vs. −1.3°±1.8, P<0.01) and torsion recoil during IVR nearly abolished (−1.7°±1.5 vs. 0.4±1.6, P<0.01).

Similarly, at WK-12 the HOLE group demonstrated a restoration from WK-01 of LV cocking during IVC (0.4°±1.4 vs. −1.8°±1.5, P<0.01) and a significant decrease in the magnitude of IVR recoil (−1.8°±1.2 vs. 0.0°±1.8, P<0.01). These changes in the HOLE group paralleled those of the CNTL group, thus are likely associated with recovery from surgery.

Global Left Ventricular Torsion – Differential Week-12 Changes

Global LV torsion during ejection, filling, and systole (ED to ES) all demonstrated significant changes between HOLE and CNTL groups at WK-12. Global ventricular torsion during ejection was decreased for HOLE at WK-12 compared to CNTL at WK-12 (3.4°±1.4 vs. 5.5°±2.7, P<0.05). Similarly, global ventricular torsion during filling was decreased in HOLE at WK-12 relative to CNTL at WK-12 (−1.7°±2.1 vs. −4.5°±2.3, P<0.01). These results indicated impaired torsion during ejection and decreased untwisting during filling for HOLE relative to CNTL at WK-12.

LV systolic (ED to ES) torsion was reduced at WK-12 in HOLE compared to both HOLE at WK-01 (1.7°±1.7 vs. 4.1°±2.8, P<0.01) and CNTL at WK-12 (1.7°±1.7 vs. 4.2°±2.7, P<0.05). Thus, LV systolic torsion was impaired after twelve weeks of chronic “pure” MR.

Rates of Global Left Ventricular Torsion – No Week-01 Changes

A summary of the rates of global left ventricular torsion during IVC, ejection, IVR, and filling is shown in Table 3 and in Figure 3. There were no significant differences in any cardiac phase component of the rate of global ventricular torsion between the CNTL and HOLE groups at WK-01, indicating that one week of “pure” mitral regurgitation did not have a deleterious impact.

Figure 3
Rate of ventricular torsion for control (CNTL) and chronic “pure” mitral regurgitation (HOLE) animals at one (WK-01) and twelve (WK-12) weeks post-op during four phases of the cardiac cycle. The rate of torsion during IVC significantly ...
Table 3
Rate of ventricular torsion for control (CNTL) and chronic “pure” mitral regurgitation (HOLE) animals at one (WK-01) and twelve (WK-12) weeks post-op during each phase of the cardiac cycle.

Rates of Global Left Ventricular Torsion – Similar Week-12 Changes

The rate of ventricular torsion during IVC, however, significantly decreases and changes sign from WK-01 to WK-12 in both CNTL (9.8°/s±12.4 vs. −14.5°/s±17.5, P<0.01) and HOLE (5.4°/s±17.0 vs. −17.2°/s±17.6, P<0.01) groups. Furthermore, the rate of ventricular torsion during IVR significantly increased from WK-01 to WK-12 in both the CNTL (−16.3°/s±11.3 vs. 3.5°/s±13.7, P<0.01) and HOLE (−17.9°/s±11.1 vs. −2.1°/s±18.4, P<0.01) groups. These similar changes in the rates of global left ventricular torsion may reflect ventricular changes associated with post-operative recovery.

Rates of Global Left Ventricular Torsion – Differential Week-12 Changes

Global left ventricular torsion within the CNTL GROUP was not significantly different during ejection or filling from WK-01 to WK-12. There was, however, a significant decrease in the rate of global LV torsion during ejection at WK-12 between the CNTL and HOLE groups (38.1°/s±11.8 vs. 25.8°/s±10.3, P<0.05). Furthermore, the rate of global LV torsion during filling was significantly decreased in magnitude at WK-12 between CNTL and HOLE (−23.7°/s±−7.9 vs. −7.9°±14.1, P<0.01). These results indicate that the rates of global LV torsion during both ejection and filling decrease in magnitude after twelve weeks of chronic “pure” MR.

Global Torsion versus Left Ventricular Volume

The least squares estimate of the slope of the torsion-volume curve during ejection for each group is compared in Table 4. The path of the torsion-volume curve during ejection was very linear. At WK-01 there was no difference between the CNTL and HOLE groups (−.35±0.12°/mL vs. −0.29±0.18°/mL). At WK-12, however, the HOLE group slope was significantly different from both CNTL at WK-12 (−0.18±0.06°/mL vs. −0.32±0.10°/mL, P<0.05) and HOLE at WK-01 (−0.18±0.06°/mL vs −0.29±0.18°/mL, P<0.05).

Table 4
Slope of the torsion versus LV volume curve during ejection for control (CNTL) and chronic “pure” mitral regurgitation (HOLE) animals at one (WK-01) and twelve (WK-12) weeks post-op.

Global Left Ventricular Torsion – PV Loop

Global LV torsion is color-mapped on mean PV-loops for the CNTL and HOLE groups at WK-01 and WK-12 in Figure 4. This method of displaying the torsion data provides a way to qualitatively appreciate global ventricular torsion during each phase of the cardiac cycle.

Figure 4
Average pressure-volume (PV) loops color coded for global ventricular torsion (degrees). A) Average CNTL and HOLE PV-loop at WK-01. There are no significant differences in any hemodynamic, physiologic, or torsion parameter at WK-01 except for MR-grade. ...

At WK-01 both the CNTL and HOLE groups demonstrated very similar patterns of global ventricular torsion during IVC, ejection, IVR, and filling phases. At WK-12, however, both CNTL and HOLE groups have regained cocking during IVC. Global LV torsion increases during ejection, but LV torsion at ES is reduced in HOLE relative to CNTL. Both groups demonstrate a continued increase in LV torsion during IVR that peaks during mid-IVR, but the net change in torsion during IVR was nearly zero. The magnitude of torsion at the end of IVR was higher in the CNTL group than the HOLE group and the CNTL group exhibited greater negative torsion during the subsequent filling phase.

Discussion

Depressed ventricular torsion

Twelve weeks of chronic “pure” mitral regurgitation (PMR) resulted in significant reductions in torsion during systole (ED to ES), ejection (end-IVC to ES), and during filling (end-IVC to ED) relative to CNTL. Only decreases in systolic torsion, however, were significantly different in HOLE at WK-12 compared to both HOLE at WK-01 and CNTL at WK-12. The small difference in aortic cross-clamp time though statistically significant is not likely to induce differences in ventricular torsion between the two groups.

Changes in systolic torsion must be understood in the context of the restoration of ventricular cocking during IVC. Cocking, an expected mode of torsion during IVC1820, is restored in both CNTL and HOLE at WK-12. The notable change in systolic torsion in HOLE at WK-12 is primarily due to a persistent reduction in torsion during ejection, combined with increased cocking during IVC.

In order to account for changes associated with the recovery from the operation we focused on measures of torsion during ejection – a phase of the cardiac cycle during which the CNTL group undergoes no significant change from WK-01 to WK-12. Ejection torsion in the HOLE group is significantly reduced at WK-12 compared to CNTL, but is not statistically different between WK-01 and WK-12 within HOLE indicating that it is not further depressed after twelve weeks of PMR.

Furthermore, during ejection the slope of the torsion-volume curve is not different within the CNTL group, or between CNTL and HOLE at WK-01. All slopes are approximately −0.32°/mL indicating that this relationship is preserved during acute PMR and in the post-op recovery period. This measure, however, is significantly decreased in the HOLE group at WK-12 (−0.18°/mL) compared to both CNTL at WK-12 and HOLE at WK-01. This significant difference within the HOLE group may indicate that the torsion-volume relationship continues to decline with persistent PMR.

Physiologic adaptation to pure MR

The physiologic adaptation to chronic “pure” MR primarily included a significant increase in LVMi. LVEDVi and LVESVi were both increased 15% in HOLE compared to CNTL at WK-12, but the results were not statistically significant. The LV changes after twelve weeks of pure MR in HOLE substantially differ from the larger LVMi, LVEDVi, and LVESVi changes that are known to occur in patients with longstanding chronic MR due, at least in part, to the duration of the disease process. Our measures of LVEDVi and LVESVi are derived from sub-epicardial markers which results in larger measures when compared to endocardial derived values as they include changes in myocardial mass. Furthermore, though sphericity index in HOLE tended to be lower (more spherical) no significant differences between CNTL and HOLE were observed. The results derived from endocardial measures may demonstrate significant differences if there are significant differences in wall thickness between CNTL and HOLE. One strength of this study and previous work10 is that it reflects a very early stage of the disease process, shown by only mild signs of LV remodeling (LV mass index), prior to overt changes in traditionally measured clinical indices.

The decreased global systolic torsion may be attributed to the increase in LV end-diastolic volume index and the significant increase in LV myocardial mass index. Because our results are definitive with regards to changes in systolic torsion and equivocal in regards to changes in LVEDVi, it may be the case that ventricular torsion or the torsion-volume slope during ejection is a more sensitive measure of LV dysfunction and remodeling using the experimental conditions and quantitative methods reported herein.

While parameters like LVEDVI and LVESVI trend toward significant increases in the HOLE group, power calculations demonstrate that a group size of 44 and 38 respectively, would be needed attain a power of 0.9 for an alpha of 0.05. In comparison, the systolic torsion results have a power of 0.96.

Post-op Recovery

Changes in global left ventricular torsion and rates of torsion reflect a combination of the deleterious effects of chronic “pure” mitral regurgitation and the recovery from the operation. A primary advantage of this study, compared to previous work21, is the inclusion of a sham operated control group. Changes in torsion during IVC and IVR are very similar between the CNTL and HOLE groups and such changes may be attributed to recovery from the surgical procedure between WK-01 and WK-12.

Tibayan et al.22 in a model of dilated cardiomyopathy that also required the use of bypass observed an increase in the magnitude of early systolic cocking from −0.7° in animals 1-week post-surgery to −1.5° after 15-days of rapid pacing. In the context of our results it appears that this may be attributed to post-surgery recovery time. Similarly, Tibayan et al. also observed that early diastolic recoil (torsion from end-ejection to 5% of LV-filling, similar to our IVR interval) was diminished 1-week after surgery, but recovered after 15-days of rapid pacing (−0.5° vs. −1.2°). Our results are similar, suggesting that this change in torsion may be largely attributable to post-operative recovery time and not dilated cardiomyopathy or chronic mitral regurgitation. The surgical intervention, which includes cardiopulmonary bypass and cardiac arrest, may however alter the measured torsion as compared to animals that forego this procedure, but still have epicardial makers implanted. Additional work is needed to compare two such groups.

Alternate mechanism of reduced torsion dynamics

Global LV torsion can be decreased by alterations in electrical conduction patterns. In normal myocardium the endocardium is electrically and mechanically activated before the epicardium23. It is understood that early activation of the endocardium may cause LV cocking during IVC24, 25. If the pattern of electrical activation was altered and endocardial activation was made more coincident with epicardial activation then early LV cocking would be reduced. It is possible that this is the mechanism underlying the loss of LV cocking in both CNTL and HOLE at WK-01 during the early post-operative period, but further study is needed to confirm this. If this were the case then it would seem that normal electrical conduction patterns were restored at WK-12 in both CNTL and HOLE and it would be difficult to ascribe such alterations to the MR alone.

Another possible cause of reduced ventricular torsion in chronic “pure” MR is heterogeneous transmural remodeling. It is possible that at WK-12 the HOLE animals have undergone eccentric hypertrophy and maintained the radius to wall-thickness ratio, but non-uniformly redistributed myocardial mass. If the epicardial fibers atrophy, while endocardial fibers hypertrophy, wall thickness could be maintained or increased, but ventricular torsion would decrease as the balance between epicardial and endocardial force generation remodeled. Investigations into the heterogeneity of transmural remodeling are on-going. Also, it should be noted that reduced torsion during ejection may also arise from paradoxical septal wall motion as a result of cardiopulmonary bypass26. Lastly, changes in torsion and torsion rates during IVR and filling may not be attributed to the MR per se, but rather to the effects of ventricular remodeling that deleteriously impacts torsion during diastole.

Each of these alternative mechanisms of reduced global ventricular torsion warrants further investigation.

Comparison to Previous Results

Caution must be used when comparing observations of ventricular rotation and ventricular torsion. Furthermore, it is imperative to account for the definitions used by each author to avoid confusion in interpreting and comparing results.

Tibayan et al. demonstrated in a different model of non-ischemic chronic mitral regurgitation (Cope biopsy needle disruption of the leading edge of the posterior leaflet) that maximum torsional deformation decreased (6.3° to 4.7°), and torsion during early diastolic recoil (first 5% of filling) decreased in magnitude (−3.8° to −1.5°) from 1-week to 7-weeks21. Though the methods of chronic MR creation differ, the salient aspects of the experiments are very similar. Most importantly the MR-grade (“moderate to severe”) and duration were similar. The presence of a sham operated control group in our study, however, allowed us to control for the impact of MR alone.

Another study by Tibayan et al.3, in a sheep model of inferior myocardial infarction (MI), describes the differences in ventricular torsion for animals that developed chronic ischemic MR (CIMR) compared to those that did not show significant MR. In the CIMR animals ventricular torsion was significantly decreased in the posterior wall 7 weeks after infarction, but the decrease in torsion in the non-MR infarct group was not large enough to be statistically significant. Hence, the decrease in LV torsion is greater in CIMR than in MI alone. This indicates that MR is an important determinant of decreased ventricular torsion. Our study specifically isolates the effects of MR alone and shows that ventricular torsion is significantly decreased. Therefore, decreases in ventricular torsion in CIMR or in “pure” MR are largely due to the MR alone.

Limitations

This model provides a unique way to study the isolated effects of “pure” MR without confounding factors such as ischemia, infarction, disruption of sub-valvular apparatus (chordae tendinae or papillary muscles), or disruption of the leaflets along the line of coaptation.

The invasiveness of the surgical procedure may acutely and chronically alter ventricular function. While the implanted myocardial marker technique affords certain advantages (high spatial and temporal resolution and the ability to track precise tissue locations over long experimental durations) it also requires a thoracotomy and disruption of the pericardium. The presence of a sham operated control groups provides a way to account for the impact of surgery and bypass. While the only experimental difference between the CNTL and HOLE groups is the surgical creation of the mitral valve leaflet hole in the HOLE group, the study is not without other confounding factors. In fact, parallel changes in the CNTL and HOLE group over the course of twelve weeks indicate that the effects of the operation on the ventricle are still present at WK-01. Furthermore, the measures made with sub-epicardially implanted markers in this study are precise, but do not afford the ability to quantify transmural differences in the myocardial kinematics. Therefore, with the current technique we are also unable to report on regional septal dysfunction that may result from the bypass procedure. Interestingly, MRI may not be able to capture the motion of early systolic events due to ECG gating and tagging pulse delays27 and its lower temporal resolution compared to echocardiography25 unless specialized pulse sequences are employed.

Conclusions

Twelve weeks of chronic “pure” MR resulted in significantly reduced global LV torsion during systole, ejection, and filling. The decreases in global torsion may be attributed to subtle increases in LVEDVI and LVMI, but may also be due to ventricular microstructural remodeling. These data suggest that in patients with CIMR the MR itself may promote deterioration of LV torsion. MV repair in patients with CIMR concomitant with CABG may therefore help to slow down, stop or even restore physiological LV torsion.

Uncorrected chronic ischemic mitral regurgitation (CIMR) is associated with decreased LV torsion and may contribute to a vicious cycle wherein “MR begets MR”. It has not been definitively established, however, whether the reduced LV torsion in CIMR is a direct result of the infarct, the MR, or a combination of the two. Currently, the most common technique to restore valve competence in patients with CIMR is the placement of an undersized annuloplasty ring. There exists, however, an ongoing debate about whether or not mitral valve annuloplasty at the time of CABG improves outcomes over and above CABG alone. Hypothetically, if the reduction in LV torsion can be attributed to MR alone, then mitral valve repair concomitant with CABG may be important for minimizing the deterioration in LV torsion. Chronic “pure” MR was created in thirteen sheep by surgically punching a 3.5–4.8mm hole (HOLE) in the mitral valve posterior leaflet. Nine control (CNTL) sheep were operated upon concurrently. At one (WK-01) and twelve weeks (WK-12) postoperatively, the 4-D motion of implanted radiopaque markers was used to calculate global LV torsion. Global LV systolic torsion decreased in HOLE from WK-01 to WK-12 (4.1°±2.8° vs. 1.7°±1.7°, P<0.01), but did not change in CNTL (5.5°±1.8° vs. 4.2°±2.7°, P=N.S.). These data suggest that in patients with CIMR the MR itself may promote deterioration of LV torsion. MV repair in patients with CIMR concomitant with CABG may therefore help to slow down, stop or even restore physiological LV torsion.

Acknowledgments

Funding Sources

The authors gratefully acknowledge research support from NIH/NHLBI Grants R01 HL-29589 and R01 HL-67025 to DCM and K99-R00 HL-087614 to DBE.

Footnotes

Disclosures

None relevant to the topic of this manuscript.

References

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