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Ann Thorac Surg 1995;60:1652-1658
© 1995 The Society of Thoracic Surgeons

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Original Articles: Cardiovascular

Exploring Better Methods to Preserve the Chordae Tendineae During Mitral Valve Replacement

Department of Cardiovascular and Thoracic Surgery and Division of Cardiovascular Medicine, Stanford University School of Medicine, Stanford, Palo Alto Veterans Affairs Medical Center, Palo Alto, and Department of Cardiovascular Physiology and Biophysics, Research Institute, Palo Alto Medical Foundation, Palo Alto, California

	Abstract

Top Footnotes Abstract Introduction Material and Methods Results Comment Acknowledgments References

 
Background. It is not known how best to resuspend the mitral chordae tendineae during mitral valve replacement to optimize postoperative left ventricular (LV) systolic and diastolic function.

Methods. Six different techniques to preserve the chordae during mitral valve replacement were compared in 12 dogs using a nondistorting isovolumic technique: conventional, all chordae severed; anterior, all chordae preserved anteriorly; partial, anterior papillary muscle chordae preserved anteriorly; posterior, all chordae preserved posteriorly; oblique, anterior papillary muscle chordae directed anteriorly and posterior papillary muscle chordae posteriorly; and counter, opposite of oblique chordal direction. Control measurements (no chordal tension) were recorded between each experimental condition.

Results. The oblique method tended to have the best LV systolic function versus the conventional method (E_max = 4.0 ± 1.8 versus 3.3 ± 1.2 mm Hg/mL [mean ± standard deviation]; p = 0.08 by repeated-measures analysis of variance; physiologic intercept E_es100 = 20.3 ± 8.6 mL [p < 0.05 versus control]), with no major change in LV diastolic stiffness. The posterior method had a lower E_max (3.3 ± 1.2 mm Hg/mL) than the oblique method, but a similar E_es100 (20.8 ± 8.1 mL; p < 0.05 versus control) and the best diastolic LV performance (LV diastolic stiffness = 0.46 ± 0.23 mm Hg/mL). The counter method also had good systolic function (E_max = 3.8 ± 1.2 mm Hg/mL; E_es100 = 19.7 ± 7.5 mL; p < 0.05 versus control), but had less favorable diastolic properties (0.65 ± 0.37 mm Hg/mL; p < 0.05 by repeated-measures analysis of variance versus posterior).

Conclusions. In this isovolumic preparation in normal canine hearts, the oblique method of chordal resuspension was associated with the best LV systolic function, whereas the counter technique impaired LV diastolic function. These preliminary results warrant further study in ejecting and failing hearts to determine conclusively which chordal orientation best preserves LV performance after mitral valve replacement.

	Introduction

Top Footnotes Abstract Introduction Material and Methods Results Comment Acknowledgments References

 
See also page 1658.

Preservation of chordae tendineae (CT) and papillary muscles (PM) during mitral valve replacement (MVR) [1] is known to enhance left ventricular (LV) systolic function [2–4], both early and late after MVR [5]. In an increasing number of institutions, MVR with chordal preservation is now performed in patients with diseased subvalvular apparati, such as those with mitral stenosis, which may involve using expanded polytetrafluoroethylene sutures to replace the native chordae [6]. In this case, the surgeon decides which direction to resuspend the CT. The optimal direction to resuspend the chordae, however, has not been clearly elucidated; studies to date have compared only the native CT in the anterior and posterior configurations [7–10] or anterior leaflet CT with posterior leaflet CT. Mitral valve replacement with placement of expanded polytetrafluoroethylene chordae does not seem to be as beneficial as native chordal preservation in terms of LV systolic performance [6], which may be related to the direction of CT reattachment. In this study, we compared six different directions of chordal preservation during MVR (including the conventional technique where no CT are preserved) in an attempt to identify which method resulted in the best LV systolic and diastolic function. We developed an isovolumic heart preparation (``double-balloon'') [11] that does not distort the preserved CT, and studied all 6 methods in each dog, allowing each animal to serve as its own control.

	Material and Methods

Top Footnotes Abstract Introduction Material and Methods Results Comment Acknowledgments References

 
Surgical Preparation
Twelve healthy adult mongrel dogs (27.7 ± 2.7 kg) were premedicated with acepromazine (0.01 to 0.05 mg/kg intramuscularly) and atropine (0.05 mg/kg intravenously). They were anesthetized with sodium pentobarbital (20 to 25 mg/kg intravenously), intubated, and placed on artificial ventilation (Ohio Anesthesia Ventilator, Madison, WI). General anesthesia was maintained with inhalational isoflurane at 1.5% to 2.2%. A catheter-tipped manometer (Millar Instruments Inc, Houston, TX) was zeroed in a 37°C water bath. Systemic arterial pressure was monitored using the manometer through the side port of the left femoral introducer. The left side of the chest was opened at the fifth intercostal space, and the fifth rib was resected. The heart was exposed and suspended in a pericardial cradle.

A latex balloon cut to the length of the LV long axis was tied onto a cannula that allowed passage of a catheter-tipped manometer into the balloon to measure the intraballoon pressure directly (Fig 1). Another latex balloon half as long as the first was made in the same fashion. These two balloons were connected to a Y-shaped circuit, which was connected to a 100-mL syringe mounted on a Harvard pump (model 55-1341; Harvard Apparatus). The circuit and the balloons were filled with water and completely deaired. In our preliminary study with porcine cadaver hearts, we confirmed that this double-balloon method provided accurate pressure-volume information with no or minimal distortion of the CT geometry; LV volume at pressures of 50 and 70 mm Hg with the double-balloon technique correlated well with LV volume by natural filling (r = 0.999). Left ventricular volume at the same pressures with use of a conventional balloon also correlated well (r = 0.923) but distorted the CT geometry [11].

View larger version (31K): [in this window] [in a new window]   Fig 1. . Schematic illustration of the circuit and the double left ventricular (LV) balloons. Two balloons (A and B) are connected to an infusion syringe via a Y-circuit. A disk occluder was sewn to the mitral annulus to avoid balloon herniation; the cannulas to the balloons were placed through small slits in the edge of the mitral disc occluder. The LV volume data were derived from the volume sensor on the syringe, and the simultaneous LV pressure data from the Millar micromanometer catheters in the two LV balloons. (ECG = electrocardiogram; PM = papillary muscle.)

The left atrium was opened under electrically induced ventricular fibrillation to avoid ejecting air into the aortic root. As soon as both mitral leaflets were incised and detached from the mitral annulus, the heart was defibrillated with direct-current countershock at 10 to 20 J. Infusion via the second arterial cardiopulmonary bypass cannula was started at 100 mL/min. The ascending aorta was snared and the cardiopulmonary bypass flow in the aortic root cannula was increased to keep the mean aortic root pressure 80 to 140 mm Hg, to minimize LV subendocardial ischemia in the empty heart [13]. Six pledgeted 2-0 braided Dacron sutures were placed around the mitral annulus. Each mitral leaflet was then divided in the midline, and the bundles of CT from each papillary muscle were grouped together with 2-0 sutures, which were later used to pull the CT in the various directions.

A specially designed transparent plastic disk occluder with the size of the canine mitral annulus at end-systole was selected, and the six annular sutures and four chordal sutures described above were passed through the occluder (Fig 2). As a result, each papillary muscle was connected to the disk in four different directions. Consequently, by pulling on a particular combination of sutures, any one of the six predetermined directions was created immediately without manipulating the occluder or the papillary muscles. The two balloons were then inserted into the LV through holes in the occluder: the larger balloon anteriorly down to the LV apex, and the smaller balloon posteriorly between the two papillary muscle bases. A Millar catheter in each balloon served as a guidewire to keep the balloons in the proper location, especially during empty beating. The disk was then snared down to the annulus.

View larger version (34K): [in this window] [in a new window]   Fig 2. . Mitral disk occluder and the layout of the annular and chordal sutures (left), and the six directions of simulated chordal resuspension as seen from the left atrium (middle and right). Once the two balloons were inserted into the left ventricle and the disk occluder was secured, the chordal direction was changed by just pulling the two appropriate sutures (eg, sutures 1a and 2p were placed on tension for the anterior method). For the conventional method, all chordae were kept redundant. (APM = anterior papillary muscle; PPM = posterior papillary muscle.)

The LV pressure-volume relationship was measured by infusing fluid into both LV balloons at a constant rate of 72 mL/min until the balloon pressure reached 120 mm Hg; the balloons were then deflated by fluid withdrawal. During each measurement, heart rate was maintained in a normal range (110 to 130 beats/min). One to two milligrams of UL-FS49 was given as needed to avoid excessive tachycardia. A preliminary study showed that a stable preparation was achieved with three inflation/deflation cycles; in this study three to six inflation/deflation cycles were carried out, and data from the last infusion were analyzed. The pressure in both balloons was measured at the start of each infusion to assure equalization. The geometry of the preserved CT remained undisturbed by the ``sandwich'' effect of the double-balloon method as assessed by two-dimensional epicardial echocardiography.

To avoid possible time-dependent changes in the experimental preparation, the following two steps were taken: (1) randomization of the study order for the six methods in each dog and (2) control runs using the conventional method (ie, no CT preservation) immediately before and after each experimental technique to allow intragroup comparisons. The data obtained in the before and after control runs were averaged, and this mean value was defined as each individual group's control measurements at that point in time.

All animals received humane care in compliance with the ``Principles of Laboratory Animal Care'' formulated by the National Society for Medical Research and the ``Guide for the Care and Use of Laboratory Animals'' prepared by the National Academy of Sciences and published by the National Institutes of Health (DHEW [NIH] publication 85-23, revised 1985). The study was approved by the Stanford University Medical Center Laboratory Research Animal Review Committee and conducted according to Stanford University policy.

Data Acquisition
The volume of the syringe was measured using a linear differential transform sensor, which detected the relative location of the inner barrel of the syringe. All analog data, such as electrocardiogram, instantaneous syringe volume, and pressure in the two LV balloons, were acquired and digitized simultaneously at 240 Hz using a 486-based microcomputer (486-50 MHz; JDR Microdevices Inc, San Jose, CA) with a high-speed data acquisition card (DT3831-G; Data Translation Inc, Marlboro, MA) controlled by commercially available software (Labtech Control 3.2.0; Laboratory Technology Corp, Wilmington, MA).

Data Analysis
SYSTOLIC FUNCTION.
The digitized data files were imported into a commercial data analysis program (Origin, version 3.0; MicroCal Inc, Northampton, MA). The data from beats that generated systolic LV pressures between 80 mm Hg and 120 mm Hg were analyzed; data from premature contractions and the subsequent two beats were excluded. Linear regression analysis (least-squares) was performed on the peak LV systolic pressure and volume data. The slope of the peak systolic pressure-volume relationship was calculated as E_max (mm Hg/mL) (Fig 3).

View larger version (47K): [in this window] [in a new window]   Fig 3. . Representative raw data as well as computed Emax, physiologic intercept (Ees100), and diastolic stiffness (Sd) of the left ventricle (LV). Both the top and the bottom pressure-volume points were calculated using least-square linear regression. (DPVR = LV diastolic pressure-volume relationship; ESPVR = LV end-systolic pressure volume relationship.)

DIASTOLIC FUNCTION.
Diastolic LV dynamic function was assessed as end-diastolic stiffness. In a similar fashion, the minimum LV pressure-volume data points were identified, and linear regression analysis was performed. The slope of this line (S_d; mm Hg/mL), calculated within the same LV pressure range as used to derive E_max, indicated relative changes in LV diastolic function (see Fig 3).

Statistical Analysis
All data are reported as mean ± one standard deviation. Data obtained for all six methods were compared using repeated measures analysis of variance (rm-ANOVA). Post hoc intergroup comparisons were performed using Bonferroni's method. Intragroup comparison with each group's own control was done using paired t-tests. Statistical significance was inferred if the p value was less than 0.05; p values between 0.05 and 0.1 were judged to be nearly significant. Ordinal data were analyzed by Wilcoxon's test. For all statistical analyses, SYSTAT (version 5.02; SYSTAT Inc, Cary, NC) was employed.

	Results

Top Footnotes Abstract Introduction Material and Methods Results Comment Acknowledgments References

 
Left Ventricular Systolic Function: E_max
The oblique group had the highest E_max, whereas the conventional and the posterior methods had the lowest E_max values, but these differences were only nearly significant (p = 0.08 by rm-ANOVA); post hoc testing revealed the major difference to be between the oblique and conventional methods. In general, most of the chordal preservation methods tended to have higher E_max values than the conventional group (Table 1); however, only the oblique and counter-oblique groups had higher E_max values compared with their own control values (p < 0.06 by paired t test). There was no significant difference among the control values between methods or over time by rm-ANOVA.

	Comment

Top Footnotes Abstract Introduction Material and Methods Results Comment Acknowledgments References

Older studies reported the following directions for chordal-sparing MVR: posterior direction of posterior leaflet CT by Lillehei and associates [1], Lillehei's method and anterolateral direction of the anterior leaflet CT by David [17], lateral by Miki and colleagues [4], circumferential (anterior and posterior, or quasianatomic) by Khonsari and Sintek [18], and a simplified form of the Khonsari II technique by Rose and Oz [19]. Few comparative studies have been performed pertaining to the best direction to resuspend the CT, let alone comparison between anterior versus posterior leaflet CT [7–10]. These experiments did not focus on the specific directions of synthetic CT resuspension; they only investigated natural chordae tendineae, where altering specific reattachment sites was not controllable.

In this experimental protocol we compared six different methods of chordal preservation: conventional, anterior, partial, posterior, oblique, and counter-oblique. The conventional, anterior, and posterior methods were studied because they are all widely employed clinically. We designed the oblique method because a preliminary anatomic study from our laboratory in a canine model revealed mechanical linkage of the anterior leaflet and associated CT from the anterior papillary muscle to the central fibrous body; this suggests the possible presence of physiologic structural integrity in this direction. In fact, van Rijk-Zwikker and associates [16], in an endoscopic study in a beating porcine heart model, described that the central CT, which connects the anterior papillary tip and central fibrous body, were under tension throughout the cardiac cycle, and suggested that these chordae may play an important role in LV function. In the current experiment, the CT were suspended from the anterior papillary muscle to the annulus close to the right trigone, as shown in Figure 2, for possible clinical application in the future (ie, to avoid possible LV outflow tract obstruction by CT underneath the aortic valve). Moreover, on the basis of studying LV mechanics [20], we hypothesized that due to the oblique orientation of the LV subepicardial myocardial fibers, a similar (oblique) CT preservation method might allow papillary muscle contraction to augment systolic LV twist. For contrast, we also studied the counter-oblique method, which theoretically would act in the opposite direction to the forces generated by the LV subepicardial fibers during ejection. The partial method was studied because previous experiments suggested that the anterior PM may have a dominant role in terms of preserving postoperative LV systolic pump function compared with the posterior PM. Another implication of the partial method is simulation of chordal-sparing MVR in the setting of an injured, infarcted, or ruptured posterior PM.

In this study, the overall difference in E_max between the six groups did not reach significance (p = 0.08 by rm-ANOVA). We assumed two possible reasons were responsible for these findings: (1) this experimental preparation contained a large number of techniques (six), which made it less statistically powerful, and (2) we used normal canine hearts. Indeed, when only five groups were compared (excluding the partial method, which is anatomically in-between the anterior and conventional methods), the overall difference in E_max was significant (p < 0.05 by rm-ANOVA), favoring the oblique method. Although LV systolic twist and early diastolic recoil were not measured in this experiment, we speculate that the better LV systolic function in the oblique group may have been related to enhanced LV systolic twist. Theoretically, improved LV systolic twist will minimize transmural oxygen metabolic demands by virtue of equalizing sarcomere stresses and increase potential energy storage in the myocardium and LV interstitium at end-systole; release of this stored energy during diastole will then augment diastolic recoil, which should enhance early LV diastolic filling. The oblique method, however, was not associated with improved diastolic LV function in this experiment, perhaps in part due to fixation of the CT by clamping of the chordae at their end-diastolic length. In the normal heart, the mitral leaflets move toward the LV chamber during diastole, and the CT thereby become redundant; fixing the chordae at their end-diastolic length could potentially suppress LV recoil. This design is, however, relevant in the clinical setting, where CT are usually tacked to the mitral annulus. Alternatively, the isovolumic experimental preparation and the relatively crude assessment of LV diastolic properties we employed could have obscured such effects.

The counter method unexpectedly showed good systolic LV function. A possible explanation for this result is that this theoretically unnatural method may contribute to preload redistribution in the direction of the circumferentially aligned LV midwall fibers [3]. Another possible explanation may be that the counter technique increased transmural LV myocardial shear between the epicardium and the endocardium in the circumferential direction, which might have thereby increased LV wall thickening. The relatively poor diastolic LV function seen in the counter group, however, indicates that this method probably has no overall merit. The anterior and the partial methods seemed to provide some advantage in terms of LV systolic function, but the differences were not marked between the six different groups.

This experiment had several potential limitations, which should be mentioned. First, the protocol was performed under general anesthesia in an acute isovolumic preparation on cardiopulmonary bypass with the chest and the pericardium open. Second, although two-dimensional echocardiography confirmed that the preserved chordae were not markedly disturbed by the two LV balloons, this experimental preparation is certainly a nonphysiologic one compared with closed-chest, ejecting conditions. Third, LV remodeling over time cannot be studied in an acute model, and such postoperative remodeling may likely have important effects on cardiac function. Fourth, our experimental techniques and analytic methods were simplified; in clinical situations, native mitral leaflet tissue (with multiple chordal insertions) are usually preserved. We only examined one principal direction for each PM at each time. Fifth, this study used normal hearts; patients with mitral valve disease (especially chronic mitral regurgitation) and LV dysfunction may respond differently to these various methods of chordal preservation.

In spite of these limitations, these initial results suggest that possible benefits may be associated with the oblique CT preservation method, and this approach warrants further consideration. Future investigations in more physiologic, as well as pathophysiologic, ejecting heart models, including computation of LV systolic twist and early diastolic LV recoil employing myocardial markers or magnetic resonance imaging using transient radiofrequency myocardial tags, are clearly necessary to answer fully these questions.

	Acknowledgments

Top Footnotes Abstract Introduction Material and Methods Results Comment Acknowledgments References

 
We gratefully acknowledge Geraldine C. Derby, RN, BSN, Cynthia E. Handen, BA, Erin M. Schultz, BS, and Mary K. Zasio, BA in the performance of this work and Ms. Phoebe E. Taboada for her assistance in the preparation of the manuscript.

Supported by grants HL-29589 and HL-48837 from the National Heart, Lung, and Blood Institute and the Veterans Affairs Medical Research Service.

Doctors Komeda, DeAnda, and Glasson are Carl and Leah McConnell Cardiovascular Surgical Research Fellows. Doctor DeAnda was also supported by Individual National Research Service Award HL-08928 from the National Heart, Lung, and Blood Institute, and Dr Glasson by The Thoracic Surgery Foundation Research Fellowship Award.

	Footnotes

Top Footnotes Abstract Introduction Material and Methods Results Comment Acknowledgments References

 
Presented at the Thirty-first Annual Meetings of The Society of Thoracic Surgeons, Palm Springs, CA, Jan 30–Feb 1, 1995.

Address reprint requests to Dr Miller, Department of Cardiovascular and Thoracic Surgery, Falk Cardiovascular Research Center, Stanford University School of Medicine, Stanford, CA 94305-5247.

	References

Top Footnotes Abstract Introduction Material and Methods Results Comment Acknowledgments References

 

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This Article Abstract Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Masashi Komeda Abe DeAnda, Jr D. Craig Miller Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Komeda, M. Articles by Miller, D. C. Search for Related Content PubMed PubMed Citation Articles by Komeda, M. Articles by Miller, D. C. Related Collections Related Article