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J Thorac Cardiovasc Surg 1998;116:148-153
© 1998 Mosby, Inc.
Cardiopulmonary Support And Physiology |
This work was supported by an operating grant from the Medical Research Council of Canada.
Received for publication June 27, 1997 Revisions requested Sept. 15, 1997. Revisions received Feb. 4, 1998. Accepted for publication Feb. 4, 1998. Address for reprints: Ray C.-J. Chiu, MD, The Montreal General Hospital, 1650 Cedar Ave., Room C9.169, Montreal, Quebec, Canada H3G 1A4.
| Abstract |
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| Introduction |
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Our concept of using prosthetic material to wrap around the ventricles to reduce ventricular dilatation in a failing heart is an offshoot from our study on the mechanisms of dynamic cardiomyoplasty, a procedure that is currently undergoing a phase III prospective randomized clinical trial.
5 These studies have shown that although dynamic contractions of skeletal muscle wrapped around the ventricles in synchrony with cardiac systole can reduce myocardial stress and may improve systolic function, the passive, that is, adynamic, wrapping of the failing ventricles with skeletal muscle may by itself reduce the progression of ventricular dilatation.
6 Such observation in adynamic cardiomyoplasty has been called "the girdling effect."
7 To further elucidate the mechanisms of the girdling effect, we compared adynamic cardiomyoplasty with a cardiac binding procedure. In the latter, a prosthetic membrane such as Marlex mesh (C. R. Bard, Inc., Murray Hill, N.J.) is used to wrap the ventricles. The effects of these procedures in a canine model of rapid-pacing heart failure were compared. The results indicate that cardiac binding may have a role in the management of selected patients with heart failure.
| Materials and methods |
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Cardiac binding
Anesthesia was induced with sodium pentobarbital (30 mg/kg) and maintained with 1% isoflurane and oxygen delivered by a mechanical ventilator (Muffled Anesthesia Ventilator series 200, Intermed, Penlon, United Kingdom). A median sternotomy was performed for pacemaker insertion and for cardiac binding with Bard Marlex mesh. A wide pericardiotomy was carried out and the Marlex mesh was wrapped around both ventricles below the atrioventricular groove, snugly against the epicardium but without tension, and sewn to itself anteriorly to complete the wrap.
Adynamic cardiomyoplasty
Under general anesthesia, the left latissimus dorsi muscle was harvested with the animal in the right lateral decubitus position. The distal and proximal muscular tendinous attachments were divided and the thoracodorsal neurovascular pedicle was preserved. No cardiomyostimulator was implanted. The muscle was then transposed into the left pleural cavity through a minithoracotomy created by resection of the second rib anteriorly. The proximal tendon was reattached to the periosteum of the resected rib to anchor it in place, taking care not to compromise the thoracodorsal nerve and vessels. The incision was closed in layers. The animal was then repositioned in the supine position and a median sternotomy performed. A wide pericardiotomy was carried out and a pacemaker for rapid pacing was implanted as described earlier. After this, the left latissimus dorsi muscle was retrieved from the pleural space and wrapped snugly in a posterior clockwise direction around both ventricular surfaces, but without excessive tension. The muscle was sewn to the pericardium along the posterior atrioventricular groove.
8 The epicardial pacing lead would be used for rapid pacing. The mediastinum was drained and the sternum closed with wires. These animals were allowed to recover for 4 weeks before rapid pacing was initiated.
Induction of heart failure
Heart failure was induced during the 4-week period of continuous rapid pacing with a modified Medtronic 8329 pacemaker (Medtronic, Inc., Minneapolis, Minn.). The pacemaker was placed in the subcutaneous pocket in the anterior abdominal wall, and a Medtronic 5071 myocardial lead was placed near the apex of the left ventricle. These pacemakers were programmed transcutaneously by telemetry to a rate of 250 beats/min with a Medtronic model 9710 programmer.
Hemodynamic studies
Cardiac dimensions were measured before and 4 weeks after the rapid pacing with a two-dimensional transthoracic echocardiogram (model 77020 AC, Hewlett-Packard Company, Andover, Mass.) with the animal under general anesthesia. Both end-diastolic volume and end-systolic volume were measured and ejection fraction was calculated. Other hemodynamic data were obtained with Swan-Ganz pulmonary catheters (Baxter Healthcare Corporation, Edwards Division, Santa Ana, Calif.). The stroke volume was measured by the thermodilution method (model 90303A, Space Labs Inc., Redmond, Wash.). Hemodynamic parameters measured included central venous pressure, right ventricular pressure, main pulmonary artery pressure, and pulmonary capillary wedge pressure. Systemic blood pressure was monitored with a femoral artery catheter. The left side of the heart was catheterized with a 5F pigtail catheter (Selector, Namic Corp., Glens Falls, N.Y.) for the cardiac binding group, specifically to rule out the possibility of constrictions around the heart. During the catheterization, the left ventricular end-diastolic pressure and rate of left ventricular pressure rise were measured.
Each measurement was repeated three times. Statistical analysis was carried out by means of SPSS version 7.5 (SPSS, Inc., Chicago, Ill.) with multivariate analysis of variance, treating two time points (baseline and failure) as repeated measures, in conjunction with the Scheffe test to compare the differences between the cardiac binding and adynamic cardiomyoplasty groups. The data were expressed as mean ± standard deviation.
| Results |
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| Discussion |
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A series of experiments carried out by Capouya and coworkers
7 and by our group
13 confirmed that adynamic cardiomyoplasty can delay ventricular dilatation and sustain ejection fraction in a canine model of rapid-pacing heart failure. Rapid pacing is a well-defined experimental model that leads to severe dilated cardiomyopathy and heart failure in 3 to 4 weeks.
14
How can the passive presence of wrapped skeletal muscle prevent cardiac dilatation without constriction? The pericardial sac is known to restrict acute cardiac dilatation such as that induced by a rapid increase in preload. However, it shows virtually no restraining effect on chronic cardiac dilatation. It was proposed that elastic stretchability of the skeletal muscle allowed it to provide dynamic constraint against progressive cardiac dilatation.
12 We therefore attempted to compare adynamic cardiomyoplasty with cardiac binding using Marlex mesh, which does not share such characteristics of the skeletal muscle wrap.
Vaynblat and associates
15 studied the cardiac binding effect with polytetrafluoroethylene
* in a canine model of doxorubicin-induced heart failure and showed that cardiac binding reduced ventricular dilatation, as we saw in our animals. They did not demonstrate improvement in cardiac function. We also observed better preservation of ventricular volume in the cardiac binding and adynamic cardiomyoplasty animals compared with that in the control group. In addition, we found that ejection fraction was better preserved with adynamic cardiomyoplasty and cardiac binding with Marlex mesh than in the control group. Although not always reaching statistical significance owing to the relatively small sample size, our data indicate a trend that the effects of cardiac binding with Marlex mesh on changes in cardiac dimensions and ejection fraction may be between those observed in the control and in the adynamic cardiomyoplasty groups. Marlex mesh is a synthetic knitted polypropylene monofilament that can stretch somewhat in both directions and induces more fibrotic responses than the polytetrafluoroethylene membrane. Skeletal muscle flap used for adynamic cardiomyoplasty is certainly much more stretchable than these prosthetic membranes. We were therefore concerned about the possibility of pericardial constrictions by Marlex mesh and epicardial fibrosis. Thus we carried out left heart catheterization in the cardiac binding group. Our findings of lack of equalization between left and right ventricular end-diastolic pressures in the animals studied suggest that there were no significant restrictions to their ventricular filling. Finally, the mean pulmonary capillary wedge pressures were slightly higher in the cardiac binding group than in the other groups, but this difference did not reach statistical significance. The apparent difference in pulmonary capillary wedge pressure may also reflect some variation in volume status in each animal at the time of study.
Although cardiac binding with Marlex mesh demonstrated its ability to reduce progressive cardiac dilatation and deterioration in ejection fraction during rapid pacing, the effects appeared to be less than those observed with adynamic cardiomyoplasty. We speculate that this is indeed due to the unique characteristics of a living muscle tissue used for cardiomyoplasty. A study on the passive stretch properties of the canine diaphragm muscle suggests a possible advantage for using muscle rather than prosthetic material.
16 The passive stress-stretch behavior of the muscle was nonlinear, with the muscle getting stiffer as stretch increased. This ability to both yield and support would be a unique advantage in providing passive restraint against left ventricular dilatation without significantly compromising diastolic filling. In contrast, the fibrotic reaction induced by Marlex mesh would provide minimal compliance. In addition to the stretchability, skeletal muscle is known to be able to undergo "conformational change," adapting to chronic changes in resting tension by the addition or deletion of the number of sarcomeres in the muscle fibers.
17,18 This process allows for the skeletal muscle to restore optimal resting tension, without altering the normal dimension of individual sarcomere units. Although it has not been possible to measure the resting tension of the wrapped muscle per se, serial determination of the Frank-Starling curve in the skeletal muscle ventricle has yielded data consistent with this hypothesis.
19 By being more versatile in adapting to a changing dimension of the failing heart, cardiomyoplasty can be superior to a prosthetic wrap.
On the other hand, adynamic cardiomyoplasty is a more invasive and complex surgical procedure than cardiac binding with a prosthetic membrane. Dissection of the latissimus dorsi muscle may take 1 to 2 hours, usually done with the patient in the lateral decubitus position. Additional time is required to close and drain the wound and to reposition the patient for sternotomy for the second stage of the procedure, namely, the wrapping of the ventricles with the muscle pedicle that had been inserted into the left pleural cavity.
8 In part because of such invasiveness and the length of procedure required, dynamic cardiomyoplasty is currently recommended for patients in New York Heart Association Functional class III, rather than in the more seriously ill patients with class IV disease.
5 In contrast, cardiac binding is a much simpler procedure, which can be carried out rapidly. The procedure lends itself readily to the video-assisted thoracoscopic approach, which makes it even less invasive.
The main benefit of cardiac binding appears to be the process of attenuating the deleterious ventricular remodeling process. Nevertheless, dynamic cardiomyoplasty does offer the added advantage of being able to induce reverse remodeling of dilated cardiomyopathy, as demonstrated clinically.
12,20 We believe that passive restraint against further dilatation is not alone sufficient to reverse remodeling and that this may necessitate the added benefit of active systolic assist available through dynamic cardiomyoplasty. The role of cardiac binding in the surgical management of heart failure instead may be as an adjunct to other more definitive procedures, with the simple goal of reducing subsequent progressive dilatation of a failing heart. One such example is to follow Batista's partial ventriculectomy for dilated cardiomyopathy with cardiac binding, thus preventing or delaying the redilatation of the heart.
21 Although we did not detect deleterious side effects of cardiac binding in this study, both the potential complications and the clinical efficacy of cardiac binding as an adjunct procedure in the management of heart failure require further experimental and clinical explorations.
| Footnotes |
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*Gore-Tex graft, registered trade
name of W. & L. Gore & Associates, Inc., Elkton, Md. ![]()
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