Eur J Cardiothorac Surg 2002;21:245-248
© 2002 Elsevier Science NL
Intracardiac ultrasonic suture welding for knotless mitral valve replacement
Marc Ruel,
Richard B. Streeter,
Ralph de la Torre,
John R. Liddicoat,
William E. Cohn*
Division of Cardiothoracic Surgery, Beth Israel Deaconess Medical Center, LMOB 2A, 110 Francis Street, Boston, MA 02215, USA
Received 13 September 2001;
received in revised form 5 November 2001;
accepted 8 November 2001.
* Corresponding author. Tel.: +1-617-632-8383; fax: +1-617-632-7562
e-mail: wcohn{at}caregroup.harvard.edu
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Abstract
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Objective: The difficulty in tying multiple knots with endoscopic instruments constitutes a technical obstacle to the development of closed-chest valve surgery. The following set of experiments was undertaken to ascertain the in-vivo feasibility of using an intracardiac ultrasonic welding device for knotless suture fixation during mitral valve replacement (MVR). Methods: Five adult sheep weighing 48–52 kg underwent MVR with a commercially available mechanical prosthesis, using pledgetted interrupted polypropylene sutures. An ultrasonic suture welder designed for intracardiac use was used to adjust suture tension and fuse strands together without knots. Echocardiographic assessment of the mitral prosthesis was carried out at baseline and after maintenance of supraphysiologic arterial pressures for 60 min. Subsequently, the animals’ explanted hearts were assessed under sustained left ventricular (LV) pressurization to 180 mmHg in an ex-vivo pressure-loop system. Results: MVR was successfully performed in all animals and welds reliably completed in less than 1 s. One sheep could not successfully be weaned off cardiopulmonary bypass; however, a normal prosthetic valve implant was confirmed at post-mortem examination. Echocardiographic assessment prior to and during LV pressurization revealed normal seating and function of the prosthesis in all cases. At post-mortem examination all valves were adequately implanted, suture tails laid flat on the surface of the prosthesis’ sewing ring, welded suture strands were intact and accurately point-fused together, and no evidence of perivalvular leak was found around any of the prostheses despite sustained LV pressurization. Conclusions: This new modality proved reliable in an acute sheep model of MVR and could constitute a promising avenue towards facilitation of total endoscopic valve procedures in humans.
Key Words: Minimally invasive valve surgery • Ultrasonic welding • Mitral valve replacement
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1. Introduction
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Recent developments in videoscopic and robotic instrumentation have made closed-chest open-heart surgery possible, with endoscopic mitral valve replacement and repair representing some its most promising applications [1–4]. However, tying the multiple knots required for these procedures using videoscopic instrumentation is not only technically demanding but time-consuming and potentially unreliable [5].
Ultrasonic welding is a fixation technique commonly used in the manufacturing of plastic products that could conceivably constitute an alternative to the tying of polypropylene, a semicrystalline thermoplastic material with a sharply defined melting point [6]. This process involves high frequency (50–70 kHz) small amplitude vibrations of individual plastic parts against one another that generate frictional heat and cause fusion at their points of contact (Fig. 1)
. Ultrasonic welds of thermoplastics take a fraction of a second to perform, and have high reliability and strength [6]. Recent ex-vivo data evaluating this modality for the joining of clinical-grade polypropylene suture material indicated that the strength, elongation, and fatigue characteristics of welded loops compare favorably with that of knotted sutures [7]. For these reasons, one may envision that a welding device adapted for intracardiac use could ultimately facilitate minimally invasive valve procedures, and we consequently conducted this study to evaluate the feasibility of using an intracardiac ultrasonic suture welder as a sole, knotless means of suture fixation during mitral valve replacement.
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Fig. 1. Photomicrograph of two polypropylene strands joined together in a weld. Short duration, high-frequency ultrasonic vibrations cause fusion to take place locally over a 5 mm segment.
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2. Materials and methods
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After Institutional Animal Care and Use Committee approval, five adult sheep weighing 48–52 kg were studied. All procedures were conducted in compliance with the Guide for the Care and Use of Laboratory Animals published by the National Institutes of Health (NIH Publication no. 86–23, revised 1985). Anesthesia was induced with pentothal and maintained with 1–2% isoflurane. A left thoracotomy was performed and the animal was placed on cardiopulmonary bypass using standard techniques. Blood was circulated via a roller pump through a standard combined heat exchanger-oxygenator. The aorta was not clamped and the heart was kept beating throughout the procedure.
A vent was placed in the left ventricular apex, and the mitral valve was approached via the left atrial appendage. The anterior leaflet and supporting chordae were completely excised and all posterior structures were preserved. Pledgetted polypropylene sutures (2-0 and #2) were placed in an everting fashion around the circumference of the mitral annulus. Sutures were then passed through the sewing ring of an appropriately sized bileaflet mechanical prosthesis (Carbomedics, Austin, TX), and the valve was seated in the usual fashion. The two ends of each mattress suture were threaded through the end of an intracardiac ultrasonic suture welder (Axya Medical, Beverly, MA), and suture tension adjusted using downward pressure with the tip of the welder as well as upward traction on the end of each suture strand (Fig. 2)
. No effort was made at preventing repeated contact between the tip of the welder and accumulated blood inside the left atrium. The welder was actuated, and the adequacy of weld energy confirmed by a digital readout integral to the device. Successful welding of each suture was confirmed by visual inspection, and the suture tails were cut 1–2 mm from the weld.
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Fig. 2. Intracardiac ultrasonic welder used for the joining of polypropylene strands during mitral valve replacement in a sheep model. Above and to the right of the welder's tip is a previously welded suture seen laying flat on the prosthesis’ sewing ring.
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After the completion of all welds, the left atriotomy was closed, de-airing maneuvers were performed, and the animal was weaned from cardiopulmonary bypass. M-mode and color-mode epicardial echocardiography was used to assess the adequacy of implantation and function of the prosthetic valve. Using low-dose intravenous epinephrine (0.01–0.05 µg/kg per min), the animals’ mean arterial pressure was pharmacologically maintained above 100 mmHg for 60 min and epicardial echocardiography repeated. Euthanasia was then carried out, and the explanted hearts were examined with respect to prosthesis seating and the presence of perivalvular leaks during 10 min of sustained left ventricular pressurization to 180 mmHg in an ex vivo pressure-loop system, generated by the injection of saline solution through a pressure-transduced catheter passed across the aortic valve.
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3. Results
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Adequate exposure of the mitral valve was obtained and prosthesis implantation was successfully performed in all animals. The loading and alignment of sutures strands during tensioning was facilitated by the use of heavier suture, an observation that led to the use of #2 polypropylene in the last three experiments. A total of 61 in vivo, intracardiac welds were performed in the study, each completed in a fraction of a second. Of these, three welds had to be redone on one of the animals due to an ultrasonic frequency problem with one prototype iteration of the welder; all other welds were however performed successfully and reliably. All 61 welded sutures loops laid flat, without knots, on the surface of the prosthesis’ sewing ring (Fig. 2).
Four of the five sheep were successfully weaned from bypass and pharmacologically maintained at supraphysiologic arterial pressures (MAP >100 mmHg) over the subsequent hour. One sheep could not successfully be weaned from bypass for reasons unrelated to the valve prosthesis per se, and adequacy of implantation was confirmed at post-mortem examination. Epicardial echocardiography after atrial closure and following maintenance of supraphysiologic arterial pressures for 1 h revealed normal seating and function of the prosthesis in all cases, and no perivalvular leak was detected. Following sacrifice, absence of perivalvular leak was confirmed in all animals by direct visual assessment of the mitral prosthesis during LV pressurization to 180 mmHg for 10 min in an ex vivo pressure-loop system. Each of the 61 constructed welds was also manipulated with forceps and confirmed to be intact.
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4. Discussion
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In this set of experiments, replacement of the mitral valve was successfully performed in a sheep model using an intracardiac ultrasonic suture welder, a novel tool that allows one to secure interrupted sutures under tension without tying knots. Welds were constructed rapidly and reliably under in vivo conditions that involved repeated contact with blood, intracardiac use, and limited access. After completion of the welds and valve implantation, adequacy of prosthesis seating was confirmed with epicardial echocardiography conducted under sustained supraphysiologic arterial pressure conditions, as well as with direct visual assessment and manipulation during ex vivo pressurization of the left ventricle.
Suture welding has recently been introduced in clinical medicine as a means of fixating the head of the humerus to the glenoid fossa in rotator cuff injuries, as well as for videoscopic bladder suspension and Nissen fundoplication. High-frequency low amplitude vibrations of suture ends against one-another cause precise localized melting that generates welds. Industrial data pertaining to the prototype used in the present series indicate that the ultrasonic welder's tip displays a temperature increase of less than 1°C over the welding cycle, without histologic alteration of neighboring tissues (personal communication; Dennis Hubbard, Axya Medical, Beverly, MA). Further, a study by Richmond showed that the tensile strength of welds performed ex-vivo using #2 polypropylene was 13.8±0.9 kg, largely exceeding the 7.04 kg USP knot strength for this particular suture [7]. Cyclic fatigue at 37°C was also comparable to that of knotted sutures (27.137±19.746 cycles to failure versus 22.866±24.151 for welded versus knotted sutures, respectively; P=NS), and significantly greater force was required to create a 3.0 mm elongation in welded versus knotted loops (13.2±0.6 versus 6.1±2.1 kg, welded versus knotted loops, respectively; P<0.001) [7].
Ultrasonic suture welding requires monofilament polymer, and appeared in our experience to be facilitated by the use of larger suture sizes, such as 0, #1, or #2 polypropylene. Monofilament suture is currently not widely employed for valve implantation with conventional techniques, presumably due to the need for additional knots and the shape-memory that can make handling these sutures more challenging. Sutures larger than 2-0 have also not traditionally been used for valve implantation due to the sub-optimal aspects of seating large sutures under tension in a restricted working space, in addition to the drawback of leaving a bulkier stack of knots. Intracardiac ultrasonic suture welding, by allowing fixation under tension without knots, avoids these pitfalls. The geometry of the welded suture is compact, the suture tails lie tangential to the suture loop and flat with respect to the sewing ring, and the complete absence of knots results in a reduction in the amount and profile of suture material, as well as in the elimination of interstices that can harbor bacteria or promote thrombus formation [8–13].
This study, undertaken to assess the feasibility of this new modality in vivo, has some limitations in that it consists of acute experiments. Although industrial and ex-vivo data indicate that plastic welds are not particularly susceptible to degeneration or weakening over time [6,7], chronic animal series will be required before this tool may be considered for use in cardiac surgical patients. Nevertheless, with the advent of totally endoscopic open-heart surgery, intracardiac suture welding devices have the potential to help bring technical ease, shorten procedural times, and thus contribute significantly to the refinement and widespread adoption of closed-chest valve replacement procedures.
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Acknowledgments
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This work was supported in part by a grant from Axya Medical, Beverly, MA.
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Abstract
1. Introduction
