- Email: [email protected]
Clinically available extracorporeal assist devices
Clinically Available Extracorporeal Assist Devices Robert D. Dowling and Steven W. Etoch
Mechanical circulatory support has been shown to be of benefit to allow recovery after conventional heart surgery and as a successful bridge to heart transplantation. Recent clinical trials with implantable left ventricular assist devices (LVADs) have been completed with these devices showing restoration of normal hemodynamics and successful bridge to transplantation. A major advantage of the implantable devices is the ability for the patient to be discharged and followed up at an outpatient setting. However, multiple advantages to extracorporeal devices still remain, which are the focus of this review. One advantage of the extracorporeal devices is that they can be placed in much smaller patients than currently available implantable LVADs. Also, because of differences in design of the assist devices, the extracorporeal devices can be placed without the need for the cardiopulmonary bypass and with decreased operative time and dissection. Perhaps the biggest advantage of the extracorporeal devices is that they can provide a support for both the right and left side of the heart as opposed to the implantable LVADs, which are only used as left ventricular assist devices. This article describes in detail the advantages and disadvantages of the extracorporeal devices as well as the operative techniques used to implant them. As the number of patients with heart failure continues to rise, so will the need for mechanical circulatory support. Though the majority of these patients will be served by a long-term implantable device, there will remain a subset of patients that will be best suited for treatment with extracorporeal devices. Copyright 娀 2000 by W.B. Saunders Company
M
echanical cardiac assistance with extracorporeal devices began in 1966 when Debakey first used a pneumatic device in a patient with a failing left ventricle. The ability of pneumatically driven devices to support the circulation was further shown by the experience with the implantation of the Jarvik 7100 pneumatic total artificial
heart (Symbion Inc, Salt Lake City, UT). These and other clinical trials showed the ability of the pneumatic devices to maintain normal circulation with normal end-organ function. Despite the emphasis on the development of totally implantable devices, the use and development of extracorporeal devices also continued because there were clearly certain advantages to these heterotopic devices. This article outlines the current indications and advantages of extracorporeal assist devices and describes the clinical experience with the 2 currently available extracorporeal pulsatile LVAD systems (ie, ABIOMED BVS-5000 [ABIOMED Inc, Danvers, MA] and Thoratec ventricular assist device system [Thoratec Laboratories Corp, Pleasanton, CA]).
Advantages of Extracorporeal Circulatory Support Extracorporeal pneumatic devices offer certain clear advantages over implantable devices. One major limitation of the implantable devices is the size of patients that are suitable candidates. The clinical trials of all currently available implantable devices were performed in adult patients and were further limited to patients with a body surface area of at least 1.5 m2. Our group has implanted the Novacor left ventricular assist device (LVAD) (Novacor Division, Baxter Health-
From the Department of Surgery, Division of Cardiothoracic Surgery, University of Louisville, Louisville, KY; and the Jewish Hospital Heart and Lung Institute, Louisville, KY. Address reprint requests to Robert D. Dowling, MD, Division of Cardiothoracic Surgery, 201 Abraham Flexner Way, Suite 1200, Louisville, KY 40202; e-mail: [email protected]. Copyright 娀 2000 by W.B. Saunders Company 0033-0620/00/4301-0004$10.00/0 doi:10.1053/pcad.2000.7192
Progress in Cardiovascular Diseases, Vol. 43, No. 1 (July/August), 2000: pp 27-36
27
28 care Corp, Oakland, CA) in smaller adults. However, these patients have had problems with discomfort associated with the device as well as early satiety. Conversely, the extracorporeal devices have been used in smaller patients. The ABIOMED BVS-5000 system has been used in pediatric patients with a body surface area (BSA) as low as 0.85 m2 with a weight of 20 kg. Sixteen patients with a BSA of less than 1.3 m2 have had placement of the Thoratec VAD system. The youngest patient was 7 years old and the smallest had a BSA of 0.73 m2. Another distinct advantage of the extracorporeal devices is that they do not require the additional dissection needed for placement of an implantable device. The only additional dissection that is needed is to create tunnels for placement of the inflow and outflow cannulas. In patients that are in severe congestive heart failure with end-organ dysfunction including hepatic dysfunction, there can be a significant advantage achieved by avoiding the additional surgical time and trauma to create a ventricular assist device (VAD) pocket. There is also a patient population that will become unstable in which the institution of cardiopulmonary bypass is performed on an urgent basis. The additional dissection to create a VAD pocket in these patients will be associated with significant increases in time on cardiopulmonary bypass with its attendant morbidity rates. Also, less surgical dissection and time are required at transplantation in patients on extracorporeal devices. However, removal of the implantable devices is quite straightforward and, therefore, this should not play a role in determining whether an extracorporeal or implantable LVAD is used. Another advantage of the extracorporeal devices is that there are multiple choices for placement for the LVAD inflow cannulas. The 4 choices for removing blood from the left side of the heart are (1) the left atrial appendage, (2) the reflected interatrial groove adjacent to the right superior pulmonary vein, (3) the dome of the left atrium with positioning of the cannula between the superior vena cava (SVC) and the aorta, and (4) the left ventricular apex. This contrasts sharply with the Novacor LVAD and TCI Heartmate (Thermo Cardiosystems Inc, Woburn, MA) implantable LVADs in which the design of the inflow cannulas only allow for left ventricular apical cannulation. Holman et al1 reported on 5 patients
DOWLING AND ETOCH
that had left atrial cannulation and 5 patients that had left ventricular cannulation. They concluded that although the flow provided by left atrial cannulation was somewhat less than that achieved with LV cannulation, flow was adequate to restore normal hemodynamics and normal end-organ function in all patients. The patients that had left atrial cannulation had a BSA ranging from 1.79 to 2.16 m2. There are a number of situations in which left atrial cannulation provides an advantage. In the setting of acute myocardial infarction, it is possible that there has been enough damage to the left ventricle (LV) apex that the tissues are not able to hold sutures. Attempts at apical cannulation in this setting may result in lifethreatening hemorrhage. The other situation in which left atrial cannulation would be indicated is in a patient who would benefit from avoidance of cardiopulmonary bypass. We frequently see patients with significant renal or hepatic dysfunction or significant thrombocytopenia in which the morbidity and the potential mortality of the surgery could be significantly decreased by the avoidance of cardiopulmonary bypass. Moreover, cannulation of the interatrial groove or the dome of the left atrium also eliminates the need for complete dissection of the left side of the heart and thereby has the potential to decrease the surgical time, reduce intraoperative blood loss, and eliminate the potential for injury of patent bypass grafts. Our preferred site for left atrial cannulation has been the dome of the left atrium as described by Jett.2 Advantages of this approach are that it requires minimal dissection, no manipulation of the heart is required, and removal of the cannula is quite easily achieved. Also, if there is bleeding from the cannulation site, it can be readily controlled and repaired, again, without the need for manipulation of the heart. One potential disadvantage of cannulation through the dome of the left atrium is compression of right-sided saphenous vein grafts especially when the vein graft comes off of the aorta anteriorly.2 However, if the saphenous vein graft comes off of the right side of the aorta, then the graft will be well away from the cannula. Our experience has been that the cannula or vein graft can be positioned to avoid compression if one is aware of this potential hazard. Our second choice for left atrial cannulation
EXTRACORPOREAL ASSIST DEVICES
has been posterior to the interatrial groove between the right superior and inferior pulmonary veins. Although this does require some manipulation of the heart and dissection of the interatrial groove, we have been able to consistently place adequate size cannulas and achieve adequate inflow drainage through this approach. Bleeding at the time of decannulation is a potential problem and can often be difficult to control. A potential disadvantage of left atrial cannulation is the development of thrombus around the cannula adjacent to the wall of the left atrium, which can be dislodged at the time of decannulation with the potential for a significant thromboembolic event. Another potential disadvantage of left atrial cannulation is stasis of blood in the left ventricle with clot formation and a potential for an increased incidence of thromboembolic events. The extracorporeal devices have received approval for use both as a bridge to transplant and for postcardiotomy support. When faced with patients that are unable to be weaned from cardiopulmonary bypass (CPB), we always prefer placement of an extracorporeal device. In these situations it is often of paramount importance to rapidly institute mechanical circulatory support because these patients invariably have been on CPB for prolonged periods with its attendant sequelae. This is best achieved with expeditious placement of a Thoratec or an ABIOMED device. Perhaps the most significant advantage of the extracorporeal devices is the ability to provide biventricular support. With improved patient management techniques and the use of inhaled nitric oxide, the percentage of patients that require biventricular support has significantly decreased, although a subset of patients will still require biventricular support because of severe right ventricular failure or refractory arrhythmias. Many centers have had patients successfully supported on biventricular devices despite prolonged periods of ventricular fibrillation. The significant disadvantage of the extracorporeal devices is the decreased patient mobility and inability to be discharged from the hospital. With the ABIOMED BVS 5000 system, the patient is essentially restricted to bed rest and any significant change of position will potentially have a significant effect on VAD function. Patients on the Thoratec VAD system are able to ambulate and
29 perform exercise on stationary machines. However, their mobility is quite limited by the large size of the drive console. Recently, a CE Mark was granted in Europe on the Thoratec TLC-II portable VAD driver. The Thoratec TLC-II portable VAD driver is a briefcase-size battery-powered pneumatic control unit that weighs 9.1 kg3,4 (Figs 1 and 2). The device can either be carried by hand with a shoulder strap or pushed with a mobile cart. Food and Drug Administration (FDA) clinical trials are now underway in the United States. Obviously, the limited mobility has precluded these patients from being discharged to home and, therefore, these patients are sentenced to confinement in a hospital setting until an appropriate donor heart is identified.
The Thoratec Left Ventricular Assist System The Thoratec VAD system is a pneumatic assist device that uses displacement of air to compress and empty a blood pumping sac (Figs 3 and 4). An extracorporeal prosthetic ventricle is used that consists of a seamless polyurethane blood sac enclosed within rigid casings. Tilting disc inlet and outlet valves provide for unidirectional blood flow. A thin tubing carries compressed air from the console to the pump’s air chamber. Adjustments can be made both in the magnitude of the drive pressure required to achieve systole and also the amount of negative pressure (ie, vacuum) that can be used to assist filling during VAD diastole. A Thoratec VAD has a stroke volume of 65 mL and is capable of delivering flow rates of 6.5 L/min. Appropriate emptying of the LVAD requires a drive line pressure of approximately 200 mm Hg or 75 to 100 mm Hg above the patient’s systolic blood pressure. In addition to an adequate systolic ejection time (⬎300 ms), we have found that optimal LVAD filling is achieved when the vacuum is set to ⫺30 to ⫺40 mm Hg. Inflow to the LVAD is achieved through a cannula placed either in the apex of the left ventricle or in the left atrium. Our standard surgical approach for the placement of a Thoratec LVAD or biventricular device is through a median sternotomy. A sternal retractor is placed with the cross bar toward the patient’s head to allow easy
30
DOWLING AND ETOCH
Fig 1. The Thoratec TLC-II portable VAD driver unit is a briefcase-size battery-powered pneumatic control unit that weighs 9.1 kg. Courtesy of Thoratec Laboratories Corporation.
access to the subcostal area for placement of the VAD cannulas and multiple chest tubes. After creation of a pericardial cradle, the VAD cannula sites are chosen on the left/right abdominal wall. Care is taken to ensure that the cannulas will not lie adjacent to the ribs. The distance between the midpoint of the inflow and outflow of the VAD is 4 cm and, therefore, 2 small circular ellipses of skin are excised 4 cm apart and tunnels for the VAD cannulas are created. Heparin is given after creation of the cannula tunnels. Aortic and atrial pursestring sutures are then placed but cannulation is not performed at this time. Rather, the outflow conduit from the LVAD is sewn end-toside to the aorta by using a partial side-biting clamp. Careful placement of this clamp has not resulted in any worsening of the hemodynamics even in this patient population. Outflow from the Thoratec device is achieved through a cannula that is made of Dacron (Ethibon; US Surgical, Norwalk, CT) woven polyester and is 14 to 18 mm. The outflow cannula is sewn end-to-side to the aorta or the pulmonary artery. Position of the outflow cannula on the aorta is often dictated by the presence of previously placed bypass grafts or cannulation sites. In patients that have not had previous surgery, our approach has been to place the outflow cannula lower on the aorta so it can
be entirely removed at the time of heart transplantation. Similarly, we attempt to position the outflow cannula of the pulmonary artery relatively proximal with the only caveat that too proximal of a position may injure or distort the pulmonary valve. After completing the anastomosis of the outflow conduit to the aorta, a cross-clamp is placed across the outflow conduit, the partial clamp is removed, and hemostasis is obtained from the aortic suture line. A pledgeted 4-0 mattress suture is placed at the highest point of the outflow conduit for later placement of a root vent, which will facilitate deairing. Aortic and atrial cannulation is then performed and CPB is initiated. The apex of the left ventricle is elevated in a routine manner. A 14-French coring knife is used to remove a circular core of tissue at the LV apex. Twelve pledgeted Ethibon sutures (US Surgical) are placed full thickness circumferentially around the ventriculotomy and then through the sewing flange of the inflow conduit. After the conduit is seated and the sutures are tied, a second running suture is placed completely around the suture line and a felt strip is used. This is performed to provide hemostasis and aerostasis. Recently, we have been placing tissue glue at this site, also for hemostasis and aerostasis. The VAD is filled with saline. Then with partial
EXTRACORPOREAL ASSIST DEVICES
31 this level may impair drainage of the SVC and may make placement of a pulmonary artery catheter from a cephalad approach difficult, if not impossible (Fig 5). Recently, we have placed the inflow cannula of the RVAD into the right ventricle as described by Arabia et al.5 We have found this to be a straightforward technique that has resulted in more consistent filling of the RVAD. There are 3 modes of support with the Thoratec VAD; volume or fill mode, R wave synchronous mode, and asynchronous mode. The asynchronous mode is most commonly used at the time of implantation for deairing and can also be used at lower rates in conjunction with echocardiographic monitoring to determine if there has been
Fig 2. The Thoratec TLC-II portable VAD driver unit can either be carried by hand with a shoulder strap or pushed with a mobile cart as shown in this figure. Courtesy of Thoratec Laboratories Corporation.
occlusion of venous return, the inflow cannula is connected to the VAD. This results in blood flow through the VAD, which is collected in a kidney basin and returned to the bypass circuit via a pump sucker. The outflow cannula is then filled retrograde with blood from the aorta and connected to the VAD. The aortic root vent is placed in the outflow conduit and extensive deairing maneuvers are performed with echocardiographic guidance to ensure complete absence of intracavitary air. After adequate deairing, the cross-clamp is removed from the outflow conduit and the patient is weaned from CPB. Atrial cannulation for a Thoratec right ventricular assist device (RVAD) should be at the mid-to-lower portion of the right atrium. Cannulation of the atrium above
Fig 3. Thoratec VAD. Courtesy of Thoratec Laboratories Corporation.
32
DOWLING AND ETOCH
Clinical Results of the Thoratec VAD System
Fig 4. Thoratec dual drive console. Courtesy of Thoratec Laboratories Corporation.
any evidence of LV recovery. The R wave synchronous mode allows timing of the VAD systole during natural heart diastole. The VAD can be programmed to eject with every diastole in a 1 to 2 mode or in a 1 to 3 mode similar to an intra-aortic balloon pump.6 We have not found this mode to be clinically useful. The fill mode or volume mode is the mode most commonly used after implantation when weaning from the device is not being considered. This mode offers optimal ventricular support and more complete washing of the blood sac, thereby theoretically decreasing the risk of thrombus formation.
The first clinical use of the Thoratec LVAD as a bridge to transplant was in September 1984 at the Pacific Presbyterian Medical Center in San Francisco.7 The device was used for left ventricular support in a 40-year-old man who had a massive myocardial infarction. This patient subsequently underwent heart transplantation and was the first patient to be discharged from the hospital after a bridge to transplant procedure. The Thoratec VAD system was also the first system to be used in a successful bridge to transplantation in a patient requiring biventricular support. Laman Gray, Jr, implanted both an RVAD and an LVAD in a 16-year-old boy with end-stage viral cardiomyopathy at Jewish Hospital in Louisville, KY, in March 1985.7 This patient also underwent successful transplantation and was discharged to home and is currently doing well 14 years after his heart transplant. The first multicenter report on the Thoratec VAD system as a bridge to transplant appeared in the New England Journal of Medicine in 1988. Twenty-nine patients underwent VAD placement, 21 survived to undergo heart transplantation, and 19 were discharged alive.8 In 1990, Farrar et al7 reported on 72 heart transplant candidates that had received Thoratec devices at 20 medical centers in 5 countries. Fifty-eight (81%) of the patients received a biventricular device and 14 (19%) received LVAD support only. Fifty-four (75%) patients recovered sufficiently to undergo heart transplant with an average duration of support of 4.4 days with a range of 8 hours to 81 days. Forty-five percent of patients were discharged from the hospital for a 63% overall survival rate from implant to discharge. They also showed that the actual 1- and 2-year posttransplant survival rates were similar to that after conventional heart transplantation. In 1993, Farrar and Hill9 reported on the use of the Thoratec VAD system in 154 transplant candidates. Biventricular support was used in 120 patients and isolated LVAD support was used in 34 patients. Average duration of support was 17.5 days with a range of 8 hours to 226 days. Sixty-five (48%) patients recovered and were able to undergo heart transplantation. The overall survival rate for the
EXTRACORPOREAL ASSIST DEVICES
33
Fig 5. Standard cannulation sites for the Thoratec VAD system. Alternative sights for left atrial cannulation are discussed in the text as is cannulation of the right ventricle for inflow to an RVAD. Courtesy of Thoratec Laboratories Corporation.
group was 54%. This study, again, showed that the survival rate 1 year after transplant is comparable with that seen after conventional nonbridged patients. This has also been shown by Pifarre et al.10 In a review of success rates of long-term VADs used as a bridge to transplantation, Arabia et al11 reported a success rate of 93% with the Thoratec LVAD and 81% with a Thoratec biventricular assist device (BIVAD). These data clearly show that bridge to transplant results were as good with the Thoratec VAD system as with any other available system. The decreased survival rate with patients requiring BIVAD reflects the need for biventricular support in patients with increasing severity of illness and organ dysfunction.
The ABIOMED BVS-5000 VAD The ABIOMED BVS-5000 is an external asynchronous pulsatile device capable of providing shortterm univentricular or biventricular circulatory support. There are 3 components to the system.
The pneumatic drive console, a single-use dual chamber blood pump, and the inflow and outflow cannulas (Fig 6). The pump is a dual chamber pump contained in a hard polycarbonated housing. The upper chamber is a passive gravity-filled reservoir (atrial chamber) and the lower chamber is the (ventricular) pumping chamber. Each chamber contains a smooth surface polyurethane bladder with a volume of 100 mL. There is a trileaflet polyurethane valve between the upper and lower chamber (inflow valve) and between the lower chamber and the outflow conduit (outflow valve). The atrial chamber fills throughout both pump systole and diastole, which ensures continuous drainage to the VAD (Fig 7). There is no vacuum and, therefore, filling of the VAD is merely by gravity. Filling can be altered by adjusting the level of the pump relative to the level of the patient. As the pumping chamber is filled with blood, all of the surrounding air is returned to the console. When the volume of displaced air indicates that the pumping chamber is full, the console immediately sends compressed air back
34
DOWLING AND ETOCH
Fig 6. (A) Schematic of the ABIOMED BVS 5000 console and blood pump set. (B) BVS 5000i and 5000t backup/transport consoles. Use by permission of ABIOMED.
to the pumping chamber, resulting in VAD systole. The drive console operates independently of the native cardiac rhythm. Control systems contained in the drive console automatically adjust the duration of pump systole and diastole to compensate for changes in preload and afterload. The control systems maintain a relatively constant stroke volume of approximately 88 mL. Beat rate of the VAD and VAD output are displayed on the console. The system operation is fully automated and does not require operator input during normal operation. A separate control is included to allow weaning from the device. These controls allow the operator to rapidly go to a manual mode in which the VAD output can be set at a fixed rate. The outflow cannula consists of a preclotted 14mm woven Dacron graft with an external Dacron velour sleeve to promote tissue incorporation at the exit site. The small size of the outflow conduit is an advantage as is the lack of bleeding through the graft interstices.12 As noted previously, early clinical experience and traditional teaching has been that the inflow cannulas are placed either through the lateral wall of the right atrium for an RVAD or in the left atrium for an LVAD. More recently, cannulation of the right ventricle on the diaphramatic surface between the posterior descending artery (PDA) and the acute margin of the heart has been performed with excellent RVAD filling. Use of the atrial cannulas to cannulate the LV apex has also been performed with
excellent LVAD function (E. Summers, personal communication, October, 1998). If replacement of the VAD is necessary due to thrombus formation, this procedure can be performed either at the bedside or in the operating room and can usually be performed in under 1 minute.
Clinical Results of the ABIOMED BVS-5000 System The first clinical use of the ABIOMED BVS-5000 system was by Dreyfus et al.13 They reported successfully bridging 2 patients to transplant for 5 and 21 days with VAD outputs ranging from 4.3 to 5.1 L/min. They were able to identify the key advantages to the system: (1) the heterotopic position allowed for rapid implantation without the need for cardiopulmonary bypass, (2) the ability to provide biventricular support, (3) automatic regulation of the device without the need for operator input, and (4) acceptable cost. After this report, multiple other centers reported their experience with successful bridge to transplantation. In 1994, Gray and Champsaur14 reported on the world-wide registry experience with the ABIOMED BVS system as both a bridge to transplant and for postcardiotomy support. Four hundred and twenty patients had undergone placement of the BVS system between February 1994 and June 1997. Indications for device implantation were postcardiotomy shock in 211 patients,
35
EXTRACORPOREAL ASSIST DEVICES
Fig 7. Cross section of the ABIOMED BVS 5000 system showing the diastolic and systolic phases of the assist device. Use by permission of ABIOMED.
cardiomyopathy in 94 patients, acute myocardial infarction and cardiogenic shock in 44 patients, failed orthotopic heart transplant in 45 patients, and multiple other indications in 26 patients. Sixty-five percent of patients had biventricular support, 29% had placement of LVAD alone, and 5% had placement of RVAD alone. Mean length of support was 5.2 days. In the postcardiotomy group, 85% of patients were weaned from support or bridged to transplant with 27% of these patients being discharged from the hospital. Seventy percent of the cardiomyopathy patients were successfully bridged to transplant with 58% of this entire group being discharged to home. Of the patients with acute myocardial infarction, 52% underwent transplantation with 70% of these patients being discharged to home. In 1993, Guyton et al15 reported on the results of the multicenter evaluation of the use of the BVS 5000 system for postcardiotomy shock. Fifty-five patients who were hemodynamically unstable despite maximal pharmacological and intra-aortic balloon therapy, and were less than 6 hours from the first attempt to wean from cardiopulmonary bypass, were enrolled in the study. The BVS
system was successful in restoring hemodynamics as evidenced by significant improvements in arterial pressure, cardiac index, and left-sided filling pressure. Of the 31 patients who met all of the appropriate criteria for enrollment in the study, 17 (55%) were weaned from support and 9 (29%) were discharged. Survival in patients that did not experience a cardiac arrest before placement of the device was 47%. One patient was bridged to a long-term device before transplantation. Overall survival rate to heart transplantation and subsequent hospital discharge was 62%. As the number of patients with intractable heart failure continues to rise so does the need for mechanical circulatory assist devices. Although patient mobility with implantable LVADs is desirable, there remains a subset of patients that will require biventricular support or whose likelihood of survival will be significantly increased if surgical trauma can be minimized. It is for these patients that extracorporeal assist devices remain an important tool in the armamentarium to treat end-stage heart disease.
References 1. Holman WL, Bourge RC, Murrah CP, et al: Left atrial or ventricular cannulation beyond 30 days for a thoratec ventricular assist device. ASAIO 41:M517-M522, 1995 2. Jett GK: Atrial cannulation for left ventricular assistance: Superiority of the dome approach. Ann Thorac Surg 61:1014-1015, 1996 3. Farrar D, Buck K, Coulter J, et al: Portable pneumatic biventricular driver for the thoratec ventricular assist device. ASAIO 43:M631-M634, 1997 4. Von Segesser L, Tkebuchava T, Leskosek B, et al: Biventricular assist using a portable driver in combination with implanted devices: Preliminary experience. Artif Organs 21:72-75, 1997 5. Arabia F, Paramesh V, Toporoff B, et al: Biventricular cannulation for the thoratec ventricular assist device. Ann Thorac Surg 66:2119-2120, 1998 6. Ley SJ, Hill JD: Thoratec ventricular assist device, in Quall S (ed): Cardiac Mechanical Assistance Beyond Balloon Pumping. St Louis, MO, Mosby-Year Book, 1993 7. Farrar D, Lawson J, Litwak P, et al: Thoratec VAD system as a bridge to heart transplantation. J Heart Transplant 9:415-423, 1990 8. Farrar D, Hill J, Gray L Jr, et al: Heterotopic prosthetic ventricles as a bridge to cardiac transplantation: A
36 multicenter study in 29 patients. N Engl J Med 318:333340, 1988 9. Farrar D, Hill J: Univentricular and biventricular thoratec VAD support as a bridge to transplantation. Ann Thorac Surg 55:276-282, 1993 10. Pifarre R, Sullivan H, Montoya A, et al: Comparison of results after heart transplantation: Mechanically supported versus nonsupported patients. J Heart Lung Transplant 11:235-239, 1992 11. Arabia F, Smith R, Rose D, et al: Success rates of long-term circulatory assist devices used currently for bridge to heart transplantation. ASAIO 42:M542M546, 1996
DOWLING AND ETOCH 12. Shook B: The Abiomed BVS 5000 biventricular support system. J Card Surg 7:309-316, 1993 13. Dreyfus G, Guillemain R, Couetel J, et al: First clinical use of a new biventricular bridging support: The Abiomed’s BVS system 5000. J Heart Transplant 7:84, 1988 14. Gray LA, Champsaur GG: The BVS 5000 biventricular assist device—the Worldwide Registry Experience. ASAIO 40:M460-M464, 1994 15. Guyton RA, Schonberger JP, Everts PA, et al: Postcardiotomy shock: Clinical evaluation of the BVS 5000 biventricular support system. Ann Thorac Surg 56:346356, 1993
Mechanical circulatory support has been shown to be of benefit to allow recovery after conventional heart surgery and as a successful bridge to heart transplantation. Recent clinical trials with implantable left ventricular assist devices (LVADs) have been completed with these devices showing restoration of normal hemodynamics and successful bridge to transplantation. A major advantage of the implantable devices is the ability for the patient to be discharged and followed up at an outpatient setting. However, multiple advantages to extracorporeal devices still remain, which are the focus of this review. One advantage of the extracorporeal devices is that they can be placed in much smaller patients than currently available implantable LVADs. Also, because of differences in design of the assist devices, the extracorporeal devices can be placed without the need for the cardiopulmonary bypass and with decreased operative time and dissection. Perhaps the biggest advantage of the extracorporeal devices is that they can provide a support for both the right and left side of the heart as opposed to the implantable LVADs, which are only used as left ventricular assist devices. This article describes in detail the advantages and disadvantages of the extracorporeal devices as well as the operative techniques used to implant them. As the number of patients with heart failure continues to rise, so will the need for mechanical circulatory support. Though the majority of these patients will be served by a long-term implantable device, there will remain a subset of patients that will be best suited for treatment with extracorporeal devices. Copyright 娀 2000 by W.B. Saunders Company
M
echanical cardiac assistance with extracorporeal devices began in 1966 when Debakey first used a pneumatic device in a patient with a failing left ventricle. The ability of pneumatically driven devices to support the circulation was further shown by the experience with the implantation of the Jarvik 7100 pneumatic total artificial
heart (Symbion Inc, Salt Lake City, UT). These and other clinical trials showed the ability of the pneumatic devices to maintain normal circulation with normal end-organ function. Despite the emphasis on the development of totally implantable devices, the use and development of extracorporeal devices also continued because there were clearly certain advantages to these heterotopic devices. This article outlines the current indications and advantages of extracorporeal assist devices and describes the clinical experience with the 2 currently available extracorporeal pulsatile LVAD systems (ie, ABIOMED BVS-5000 [ABIOMED Inc, Danvers, MA] and Thoratec ventricular assist device system [Thoratec Laboratories Corp, Pleasanton, CA]).
Advantages of Extracorporeal Circulatory Support Extracorporeal pneumatic devices offer certain clear advantages over implantable devices. One major limitation of the implantable devices is the size of patients that are suitable candidates. The clinical trials of all currently available implantable devices were performed in adult patients and were further limited to patients with a body surface area of at least 1.5 m2. Our group has implanted the Novacor left ventricular assist device (LVAD) (Novacor Division, Baxter Health-
From the Department of Surgery, Division of Cardiothoracic Surgery, University of Louisville, Louisville, KY; and the Jewish Hospital Heart and Lung Institute, Louisville, KY. Address reprint requests to Robert D. Dowling, MD, Division of Cardiothoracic Surgery, 201 Abraham Flexner Way, Suite 1200, Louisville, KY 40202; e-mail: [email protected]. Copyright 娀 2000 by W.B. Saunders Company 0033-0620/00/4301-0004$10.00/0 doi:10.1053/pcad.2000.7192
Progress in Cardiovascular Diseases, Vol. 43, No. 1 (July/August), 2000: pp 27-36
27
28 care Corp, Oakland, CA) in smaller adults. However, these patients have had problems with discomfort associated with the device as well as early satiety. Conversely, the extracorporeal devices have been used in smaller patients. The ABIOMED BVS-5000 system has been used in pediatric patients with a body surface area (BSA) as low as 0.85 m2 with a weight of 20 kg. Sixteen patients with a BSA of less than 1.3 m2 have had placement of the Thoratec VAD system. The youngest patient was 7 years old and the smallest had a BSA of 0.73 m2. Another distinct advantage of the extracorporeal devices is that they do not require the additional dissection needed for placement of an implantable device. The only additional dissection that is needed is to create tunnels for placement of the inflow and outflow cannulas. In patients that are in severe congestive heart failure with end-organ dysfunction including hepatic dysfunction, there can be a significant advantage achieved by avoiding the additional surgical time and trauma to create a ventricular assist device (VAD) pocket. There is also a patient population that will become unstable in which the institution of cardiopulmonary bypass is performed on an urgent basis. The additional dissection to create a VAD pocket in these patients will be associated with significant increases in time on cardiopulmonary bypass with its attendant morbidity rates. Also, less surgical dissection and time are required at transplantation in patients on extracorporeal devices. However, removal of the implantable devices is quite straightforward and, therefore, this should not play a role in determining whether an extracorporeal or implantable LVAD is used. Another advantage of the extracorporeal devices is that there are multiple choices for placement for the LVAD inflow cannulas. The 4 choices for removing blood from the left side of the heart are (1) the left atrial appendage, (2) the reflected interatrial groove adjacent to the right superior pulmonary vein, (3) the dome of the left atrium with positioning of the cannula between the superior vena cava (SVC) and the aorta, and (4) the left ventricular apex. This contrasts sharply with the Novacor LVAD and TCI Heartmate (Thermo Cardiosystems Inc, Woburn, MA) implantable LVADs in which the design of the inflow cannulas only allow for left ventricular apical cannulation. Holman et al1 reported on 5 patients
DOWLING AND ETOCH
that had left atrial cannulation and 5 patients that had left ventricular cannulation. They concluded that although the flow provided by left atrial cannulation was somewhat less than that achieved with LV cannulation, flow was adequate to restore normal hemodynamics and normal end-organ function in all patients. The patients that had left atrial cannulation had a BSA ranging from 1.79 to 2.16 m2. There are a number of situations in which left atrial cannulation provides an advantage. In the setting of acute myocardial infarction, it is possible that there has been enough damage to the left ventricle (LV) apex that the tissues are not able to hold sutures. Attempts at apical cannulation in this setting may result in lifethreatening hemorrhage. The other situation in which left atrial cannulation would be indicated is in a patient who would benefit from avoidance of cardiopulmonary bypass. We frequently see patients with significant renal or hepatic dysfunction or significant thrombocytopenia in which the morbidity and the potential mortality of the surgery could be significantly decreased by the avoidance of cardiopulmonary bypass. Moreover, cannulation of the interatrial groove or the dome of the left atrium also eliminates the need for complete dissection of the left side of the heart and thereby has the potential to decrease the surgical time, reduce intraoperative blood loss, and eliminate the potential for injury of patent bypass grafts. Our preferred site for left atrial cannulation has been the dome of the left atrium as described by Jett.2 Advantages of this approach are that it requires minimal dissection, no manipulation of the heart is required, and removal of the cannula is quite easily achieved. Also, if there is bleeding from the cannulation site, it can be readily controlled and repaired, again, without the need for manipulation of the heart. One potential disadvantage of cannulation through the dome of the left atrium is compression of right-sided saphenous vein grafts especially when the vein graft comes off of the aorta anteriorly.2 However, if the saphenous vein graft comes off of the right side of the aorta, then the graft will be well away from the cannula. Our experience has been that the cannula or vein graft can be positioned to avoid compression if one is aware of this potential hazard. Our second choice for left atrial cannulation
EXTRACORPOREAL ASSIST DEVICES
has been posterior to the interatrial groove between the right superior and inferior pulmonary veins. Although this does require some manipulation of the heart and dissection of the interatrial groove, we have been able to consistently place adequate size cannulas and achieve adequate inflow drainage through this approach. Bleeding at the time of decannulation is a potential problem and can often be difficult to control. A potential disadvantage of left atrial cannulation is the development of thrombus around the cannula adjacent to the wall of the left atrium, which can be dislodged at the time of decannulation with the potential for a significant thromboembolic event. Another potential disadvantage of left atrial cannulation is stasis of blood in the left ventricle with clot formation and a potential for an increased incidence of thromboembolic events. The extracorporeal devices have received approval for use both as a bridge to transplant and for postcardiotomy support. When faced with patients that are unable to be weaned from cardiopulmonary bypass (CPB), we always prefer placement of an extracorporeal device. In these situations it is often of paramount importance to rapidly institute mechanical circulatory support because these patients invariably have been on CPB for prolonged periods with its attendant sequelae. This is best achieved with expeditious placement of a Thoratec or an ABIOMED device. Perhaps the most significant advantage of the extracorporeal devices is the ability to provide biventricular support. With improved patient management techniques and the use of inhaled nitric oxide, the percentage of patients that require biventricular support has significantly decreased, although a subset of patients will still require biventricular support because of severe right ventricular failure or refractory arrhythmias. Many centers have had patients successfully supported on biventricular devices despite prolonged periods of ventricular fibrillation. The significant disadvantage of the extracorporeal devices is the decreased patient mobility and inability to be discharged from the hospital. With the ABIOMED BVS 5000 system, the patient is essentially restricted to bed rest and any significant change of position will potentially have a significant effect on VAD function. Patients on the Thoratec VAD system are able to ambulate and
29 perform exercise on stationary machines. However, their mobility is quite limited by the large size of the drive console. Recently, a CE Mark was granted in Europe on the Thoratec TLC-II portable VAD driver. The Thoratec TLC-II portable VAD driver is a briefcase-size battery-powered pneumatic control unit that weighs 9.1 kg3,4 (Figs 1 and 2). The device can either be carried by hand with a shoulder strap or pushed with a mobile cart. Food and Drug Administration (FDA) clinical trials are now underway in the United States. Obviously, the limited mobility has precluded these patients from being discharged to home and, therefore, these patients are sentenced to confinement in a hospital setting until an appropriate donor heart is identified.
The Thoratec Left Ventricular Assist System The Thoratec VAD system is a pneumatic assist device that uses displacement of air to compress and empty a blood pumping sac (Figs 3 and 4). An extracorporeal prosthetic ventricle is used that consists of a seamless polyurethane blood sac enclosed within rigid casings. Tilting disc inlet and outlet valves provide for unidirectional blood flow. A thin tubing carries compressed air from the console to the pump’s air chamber. Adjustments can be made both in the magnitude of the drive pressure required to achieve systole and also the amount of negative pressure (ie, vacuum) that can be used to assist filling during VAD diastole. A Thoratec VAD has a stroke volume of 65 mL and is capable of delivering flow rates of 6.5 L/min. Appropriate emptying of the LVAD requires a drive line pressure of approximately 200 mm Hg or 75 to 100 mm Hg above the patient’s systolic blood pressure. In addition to an adequate systolic ejection time (⬎300 ms), we have found that optimal LVAD filling is achieved when the vacuum is set to ⫺30 to ⫺40 mm Hg. Inflow to the LVAD is achieved through a cannula placed either in the apex of the left ventricle or in the left atrium. Our standard surgical approach for the placement of a Thoratec LVAD or biventricular device is through a median sternotomy. A sternal retractor is placed with the cross bar toward the patient’s head to allow easy
30
DOWLING AND ETOCH
Fig 1. The Thoratec TLC-II portable VAD driver unit is a briefcase-size battery-powered pneumatic control unit that weighs 9.1 kg. Courtesy of Thoratec Laboratories Corporation.
access to the subcostal area for placement of the VAD cannulas and multiple chest tubes. After creation of a pericardial cradle, the VAD cannula sites are chosen on the left/right abdominal wall. Care is taken to ensure that the cannulas will not lie adjacent to the ribs. The distance between the midpoint of the inflow and outflow of the VAD is 4 cm and, therefore, 2 small circular ellipses of skin are excised 4 cm apart and tunnels for the VAD cannulas are created. Heparin is given after creation of the cannula tunnels. Aortic and atrial pursestring sutures are then placed but cannulation is not performed at this time. Rather, the outflow conduit from the LVAD is sewn end-toside to the aorta by using a partial side-biting clamp. Careful placement of this clamp has not resulted in any worsening of the hemodynamics even in this patient population. Outflow from the Thoratec device is achieved through a cannula that is made of Dacron (Ethibon; US Surgical, Norwalk, CT) woven polyester and is 14 to 18 mm. The outflow cannula is sewn end-to-side to the aorta or the pulmonary artery. Position of the outflow cannula on the aorta is often dictated by the presence of previously placed bypass grafts or cannulation sites. In patients that have not had previous surgery, our approach has been to place the outflow cannula lower on the aorta so it can
be entirely removed at the time of heart transplantation. Similarly, we attempt to position the outflow cannula of the pulmonary artery relatively proximal with the only caveat that too proximal of a position may injure or distort the pulmonary valve. After completing the anastomosis of the outflow conduit to the aorta, a cross-clamp is placed across the outflow conduit, the partial clamp is removed, and hemostasis is obtained from the aortic suture line. A pledgeted 4-0 mattress suture is placed at the highest point of the outflow conduit for later placement of a root vent, which will facilitate deairing. Aortic and atrial cannulation is then performed and CPB is initiated. The apex of the left ventricle is elevated in a routine manner. A 14-French coring knife is used to remove a circular core of tissue at the LV apex. Twelve pledgeted Ethibon sutures (US Surgical) are placed full thickness circumferentially around the ventriculotomy and then through the sewing flange of the inflow conduit. After the conduit is seated and the sutures are tied, a second running suture is placed completely around the suture line and a felt strip is used. This is performed to provide hemostasis and aerostasis. Recently, we have been placing tissue glue at this site, also for hemostasis and aerostasis. The VAD is filled with saline. Then with partial
EXTRACORPOREAL ASSIST DEVICES
31 this level may impair drainage of the SVC and may make placement of a pulmonary artery catheter from a cephalad approach difficult, if not impossible (Fig 5). Recently, we have placed the inflow cannula of the RVAD into the right ventricle as described by Arabia et al.5 We have found this to be a straightforward technique that has resulted in more consistent filling of the RVAD. There are 3 modes of support with the Thoratec VAD; volume or fill mode, R wave synchronous mode, and asynchronous mode. The asynchronous mode is most commonly used at the time of implantation for deairing and can also be used at lower rates in conjunction with echocardiographic monitoring to determine if there has been
Fig 2. The Thoratec TLC-II portable VAD driver unit can either be carried by hand with a shoulder strap or pushed with a mobile cart as shown in this figure. Courtesy of Thoratec Laboratories Corporation.
occlusion of venous return, the inflow cannula is connected to the VAD. This results in blood flow through the VAD, which is collected in a kidney basin and returned to the bypass circuit via a pump sucker. The outflow cannula is then filled retrograde with blood from the aorta and connected to the VAD. The aortic root vent is placed in the outflow conduit and extensive deairing maneuvers are performed with echocardiographic guidance to ensure complete absence of intracavitary air. After adequate deairing, the cross-clamp is removed from the outflow conduit and the patient is weaned from CPB. Atrial cannulation for a Thoratec right ventricular assist device (RVAD) should be at the mid-to-lower portion of the right atrium. Cannulation of the atrium above
Fig 3. Thoratec VAD. Courtesy of Thoratec Laboratories Corporation.
32
DOWLING AND ETOCH
Clinical Results of the Thoratec VAD System
Fig 4. Thoratec dual drive console. Courtesy of Thoratec Laboratories Corporation.
any evidence of LV recovery. The R wave synchronous mode allows timing of the VAD systole during natural heart diastole. The VAD can be programmed to eject with every diastole in a 1 to 2 mode or in a 1 to 3 mode similar to an intra-aortic balloon pump.6 We have not found this mode to be clinically useful. The fill mode or volume mode is the mode most commonly used after implantation when weaning from the device is not being considered. This mode offers optimal ventricular support and more complete washing of the blood sac, thereby theoretically decreasing the risk of thrombus formation.
The first clinical use of the Thoratec LVAD as a bridge to transplant was in September 1984 at the Pacific Presbyterian Medical Center in San Francisco.7 The device was used for left ventricular support in a 40-year-old man who had a massive myocardial infarction. This patient subsequently underwent heart transplantation and was the first patient to be discharged from the hospital after a bridge to transplant procedure. The Thoratec VAD system was also the first system to be used in a successful bridge to transplantation in a patient requiring biventricular support. Laman Gray, Jr, implanted both an RVAD and an LVAD in a 16-year-old boy with end-stage viral cardiomyopathy at Jewish Hospital in Louisville, KY, in March 1985.7 This patient also underwent successful transplantation and was discharged to home and is currently doing well 14 years after his heart transplant. The first multicenter report on the Thoratec VAD system as a bridge to transplant appeared in the New England Journal of Medicine in 1988. Twenty-nine patients underwent VAD placement, 21 survived to undergo heart transplantation, and 19 were discharged alive.8 In 1990, Farrar et al7 reported on 72 heart transplant candidates that had received Thoratec devices at 20 medical centers in 5 countries. Fifty-eight (81%) of the patients received a biventricular device and 14 (19%) received LVAD support only. Fifty-four (75%) patients recovered sufficiently to undergo heart transplant with an average duration of support of 4.4 days with a range of 8 hours to 81 days. Forty-five percent of patients were discharged from the hospital for a 63% overall survival rate from implant to discharge. They also showed that the actual 1- and 2-year posttransplant survival rates were similar to that after conventional heart transplantation. In 1993, Farrar and Hill9 reported on the use of the Thoratec VAD system in 154 transplant candidates. Biventricular support was used in 120 patients and isolated LVAD support was used in 34 patients. Average duration of support was 17.5 days with a range of 8 hours to 226 days. Sixty-five (48%) patients recovered and were able to undergo heart transplantation. The overall survival rate for the
EXTRACORPOREAL ASSIST DEVICES
33
Fig 5. Standard cannulation sites for the Thoratec VAD system. Alternative sights for left atrial cannulation are discussed in the text as is cannulation of the right ventricle for inflow to an RVAD. Courtesy of Thoratec Laboratories Corporation.
group was 54%. This study, again, showed that the survival rate 1 year after transplant is comparable with that seen after conventional nonbridged patients. This has also been shown by Pifarre et al.10 In a review of success rates of long-term VADs used as a bridge to transplantation, Arabia et al11 reported a success rate of 93% with the Thoratec LVAD and 81% with a Thoratec biventricular assist device (BIVAD). These data clearly show that bridge to transplant results were as good with the Thoratec VAD system as with any other available system. The decreased survival rate with patients requiring BIVAD reflects the need for biventricular support in patients with increasing severity of illness and organ dysfunction.
The ABIOMED BVS-5000 VAD The ABIOMED BVS-5000 is an external asynchronous pulsatile device capable of providing shortterm univentricular or biventricular circulatory support. There are 3 components to the system.
The pneumatic drive console, a single-use dual chamber blood pump, and the inflow and outflow cannulas (Fig 6). The pump is a dual chamber pump contained in a hard polycarbonated housing. The upper chamber is a passive gravity-filled reservoir (atrial chamber) and the lower chamber is the (ventricular) pumping chamber. Each chamber contains a smooth surface polyurethane bladder with a volume of 100 mL. There is a trileaflet polyurethane valve between the upper and lower chamber (inflow valve) and between the lower chamber and the outflow conduit (outflow valve). The atrial chamber fills throughout both pump systole and diastole, which ensures continuous drainage to the VAD (Fig 7). There is no vacuum and, therefore, filling of the VAD is merely by gravity. Filling can be altered by adjusting the level of the pump relative to the level of the patient. As the pumping chamber is filled with blood, all of the surrounding air is returned to the console. When the volume of displaced air indicates that the pumping chamber is full, the console immediately sends compressed air back
34
DOWLING AND ETOCH
Fig 6. (A) Schematic of the ABIOMED BVS 5000 console and blood pump set. (B) BVS 5000i and 5000t backup/transport consoles. Use by permission of ABIOMED.
to the pumping chamber, resulting in VAD systole. The drive console operates independently of the native cardiac rhythm. Control systems contained in the drive console automatically adjust the duration of pump systole and diastole to compensate for changes in preload and afterload. The control systems maintain a relatively constant stroke volume of approximately 88 mL. Beat rate of the VAD and VAD output are displayed on the console. The system operation is fully automated and does not require operator input during normal operation. A separate control is included to allow weaning from the device. These controls allow the operator to rapidly go to a manual mode in which the VAD output can be set at a fixed rate. The outflow cannula consists of a preclotted 14mm woven Dacron graft with an external Dacron velour sleeve to promote tissue incorporation at the exit site. The small size of the outflow conduit is an advantage as is the lack of bleeding through the graft interstices.12 As noted previously, early clinical experience and traditional teaching has been that the inflow cannulas are placed either through the lateral wall of the right atrium for an RVAD or in the left atrium for an LVAD. More recently, cannulation of the right ventricle on the diaphramatic surface between the posterior descending artery (PDA) and the acute margin of the heart has been performed with excellent RVAD filling. Use of the atrial cannulas to cannulate the LV apex has also been performed with
excellent LVAD function (E. Summers, personal communication, October, 1998). If replacement of the VAD is necessary due to thrombus formation, this procedure can be performed either at the bedside or in the operating room and can usually be performed in under 1 minute.
Clinical Results of the ABIOMED BVS-5000 System The first clinical use of the ABIOMED BVS-5000 system was by Dreyfus et al.13 They reported successfully bridging 2 patients to transplant for 5 and 21 days with VAD outputs ranging from 4.3 to 5.1 L/min. They were able to identify the key advantages to the system: (1) the heterotopic position allowed for rapid implantation without the need for cardiopulmonary bypass, (2) the ability to provide biventricular support, (3) automatic regulation of the device without the need for operator input, and (4) acceptable cost. After this report, multiple other centers reported their experience with successful bridge to transplantation. In 1994, Gray and Champsaur14 reported on the world-wide registry experience with the ABIOMED BVS system as both a bridge to transplant and for postcardiotomy support. Four hundred and twenty patients had undergone placement of the BVS system between February 1994 and June 1997. Indications for device implantation were postcardiotomy shock in 211 patients,
35
EXTRACORPOREAL ASSIST DEVICES
Fig 7. Cross section of the ABIOMED BVS 5000 system showing the diastolic and systolic phases of the assist device. Use by permission of ABIOMED.
cardiomyopathy in 94 patients, acute myocardial infarction and cardiogenic shock in 44 patients, failed orthotopic heart transplant in 45 patients, and multiple other indications in 26 patients. Sixty-five percent of patients had biventricular support, 29% had placement of LVAD alone, and 5% had placement of RVAD alone. Mean length of support was 5.2 days. In the postcardiotomy group, 85% of patients were weaned from support or bridged to transplant with 27% of these patients being discharged from the hospital. Seventy percent of the cardiomyopathy patients were successfully bridged to transplant with 58% of this entire group being discharged to home. Of the patients with acute myocardial infarction, 52% underwent transplantation with 70% of these patients being discharged to home. In 1993, Guyton et al15 reported on the results of the multicenter evaluation of the use of the BVS 5000 system for postcardiotomy shock. Fifty-five patients who were hemodynamically unstable despite maximal pharmacological and intra-aortic balloon therapy, and were less than 6 hours from the first attempt to wean from cardiopulmonary bypass, were enrolled in the study. The BVS
system was successful in restoring hemodynamics as evidenced by significant improvements in arterial pressure, cardiac index, and left-sided filling pressure. Of the 31 patients who met all of the appropriate criteria for enrollment in the study, 17 (55%) were weaned from support and 9 (29%) were discharged. Survival in patients that did not experience a cardiac arrest before placement of the device was 47%. One patient was bridged to a long-term device before transplantation. Overall survival rate to heart transplantation and subsequent hospital discharge was 62%. As the number of patients with intractable heart failure continues to rise so does the need for mechanical circulatory assist devices. Although patient mobility with implantable LVADs is desirable, there remains a subset of patients that will require biventricular support or whose likelihood of survival will be significantly increased if surgical trauma can be minimized. It is for these patients that extracorporeal assist devices remain an important tool in the armamentarium to treat end-stage heart disease.
References 1. Holman WL, Bourge RC, Murrah CP, et al: Left atrial or ventricular cannulation beyond 30 days for a thoratec ventricular assist device. ASAIO 41:M517-M522, 1995 2. Jett GK: Atrial cannulation for left ventricular assistance: Superiority of the dome approach. Ann Thorac Surg 61:1014-1015, 1996 3. Farrar D, Buck K, Coulter J, et al: Portable pneumatic biventricular driver for the thoratec ventricular assist device. ASAIO 43:M631-M634, 1997 4. Von Segesser L, Tkebuchava T, Leskosek B, et al: Biventricular assist using a portable driver in combination with implanted devices: Preliminary experience. Artif Organs 21:72-75, 1997 5. Arabia F, Paramesh V, Toporoff B, et al: Biventricular cannulation for the thoratec ventricular assist device. Ann Thorac Surg 66:2119-2120, 1998 6. Ley SJ, Hill JD: Thoratec ventricular assist device, in Quall S (ed): Cardiac Mechanical Assistance Beyond Balloon Pumping. St Louis, MO, Mosby-Year Book, 1993 7. Farrar D, Lawson J, Litwak P, et al: Thoratec VAD system as a bridge to heart transplantation. J Heart Transplant 9:415-423, 1990 8. Farrar D, Hill J, Gray L Jr, et al: Heterotopic prosthetic ventricles as a bridge to cardiac transplantation: A
36 multicenter study in 29 patients. N Engl J Med 318:333340, 1988 9. Farrar D, Hill J: Univentricular and biventricular thoratec VAD support as a bridge to transplantation. Ann Thorac Surg 55:276-282, 1993 10. Pifarre R, Sullivan H, Montoya A, et al: Comparison of results after heart transplantation: Mechanically supported versus nonsupported patients. J Heart Lung Transplant 11:235-239, 1992 11. Arabia F, Smith R, Rose D, et al: Success rates of long-term circulatory assist devices used currently for bridge to heart transplantation. ASAIO 42:M542M546, 1996
DOWLING AND ETOCH 12. Shook B: The Abiomed BVS 5000 biventricular support system. J Card Surg 7:309-316, 1993 13. Dreyfus G, Guillemain R, Couetel J, et al: First clinical use of a new biventricular bridging support: The Abiomed’s BVS system 5000. J Heart Transplant 7:84, 1988 14. Gray LA, Champsaur GG: The BVS 5000 biventricular assist device—the Worldwide Registry Experience. ASAIO 40:M460-M464, 1994 15. Guyton RA, Schonberger JP, Everts PA, et al: Postcardiotomy shock: Clinical evaluation of the BVS 5000 biventricular support system. Ann Thorac Surg 56:346356, 1993