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Ann Thorac Surg 2001;71:S133-S138
© 2001 The Society of Thoracic Surgeons
a Department of Surgery, Baylor College of Medicine, Houston, Texas, USA
b MicroMed Technology, Inc, Houston, Texas, USA
Address reprint requests to Dr Noon, 6560 Fannin, Suite 1860, Houston, TX 77030
e-mail: gnoon{at}bcm.tmc.edu
Presented at the Fifth International Conference on Circulatory Support Devices for Severe Cardiac Failure, New York, NY, Sept 15–17, 2000.
Abstract
Background. The MicroMed DeBakey ventricular assist device (VAD) (MicroMed Technology, Inc, Houston, TX) is the first long-term axial flow circulatory assist device to be introduced into clinical trials as a bridge to transplantation. Clinical trials began in Europe in November 1998 and in the United States in June 2000.
Methods. To qualify for the study, the patients must be listed for cardiac transplantation and must have demonstrated profound cardiac failure. There were no exclusions to the MicroMed DeBakey VAD implant other than those patients who would typically be excluded from cardiac transplantation.
Results. As of September 2000, 51 patients have been implanted with the MicroMed DeBakey VAD. A detailed evaluation of the first 32 patients has been completed. With current data, the probability of survival at 30 days after VAD implant is 81%.
Conclusions. The clinical trial demonstrated that the MicroMed DeBakey VAD is capable of providing adequate circulatory support in patients with severe heart failure, sufficient to recover and return to normal activities while awaiting a heart transplantation. Much has been learned about the function of the device and its continuous flow in humans.
In 1988, a team of researchers from Baylor College of Medicine led by George P. Noon, MD, and Michael E. DeBakey, MD, met with engineers from NASA Johnson Space Center and began a comprehensive research process to develop a miniaturized axial flow blood pump for use as a ventricular assist device (VAD) (Houston, TX) in patients with end-stage heart failure. MicroMed Technology, Inc received the license for this technology in June 1996 and has continued to develop the MicroMed DeBakey VAD for commercial use.
The MicroMed DeBakey VAD human configuration and final pump design is depicted in Figure 1. The VAD system consists of four subsystems: (1) a pump system (Figs 2 and 3), (2) a controller system (Fig 4), (3) a Clinical Data Acquisition System (Fig 5), and (4) a Patient Home Support System (Fig 6).
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The controller is designed to operate the pump system. It is completely external and consists of the controller module, battery packs, and a battery charger. The 4 x 6 inch controller module has audible and visual alarms with messages, and prompts displayed on the controller module’s scrollable liquid crystal display (Fig 4). Two intelligent battery packs can be connected to the controller module to power the pump for up to 8 hours, or the controller may be connected to the Clinical Data Acquisition System or Patient Home Support System for power.
The VAD system is designed to be simple to operate by both the patient and clinician. During the development of the pump system, a VADPAK (Fig 1) was designed to provide the patient with a safe, ergonomic, small, and comfortable external transport mechanism.
The Clinical Data Acquisition System is a laptop-based system that stores pump-operating data, displays pump and physiologic information, and is used by the clinicians to stop and start the pump and to modify the pump speed (Fig 5). It also serves as a primary power source for the pump [1,2].
The Patient Home Support System (Fig 6) is a small unit that serves as a battery charger and provides wall power to the controller when the patient is at home or in the regular ward of the hospital. Additionally, it serves as an emergency battery backup for the patient at home in the event of a power outage.
Patients and methods
Patient selection
The MicroMed DeBakey VAD was the first long-term axial flow pump to be introduced into clinical trials as a bridge to transplantation. These trials began in Europe in November 1998 and in the United States in June 2000. Patients with advanced heart failure who were transplantation candidates and whose conditions were rapidly deteriorating were implanted in the clinical trial of the MicroMed DeBakey VAD. In general, to qualify for the study, the patients must have been listed for transplantation and have demonstrated profound cardiac failure. Cardiac failure was confirmed either by hemodynamics (elevated pulmonary capillary wedge pressure, low cardiac index, and other factors) or the need for extraordinary inotropic support including intra-aortic balloon pump. There were no exclusions to the MicroMed DeBakey VAD implant other than those that would typically exclude a patient from cardiac transplantation. The criteria used for the clinical trial were similar to criteria used during the clinical investigations of left ventricular assist devices currently on the market.
Patients with left ventricular failure requiring left ventricular assistance are often also in the preliminary stages of multiorgan failure. In many patients, end-organ dysfunction will require multiorgan support during and immediately after the implant surgery. A right ventricular assist device may be necessary for refractory right heart failure. If additional hemodynamic support and counterpulsation are necessary, an intraaortic balloon pump may be implemented. A continuous veno-venous hemofiltration or hemodialysis system may be necessary to correct fluid volume overload. Transesophageal or transthoracic echocardiogram is valuable for evaluating bilateral ventricular function, the presence of a patent foramen ovale, and visualizing air or clot in the left ventricle. Transesophageal echocardiography should also be used to evaluate the inflow cannula position in the left ventricle before closing the chest.
Implant procedure
For pump implant, a median sternotomy incision is performed extending several inches below the xiphoid process. A small abdominal wall pocket is formed below the rectus muscle. The size and configuration of the pocket is determined by measuring with the actual or mock pump as a model. To provide access to the left ventricular apex, the pericardium is opened, the diaphragmatic attachment to the costal margin is divided and both are extended laterally beyond the apex. Meticulous hemostasis is important.
The patient is heparinized in preparation for cardiopulmonary bypass. The ascending aorta is cannulated, then single or double cannulation of the right atrium and cava is performed depending upon presence of a patent foramen ovale. Cardiopulmonary bypass can begin when desired. If a patent foramen ovale is detected by echocardiogram or right atrial exploration, it is repaired before beginning the left ventricular assist device implant.
The left ventricular apex is elevated, the insertion site of the inflow cannula is selected, and an apical fixation ring is sewn to the apex. In preparation for insertion of the pump inflow cannula, the heart may remain beating, fibrillated, or arrested. A round bladed coring device is inserted into the left ventricle to extract a core of the left ventricular apex. Digital ventricular exploration is performed to evaluate a position of the core and to ensure absence of any potential obstruction of inflow. Visual exploration of the ventricle may be necessary for further removal of myocardium or clot. The pump outflow graft is clamped and the inflow cannula is inserted into the left ventricular apex.
Proper placement of the inflow cannula inside the ventricle is imperative. The goal is to place the inflow cannula so that it is angled toward the aortic valve without being directed toward the septum or free wall of the myocardium. The inflow cannula position can be adjusted by moving the body of the pump in the pocket. The cannula opening must be clear of any ventricular tissue. The apical insertion site is carefully checked for bleeding. A trocar is connected to the pump driveline to facilitate tunneling from the abdominal wall pocket, across the midline, to exit through the skin in a convenient position above the iliac crest.
The MicroMed DeBakey VAD is placed into the abdominal pocket and the length of the outflow graft is measured and trimmed. The graft should lie under the right sternal border without kinking or overstretching. After completion of the anastomosis to the ascending aorta, any remaining air in the system is removed through an 18-gauge needle placed in the outflow graft. Pumping is started at 7500 rpm and adjusted to maintain a pump index of approximately 2.0 L/min/m2 or greater. To maintain sufficient pump flow, it is important to ensure adequate preload. Hypovolemia and excessive pump speed could result in ventricular collapse, diminished flows, and possibly air emboli. Using echocardiography, the location of the inflow cannula is viewed and the ventricles are assessed for volume, function, and air. The inflow cannula must be free of obstruction. When placement of the cannula is noted to be satisfactory and flows are adequate, the patient is weaned from cardiopulmonary bypass and protamine is given to reverse heparin. After meticulous hemostasis, drains are placed in the mediastinum and the pump pocket and the incision is closed. The drive line exit site is approximated and the line secured in place with a suture [1, 2].
Patient management
For optimum pump and cardiac function it is important to maintain adequate preload, afterload, ventricular function, and cardiac rhythm. If right ventricular failure occurs in spite of medical management, a temporary or long-term right ventricular assist device is implemented. An intraaortic balloon device may also be inserted for temporary pulsatile flow and hemodynamic support, if desired, or for both. Coagulopathies are treated and the patient is not placed on anticoagulant therapy until postoperative bleeding is minimal and any coagulopathy is controlled, usually within 24 to 48 hours postimplant. The current recommendation for anticoagulation is to start the patient on intravenous heparin or subcutaneous low molecular weight heparin, then convert to coumadin, aspirin, and clopidogrel bisulfate.
Patients with a continuous flow pump may not have palpable pulses or blood pressures audible with a sphygmomanometer and stethoscope [1, 2]. Because the pulse is diminished, pulse oximeters may not measure peripheral oxygen saturation accurately. Indwelling arterial catheters or dopplers may be needed to evaluate blood flow, measure blood pressure and locate peripheral arteries.
Pump auscultation with a stethoscope can be used to ascertain pump function and give some indication of left ventricular contractility. This is judged by a steady versus undulating sound of pulsatility. The pulse rate can also be determined by a combination of pump and heart sounds.
Cardiac catheterization can be performed by percutaneous technique after the veins or arteries, or both, are identified. Crossing an aortic valve that is not opening may be difficult. Valve opening can be facilitated by slowing down pump flow, or by adding or increasing inotropic medications, or by a combination thereof. Because of its viscosity, injection of radiopaque contrast in the left ventricle will temporarily increase pump power and possibly decrease pump flow and speed as it passes through the pump. Thrombolytic therapy can be performed if pump thrombus is present.
Pump rpm and flow control can only be performed using the Clinical Data Acquisition System. During the implant operation and stabilization phase in the intensive care unit, changes in rpm are not uncommon. In ambulatory patients with a pump index in the range of 2.5 to 3 L/min/m2, pump rpm changes were not necessary during rest, exercise, or normal activities.
Results
As of September 2000, 51 patients (44 male, 7 female) have been implanted with the MicroMed DeBakey VAD. Four of the 51 patients have been implanted in the United States under an investigational device exemption approved clinical trial which began in June of 2000 and is conducted at the Methodist Hospital in Houston, Texas, by G.P.N. Fourteen patients have undergone successful transplantations. A detailed evaluation of the first 32 patients has been completed. Statistics presented here are for that subset of the MicroMed DeBakey VAD experience. Support duration ranged up to 133 days. Twenty-one patients were supported for more than 30 days, while 13 patients were supported for more than 60 days. The median time to transplant was 74.5 days with a median support duration of 47 days. The cumulative number of patient days of support was 1,876.
Fifty percent of the patients enrolled in the clinical trial had idiopathic dilated cardiomyopathies, while 38% had ischemic cardiomyopathies. On study entry, the mean cardiac index was 1.7 L/min/m2 with an average mean pulmonary artery pressure of 25 mm Hg. With current data, the probability of survival at 30 days after the MicroMed DeBakey VAD implant is 81%. Eleven of the 32 patients were transplanted and 10 of the 32 patients died on support. Only one death had a potential relationship to the device. Deaths in most patients occurred as a result of multiorgan failure, primarily in patients who were in early multiorgan failure requiring optimal medical support and intraaortic balloon pump prior to implant, or both, and who may not have recovered with any form of mechanical support.
Results of the European clinical trial have clearly shown the MicroMed DeBakey VAD to be capable of providing adequate circulatory support in patients with severe heart failure. Figure 7 illustrates the trend in total pump flow and pump index over 90 days of the MicroMed DeBakey VAD support. Pump flow ranged from 3.9 to 5.4 L/min. This flow was adequate to meet the perfusion needs of all patients, regardless of size, as shown by the pump index, which ranged from 2.5 to 2.8 L/min/m2. Figure 8 illustrates the speed, power, and current requirements for the device during this time and shows that adequate pump performance was maintained at relatively low energy costs.
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Comment
Many advantages of the MicroMed DeBakey VAD have been observed during the conduct of the clinical trial. Investigators have found the ease and reduced operation time for implantation and explantation of the device to be desirable when compared to larger pulsatile devices. Decreased operation times could result in fewer perioperative complications. Only 2 of the 32 patients with implantations have encountered significant perioperative bleeding. Many investigators have selected the MicroMed DeBakey VAD for implantation in patients with previous thoracic operations who have an increased risk of surgical bleeding. Investigators have found the miniature size of the MicroMed DeBakey VAD has allowed them to place the device in women or small-framed men who otherwise would have been saddled with an extracorporeal device. The majority of patients have been quickly mobilized postimplant. Several of the patients have participated in postimplant bicycle ergometer training programs and have been able to achieve workloads of greater than 50 watts without alteration of device settings. Several patients were able to spend weekends at home and make other trips outside of the hospital. One patient was moved to his home to await cardiac transplantation for nearly a month before a flu-like infection brought him back to the hospital.
The MicroMed DeBakey VAD is a continuous axial flow pump with an output that is determined by the pump’s rpm and the delta pressure between the inflow and outflow cannulation sites (Fig 11). At a fixed rpm, the pump output in patients varies depending upon left ventricular pressure changes during the cardiac cycle and the central aortic pressure. Continuous flow produced by the pump may be steady or pulsatile depending upon the delta pressure (Fig 12). In most patients on support there was some degree of pulsatile blood flow and pressure even when the aortic valve did not open.
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Most patients are stable when they have had the pump set at a fixed rpm providing a pump index of at least 2.5 L/min/m2. These patients were able to perform normal activities and exercise. An occasional patient would have a transient nocturnal decrease in flows that was related to hypovolemia.
In conclusion, data from the European Clinical Trial of the MicroMed DeBakey VAD supports safety and performance of the device. Results demonstrate that the device provides adequate left ventricular and circulatory support in patients with end-stage heart failure without unduly jeopardizing patient safety. Moreover, the device provides advantages not inherent to commercially available pulsatile devices: (1) miniature size enabling implantation in smaller patients, (2) ease of implantation, (3) reduction of surgical bleeding, (4) a low incidence of postoperative infections, and (5) less operative dissection at explantation. The MicroMed DeBakey VAD European Clinical Trial is the first demonstration of the compatibility of continuous blood flow with adequate tissue perfusion and overall maintenance of life so far for up to 6 months. This initial experience with the MicroMed DeBakey VAD suggests that the pump can provide circulatory support to bridge patients to cardiac transplantation and may provide an improved quality of life for the end-stage heart failure patient.
Footnotes
Dr Morley, Ms Abdelsayed, and Mr Benkowski are employees of MicroMed Technology, Inc, the company that manufactures the DeBakey VAD. Dr Noon has a small number of unexercised stock options in MicroMed Technology, Inc.
References
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