CORRESPONDENCE: RESEARCH CORRESPONDENCE
Safety and feasibility trial of the MicroMed DeBakey ventricular assist device as a bridge to transplantation
Daniel J. Goldstein, MD, FACC*,
Mark Zucker, MD, FACC,
Luis Arroyo, MD,
David Baran, MD, FACC,
Patrick M. McCarthy, MD,
Matthias Loebe, MD and
George P. Noon, MD, FACC
* Departments of Cardiothoracic Surgery, Newark Beth Israel Medical Center, 201 Lyons Avenue, Suite G5, Newark, NJ 07112 (Email: dgoldstein{at}sbhcs.com).
To the Editor:
Pulsatile ventricular assist device (VAD) support is an effective and widely used treatment strategy for the management of unstable cardiac transplant candidates and has recently been approved by the Food and Drug Administration as a suitable alternative treatment for patients who are not candidates for heart transplantation. The approval was borne out of results of the Randomized Evaluation of Mechanical Assistance in Treatment of Chronic Heart Failure trial, which demonstrated significant survival and quality-of-life benefits for recipients of the HeartMate VAD (Thoratec Corp., Pleasanton, California) as compared to an optimally medically treated cohort (1). This trial underscored the significant limitations of pulsatile technology, including the large size of the devices and the high incidence of device-related infection and malfunction.
In an effort to address several of these shortcomings, the MicroMed DeBakey VAD (MicroMed Technology Inc., Houston, Texas), an axial flow pump (2), has been undergoing clinical evaluation as a bridge to transplantation. The present study details the results of the recently completed safety and feasibility trial with this device in the U.S.
Study patients were in New York Heart Association functional class IV, were inotrope and/or intra-aortic balloon pump dependent, and were accepted as candidates for cardiac transplantation. The demographic and clinical profile of the patients is depicted in Table 1.
Details of the pump have been described in detail elsewhere (2). Briefly, the MicroMed DeBakey VAD is a miniaturized, implantable, electromagnetically actuated titanium pump with a single moving impeller. The rotating motion of the impeller produces continuous flow. An ultrasonic flow probe provides direct measurements of pump flow. Wiring from the pump and the flow probe exit the skin and connect to a portable controller.
Placement of the device was conducted under full cardiopulmonary bypass (CPB) support without cardioplegic arrest. In all cases, the VAD inflow was inserted into the left ventricular cavity via the apex, and the outflow was connected to the aorta.
Post-operatively, all patients received systemic anticoagulation therapy with intravenous heparin. Coumadin therapy was instituted with a target international normalized ratio of 2.5 to 3.5. Platelet inhibitors were administered to all patients.
Unpaired t tests were used to compare continuous variables between survivors and non-survivors. Contingency tables were used for categorical variables and compared in all instances with Fisher exact test. Univariate analysis was used to identify correlates of non-survival. A value of p  0.05 was considered significant.
All 30 patients survived the operation. Mean VAD support time was 42 ± 30 days (range 9 to 111 days). Cumulative support time was 42 patient-months.
Adverse events included reoperation for bleeding (n = 8, 27%), hemolysis (n = 3, 10%), device-related infection (n = 2, 6.7%), pump thrombus (n = 4, 13%), and stroke (n = 3, 10%). There was no device failure. Twenty patients (67%) were successfully bridged to transplantation, and 19 were well 30 days after transplantation.
Pulsatile VAD systems remain the gold standard for the treatment of advanced heart failure necessitating advanced mechanical support. Clinical experience with these devices (1,3,4) has underscored the limitations inherent to these large pumps— namely, the inability to support small patients, noise, and the high rates of device infection and malfunction.
Miniaturization of mechanical support technology has resulted in pumps capable of expanding the pool of patients that can be supported. Inherent in their design is a limited blood-contacting surface and the lack of valves, air vents, or compliance chambers. The MicroMed DeBakey VAD is the most widely tested of these new miniaturized devices.
Considerations for participation in the trial included degree of end-organ dysfunction, condition of the right ventricle, ability to tolerate chronic anticoagulation, expected transplantation waiting time, and the patient's desire to participate in the trial, among others.
End-organ perfusion was maintained, and despite continuous flow physiology, all recipients regained some pulsatility owing to improvement of the left ventricle with unloading.
Bleeding requiring reoperation was the most common adverse event seen. Hemolysis occurred rarely and was transient in all three instances. Remarkably, the incidence of significant device-related infection or malfunction, the two main limitations of pulsatile technology, was almost negligible. The low incidence of infection is likely due to the presence of a low-caliber flexible driveline that readily incorporates to the integument, less torque motion, and the absence of a large preperitoneal pocket.
Pump thrombus is a problem more often seen with axial flow technology and was identified in four patients. In this early experience, pump exchange and high doses of plasminogen activator were used for treatment. None of the patients who developed pump thrombus suffered a stroke or a clinically obvious peripheral embolus.
Not surprisingly, pre-operative dependence on intra-aortic balloon support and mechanical ventilation were strong univariate predictors of death before transplantation. Longer CPB time was also a predictor of death and likely reflects previous surgery, technical difficulties, excessive bleeding, and/or right heart dysfunction.
Patients were able to ambulate connected to their portable controller and battery packs, but otherwise were untethered. The nursing staff readily became proficient with idiosyncrasies of axial flow, including the lack of an arterial waveform or a palpable pulse and the treatment algorithms for abnormal wave patterns suggestive of ventricular suction.
The trial protocol required that patients remain hospitalized until a suitable donor organ became available. This explains the relatively short mean support time of 42 days observed in the trial. Overall success with bridging to transplantation was 67%, and 95% of these patients survived to 30 days after transplantation.
Skepticism surrounding the hemodynamics of continuous flow and the ability to maintain end-organ function has abated as clinical experience with axial flow pumps has grown. Indeed, a number of these devices, including the Jarvik Flowmaker (New York, New York) (5) and the Incor pump (Berlin, Germany) (6), are entering clinical trials.
In summary, the results of this first trial of an axial flow pump in the U.S. suggest that the MicroMed DeBakey VAD can provide effective circulatory support in patients with end-stage heart failure. Successful bridging to transplantation occurred with a rate similar to that currently achieved with time-tested pulsatile technology (4).
As a result of this initial feasibility trial, the FDA has approved an expanded multicenter evaluation of the MicroMed DeBakey pump as a bridge to transplantation.
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References
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- Rose EA, Gelijns A, Moskowitz AJ, et al. Long-term use of a left ventricular assist device for end-stage heart failure N Engl J Med 2001;345:1435-1443.[Abstract/Free Full Text]
- DeBakey M. A miniature implantable axial flow ventricular assist device Ann Thorac Surg 1999;68:637-640.[Abstract/Free Full Text]
- Goldstein DJ, Oz MC, Rose EA. Implantable left ventricular assist devices N Engl J Med 1998;339:1522-1533.[Free Full Text]
- Frazier OH, Rose EA, Oz MC, et al. Multicenter clinical evaluation of the HeartMate vented left ventricular assist system in patients awaiting transplantation J Thorac Cardiovasc Surg 2001;122:1186-1195.[Abstract/Free Full Text]
- Frazier OH, Myers TJ, Westaby S, Gregoric ID. Clinical experience with an implantable, intracardiac, continuous flow circulatory support device: physiologic implications and their relationship to patient selection Ann Thorac Surg 2004;77:133-142.[Abstract/Free Full Text]
- Hetzer R, Weng Y, Potapov EV, et al. First experiences with a novel magnetically suspended axial flow left ventricular assist device Eur J Cardiothorac Surg 2004;25:964-970.[Abstract/Free Full Text]
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