The modern era of mechanical circulatory support can be traced back to 1953, when cardiopulmonary bypass was first used in a clinical setting and ushered in the possibility of brief periods of circulatory support to permit open-heart surgery. Subsequently, a variety of extracorporeal pumps to provide circulatory support for brief periods of time have been developed. The use of a mechanical device to support the circulation for more than a few hours initially developed slowly, with the implant of a total artificial heart in 1969 in Texas by Cooley. This patient survived for 60 hours until a donor organ became available, at which point he was transplanted. Unfortunately, the patient died of pulmonary complications after transplantation. The entire field of mechanical replacement of the heart took a decade-long hiatus until the 1980s, when total artificial hearts were reintroduced with much publicity; however, they failed to produce the hoped-for treatment of end-stage heart disease. Starting in the 1970s, parallel to the development of the total artificial heart, there was intense research in the development of ventricular assist devices, which provide mechanical assistance to (rather than replacement of) the failing ventricle (currently, newer versions of the total artificial heart are in preliminary clinical trials).
Although conceived of initially as alternatives to biologic replacement of the heart, LVADs were introduced as, and are still employed primarily as, temporary “bridges” to heart transplantation in candidates who begin to fail medical therapy before a donor heart becomes available. Several devices are approved by the U.S. Food and Drug Administration (FDA) and are currently in widespread use. Those that are implantable within the body are compatible with hospital discharge and offer the patient a chance for life at home while waiting for a donor heart. However successful such “bridging” is for the individual patient, it does nothing to alleviate the scarcity of donor hearts; the ultimate goal in the field remains that of providing a reasonable alternative to biologic replacement of the heart—one that is widely and easily available and cost-effective.
Current Indications and Applications of Ventricular Assist Devices
Currently, there are two major indications for long-term ventricular assistance. First, patients with chronic end-stage heart failure are eligible for mechanical support if they are at risk of imminent death from cardiogenic shock. Second, if patients have a left ventricular ejection fraction < 25%, peak VO2 < 14 mL/kg/min or are dependent on inotropic therapy or support with intra-aortic balloon counterpulsation, they may be eligible for mechanical support. If they are eligible for heart transplantation, the mechanical circulatory assistance is termed “bridge to transplantation.” By contrast, if the patient has a contraindication to heart transplantation, the device therapy is deemed to be “destination” left ventricular assistance therapy.
In the United States, there are currently four FDA-approved devices that are used as bridges to transplantation in adults. Of these four devices, one is also approved for use as destination therapy or long-term mechanical support of the heart. There are a number of other devices that are approved only for short-term support for post-cardiac surgery shock or for patients with cardiogenic shock secondary to acute myocardial infarction or fulminant myocarditis; these will not be considered here. None of the long-term devices as yet are totally implantable and, because of this need for transcutaneous connections, all share a common problem with infectious complications. Likewise, all share some tendency to thromboembolic complications as well as the expected possibility of mechanical device failure common to any machine.
The CardioWest total artificial heart (TAH) (Syncardia, Tucson, AZ) is a pneumatic, biventricular, orthotopically implanted total artificial heart with an externalized driveline connecting it to its console. It consists of two spherical polyurethane chambers with polyurethane diaphragms. Inflow and outflow conduits are constructed of Dacron and contain Medtronic-Hall (Medtronic, Inc., Minneapolis, MN) valves. It is currently the only FDA-approved device for use as a bridge to transplantation in patients who have severe biventricular failure.
The Thoratec LVAD (Thoratec Corp., Pleasanton, CA) is an extracorporeal pump that takes blood from a large cannula placed in the left ventricular apex and propels it forward through an outflow cannula inserted into the ascending aorta. The pump itself sits in the paracorporeal position on the abdomen and is attached to a device console cart with wheels, allowing for limited ambulation. The extracorporeal nature of this pump allows it to be used in small adults for whom intracorporeal pumps would be too large.
The Novacor LVAD (WorldHeart, Inc., Oakland, CA) also takes blood from the left ventricular apex through a cannula and propels it into the ascending aorta through a second cannula. With this device, the pump itself is placed in a surgically created pocket in the peritoneal fascia in the abdomen. A driveline that connects to the power source is tunneled subcutaneously and usually exits in the right upper quadrant of the abdomen.
The HeartMate XVE LVAD (Thoratec Corp., Pleasanton, CA) is an intracorporeal left ventricular assist device that has an externalized driveline. The pump sits in the anterior abdominal wall with cannulae that traverse across the diaphragm. There is a drainage cannula in the left ventricular apex, and the blood is expelled from the pump into the ascending aorta via a synthetic vascular graft. This device may be used as a bridge to transplantation and patients may be discharged from the hospital with this device to await transplantation. The HeartMate XVE LVAD is now one of two FDA-approved devices for destination therapy.
The HeartMate II LVAS (Thoratec Corp., Pleasanton, CA) similarly uses a drainage cannula in the left ventricular apex to drain blood into a small chamber, where the blood is driven by an electrically powered motor that spins a rotor, accelerating blood outflow into the ascending aorta (Figure 235-2). This device is currently the only FDA-approved axial-flow pump that can be used both as a bridge to transplantation and as destination therapy. There are several other axial-flow pumps currently being investigated. These devices have fewer moving parts than previous devices and provide non-pulsatile blood flow. All current axial-flow pumps continue to require transcutaneous connections to power the electric motor. Newer, third-generation devices, which also provide non-pulsatile flow, work through a different mechanism than the axial-flow pumps and are currently being investigated. These devices are even smaller than the currently available axial-flow pumps, and their mechanism of action involves less trauma to blood cells, which may result in better durability and decreased long-term complications.
Diagram of HeartMate II left ventricular assist device. (Reprinted with permission from Thoratec Corp., Pleasanton, CA.)
The use of these devices in the United States is limited mainly to patients with post-cardiac surgery shock and to those who are bridged to transplantation. The results of bridging to transplantation with the available devices are quite good, with nearly 75% of younger patients receiving a transplant by 1 year and having excellent posttransplant survival rates.
Publication of the REMATCH (Randomized Evaluation of Mechanical Assistance in the Treatment of Heart Failure) trial in 2001 documented a somewhat improved survival rate in nontransplant candidates with end-stage heart disease randomized to a HeartMate XVE LVAD (albeit with a high rate of complications, especially neurologic ones) as opposed to continued medical therapy; this led to renewed interest in also using the devices as non- biologic permanent replacement of heart function, as well as to FDA approval of one device for this indication. This outcome, in turn, led the ISHLT to initiate a Mechanical Circulatory Support database in 2002, which collects voluntary data from 60 international centers and contained data from 655 patients in its most recent publication. Only 12% of these patients had the device placed with the intention of permanent, or “destination,” use, with survival rates of only 65% at 6 months and 34% at 1 year.
Several studies have evaluated the benefit of LVAD therapy as a bridge to transplantation, with the most recent data taken from a series of 133 patients who underwent implantation of a HeartMate II device. In this group of patients, 80% achieved the principal outcome (defined as survival to transplantation, recovery of heart function, or ongoing device support) at 180 days. With increased experience and improved outcomes using LVADs as a bridge to transplantation, the ability to maintain end-organ function and limit the progression of pulmonary hypertension, or even decrease pulmonary vascular resistance, makes mechanical unloading an attractive option when compared with continued inotropic support. The early bridge-to-transplantation experience demonstrated reduced posttransplantation survival when compared with medical management; however, more recent experience has shown equivalent outcomes following transplantation. This result is likely secondary to a trend toward earlier device implantation, prior to the onset of irreversible end-organ damage.