A ventricular assist device (VAD) is an electromechanical device that provides support for cardiac pump function, which is used either to partially or to completely replace the function of a failing heart. VADs can be used in patients with acute (sudden onset) or chronic (long standing) heart failure, which can occur due to coronary artery disease, atrial fibrillation, valvular disease, and other conditions.
Categorization of VADs VADs may be used to manage a variety of cardiac diseases and can be categorized based on which ventricle the device is assisting, and whether the VAD will be temporary or permanent.
Ventricular Assistance
First, VADs can be categorized based on whether they are designed to assist the right ventricle (RVAD) or the left ventricle (LVAD) or to both ventricles (BiVAD). The type of VAD implanted depends on the type of underlying heart disease (e.g. patients with right ventricular failure from pulmonary arterial hypertension may require an RVAD, versus those with left ventricular failure from a myocardial infarction may require an LVAD). The LVAD is the most common device applied to a defective heart (it is sufficient in most cases; the right side of the heart is then often able to make use of the heavily increased blood flow), but when the pulmonary arterial resistance is high, then an (additional) right ventricular assist device (RVAD) might be necessary to resolve the problem of cardiac circulation. If both an LVAD and an RVAD are needed a BiVAD is normally used, rather than a separate LVAD and RVAD.
Duration
VADs can further be divided by the duration of their use (i.e. temporary versus permanent). Some VADs are for short-term use, typically for patients recovering from myocardial infarction (heart attack) and for patients recovering from cardiac surgery; some are for long-term use (months to years to perpetuity), typically for patients with advanced heart failure
Temporary use of VADs may vary in scale (e.g. days to months) depending on a patient's condition. Certain types of VADS may be used in patients with signs of acute (sudden onset) heart failure or cardiogenic shock as a result of an infarction, valvular disease, among other causes. In patients with acute signs of heart failure, small percutaneous (introduced to the heart through the skin into a blood vessel rather than through an incision) VADs such as the Impella 5.5, Impella RP, and others can be introduced to either the left or right ventricle (depending on the patient-specific needs) using a wire and that is introduced through the arteries or veins of the neck, axilla, or groin.
Long-term use of VADs may also vary in its scale (i.e. months to permanently). VADs that are intended for long term use are also termed "durable" VADS, due to their design to function for longer periods of time compared to short term VADs (e.g. Impella, etc.). The long-term VADs can be used in a variety of scenarios. First, VADs may be used as bridge to transplantation (BTT) – keeping the patient alive, and in reasonably good condition, and able to await heart transplant outside of the hospital. Other "bridges" include bridge to candidacy (used when a patient has a contraindication to heart transplantation but is expected to improve with the VADs support), bridge to decision (used to support a patient while their candidacy status is decided), and bridge to recovery (used until a patient’s native heart function improves after which the device would be removed). In some instances, VADs are also used as destination therapy (DT) which indicates that the VAD will remain implanted indefinitely. VADs as destination therapy are used in circumstances where patients are not candidates for transplantation and will thus rely on the VAD for the remainder of their life."First VentrAssist Heart Recovery Featured on National TV". Ventracor.com. 19 October 2006.
Other Cardiac Support Devices
Some devices are designed to support the heart and its various components/function but are not considered VADs, below are some common examples.
Pacemakers and Internal Cardiac Defibrillators (ICDs) – the function of a VAD differs from that of an artificial cardiac pacemaker in that a VAD pumps blood, whereas a pacemaker delivers electrical impulses to the heart muscle.
Total Artificial Heart – VADs are distinct from artificial hearts, which are designed to assume cardiac function, and generally require the removal of the patient's heart.
Extracorporeal Membrane Oxygenation (ECMO) – is a form of mechanical circulatory support typically used in critically ill patients in cardiogenic shock that is established by introducing cannula into the arteries and or veins of the neck, axilla or groin. Generally, a venous cannula pulls deoxygenated blood from the patient's veins into an oxygenating device at the patient's bedside, after which a motor powered pump moves the oxygenated blood is back to the body (either into a vein or the arterial system, typically the aorta). There are different ECMO configurations (venoarterial ECMO, venovenous ECMO, etc.) the end goal remains the same; to oxygenate blood and return it to the body. In this sense, the ECMO circuit bypasses one or both ventricles and is therefore not in contact with the patient's native ventricle and is generally not considered a type of VAD.
Design
Pumps
The pumps used in VADs can be divided into two main categories – pulsatile pumps, which mimic the natural pulsing action of the heart, and continuous-flow pumps. Pulsatile VADs use positive displacement pumps. In some pulsatile pumps (that use compressed air as an energy source), the volume occupied by blood varies during the pumping cycle. If the pump is contained inside the body then a vent tube to the outside air is required.
Continuous-flow VADs are smaller and have proven to be more durable than pulsatile VADs. They normally use either a centrifugal pump or an axial flow pump. Both types have a central rotor containing permanent magnets. Controlled electric currents running through coils contained in the pump housing apply forces to the magnets, which in turn cause the rotors to spin. In the centrifugal pumps, the rotors are shaped to accelerate the blood circumferentially and thereby cause it to move toward the outer rim of the pump, whereas in the axial flow pumps the rotors are more or less cylindrical with blades that are helical, causing the blood to be accelerated in the direction of the rotor's axis.
An important issue with continuous flow pumps is the method used to suspend the rotor. Early versions used solid bearings; however, newer pumps, some of which are approved for use in the EU, use either magnetic levitation ("maglev")Smart, Frank. "Magnetic levitation heart pump implanted in first U.S. patient". "Cardiology Today". October 2008. or hydrodynamic suspension.
History
The first left ventricular assist device (LVAD) system was created by Domingo Liotta at Baylor College of Medicine in Houston in 1962. The first LVAD was implanted in 1963 by Liotta and E. Stanley Crawford. The first successful implantation of an LVAD was completed in 1966 by Liotta along with Dr. Michael E. DeBakey. The patient was a 37-year-old woman, and a paracorporeal (external) circuit was able to provide mechanical support for 10 days after the surgery. The first successful long-term implantation of an LVAD was conducted in 1988 by Dr. William F. Bernhard of Boston Children's Hospital Medical Center and Thermedics, Inc. of Woburn, MA, under a National Institutes of Health (NIH) research contract which developed HeartMate, an electronically controlled assist device. This was funded by a three-year $6.2 million contract to Thermedics and Children's Hospital, Boston, MA, from the National Heart, Lung, and Blood Institute, a program of the NIH. The early VADs emulated the heart by using a "pulsatile" action where blood is alternately sucked into the pump from the left ventricle then forced out into the aorta. Devices of this kind include the HeartMate IP LVAS, which was approved for use in the US by the Food and Drug Administration (FDA) in October 1994. These devices began to gain acceptance in the late 1990s as heart surgeons including Eric Rose, O. H. Frazier and Mehmet Oz began popularizing the concept that patients could live outside the hospital. Media coverage of outpatients with VADs underscored these arguments.
More recent work has concentrated on continuous-flow pumps, which can be roughly categorized as either centrifugal pumps or axial flow impeller driven pumps. These pumps have the advantage of greater simplicity resulting in smaller size and greater reliability. These devices are referred to as second-generation VADs. A side effect is that the user will not have a pulse,Dan Baum. "No Pulse: How Doctors Reinvented the Human Heart". Popular Science. 2012. or that the pulse intensity will be seriously reduced."VAD FAQs". Columbia University Medical Center. quote: "A patient who is implanted with a HeartMate II usually has a dampened pulse." Viewed 2016-08-27.
A very different approach in the early stages of development was the use of an inflatable cuff around the aorta. Inflating the cuff contracts the aorta and deflating the cuff allows the aorta to expand – in effect the aorta becomes a second left ventricle. A proposed refinement is to use the patient's skeletal muscle, driven by a pacemaker, to power this device – which would make it truly self-contained. However, a similar operation (cardiomyoplasty) was tried in the 1990s with disappointing results.
At one time Peter Houghton was the longest surviving recipient of a VAD for permanent use. He received an experimental Jarvik 2000 LVAD in June 2000. Since then, he completed a 91-mile charity walk, published two books, lectured widely, hiked in the Swiss Alps and the American West, flew in an ultra-light aircraft, and traveled extensively around the world. He died of acute kidney injury in 2007 at the age of 69.Patient Sets World Record for Living with Heart Assist Device. Texas Heart Institute. 6 July 2007. Since then, patient Lidia Pluhar has exceeded Houghton's longevity on a VAD, having received a HeartMate II in March 2011 at age 75, and currently continues to use the device. In August 2007 the International Consortium of Circulatory Assist Clinicians (ICCAC) was founded by Anthony "Tony" Martin, a nurse practitioner (NP) and clinical manager of the mechanical circulatory support (MCS) program at Newark Beth Israel Medical Center, Newark, N.J. The ICCAC was developed as a 501c3 organization, dedicated to the development of best practices and education related to the care of individuals requiring MCS as a bridge to heart transplantation or as destination therapy in those individuals who don't meet the criteria for heart transplantation.
Studies and outcomes Recent developments In August 2007 The International Consortium of Circulatory Assist Clinicians (ICCAC) was founded by Anthony "Tony" Martin. A nurse practitioner (NP) and clinical manager of the mechanical circulatory support (MCS) program at Newark Beth Israel Medical Center, Newark, N.J.. In July 2009 in England, surgeons removed a donor heart that had been implanted in a toddler next to her native heart, after her native heart had recovered. This technique suggests mechanical assist device, such as an LVAD, can take some or all the work away from the native heart and allow it time to heal. In July 2009, 18-month follow-up results from the HeartMate II Clinical Trial concluded that continuous-flow LVAD provides effective hemodynamic support for at least 18 months in patients awaiting transplantation, with improved functional status and quality of life.