Ventricular assist device explained

Ventricular assist device

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.[1] [2]

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,[3] 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.[4] 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.[5]

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).[6] 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.[7] [8]

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.[9]

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.[10] 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,[11] which mimic the natural pulsing action of the heart, and continuous-flow pumps.[12] Pulsatile VADs use positive displacement pumps.[13] [14] [15] In some pulsatile pumps (that use compressed air as an energy source[16]), 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.[17] 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.[18]

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")[19] [20] [21] 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.[22] 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.[23] 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.[24]

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,[25] or that the pulse intensity will be seriously reduced.[26]

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.[27] [28] 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.[29]

Studies and outcomes

Recent developments

The majority of VADs on the market today are somewhat bulky. The smallest device approved by the FDA, the HeartMate II, weighs about 1lb and measures 3inches. This has proven particularly important for women and children, for whom alternatives would have been too large.[44] As of 2017, HeartMate III has been approved by the FDA. It is smaller than its predecessor HeartMate II and uses a full maglev impeller instead of the cup-and-ball bearing system found in HeartMate II.[45]

The HeartWare HVAD works similarly to the VentrAssist—albeit much smaller and not requiring an abdominal pocket to be implanted into. The device has obtained CE Mark in Europe, and FDA approval in the U.S. The HeartWare HVAD could be implanted through limited access without sternotomy, however in 2021 Medtronic discontinued the device.[46]

In a small number of cases left ventricular assist devices, combined with drug therapy, have enabled the heart to recover sufficiently for the device to be able to be removed (explanted).[7] [8] Several surgical approaches, including interventional decommissioning, off-pump explantation using a custom-made plug and complete LVAD removal through redo sternotomy, have been described with a 5-year survival of up to 80%.[47]

HeartMate II LVAD pivotal study

A series of studies involving the use of the HeartMate II LVAD have proven useful in establishing the viability and risks of using LVADs for bridge-to-transplantation and destination therapy.

HARPS

The Harefield Recovery Protocol Study (HARPS) is a clinical trial to evaluate whether advanced heart failure patients requiring VAD support can recover sufficient myocardial function to allow device removal (known as explantation). HARPS combines an LVAD (the HeartMate XVE) with conventional oral heart failure medications, followed by the novel β2 agonist clenbuterol. This opens the possibility that some advanced heart failure patients may forgo heart transplantation.[56]

REMATCH

The REMATCH (Randomized Evaluation of Mechanical Assistance for the Treatment of Congestive Heart Failure) clinical trial began in May 1998 and ran through July 2001 in 20 cardiac transplant centers around the USA. The trial was designed to compare long-term implantation of left ventricular assist devices with optimal medical management for patients with end-stage heart failure who require, but do not qualify to receive cardiac transplantation. As a result of the clinical outcomes, the device received FDA approval for both indications, in 2001 and 2003, respectively.[57]

According to a retrospective cohort study comparing patients treated with a left ventricular assist device versus inotrope therapy while awaiting heart transplantation, the group treated with LVAD had improved clinical and metabolic function at the time of transplant with better blood pressure, sodium, blood urea nitrogen, and creatinine. After transplant, 57.7% of the inotrope group had kidney failure versus 16.6% in the LVAD group; 31.6% of the inotrope group had right heart failure versus 5.6% in the LVAD group; and event-free survival was 15.8% in the inotrope group versus 55.6% in the LVAD group.[58]

Complications and side effects

There are a number of potential risks associated with VADs. The most common of these are bleeding events, stroke, pump thrombosis, and infections.[59]

Bleeding

Because the VADs generally result in blood flowing over a non-biologic surface (e.g. metal, synthetic polymers, etc.) this can result in formation of blood clots, also referred to as thrombosis. Due to these clotting abnormalities, anticoagulation medications are used to decrease the risk of thrombosis. One device, the HeartMate XVE, is designed with a biologic surface derived from fibrin and does not require long term anticoagulation (except aspirin); unfortunately, this biologic surface may also predispose the patient to infection through selective reduction of certain types of leukocytes, however this device was phased out of use starting in 2009 in favor of newer devices.[60] [61]

Due to the use of anticoagulation, bleeding is the most common postoperative early complication after implantation or explantation of VADs, necessitating reoperation in up to 60% of recipients.[62] [63] Most commonly bleeding occurs in the gastrointestinal tract resulting in dark or bright red stools,[64] however if trauma to the head occurs, intracranial bleeding may also occur.[65] Bleeding events may require massive blood transfusions and incur certain risks including infection, pulmonary insufficiency, increased costs, right heart failure, allosensitization, and viral transmission, which can prove fatal or preclude transplantation. When bleeding occurs, it impacts the one year Kaplan-Meier mortality. In addition to complexity of the patient population and the complexity of these procedures contributing to bleeding, the devices themselves may contribute to the severe coagulopathy that can ensue when these devices are implanted.[66]

Ischemic Stroke and Pump Thrombosis

In patients with VADs, ischemic strokes and pump thrombosis occur when there is inadequate anticoagulation to counter act the blood's tendency to form blood clots when exposed to the foreign materials in a VAD. Stroke risk varies based on the type of VAD in place and other risk factors.[59] Both atrial fibriliation and high blood pressure may increase risk of stroke and high blood pressure can increase a patient's risk of stroke in the setting of VAD use.[67] However, it is difficult to measure blood pressure in LVAD patients using standard blood pressure monitoring and the current practice is to measure by Doppler ultrasonography in outpatients and invasive arterial blood pressure monitoring in inpatients.[68]

Infections

Infections in VAD patients occur because the artificial surfaces of the devices serve as a surface for bacterial and or fungal growth.[69] Most infections are classified as driveline infections, which are infections that occur where the device's power cord enters the skin (usually in the upper abdomen)[69]

VAD-related infection can be caused by a large number of different organisms:[70] [69]

Other immune system related problems include immunosuppression. Some of the polyurethane components used in the devices cause the deletion of a subset of immune cells when blood comes in contact with them. This predisposes the patient to fungal and some viral infections necessitating appropriate prophylactic therapy.[71]

Considering the multitude of risks and lifestyle modifications associated with ventricular assist device implants,[72] it is important for prospective patients to be informed prior to decision making.[73] In addition to physician consult, various Internet-based patient directed resources are available to assist in patient education.[74] [75]

List of implantable VAD devices

This is a partial list and may never be complete
Referenced additions are welcome

DeviceManufacturerTypeApproval Status as of July 2010
HeartAssist5ReliantHeartContinuous flow driven by an axial flow rotor.Approved for use in the European Union. The child version is approved by the FDA for use in children in USA. Undergoing clinical trials in USA for FDA approval.
NovacorWorld HeartPulsatile.Was approved for use in North America, European Union and Japan. Now defunct and no longer supported by the manufacturer. (HeartWare completed acquisition August 2012)
HeartMate XVEThoratecPulsatileFDA approval for BTT in 2001 and DT in 2003. CE Mark Authorized. Rarely used anymore due to reliability concerns.
HeartMate IIThoratecRotor driven continuous axial flow, ball and cup bearings.Approved for use in North America and EU. CE Mark Authorized. FDA approval for BTT in April 2008. Recently approved by FDA in the US for Destination Therapy (as at January 2010).
HeartMate IIIThoratecContinuous flow driven by a magnetically suspended axial flow rotor.Pivotal trials for HeartMate III started in 2014 and supported with CarewMedicalWear. FDA approval for BTT in 2017
IncorBerlin HeartContinuous flow driven by a magnetically suspended axial flow rotor.Approved for use in European Union. Used on humanitarian approvals on a case-by-case basis in the US. Entered clinical trials in the US in 2009.
Excor PediatricBerlin HeartExternal membrane pump device designed for children.Approved for use in European Union. FDA granted Humanitarian Device Exemption for US in December 2011.
Jarvik 2000Jarvik HeartContinuous flow, axial rotor supported by ceramic bearings.Currently used in the United States as a bridge to heart transplant under an FDA-approved clinical investigation. In Europe, the Jarvik 2000 has earned CE Mark certification for both bridge-to-transplant and lifetime use. Child version currently being developed.
MicroMed DeBakey VADMicroMedContinuous flow driven by axial rotor supported by ceramic bearings.Approved for use in the European Union. The child version is approved by the FDA for use in children in USA. Undergoing clinical trials in USA for FDA approval.
VentrAssistVentracor[76] Continuous flow driven by a hydrodynamically suspended centrifugal rotor.Approved for use in European Union and Australia. Company declared bankrupt while clinical trials for FDA approval were underway in 2009. Company now dissolved and intellectual property sold to Thoratec.
MTIHeartLVADwww.mitiheart.comContinuous flow driven by a magnetically suspended centrifugal rotor.Currently in animal testing, recently completed successful 60-day calf implant.
C-Pulse (Now "Aquadex")Sunshine Heart (Now "CHF Solutions")Pulsatile, driven by an inflatable cuff around the aorta.Currently available commercially
HVADHeartWare (now Medtronic)Miniature "third generation" device with centrifugal blood path and hydromagnetically suspended rotor that may be placed in the pericardial space.Obtained CE Mark for distribution in Europe, January 2009. Obtained FDA approval in the U.S., November 2012. Initiated US BTT trial in October 2008 (completed February 2010) and US DT trial in August 2010 (enrollment completed May 2012). FDA approval for BTT in 2012 and DT in 2017. Withdrawn from market in June 2021[77]
MVADHeartWareHeartWare's MVAD Pump is a development-stage miniature ventricular assist device, approximately one-third the size of HeartWare's HVAD pump.HeartWare Completed GLP Studies (September 2011).
DuraHeartTerumoMagnetically levitated centrifugal pump.CE approved, US FDA trials underway as at January 2010.
Thoratec PVAD (Paracorporeal Ventricular Assist Device) ThoratecPulsatile system includes three major components: Blood pump, cannulae and pneumatic driver (dual drive console or portable VAD driver).CE Mark Authorized. Received FDA approval for BTT in 1995 and for post-cardiotomy recovery (open heart surgery) in 1998.
IVAD—Implantable Ventricular Assist DeviceThoratecPulsatile system includes three major components: Blood pump, cannulae and pneumatic driver (dual drive console or portable VAD driver).CE Mark Authorized. Received FDA approval for BTT in 2004. Authorized only for internal implant, not for paracorporeal implant due to reliability issues.
FiVADLeviticus CardioVersatile wireless system for LVAD. Allow 6-hour of freedom to the patientsInvestigation device, 2 patients trial conduct in Dec 2018 with Jarvik 2000 LVAD in Astana by prof Pya.

See also

External links

Notes and References

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