Interest in mechanical circulatory support (MCS) developed concurrently with interest in cardiopulmonary bypass and open-heart surgery in the 1950s. Success in the development of long-term MCS should be credited to the collaborative approach between the National, Heart, Lung and Blood Institute; industries; and cardiology and cardiac surgery programs in the 1960s. This collaborative relationship was the origin of the first "heart team." The lack of heart donors and contraindications to heart transplantation further stimulated the necessity for the heart team.

Patients with advanced heart failure needing MCS are severely limited with symptoms on minimal exertion or rest, with circulatory insufficiency, on inotropic support or awaiting transplantation. Based on American College of Cardiology (ACC)/American Heart Association (AHA) heart failure guidelines, these patients are in refractory heart failure with peak VO₂ of <14 ml.kg-1.min-1 and a six-minute walk distance of <300 m. Many of these patients are in the hospital on support.^1,2 The waiting time for heart transplantation can be long. Patients that are deemed ineligible for heart transplantation because of underlying medical conditions related to heart failure may become candidates due to the beneficial effects of MCS. This observation led to blunting of various definitions such as bridge to transplantation (BTT) and destination therapy (DT). The most important decision to be made is whether to choose a short-term temporary device or a long-term durable device.

Short-Term Devices

Temporary support is used to support patients through a high-risk procedure to recovery or to allow time to assess prognosis and guide definitive treatment.³ Patients are usually tethered when they are on support, except the few occasions during which they can be mobilized in the hospital using specific configurations.

They vary from simple to highly complex systems:

Table 1: Devices Available for Short-Term MCS

Device	Mechanism	Duration
IABP	Counterpulsation	Days
ECMO	CPB	Days-weeks
BVS5000, AB5000	Pulsatile	Weeks
Thoratec pVAD	Pulsatile	Weeks
CentriMag	Centrifugal	Weeks
TandemHeart	Centrifugal	Days
Impella	Axial flow	Days

1. The intra-aortic balloon pump (IABP) has a long history of supporting hemodynamics by diastolic pressure augmentation and improved coronary perfusion since the early 1960s. In heart failure management, the results are conflicting. It is still an effective first-line treatment.
2. Currently, temporary support is directed towards salvaging the myocardium to cardiac recovery or protecting the multiple organs by supporting the circulation towards cardiac replacement therapy ultimately. The devices commonly used are percutaneous interventional support, such as those described below:
   1. A continuous-flow centrifugal pump: The inflow is from left atrial cannula introduced transseptally, and the outflow is to the femoral artery.
   2. An axial flow pump: This is introduced transfemorally or trans-axillary across the aortic valve, and it pumps from the left ventricle to aorta.
   3. Other temporary devices are more complex but provide more complete support due to the patient's need. Some examples include the following: a pulsatile pneumatic device⁴ and a continuous-flow centrifugal pump used as para-corporeal MCS via sternotomy approach.⁵
   4. Extra-corporeal membrane oxygenation (ECMO) is employed to support both heart and lungs when there is cardiogenic shock and poor oxygenation even on ventilator support. This is frequently employed in children and in patients being resuscitated from cardiopulmonary arrest. Short duration and occurrence of multiple vascular and bleeding complications limit ECMO support. Adding a left ventricular assist device (LVAD) in the ECMO circuit helps to off-load the left ventricle. Short-term para-corporeal support towards cardiac replacement can be provided using pulsatile and non-pulsatile devices.

All of these para-corporeal devices require sternotomy and are often used following cardiac surgery, post myocardial infarction cardiogenic shock, and in acute decompensated heart failure. Recently, these devices are occasionally being implanted in a less invasive fashion without bypass (cardiopulmonary bypass) support via mini-thoracotomy and epigastric incisions. The temporary devices, both percutaneous and operative, are generally used to support patients during high-risk interventional procedures or in emergency situations during which the patient's prognosis is indeterminate. In the latter cases, the MCS is used as a bridge to decision or bridge to recovery.^6,7 Frequently, the choice of the device depends on the availability and expertise in the center and ability to transfer the patient to an expert heart failure program.

Long-Term Support

There are basically two types of devices used: para-corporeal (percutaneous ventricular assist device [PVAD]) and totally implantable. They are either LAVDs or biventricular assist devices (BiVADs):

Table 2: Devices Approved by the U.S. Food and Drug Administration for Long-Term MCS

Device	Mechanism	Indications
Thoratec pVAD	Pulsatile	BTT, BTR
Novacor	Pulsatile	BTT, DT
Heartmate XVE	Pulsatile	BTT, DT
Heartmate II	Axial flow	BTT, DT
Abiomed TAH	Pulsatile	BTT
CardioWest TAH	Pulsatile	BTT
Berlin EXOR Pediatric	Pulsatile/pneumatic	BTT
DeBakey Child	Continuous	BTT, BTR

There are two pulsatile pneumatic paracorporeal pumps that can be used for a longer duration in terms of months.⁸ Pulsatile devices in adults and pulsatile/pneumatic devices in children are commonly used PVADs.⁹ They are also frequently employed when BiVAD is needed in any age group.

Durable and expensive medical devices usually provide really long-term support for months to years, and they are intra-corporeal. Most of these supports mainly involve LVAD. Temporary devices and/or inotropic support can provide right ventricular support in the short term during the early post-surgical period. These patients are usually bridged to transplantation or for permanent support as DT. Occasionally, the devices can be explanted after myocardial recovery.

Implantable devices are the most commonly used ones, and major scientific advances and research are directed towards improving the outcomes following their implantation. The older devices that laid the foundation for the MCS therapy were all pulsatile volume displacement devices. They were large, difficult to implant, and had significant complications. In spite of these problems, they were used in the Randomized Evaluation of Mechanical Assistance for the Treatment of Congestive Heart Failure (REMATCH) study, leading to proof that the LVADs were superior to optimal medical therapy for end-stage heart failure.¹⁰ Currently, all devices used are continuous-flow pumps based on axial or centrifugal mechanism. The axial-flow rotary pump consists of a rotating, screw-like propeller within a tube housing. The energy from the rotating element increases blood pressure and flow. The centrifugal pump with spinning blades captures and throws fluid forwards.¹¹ This essentially means the patients live without pulsatile blood flow, depending on the residual left ventricular function. The commonly used pumps are an axial flow pump and a centrifugal pump.^12,13 An axial pump is approved for use in the pediatric population; however, the complication rates have been high, and have limited its use.¹⁴ There is the only approved device for replacing the entire heart.¹⁵ It serves as a biventricular support in many special situations as BTT. There has been a significant improvement in post-transplant outcomes in BTT and two-year outcomes in DT.^16,17

Choosing the Right Patient and the Right Time

MCS is indicated to support a patient with advanced heart failure. The mortality and morbidity is significantly reduced if MCS is initiated before the onset of severe right heart failure and systemic organ failure. The Seattle Heart Failure score¹⁸ can be used to identify high-risk patients. These patients are usually the ones with progressive symptoms, repeated hospital admissions, failure, or intolerance to maximal heart failure therapy with signs of reduced systemic organ perfusion. Some of them are inotropic dependent with peak VO₂<14 ml/kg/mt. Categorizing the patients into seven clinical profiles has been possible using the Interagency for Mechanically Assisted Circulatory Support (INTERMACS) profile.¹⁹ Level 1 is cardiogenic shock, level 2 is a patient declining on inotropic support, and level 3 is stable on inotropic support. Patients in all three groups need MCS sooner. Levels 4 and 5 are patients severely limited, requiring frequent admission to the hospital with heart failure mostly resting. Levels 6 and 7 are patients in significant heart failure who need constant surveillance under a multidisciplinary heart failure team.²⁰

Table 3: The INTERMACS Clinical Profiles

Level	Symptoms	Signs and Hemodynamics	Need for LVAD
1	Critical cardiogenic shock, "crash and burn"	Persistant hypotension despite rapidly escalating inotropic support and eventually IABP, and critical organ hypoperfusion	Within hours
2	Progressive decline on inotropic support, "sliding on inotropes"	Intravenous inotropic support with acceptable values of blood pressure and continuing deterioration in nutrition, renal function, or fluid retention	Within days
3	Stable but inotrope dependent, "dependent stability"	Stability reached with mild-to-moderate doses of inotropes but demonstrating failure to wean from them because of hypotension, worsening symptoms, or progressive renal dysfunction	Elective over weeks to months
4	Resting symptoms, "frequent flyer"	Possible weaning of inotropes but experiencing recurrent relapses, fluid retention	Elective over weeks to months
5	Exertion intolerant, housebound	Severe limited tolerance for activity, comfortable at rest with some volume overload and often with some renal dysfunction	Variable urgency, dependent on nutrition and organ function
6	Exertion limited, "walking wounded"	Less severe limited tolerance for activity and lack of volume overload, fatigue easily	Variable urgency, dependent on nutrition and organ function
7	Advanced NYHA III "symptoms placeholder"	Patient without current or recent unstable fluid balance, NYHA class II or III	Not currently indicated

The results are significantly better when MCS is provided in a semi-elective or stable patient. Heart failure patients with approximately 30% one-year mortality can be considered for inclusion in MCS trials.²¹ Overall, the assessment of patients for MCS is similar to the assessment of patients for heart transplantation, but the criteria are less restrictive and reversible organ dysfunction and comorbidities are usually accepted. The majority of the risk factors are analyzed during work-up and use of a scoring through a system (Lietz-Miller Scores) created mainly towards improving survival and reducing complications. Major risk factors are hepatic and renal dysfunction, valve disorders, coagulopathy and malnutrition. Major complications following MCS using LVADs are right ventricular failure,²² valve disorders, infection, bleeding disorders, and thromboembolic complications. The overall two-year survival after MCS exceeds 60%.

Future

The future focus of MCS will be to reduce infection,²³ decrease thromboembolic and bleeding complications, reduce the size of the devices,²³ early implantation before the onset of organ failure.

References

1. Yancy CW, Jessup M, Bozkurt B, et al.2013 ACCF/AHA guideline for the management of heart failure: a report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines. J Am Coll Cardiol 2013;62:e147-e239.
2. Fang JC, Ewald GA, Allen LA, et al. Advanced (stage d) heart failure: a statement from the heart failure society of America guidelines committee. J Card Fail 2015;21:519-34.
3. Rihal CS, Naidu SS, Givertz MM, et al. 2015 SCAI/ACC/HFSA/STS Clinical Expert Consensus Statement on the Use of Percutaneous Mechanical Circulatory Support Devices in Cardiovascular Care (Endorsed by the American Heart Association, the Cardiological Society of India, and Sociedad Latino Americana de Cardiologia Intervencion; Affirmation of Value by the Canadian Association of Interventional Cardiology-Association Canadienne de Cardiologie d'intervention). J Am Coll Cardiol 2015;65:e7-e26.
4. Morgan JA, Stewart AS, Lee BJ, Oz MC, Naka Y. Role of the Abiomed BVS 5000 device for short-term support and bridge to transplantation. ASAIO J 2004;50:36-3.
5. John R, Long JW, Massey HT, et al. Outcomes of a multicenter trial of the Levitronix CentriMag ventricular assist system for short-term circulatory support. J Thorac Cardiovasc Surg 2011;141:932-9.
6. Gregoric ID, Jacob LP, La Francesca S, et al. The TandemHeart as a bridge to a long-term axial-flow left ventricular assist device (bridge to bridge). TexHeart Inst J 2008;35:125-9.
7. Idelchik GM, Simpson L, Civitello AB, et al. Use of the percutaneous left ventricular assist device in patients with severe refractory cardiogenic shock as a bridge to long-term left ventricular assist device implantation. J Heart Lung Transplant 2008;27:106-11.
8. Kirsch M, Vermes E, Damy T, et al. Single-centre experience with the Thoratec Paracorporeal Ventricular Assist Device for patients with primary cardiac failure. Arch Cardiovasc Dis 2009;102:509-18.
9. Morales DL, Almond CS, Jaquiss RD, et al. Bridging children of all sizes to cardiac transplantation: the initial multicenter North American experience with the Berlin Heart EXCOR ventricular assist device. J Heart Lung Transplant 2011;30:1-8.
10. Rose EA, Gelijns AC, Moskowitz AJ, et al. Randomized Evaluation of Mechanical Assistance for the Treatment of Congestive Heart Failure (REMATCH) Study Group. Long-term use of a left ventricular assistance device for end-stage heart failure. N Engl J Med 2001;345:1435-43.
11. Moazami N,Fukamachi K, Kobayashi M, et al. Axial and centrifugal continuous-flow rotary pumps: a translation from pump mechanics to clinical practice. J Heart Lung Transplant 2013;32:1-11.
12. Wieselthaler GM, O Driscoll G, Jansz P, Khaghani A, Strueber M; HVAD Clinical Investigators. Initial clinical experience with a novel left ventricular assist device with a magnetically levitated rotor in a multiinstitutional trial. J Heart Lung Transplant 2010;29:1218-25.
13. Pagani FD, Miller LW, Russell SD, et al. Extended mechanical circulatory support with a continuous-flow rotary left ventricular assist device. J Am Coll Cardiol 2009;54:312-21.
14. Goldstein DJ. Worldwide experience with the MicroMed DeBakey ventricular assist device as a bridge to transplantation. Circulation 2003;108(suppl 1):II-272–II-277.
15. Copeland JG, Smith RG, Arabia FA, et al. Cardiac replacement with a total artificial heart as a bridge to transplantation. N Engl J Med 2004;351:859-67.
16. Alba AC, McDonald M, Rao V, Ross HJ, Delgado DH. The effect of ventricular assist devices on long-term post-transplant outcomes: a systematic review of observational studies. Eur J Heart Fail 2011; 785-95.
17. Slaughter MS, Rogers JG, Milano CA, et al. Advanced heart failure treated with continuous-flow left ventricular assist device. N Engl J Med 2009;361:2241-51.
18. Kalogeropoulos AP, Georgiopoulou VV, Giamouzis G, et al. Utility of the Seattle Heart Failure Model in patients with advanced heart failure. J Am Coll Cardiol 2009;53:334-42.
19. Alba AC, Rao V, Ivanov J, Ross HJ, Delgado DH. Usefulness of the INTERMACS scale to predict outcomes after mechanical assist device implantation. J Heart Lung Transplant 2009;28:827-33.
20. Peura JL, Colvin-Adams M, Francis GS, et al. Recommendations for the use of mechanical circulatory support: device strategies and patient selection: a scientific statement from the American Heart Association. Circulation 2012;126:2648-67.
21. Neaton JD, Normand SL, Gelijns A, et al. Designs for mechanical circulatory support device studies. J Card Fail 2007;13:63-74.
22. Santambrogio L, Bianchi T, Fuardo M, et al. Right ventricular failure after left ventricular assist device insertion: preoperative risk factors. Interact Cardiovasc Thorac Surg 2006;5:379 -82.
23. Topkara VK, Kondareddy S, Malik F, et al. Infectious complications in patients with left ventricular assist device: etiology and outcomes in the continuous-flow era. Ann Thorac Surg 2010;90:1270-7.
24. Meyns B, Klotz S, Simon A, et al. Proof of concept: hemodynamic response to long-term partial ventricular support with the Synergy Pocket Micro-Pump. J Am Coll Cardiol 2009;54:79-86.

Keywords: American Heart Association, Aorta, Aortic Valve, Blood Pressure, Bone Screws, Burns, Cardiac Surgical Procedures, Cardiopulmonary Bypass, Comorbidity, Counterpulsation, Device Approval, Femoral Artery, Heart Arrest, Heart Failure, Heart Transplantation, Heart Ventricles, Heart-Assist Devices, Housing, Hypotension, Intra-Aortic Balloon Pumping, Malnutrition, Myocardium, Physical Exertion, Prognosis, Recurrence, Risk Factors, Shock, Cardiogenic, Sternotomy, Thoracotomy, United States Food and Drug Administration, Ventricular Function, Left, Walking, Water-Electrolyte Balance, Weaning

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