72. Durable Mechanical Circulatory Support- Review of CT Surgery

Antonia Kreso and David D’Alessandro

Mechanical circulatory support (MCS) with durable ventricular assist devices (VADs) is a rapidly evolving field. VADs provide life-saving therapy for patients with decompensating heart failure despite optimal medical management.

Terminology

Bridge to transplantation

The use of a VAD to sustain life until a donor heart becomes available is termed “bridge to transplantation.” The majority of devices are implanted to allow a patient to survive until a donor heart becomes available.

Destination therapy

A VAD can be used as an alternative to transplantation, and when used with this objective, it is termed “destination therapy.” This usually happens in patients who are considered ineligible for transplantation (Rose et al. 2001; Stevenson et al. 2004; Rogers et al. 2007). Destination therapy has become a more attractive option for elderly patients as the durability and adverse event profiles of current use devices have improved substantially (Kirklin, Naftel, Pagani, et al. 2014; Mehra et al. 2019).

Bridge to decision

In patients with acute cardiogenic shock, non-implantable VADs are frequently used to stabilize the patient and await myocardial recovery. Such patients often have potentially reversible medical conditions that may be temporary contraindications to cardiac transplantation and MCS can be used as a “bridge to decision.” For example, pulmonary pressures can be reduced with MCS therapy in patients with heart failure, who are considered to have permanently elevated pulmonary hypertension (Alba et al. 2010; Elhenawy et al. 2011; Nair et al. 2010). Thereafter, eligibility for transplant or durable MCS can be determined. 

Bridge to recovery

The strategy of device implantation to promote offloading of the heart and myocardial recovery is termed “bridge to recovery.” Some patients can recover sufficiently to allow explanation of their left sided VAD (LVAD) (Jakovljevic et al. 2017). Typically, this occurs in a minority of patients. 

Indications for use

According to the 2013 ACCF/AHA guideline for the management of heart failure (Yancy et al. 2013), general indications for referral for MCS therapy include patients with left ventricular ejection fraction that is less than 25% and New York Heart Association (NYHA) class III–IV functional status despite guideline-directed medical therapy with either high predicted 1- to 2- year mortality or dependence on continuous inotropic support. Specific recommendations outlined by ACCF/AHA are:

1. MCS is beneficial in carefully selected patients with stage D heart failure with reduced ejection fraction in whom definitive management (e.g., cardiac transplantation) or cardiac recovery is anticipated or planned.
2. Nondurable MCS, including the use of percutaneous and extracorporeal VADs, is reasonable as a “bridge to recovery” or “bridge to decision” for carefully selected patients with heart failure with reduced ejection fraction with acute, profound hemodynamic compromise.
3. Durable MCS is reasonable to prolong survival for carefully selected patients with stage D heart failure with reduced ejection fraction.

Patient selection requires a multidisciplinary team of experienced advanced heart failure and transplantation cardiologists, cardiothoracic surgeons, nurses, social workers and palliative care clinicians.

Device options

The durability of devices and the outcomes of patients with mechanical devices continues to improve and evolve as a field. VADs are differentiated by the flow characteristic (pulsatile or continuous), pump mechanism (volume displacement, axial, or centrifugal), and the ventricle that is supported (left, right, biventricular). The mechanism of operation can be used to group the different devices into first generation (pulsatile pumps), second generation (continuous flow devices), and third generation devices (centrifugal pumps).

First Generation Devices

Initial VADs were pulsatile positive displacement pumps, which provided good hemodynamic support but were limited by the lack of long-term device durability. First generation devices had large drivelines that were prone to infection, and were noisy, which adversely affected patient quality of life. More importantly, these systems required multiple moving parts that contributed to thrombogenicity and device malfunction. These factors contributed to their limited use. The early devices included the Thoratec paracorporeal ventricular assist device (PVAD) and the Novacor (Wheeldon, Jansen, and Portner 2000), which was one of the first devices to provide more than four years of continuous circulatory support (Potapov et al. 2005; Rogers et al. 2007). The use of Novacor has been discontinued due to high rates of stroke. The HeartMate I was a first generation device that was used in the landmark REMATCH (Randomized Evaluation of Mechanical Assistance for the Treatment of Congestive Heart Failure) trial, which became the basis for approval for destination therapy (Rose et al. 2001).

Second Generation Devices

The second generation of MCS devices were defined by axial pump mechanisms, rather than volume displacement, and continuous, rather than pulsatile flow. These devices were more durable, quieter, and smaller, including smaller drivelines that lowered the rates of infections.

HeartMate II: As technology evolved, the smaller, second-generation HeartMate II device had axial flow and featured inflow and outflow cannulas without valves, eliminating the reservoir necessary in pulsatile pumps. The pump weighs 350 g and is 7×4 cm in size. The pump can generate 10 L/min of flow at a pressure of 100 mm Hg (Griffith et al. 2001). The device was shown to provide excellent hemodynamic support in patients awaiting heart transplant (Miller et al. 2007). The HEARTMATE II trial showed that the HeartMate II was associated with an increased probability of 2-year survival, free from disabling stroke and LVAD failure (Slaughter et al. 2009). The INTERMACS (Interagency Registry for Mechanically Assisted Circulatory Support) analysis showed an overall survival of 80% at 1 year and 69% at 2 years (Kirklin, Naftel, Kormos, et al. 2014). Pump thrombosis was reported as high at 10%, frequently requiring re-operation (Starling et al. 2014).

Jarvik 2000: This continuous flow axial pump is positioned within the left ventricle, reducing the risk of pump infections. It weighs 85 g and measures 5.5×2.4 cm. The pump can generate 7 L/min of flow. Blood that passes through the pump can be returned to either the ascending or descending aorta.

Third Generation Devices

Technological advancements in design and mechanics has led to the third generation MCS devices that are defined by centrifugal pumps. Anticipated durability is 5–10 years. The devices have become smaller, easier to implant, and have optimized flows that minimize the risk of thrombosis and hemolysis. 

HeartWare (also known as HVAD): This device contains a small continuous flow centrifugal pump that allows for implantation in the pericardial space. It only has one moving part, the impeller, and no mechanical bearings (Larose et al. 2010). It weighs 145 g and the impeller can generate up to 10 L/min of blood flow. It has been approved as a bridge to heart transplantation based on the results of the multicenter prospective Advance BTT trial (Aaronson et al. 2012). HeartWare is approved as destination therapy based on the randomized prospective multicenter noninferiority Endurance DT trial (Mehra et al. 2017, 2019).

HeartMate III: The third iteration of the HeartMate device is a magnetically levitated centrifugal flow pump. It weighs 240 g and is inserted into the apex of the left ventricle. It can generate up to 10 L/min of flow. Compared to HeartMate II, the third generation device results in fewer strokes and a 14.6% absolute reduction in pump thrombosis, translating to a 15.4% reduction in reoperation rates for device malfunction [MOMENTUM 3 trial] (Mehra et al. 2017, 2019).

Biventricular support

Patients have been treated with implanted VADs on both sides. While this is an off-label use, it can be performed with the HeartMate II, HeartMate III and HVAD. Other device options for patients who have biventricular failure include the Thoratec PVAD or the total artificial heart (Krabatsch et al. 2011; Meyer and Slaughter 2011). The Syncardia temporary Total Artificial Heart (TAH) is a pulsatile system that has two chambers, which allow for blood pumping. The left artificial ventricle is connected via the left arterial inflow connector to the left atrium and via the aortic outflow cannula to the aorta. The artificial right ventricle is connected via the right inflow connector to the right atrium and via the pulmonary artery outflow cannula to the pulmonary artery. The size of the system has limited its use as recipients must have a body surface area of greater than 1.7 and a large enough distance within the chest for implantation.

Preoperative considerations

Patients with end stage heart failure are evaluated by a multidisciplinary team to determine transplant eligibility. Evaluation of patients for destination VAD therapy includes a similar workup. Cardiac work-up is a central component of this evaluation. This is obtained using left and right heart catheterization, transthoracic or transesophageal echocardiography and functional studies in the form of cardiopulmonary stress test. In the setting of a maximal study, peak oxygen consumption <14 mL/kg/min and/or a ventilation/carbon dioxide production slope >36 have been associated with marked impairment in cardiac reserve and poor prognosis (Kirklin et al. 2020). CT chest should also be obtained to aid in the cardio-pulmonary assessment. A comprehensive review of non-cardiac systems including renal, gastrointestinal (colonoscopy in patients older than 50 years of age), pulmonary, and hepatic are also considered. Infection needs to be ruled out with surveillance blood cultures and replacement of old lines. A nutritional assessment, neurologic evaluation and psychosocial evaluation are also performed.

Intraoperative considerations

At the time of LVAD implantation, additional cardiac procedures should be addressed. Aortic regurgitation must be corrected to prevent flow reversal from the ascending aorta to the LVAD. Inter-atrial and inter-ventricular septal defects must be corrected to avoid right to left shunting. Intra-op evaluation with transesophageal echocardiography after the implantation of the LVAD should confirm unobstructed inflow cannula positioning with the interventricular septum in the midline. 

Right heart failure is a special scenario that needs to be considered for durable support. This includes preoperative cardiopulmonary optimization and the judicious use of perioperative inotropes. Afterload should be maintained with the use of pressors for target mean pressures above 65 mmHg as pump flow is partly dependent on afterload and inadequate systolic pressure can predispose to left ventricle suck down and acute right ventricle failure. Close attention to peri-operative right heart optimization will help maintain durable cardiac function.

Postoperative considerations and complications

Postoperative intensive care management includes adequate nutritional support and anticoagulation. In general, an antiplatelet agent and an anticoagulant are used. When to initiate anticoagulation postoperatively must be decided in an individualized manner. The most common complication after implantation of an LVAD is bleeding. Sepsis occurs in a significant proportion of LVAD patients. Although thromboembolic events are less common with the current devices, stroke continues to be a feared complication.

Right sided heart failure occurs in about 20% of patients after LVAD insertion. Early signs include elevated central venous pressure, marginal LVAD flows and decreased urine output. Right sided heart failure is managed with inotropes and diuresis. In some cases of persistent right sided heart failure, RVAD may be required.

Gastrointestinal bleeding is a common complication following VAD implantation. The etiology of the bleeding is likely multifactorial and related to altered blood flow patterns from the pump as well as loss of pulsatile perfusion, which may cause distention of the submucosal venous plexus and lead to bleeding. Acquired von Willebrand syndrome has also been described in these patients. In these instances, the anticoagulation plan needs to be carefully tailored to the specific clinical situation. 

Hemolysis is a potential indicator of device malfunction. In severe cases, thrombosis can occur, and the device may need to be exchanged. For patients with mild hemolysis (LDH <2.5 normal in the absence of clinical symptoms), pump speed and blood pressure should be optimized. For patients with major hemolysis (LDH >2.5 normal or new onset of heart failure symptoms), VAD alarms and power spikes need to be evaluated. An echocardiogram should be obtained to evaluate pump function and optimal anticoagulation should be achieved. In a minority of patients, pump exchange is required.  

Long-term care

The rapid advancement of MCS technologies has enabled improved outcomes and lower device associated complications. Patients with VADs continue to be at risk for a number of complications and a multidisciplinary team of experienced advanced heart failure health care professionals is paramount in post-operative management of these patients. The experience with earlier devices has enabled the advancement of new devices and concomitant improvement in durability. These improvements in VAD therapies will continue to provide life-saving therapy for patients with decompensating heart failure.

Suggested Readings

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5. Kirklin JK, Naftel DC, Kormos RL, et al. Interagency Registry for Mechanically Assisted Circulatory Support (INTERMACS) Analysis of Pump Thrombosis in the HeartMate II Left Ventricular Assist Device. J Heart Lung Transplant. 2014;33(1):12-22.
6. Kirklin JK, Naftel DC, Pagani FD, et al. Sixth INTERMACS Annual Report: A 10,000-Patient Database. J Heart Lung Transplant. 2014;33(6):555-64.
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