Fig. 11.1
Kaplan-Meier analysis of survival for transplant-ineligible patients who received a left ventricular assist device (LVAD) versus optimal medical therapy (Reproduced, with permission, from Rose et al. [3])
This chapter will focus on the use of implantable long-term MCS devices as BTT and DT for adult patients with advanced heart failure. Specifically, we will review the landscape of BTT and DT in the United States, as well as describe considerations for device selection.
11.2 Strategies for Mechanical Circulatory Support
The strategies underlying the use of implantable MCS devices in patients with end-stage heart failure are stratified based on transplant eligibility, and include bridge to transplant (BTT) as well as destination therapy (DT) [3–9]. BTT is a strategy for patients actively listed for heart transplantation, utilizing MCS to prevent death or progressive end-organ dysfunction in the setting of low cardiac output while awaiting transplant. In contrast, DT is a strategy for patients requiring lifelong circulatory support, who are not eligible for heart transplantation because of relative or absolute contraindications. There exist additional MCS strategies including bridge to candidacy (i.e., for patients who are not currently listed for heart transplantation and have no absolute contraindications to transplant, but who have medical, social, or financial barriers to transplant candidacy at the time of evaluation) and bridge to recovery (i.e., for patients who may expect sufficient recovery of native myocardial function after temporary MCS device implantation to unload the recovering ventricle), although these will not be within the scope of our discussion below.
It is important to note that, although distinct strategies (BTT or DT) can be defined with regard to clinical intent at the time of MCS device implantation (◘ Fig. 11.2), these strategies can also evolve as a patient’s condition changes over time. For example, a DT patient who was previously ineligible for transplant may experience improvement in comorbidities after LVAD implantation and become transplant-eligible with MCS. Alternatively, a BTT patient awaiting transplant after LVAD implantation may become transplant-ineligible because of device-related complications or progression of coexistent comorbidities. Therefore, patient assignment to BTT or DT may be dynamic and is dependent on the clinical situation. Decisions about candidacy for each strategy should be made collaboratively by an experienced heart failure team, including both surgeons and cardiologists, and reassessed as dictated by the patient’s clinical course. This process is particularly important in order to ensure appropriate patient selection, proper device selection, and optimal timing for device placement.
Fig. 11.2
Strategies for mechanical circulatory support implantation. HTx indicates orthotopic heart transplant; BTT, bridge to transplant; BTC, bridge to candidacy; and DT, destination therapy
The selection of patients for MCS device implantation requires careful evaluation of heart failure history and severity. The general indicators of advanced heart failure leading to referral for MCS device implantation include New York Heart Association (NYHA) class IIIb–IV symptoms, failure of optimal medical therapies, frequent hospitalizations for heart failure exacerbation, inotrope dependence, recurrent or refractory ventricular tachyarrhythmia, unresponsiveness to cardiac resynchronization therapy, end-organ dysfunction due to low cardiac output, peak oxygen consumption less than 14 ml/kg/min, and 6 minute walk distance less than 300 meter [10]. To aid in the identification of patients who might benefit from LVAD support, the Seattle Heart Failure Model can be used to estimate a heart failure patient’s expected mortality over the subsequent 1 to 2 years [11]. A thorough assessment of operative risk and potential complications must also be performed to determine whether a patient is an appropriate surgical candidate for an implantable MCS device.
Another critical component of the preoperative evaluation for MCS involves determining the potential need for biventricular support. Previous work has clearly demonstrated a significant mortality benefit when patients at risk of failing isolated LVAD therapy and requiring biventricular support receive early, planned biventricular assist device (BiVAD) therapy instead of an LVAD followed by later conversion to BiVAD therapy. In one study of 99 patients who ultimately received BiVAD support, the survival at 1 year post-implantation was 48% among patients who were planned for direct BiVAD implantation, versus 25% for patients who received an LVAD initially and were later converted to BiVAD therapy [12]. To assist in determining which patients may ultimately require biventricular support, the 5-point CRITT score may be utilized, in which a binary scoring system (i.e., 0 or 1 point) is applied for each of five criteria identified by multivariable analysis to be associated with increased risk of requiring biventricular support: central venous pressure > 15 mmHg (C), severe right ventricular dysfunction (R), preoperative mechanical ventilation or intubation (I), severe tricuspid regurgitation (T), and tachycardia (T) [13]. According to this model, 93% of patients with a total CRITT score of 0 or 1 successfully tolerated isolated LVAD therapy, whereas 80% of patients with a score of 4 or 5 required biventricular support. As such, isolated LVAD therapy may be recommended for patients with a CRITT score of 0–1, and BiVAD therapy may be recommended for a score of 4–5. Patients with a CRITT score of 2-3 have less predictable outcomes, and may tolerate isolated LVAD therapy with appropriate pharmacologic support, or may require temporary right ventricular support.
11.2.1 Bridge to Transplant (BTT)
Historically, BTT once represented the predominant strategy for patients receiving an implantable MCS device, with 42.4% of patients in the 2006–2007 INTERMACS registry actively listed for heart transplant at the time of device implantation [5]. Although the proportion of MCS strategies declared as BTT decreased to 21.7% by 2011–2013 (with a concomitant increase seen in the use of DT), BTT continues to serve as an invaluable option for patients for whom heart transplantation remains the ideal ultimate therapy.
It is well established that patients actively awaiting heart transplant experience improved survival, functional status, and quality of life through the use of a long-term durable LVAD as BTT therapy [14–15]. In a study of 133 patients who received a continuous-flow LVAD as BTT, Miller et al. observed survival at 180 days post-implant to be 75%, including those who received a heart transplant, those who recovered native cardiac function, and those with continued survival on MCS [14]. Based on more recent data from the INTERMACS registry in 2014, the actuarial survival of BTT patients at 1 year is now expected to be >80% [5]. Miller et al. also observed recovery in renal and hepatic function after LVAD implantation, measured by significant improvements in serum creatinine, blood urea nitrogen, and transaminase levels at 3 months post-implant. Functional status improved by at least two NYHA classes in 83% of patients by 3 months post-implant.
The percentage of BTT patients receiving a heart transplant by 1 year after device implantation is currently 20–30% [6]. In light of the continued organ shortage, the BTT strategy is perhaps most beneficial in patients who are expected to have an extended time on the waiting list for heart transplantation. Reasons for an extended waiting time commonly include ABO blood type, large body habitus, or the presence of anti-HLA antibodies. However, for any patient under consideration for BTT therapy, the overall operative risk combines those associated with two surgeries instead of one. The initial LVAD implantation procedure is typically approached through a median sternotomy and performed with cardiopulmonary bypass, and the potential postoperative complications (e.g., bleeding, infection, stroke, thrombosis, and LVAD failure) can in some cases affect a patient’s ultimate transplant eligibility. The eventual transplant operation would involve a redo sternotomy and repeat cardiopulmonary bypass, both of which are associated with increased operative risk. For heart failure patients with a previous history of cardiac surgery prior to LVAD implantation, commonly including coronary artery bypass grafting, valve surgery, or congenital repairs, the operative risk is greater still, and may even be prohibitive. Furthermore, the transfusion of blood products during or after the initial LVAD implantation procedure can result in increased sensitization to HLA antibodies, which ultimately may increase the difficulty of finding a suitable donor match [16]. All of the above must be carefully considered in order to appropriately select patients for successful BTT therapy.
11.2.2 Destination Therapy (DT)
Over the past decade, LVAD therapy as DT has become a well-established option for end-stage heart failure patients with contraindications to heart transplantation [3–6, 9]. As introduced previously, the prospectively randomized REMATCH trial first illustrated the superiority of LVAD therapy (using the HeartMate XVE) over optimal medical therapy alone among transplant-ineligible patients in 2001. Survival at 1 year was 52% among LVAD recipients versus 23% among patients medically-managed [3]. Later in 2009, the pulsatile-flow HeartMate XVE was compared to the newer continuous-flow HeartMate II in a randomized study of 200 advanced heart failure patients who were ineligible for transplant [7]. Survival at 1 year was 68% in the HeartMate II group versus 52% for the HeartMate XVE group, and at 2 years, 58% compared to 24%, respectively. For reference, the 2-year survival among medically-managed, transplant-ineligible patients in the REMATCH trial was just 8% [3]. Thus, for end-stage heart failure patients with contraindications to heart transplantation, commonly including those with advanced age (over 72 years old), morbid obesity (body mass index greater than 35 kg/m2), active infection, severe diabetes mellitus, severe peripheral vascular disease, psychosocial instability, recent drug/alcohol/tobacco abuse (within 6 months), and irreversible pulmonary hypertension [15], the survival benefit associated with MCS use as DT is well established. These patients also experience significant improvements in quality of life based on assessments such as the Minnesota Living with Heart Failure Questionnaire and Kansas City Cardiomyopathy Questionnaire, as well as considerable recovery of functional status, with 80% of patients who received a HeartMate II having NYHA class I or II symptoms at 2 years after LVAD implantation [7].
Given the advancements in technology and the parallel improvements in patient care and selection, the use of MCS as DT has rapidly grown over the past decade. In the 2006–2007 INTERMACS registry, only 14.7% of patients who received an LVAD were declared for DT, but by 2011–2014, the DT proportion had increased dramatically to 46%, becoming the predominant strategy among all LVAD implantations [5–6]. In fact, because DT LVADs have demonstrated such effectiveness under its original indication (i.e., for transplant-ineligible patients with less than 2 years life expectancy on maximal heart failure medication), some surgeons and cardiologists have advocated expanding the use of LVAD therapy as DT to include heart failure patients who are less ill and who have not yet developed sequelae of end-stage cardiac insufficiency. To this end, the Randomized Evaluation of VAD InterVEntion before Inotropic Therapy (REVIVE-IT) trial has been designed to evaluate all-cause mortality, functional status, and quality of life among heart failure patients who are not dependent on inotropes and who do not exhibit end-organ dysfunction due to heart failure. Patients will be randomized to early LVAD therapy or optimal medical therapy [17]. Overall, with further optimizations in patient care and selection strategy moving forward, and continued innovations in MCS device technology to enhance durability, safety, and convenience, DT LVADs have the potential to reduce the burden of the current donor organ shortage, and eventually may even challenge heart transplantation as the standard of care for patients with end-stage heart failure [18].
11.3 Device Selection for Mechanical Circulatory Support
Currently in the United States, several different durable MCS devices are FDA approved and actively used either as BTT or DT (◘ Table 11.1). As previously discussed regarding selection of a specific MCS strategy (BTT versus DT), the selection of a specific device for MCS should also be made collaboratively by an experienced heart failure team, and only after careful evaluation of the patient’s heart failure history. Patient-specific factors that should be considered include the patient’s overall prognosis, the desired MCS strategy (BTT versus DT), the expected duration of MCS, and the need for left, right, or biventricular support. Practical considerations, such as the patient’s body habitus and anticoagulation status, the surgeon’s preference and familiarity with various devices, or MCS device availability, may all impact the ultimate choice of device as well.
Table 11.1
Mechanical circulatory support devices approved by the FDA and currently in use
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