Tools
Left Ventricular Assist Devices
Introduction
Heart failure (HF) is a frequent cause of inpatient admissions. The Framingham study in 1993 described the risk factors for heart failure and showed unacceptably high five-year mortality rates of 25% in men and 38% in women [1]. The American Heart Association reported the prevalence of HF to be 5.1 million in the United States in 2006 [2]. The worldwide prevalence has been estimated to be 23 million [3].
HF can be categorized based on the left ventricular ejection fraction (LVEF) into systolic and diastolic HF. The former group includes patients with LVEF less than or equal to 40%, also termed heart failure with reduced ejection fraction (HFrEF). Heart failure with preserved ejection fraction (HFpEF) includes those with LVEF greater than or equal to 40%.
The multiple modalities of treatment available to treat HF include but are not limited to lifestyle modifications, pharmacologic agents, device therapies such as implantable cardioverter-defibrillator (ICD), and cardiac resynchronization therapy (CRT). At times, failure to improve could necessitate short-term mechanical circulatory support with the use of an intra-aortic balloon pump (IABP) or even extracorporeal membrane oxygenation (ECMO). However, a large population of patients continues to have advanced heart failure with worsening LVEF despite maximal therapy.
Circulatory support with the use of a left ventricular assist device (LVAD) is an emerging field. The landmark REMATCH trial that compared LVADs with optimal medical therapy in class IV HF patients found a 48% reduction in mortality from any cause [4]. There was also a significant increase in the survival rates at one year (52% versus 25%) and two years (23% versus 8%). The definitive treatment for advanced HF (class II and IV) is cardiac transplantation [5]. However, with the limited number of donor hearts available, LVADs are life-saving.
Anatomy and Physiology
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Anatomy and Physiology
The basic design of LVADs since their inception has stayed similar. The inlet cannula is placed in the apex of the left ventricle (LV). The blood subsequently enters the pump, the structure of which has undergone significant changes over time. The outflow graft then leads to the ascending (most common) or descending aorta [6].
The first-generation LVADs were approved by the United States Food and Drug Administration (FDA) for clinical use in 1994. These were pulsatile-flow LVADs used for circulatory support as a bridge to transplantation (BTT) for patients awaiting donor's hearts. However, the second and third-generation continuous-flow devices have had modifications in structure and function, with improved durability, which has expanded their use as destination therapy (DT) in patients who are not eligible for cardiac transplantation [7][8]. The eighth annual report of the Interagency Registry for Mechanically Assisted Circulatory Support (INTERMACS) in 2017 showed one-year and two-year survival rates with currently used continuous-flow devices to be greater than 80% and greater than 70%, respectively [9].
First-Generation Devices
These were pulsatile volume-displacement pumps. The Heartmate I (HM I) was used in the REMATCH trial and underwent many modifications but is no longer manufactured. Another of its generation, the Novacor also, is now obsolete due to a high risk of stroke. The Thoratec paracorporeal ventricular assist device (PVAD) is still manufactured but rarely used. The first-generation devices required significant surgical dissection for placement and were suited to those with a large body habitus. They also had a high rate of infection at the external lead. Also, the pump was audible and caused discomfort. These limitations resulted in the discontinuation of their use.
Second-Generation Devices
These are continuous flow devices that use axial flow pumps. The Heartmate II (HM II) and Jarvik 2000 are the two most commonly used devices, with the latter characterized by outflow graft anastomosis to the descending aorta. The rotor is the only moving part of these devices, making them more durable. These devices are smaller and easier to implant. They are quieter and also associated with lower rates of driveline infection than first-generation devices. The sixth INTERMACS annual report showed that, since 2010, all patients receiving DT had continuous flow devices [10].
Third Generation Devices
These are continuous flow centrifugal pumps, of which the HeartWare and Heartmate 3 are the most popular. They are designed for long durability (5 to 10 years), easy surgical placement, and low risk of hemolysis or thrombosis. Smaller devices are currently in the testing phase.
Biventricular Assist Device (BiVAD)
This device is used for patients with either biventricular failure or right ventricular failure associated with left ventricular disease. The total artificial heart (TAH) has been a revolution. The SynCardia TAH is the most widely used TAH, with over 1600 patients benefiting.
Indications
Bridge-to-Transplantation
The purpose of Bridge-to-Transplantation (BTT) is to provide circulatory support to transplant-eligible patients with HFrEF until a donor's heart becomes available.
Destination therapy
Destination therapy (DT) is used in patients with HFrEF who are ineligible for cardiac transplantation. The newer devices with improved durability have shown increased survival rates in this category of patients.
Bridge-to-the-Decision
LVADs have been used as a temporary measure in patients with end-organ dysfunction due to HF (relative contraindication to transplantation). Stabilization of hemodynamics with improvement in renal function, nutritional status, and reduction in pulmonary hypertension can help make them transplant-eligible.
Bridge-to-Recovery
Bridge-to-Recovery (BTR) provides temporary ventricular support in some HF patients and has been shown to improve myocardial function and promote recovery [11].
Strong indications for either BTT, DT, or BTR are as follows. All must be applicable [12]:
- NYHA class IV for 60 to 90 days
- Maximal tolerated medical therapy and certified respiratory therapy/implantable cardioverter-defibrillator if indicated
- Chronic dependence on inotropic agents
- LVEF less than 25%
- Pulmonary capillary wedge pressure greater than or equal to 20 mmHg
- Systolic BP less than or equal to 80 to 90 mmHg or cardiac index less than or equal to 2 L/min/m2, evidence of declining renal or right ventricular function
Contraindications
These have been well summarized in a review that was derived from entry criteria in several studies [12].
- Right, ventricular dysfunction: Either primarily or right heart failure, not secondary to left heart failure. Improper function of the right ventricle leads to insufficient filling of the left heart, which may lead to inadequate inflow in the device and, ultimately, device malfunction.
- Acute cardiogenic shock with a neurological compromise: Without adequate higher functions, LVAD placement is not recommended as it would only increase morbidity and decrease the quality of life.
- Coexisting severe terminal comorbidity: Severe renal, pulmonary, liver, or neurological disease or evidence of advanced metastatic cancer are considered contraindications.
- Bleeding: Active bleeding or thrombocytopenia (platelet count less than 50000 x 10 per L) or confirmed heparin-induced thrombocytopenia. Not just the bleeding but also the inability to be placed on anticoagulation makes this a contraindication.
- Anatomical factors: Hypertrophic cardiomyopathy or a large ventricular septal defect are a hindrance to device placement and function.
- Technical limitations: Body surface area less than 1.2 to 1.5 m2 or any other factor.
- Social considerations: LVAD management requires a high degree of patient compliance, which necessitates adequate psychological function to comply with medications and device maintenance. It also requires family education in interpreting basic functions and alarms. Thus, any difficulty posed by such factors could pose a contraindication to LVAD placement.
Complications
Hematological
Bleeding is the most common complication, occurring in both the perioperative period as well as later due to the need for anticoagulation with warfarin [13]. Cardiopulmonary bypass perioperatively alters the coagulation cascades and impairs the normal clotting mechanism, leading to bleeding. Also, bleeding has been attributed to the association of acquired von Willebrand disease in LVAD patients, typically more than a week after the procedure [14]. This is usually reversible if the LVAD is removed [15].
Bleeding may occur due to a leak at the pump site from polyester grafts in the conduits, the gastrointestinal mucosal surfaces, and intracranial vessels. The target INR in outpatients is usually 1.5 to 2.5 [16].
Thrombosis is another significant hematological complication Patients may develop pump thrombosis, embolic events, or stroke. It is usually due to subtherapeutic anticoagulation, atrial fibrillation, or infection that predisposes to a hypercoagulative state.
Hemolysis is another possibility due to technical complications involving the design of the pump, malpositioned cannulae, or the development of heparin-induced thrombocytopenia and pump thrombosis [17].
Right Heart Failure
Anatomic changes following LVAD placement cause right ventricular geometric alterations. With left ventricular (LV) unloading, the septum shifts to the left. The increased cardiac output from the LVAD results in an increased venous return to the right ventricle, which now has improved compliance. However, in patients with chronic heart failure, there is pre-existing pulmonary hypertension. This can result in right ventricular (RV) failure [18][19][18].
This may necessitate the use of milrinone to reduce pulmonary vascular resistance or epoprostenol as a selective pulmonary vasodilator [20][21][20]. In some circumstances, the use of RV mechanical support or ECMO may be required [22][23][22].
Infection
The International Society of Heart and Lung Transplantation has classified infections based on their relationship to LVAD [24]. Infections usually occur at the pump site, pump pocket, or driveline. They typically present with localized warmth and erythema at the pump site, along with fever and leukocytosis. Ultrasound of the local region can diagnose such collections and also guide aspiration. Swabs are helpful in guiding treatment.
Most commonly, the gram-positive Staphylococcus aureus is isolated, but Enterococcus and other Staphylococcal species may be present. The most common gram-negative organism is Pseudomonas aeruginosa [25]. Aggressive treatment is indicated with the use of appropriate antibiotics to cover the involved pathogen. Surgical revision of the driveline away from the infection may be needed. However, the pump usually needs to be replaced. Surgical debridement may be needed for deeper infections, with the use of omental or muscle flaps or vacuum-assisted closure techniques described [26][27][28]. Infection is associated with significantly increased mortality rates. Hence, severe infections may warrant device explantation with the use of ECMO or cardiac transplantation as definitive treatment.
Neurological
Stroke is one of the most dreaded complications of LVAD placement. Both ischemic and hemorrhagic strokes are known to occur, either immediately postoperatively or after several months [29]. Strokes more commonly affect the right hemisphere, indicating a cardioembolic source [30]. Ischemic events have been attributed to partial obstruction of the inflow cannula, deformation of blood in the pump apparatus, outflow graft obstruction, and subtherapeutic anticoagulation or infection. The risk of hemorrhagic stroke is due to anticoagulation. Hence, a fine balance is necessary to achieve optimum anticoagulation.
Arrhythmias
Ventricular arrhythmias are common after the procedure. Placement of the cannula can cause reentrant circuits [31]. Suction can lead to contact between the cannula and ventricular septum, triggering an arrhythmia. Significant changes in weight or the development of scar tissue can create malposition of the cannula, leading to arrhythmias [32]. Usually, the development of such arrhythmias can be managed by a change in device settings, such as reducing the speed of the LVAD to allow adequate ventricular filling. Management with a variety of medications is usually successful; however, refractory cases require catheter ablation or device exchange.
Clinical Significance
LVADs are life-saving in patients with end-stage cardiac disease. They offer an intermediate to long-term solution for those with HFrEF. They are most commonly used as a BTT or as DT. The structure and function of these devices have evolved with time, and further developments and advances in this field should help to further reduce the mortality rates from advanced heart failure.
Enhancing Healthcare Team Outcomes
Dedicated LVAD setups are required for the management of patients who have received the devices. An integrated interprofessional approach between cardiac surgeons, intensivists, and special LVAD nurses is necessary for adequate monitoring and follow-up of patients to produce the best clinical outcome. Specialty-trained nurses managing patients with LVAD need to be familiar with expected complications and have open communication with the clinicians. The nursing staff should also assist in patient and family education in regard to the device, its purpose, and management requirements. [Level 5]
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