Ventricular Assist Devices

Introduction¹

• Mechanical circulatory support via VADs emerged in the 1990s, as the growth of heart transplantation accelerated.
  • VADs were originally conceived as a temporary, bridge-to-transplantation (BTT) alternative.
• As heart transplants reached an epidemiological plateau of around 3,000/year, VADs took on a new indication: destination therapy (DT).
  • Currently, ~50% of VAD recipients are DT, whereas ~26% are BTT.
• Advancements in technology have led to more reliable and effective continuous-flow devices, thereby improving short- and medium-term outcomes.
• Given the increasing number of patients with left ventricular assist devices (LVADs), understanding the interactions between these devices and native right- and left-ventricular function is of paramount importance.

LVADs¹

• LVADs provide an alternate parallel route of blood flow from the left ventricle into the aorta.

Anatomy of LVADs

• The early-generation devices were pneumatically driven and generated pulsatile flow.
  • The mechanical components of these devices were prone to breakdown, and the mechanics of pulsatility led to disrupted blood flow and thrombus formation.
  • For these reasons, these first-generation devices were associated with significant morbidity.
• Continuous-flow devices (CF-VADs) were subsequently developed and were smaller, more durable, and demonstrated improved outcomes in both DT and BTT populations.
• CF-VADs were subsequently transitioned to two main pump types: axial and centrifugal.
  • Axial-flow is generated by a propeller in a tube (e.g., HeartMate II), while centrifugal-flow is generated by a bladed disk spinning in a cavity (e.g., HeartWare HVAD).
  • Both devices can directly unload the left ventricle (LV) and provide full cardiac output support.
  • Both devices require an external power source via a driveline tunneled subcutaneously and exiting through the upper abdomen.

• The Medtronic HeartWare HVAD device was recalled in July 2021 due to a higher risk of strokes and pump failure malfunctions due to internal pump delays or restart failure, which led to a higher morbidity and mortality.
• The concomitant development and success of the Abbott Heartmate III (HM3) LVAD contributed to the HeartWare HVAD discontinuation.⁸
  • The HM3 received FDA approval in August 2017 for short-term support and in October 2018 for use as DT. It is currently the most commonly implanted VAD.
  • It consists of a centrifugal pump with a magnetically levitated rotor.
  • The magnetic levitation system is able to withstand 7 times the gravitational acceleration of previous VAD models.
  • This system maintains a wide blood flow path, thereby improving flow and reducing shear stress on blood elements.
  • The presence of an internal pulsatility function causes automatic variation in pump speeds that helps prevent blood stasis.

Determinants of Flow¹

• Pump differential pressure (left ventricle minus aortic pressure) is the primary driver of flow in continuous-flow VADs.
  • The relationship between pump differential pressure and flow across the VAD is inversely proportional (i.e., higher differential pressure yields lower VAD flows).
  • High pump differential pressure can result from either high afterload or low ventricular pressure.
    • High afterload and low left ventricular (LV) pressure (preload) decrease forward flow.
  • Pump differential pressure varies with the cardiac cycle, being smaller in systole and larger in diastole.
    • Total systemic blood flow during systole is determined by left ventricular contractility and LVAD output, whereas during diastole it is fully attributable to LVAD output.
  • LVADs are 3-4 times more sensitive than a VAD naïve ventricle to increases in afterload.
• At a given set VAD speed (rpm), flow through the device is directly proportional to LV filling or preload.
  • LV filling is determined by volume status and right heart function.
  • It is of utmost importance to understand that for an LVAD to work appropriately, RV function needs to be adequate to support LV filling.
  • “Suck-down” or “suction” events are episodes that can be seen during low LV filling, in which the LV inflow cannula “sucks-down” or latches onto a ventricular wall.
    • Centrifugal pumps tend to maintain a relatively constant pump differential pressure, thereby reducing the risk of suction events compared with axial-flow VADs.
    • During suction events, flow through the VAD decreases, and ventricular arrhythmias can be precipitated.

Heart and LVAD Interaction¹

• Output from VADs depends on the interaction between preload, afterload, and device pump speed.
  • Pump speed is a fixed value that does not automatically adjust to changes in loading conditions.

VAD Interaction with the Left Ventricle

• The left ventricle and LVAD compete for the same preload, which is determined by the RV function, transpulmonary blood flow, and pulmonary vascular resistance.
• Poor transpulmonary blood flow will lead to “suck-down” or “suction” events.
  • During a suction event, newer devices automatically reduce the pump speed to a preprogrammed level, thereby improving ventricular filling and resolving the event.
• As a result of LVAD implantation and improved left ventricular offloading, ventricular dimensions decrease almost immediately, which continues for the following weeks and months.
  • Pulmonary artery wedge pressures and left ventricular end-diastolic dimensions both decrease within two minutes of LVAD support initiation.
  • This reduction in ventricular chamber size leads to a decrease in myocyte size, increased expression of sarcoplasmic endoplasmic reticulum calcium ATPase 2, and improvement in the passive pressure-volume relationship after 40 days of support.

VAD Interaction with the Right Ventricle

• The decrease in LV filling upon implantation of an LVAD leads to an almost immediate decrease in RV afterload, improvement in RV geometry, and function.
• A longstanding reduction in LV filling pressures reduces pulmonary hypertension and normalizes pulmonary vascular resistance.
• Increased LVAD systemic blood flow increases RV preload, which, in the setting of good RV contractility, leads to increased RV output.
  • In the setting of baseline reduced RV function, increased venous return can lead to worsening tricuspid regurgitation and further reduced RV function.
  • Other factors that contribute to early RV dysfunction during LVAD implantation include prolonged cardiopulmonary bypass time, iatrogenic injury to the RV, acute lung injury, protamine-induced pulmonary hypertension, and excessive volume or blood product administration.
• Due to ventricular interdependence, LVADs can cause septal wall ischemia, reduce septal thickening, and alter septal geometry, thereby affecting RV function.
  • Increasing LVAD pump speeds progressively reduces the septal wall’s contribution to RV contractility, thereby increasing RV work.
• In cases of severe RV dysfunction or acute RV failure upon LVAD implantation, nondurable mechanical circulatory support can be considered for the RV.²
  • A device such as the Thoratec Centrimag can be inserted percutaneously into the right side of the heart.
  • The device is positioned at a depth in which the inlet is at the level of the right atrium and the outlet within the pulmonary artery.
  • In cases of pulmonary compromise, an oxygenator can be attached to the device to compensate for reduced oxygenation.

Pulsatility in Continuous-Flow VADs¹

• The presence of continuous flow throughout the cardiac cycle increases diastolic pressure, while systolic pressure remains stable.
  • This diminishes the pulse pressure on invasive blood pressure (BP) monitoring.
  • As the pump speeds and systemic flow are increased, the diminishing of the pulse pressure is more pronounced.
• Although in the inpatient setting, arterial lines are the standard method of measuring blood pressure, this is not practical in the outpatient setting.
  • Doppler-assisted BP measurement is commonly used to estimate BP measurements in the absence of pulsatility.
  • Doppler measurements tend to reflect systolic pressure more than mean arterial pressure, which can lead to an overestimation of BP.
  • The International Society of Heart and Lung Transplantation has recommended aiming for a Doppler-derived BP target of 80 mmHg.
• Pulsatility in patients with LVADs reflects the degree of aortic valve opening.
  • The degree of pulsatility depends directly on LVAD pump speed.
  • Increasing pump speeds will empty the left ventricle, thereby reducing the volume of blood flowing through the aortic valve during ventricular contraction.
  • Some degree of pulsatility is beneficial, as the absence of aortic valve opening can lead to blood stasis and thrombosis.
    • Over time, the aortic valve commissures fuse, leading to aortic valve insufficiency in approximately 30% of patients.
• A lack of pulsatility has been attributed to the development of gastrointestinal (GI) arteriovenous malformations, which commonly lead to GI bleeding.²
  • Bleeding can be exacerbated by acquired Von Willebrand syndrome.
  • Sheer stress-induced activation of ADAMTS-13 metalloprotease cleaves the multimers necessary for platelet aggregation by Von Willebrand factor.
  • Newer devices, such as the HeartMate III, enable pulsatility by modulating pump speed, thereby reducing stasis within the left ventricle.²

Interpreting Pump Parameters¹

• Contemporary continuous-flow LVADs display pump flow in a console.
• To avoid issues with long-term reliability, VADs don’t have an embedded flow sensor.
  • Pump flows displayed in the console are calculated from the pump power consumption, due to the assumption that as pump power increases, flows will increase respectively.
  • The estimated displayed flow correlates well with actual blood flow, yet there is a 15-20% difference between the two.
• Obstruction of the VAD inflow cannula mimics low preload conditions, causing decreased pump flows.
• Obstruction of the VAD outflow cannula or increased afterload will also decrease pump flow and work.
  • In this case, the in-pump impeller will continue rotating without generating flow, while the blood will swirl within the pump housing.
• Therefore, a drop in pump flows likely indicates low pump speed, reduced left ventricular filling (e.g., hypovolemia, right ventricular dysfunction, outflow cannula obstruction), or increased afterload (e.g., outflow obstruction, high blood pressure).
• High pump flow indicates high pump speed, increased preload, or decreased afterload (e.g., vasodilation decreases the pump differential pressure and increases flow without increasing pump speed).
• In HeartMate II and III devices, pulsatility of pump flow is indicated on the console by the pulsatility index (PI).
  • PI = maximum pump flow minus minimum pump flow/average pump flow x 10.
  • PI is utilized as a surrogate of the degree of LVAD support.
  • The PI range is typically from 1 to 10.
  • In a stable, resting patient, the PI will likely fall between 3-6.
  • Lower PI values indicate an increase degree of LVAD support.
  • Higher PI values indicate a decrease in degree of support.
  • High pump speed, cardiac tamponade, right ventricular failure and hypovolemia will lead to decreased LV preload and native LV ejection, hence increasing the dependency on the pump (lower PI values).
  • Higher PI values may indicate lesser degree of pump mediated LV unloading and more native LV function, an indicator of improving underlying contractility.

Indications and Contraindications

Adverse Effects Associated with LVADs⁶

Right Heart Failure

• The combination of preexisting RV dysfunction, pulmonary hypertension, and acute hemodynamic shifts precipitates acute RV failure in 15-25% of LVAD implantations.
  • Approximately 4% of patients require implantation of a right ventricular assist device (RVAD).
• Patients with existing RV dysfunction on echocardiography, increased pulmonary vascular resistance, increased pulmonary capillary wedge pressure to central venous pressure ratio, high INTERMACS class, and presence of other measures of end-organ dysfunction are all at higher risk of acute RV failure.
• Inotropic support of the RV improves function, but in certain cases, implantation of an RVAD or Impella RP is necessary.

Pump Thrombosis

• A multifactorial phenomenon that can cause rapid clinical deterioration and require emergent pump exchange.
• Typically caused by suboptimal anticoagulation in the setting of turbulent or decreased blood flow through the device.
• Diagnosis can be suspected in the presence of pump power spikes, elevated hemolytic markers, and reduced ventricular unloading.

Bleeding

• This is the most common complication and reason for readmission in patients with LVADs.
• Bleeding has been attributed to the development of gastrointestinal angiodysplasia, acquired Von Willebrand factor deficiency, and the need for anticoagulation.
• Management includes holding anticoagulation, resuscitation, use of proton pump inhibitors (PPI), and endoscopic cauterization.
  • Blood product requirements are problematic in the bridge-to-transplant population, which may sensitize their antibody panel to future donors.
• The intrinsic pump pulsatility of devices such as the HeartMate III has not improved the rates of GI bleeding in this population.

Stroke

• Likely the most debilitating complication associated with LVAD implantation occurs in 13-30% of patients.
• The more common ischemic subtype is likely due to embolic sources such as thrombi in the pump, aortic valve, inflow, or outflow VAD cannulas.
• Thrombolytic therapy should be carefully considered, given the high risk of hemorrhagic stroke conversion in this population.
• Hemorrhagic stroke is more frequently associated with endocarditis, hypertension, and hemorrhagic conversion of ischemic strokes.
• The ENDURANCE II trial demonstrated equivalent stroke rates in patients with centrifugal HVAD versus HeartMate II devices, if hypertension is well-controlled.
• In the MOMENTUM trial, there was a lower incidence of stroke in patients with HeartMate III devices compared with HeartMate II devices.

Aortic Insufficiency

• Over 30% of LVAD recipients develop moderate or worse aortic insufficiency (AI) within 2 years of implantation.
  • Aortic closure leads to commissural fusion and an incompetent valve.
• Significant aortic insufficiency without correction precludes long-term LVAD support.
  • An incompetent aortic valve causes blood to flow back from the outflow cannula into the left ventricle, leading to recirculation and reduced systemic flow.
• Maintaining pulsatility allows the aortic valve to open intermittently and protects it from degeneration.
• Surgical options available to address acquired aortic insufficiency include aortic valve closure using a Park stitch, aortic valve replacement, and, in some experimental cases, transcatheter aortic valve replacement.
  • If aortic insufficiency is present at the time of LVAD implantation, surgical aortic valve replacement can be performed concomitantly.

Driveline Infections

• Presence of driveline infection is an independent predictor of mortality.
• Infections can occur in any part of the device, but the most common site is the soft-tissue and driveline-outlet junction.
  • The percutaneous driveline serves as a portal of entry for pathogens, particularly in this critically ill population, which is commonly immunocompromised and malnourished.
  • Hygiene is essential for preventing infection, as direct contamination is the most common mechanism.
• Gram-positive cocci, particularly Staphylococcus epidermidis and aureus, are the most common identified pathogens.
• Gram-negative rods, such as Pseudomonas and Klebsiella, fungi, and mycobacteria are the most common isolated pathogens.
• Organisms capable of forming biofilms are particularly virulent.
• In cases of severe, persistent infections that have spread beyond the driveline site, device exchange should be considered.

Perioperative Management of VADs⁵

Preoperative Assessment

• The type of implanted VAD should be confirmed.
• Determine if the device was implanted as a bridge-to-transplant (BTT) or destination therapy (DT).
  • Blood transfusions should be minimized in patients listed for transplantation to decrease alloimmunization and the potential difficulty of donor-organ match.
• Anticoagulation medications should be reviewed and managed perioperatively based on surgical bleeding risk, the emergent nature of the procedure, patient risk factors, and institutional policy.
  • Active reversal of anticoagulation is recommended in emergent cases based on the above factors.
• Preoperative RV function assessment
  • Consider echocardiography and a history of RV failure (e.g., inotrope requirements, temporary mechanical support).
  • The more tenuous the RV function, the higher the risk of perioperative RV failure.

Intraoperative Management

• Invasive hemodynamic monitors should be considered to evaluate RV function.
  • Central venous line, arterial line, pulmonary artery catheter, TEE.
• VAD Power Source
  • Backup batteries (x2) fit into the holster, providing 6-10 hours of power.
  • VAD should be connected to an alternating-current source, with batteries available for emergencies.
• VAD Monitor
  • Speed (revolutions per minute (RPMs)): 5000-6200 RPMs for HeartMate III, varies by type of device.
  • Power (Watts): determined by RPMs, typically 4.5-6.5W for HeartMate III.
    • The pump power increases in proportion to increases in the pump speed.
    • Increased pump speed without an increase power is suggestive of pump thrombosis.
  • Pump flow is calculated from the pump speed and power.
  • Pulsatility Index (PI)
    • Dimensionless measure of flow through the pump.
      • Relative increases in LVAD support or decreases in native function will lead to decreased pulsatility.
      • Some degree of pulsatility is recommended, as flow through the aortic valve prevents stasis and associated thrombus formation.
• Pulse oximetry
  • Pulse oximetry is unreliable during periods of low pulsatility.
  • Consider arterial blood gases or cerebral oximeter, which doesn’t require pulsatility.
• Blood Pressure
  • Noninvasive blood pressure is erratic and unreliable during periods of low pulsatility.
  • Arterial cannulation can be challenging in the setting of low pulsatility, yet it is often warranted based on patient factors and surgical risk.
  • Ultrasound guidance may be required.
  • Consider Doppler mean arterial pressure (MAP) measurements for short cases.

Hemodynamic Goals

• Maintain afterload (MAPs 70-90 mmHg) and adequate preload.
• Prevent elevation of pulmonary vascular resistance by limiting hypoxia and hypercarbia.
  • Spontaneous is better than mechanical ventilation.
  • If mechanical ventilation is required, avoid excessive tidal volumes and positive end-expiratory pressure.
• Avoid overaggressive fluid resuscitation in patients who are not volume responsive.
• If right ventricular failure is suspected, chemical or mechanical support should be initiated.
  • Pulmonary vasodilators: inhaled epoprostenol, inhaled nitric oxide.
  • Inotropic support: epinephrine, milrinone, dobutamine.
  • Vasopressor therapy to prevent ischemia: vasopressin, norepinephrine.
• Other issues include:
  • Advanced Cardiac Life Support (ACLS)
    • Do not rely on palpable pulse; assess perfusion by other signs; if not present, proceed with standard ACLS.
    • If shockable rhythm, proceed with cardioversion/defibrillation.
    • Assess the VAD for malfunction – look out for device “humming”.
    • Among reversible causes, consider RV failure first.