Advanced mechanical circulatory support with the HeartMate left ventricular assist device in the year 2000

Left ventricular assist devices: A historical perspective at the intersection of medicine and engineering

Over the last half-century, left ventricular assist device (LVAD) technology has progressed from conceptual therapy for failed cardiopulmonary bypass weaning to an accepted destination therapy for advanced heart failure. The history of LVAD engineering is defined by an initial development phase, which demonstrated the feasibility of such an approach, to the more recent three major generations of commercial devices. In this review, we explore the engineering challenges of LVADs, how they were addressed over time, and the clinical outcomes that resulted from each major technological development. The first generation of commercial LVADs were pulsatile devices, which lacked the appropriate durability due to their number of moving components and hemocompatibility. The second generation of LVADs was defined by replacement of complex, pulsatile pumps with primarily axial, continuous-flow systems with an impeller in the blood passageway. These devices experienced significant commercial success, but the presence of excessive trauma to the blood and in-situ bearing resulted in an unacceptable burden of adverse events. Third generation centrifugal-flow pumps use magnetically suspended rotors within the pump chamber. Superior outcomes with this newest generation of devices have been observed, particularly with respect to hemocompatibility-related adverse events including pump thrombosis, with fully magnetically levitated devices. The future of LVAD engineering includes wireless charging foregoing percutaneous drivelines and more advanced pump control mechanisms, including synchronization of the pump flow with the native cardiac cycle, and varying pump output based on degree of physical exertion using sensor or advanced device-level data triggers.

A review of implantable pulsatile blood pumps: Engineering perspectives

It has been reported that long-term use of continuous-flow mechanical circulatory support devices (CF-MCSDs) may induce complications associated with diminished pulsatility. Pulsatile-flow mechanical circulatory support devices (PF-MCSDs) have the potential of overcoming these shortcomings with the advance of technology. In order to promote in-depth understanding of PF-MCSD technology and thus encourage future mechanical circulatory support device innovations, engineering perspectives of PF-MCSD systems, including mechanical designs, drive mechanisms, working principles, and implantation strategies, are reviewed in this article. Some emerging designs of PF-MCSDs are introduced, and possible elements for next-generation PF-MCSDs are identified.

A review of clinical ventricular assist devices

Given the limited availability of donor hearts, ventricular assist device (VAD) therapy is fast becoming an accepted alternative treatment strategy to treat end-stage heart failure. The field of mechanical ventricular assistance is littered with novel and unique ideas either based on volume displacement or rotary pump technology, which aim to sufficiently restore cardiac output. However, only a select few have made the transition to the clinical arena. Clinical implants were initially dominated by the FDA approved volume displacement Thoratec HeartMate I, IVAD, and PVAD, whilst Berlin Heart's EXCOR, and Abiomed's BVS5000 and AB5000 offered suitable alternatives. However, limitations associated with an inherently large size and reduced lifetime of these devices stimulated the development and subsequent implantation of rotary blood pump (RBP) technology. Almost all of the reviewed RBPs are clinically available in Europe, whilst many are still undergoing clinical trial in the USA. Thoratec's HeartMate II is currently the only rotary device approved by the FDA, and has supported the highest number of patients to date. This pump is joined by MicroMed Cardiovascular's Heart Assist 5 Adult VAD, Jarvik Heart's Jarvik 2000 FlowMaker and Berlin Heart's InCOR as the axial flow devices under investigation in the USA. More recently developed radial flow devices such as WorldHeart's Levacor, Terumo's DuraHeart, and HeartWare's HVAD are increasing in their clinical trial patient numbers. Finally CircuLite's Synergy and Abiomed's Impella are two mixed flow type devices designed to offer partial cardiac support to less sick patients. This review provides a brief overview of the volume displacement and rotary devices which are either clinically available, or undergoing the advanced stages of human clinical trials.

Nursing Education and Implications for Left Ventricular Assist Device Destination Therapy

The HeartMate VE Left Ventricular Assist Device (vented electric abdominally positioned pulsatile blood pump; Thoratec Corp., Pleasanton, CA), approved as a permanent support, or destination therapy, by the US Food and Drug Administration in 2002 and Medicare in 2003, is now a potential therapy for numerous patients. Postimplantation nursing care is crucial to the success of left ventricular support device therapy and long-term recipient outcome. Nurses also contribute to cost containment, making this a viable treatment for the facility and the patient. Consequently, nurses must be educated about left ventricular assist device concepts and challenges, the benefits of device placement, intensive care unit and postintensive care unit daily care requirements, and outpatient preparation. This knowledge will enable nurses to provide necessary care and to educate recipients, families, and community health care providers on how to give appropriate posthospital care.

Mechanical circulatory support: a clinical reality

Mechanical circulatory support is becoming an alternative therapeutic option for patients in cardiogenic shock or advanced cardiac failure who cannot be improved by maximal medical therapy. More than 30 years of engineering development and clinical research have led to a level of efficacy and reliability of ventricular assist devices, which allows promotion of this approach for the most difficult patients. Uses include a gaining-time strategy as a bridge to cardiac transplantation or recovery of native cardiac function, as well as permanent support with the device. The large variety of devices permits every cardiac surgical unit, even those not used to cardiac transplantation, to propose this option to the patient. Recent experience with small silent implantable pumps suggests that the pioneering period of mechanical circulatory support is probably over, and the time has come for precise prospective trials to optimize both patient selection and the timing for utilization. In countries where cardiac transplantation has not developed, there is now an easily accessible technique for management of patients with cardiac failure.

In vitro evaluation of control strategies for an artificial vasculature device

Ventricular assist devices (VADs) have been used successfully as a bridge to transplant in heart failure patients by unloading ventricular volume and restoring the circulation. An artificial vasculature device (AVD) that may better facilitate myocardial recovery than VAD by controlling the afterload seen by the ejecting heart is being developed. The AVD concept is to enable any user-defined input impedance (IM) with resistance (R) and compliance (C) components. In this study, a pulse duplicator was used to test the efficacy of the AVD concept for two control strategies in an adult mock circulation: (1) R-C in series and (2) 2-element Windkessel (R-C in parallel) using instantaneous impedance position control (IIPC) to maintain a desired value or profile of R and C. In vitro experiments were performed and the resulting cardiovascular pressures, volumes, flows, and the afterload (R and C) seen by the LV during ejection for simulated cardiac failure were recorded and analyzed. Our results indicate that setting the AVD to lower IM reduced LV volume and pressure, restored LV stroke volume, and increased coronary flow. The IIPC control algorithms are better suited to maintain any instantaneous IM or an IM profile, but are susceptible to measurement noise.

Hemodynamic and left ventricular pressure-volume responses to counterpulsation in mock circulation and acute large animal models

Alternative therapies for treating heart failure patients are being explored to provide effective options for patients with progressive heart failure. Cardiac assist devices that promote myocardial recovery may be a potential solution. Ventricular assist devices (VAD) have demonstrated long-term efficacy and intraaortic balloon pumps (IABP) have shown short-term successes. In this paper, testing of a hybrid counterpulsation device (CPD) that couples the attributes of device longevity (VAD) with less invasive surgery (IABP) is presented. Hemodynamic and ventricular pressure-volume responses to a 40 ml CPD and 40 ml IABP were evaluated in vitro in an adult mock circulation and in vivo in a large animal heart failure model. The CPD is a flexing diaphragm ventricle with a controlled stroke volume up to 85 cc through a single, valveless cannula. In this study, the CPD was cannulated to the brachiocephalic artery to provide 40 ml of counterpulsation support. The CPD effectively provided diastolic augmentation increasing coronary flow and afterload reduction. These results were comparable to IABP. These preliminary studies suggest that CPD may be an effective therapy for treating patients with early stage heart failure.

Clinical comparison between HeartMate VE auto-mode and HeartMate XVE auto-mode with Opti-Fill and the effect of stroke volume on blood chamber and inflow valve peak pressures

We determined the difference between HeartMate (HM) VE auto mode, average filling 76 mL, and HM XVE Opti-Fill, average filling 79 mL, regarding blood chamber and inflow valve peak pressure pulses (BCPP and IVPP). The relation between stroke volume (SV) and peak pressures was investigated by using a circulatory mock loop. At high SVs, 79 to 83 mL, BCPP and IVPP never exceeded 400 mm Hg. For lower SVs, down to 50 mL, the peak pressures increased to 788 mm Hg for BCPP and 416 mm Hg for IVPP. Distribution of SV was measured in 2 VE and 6 XVE patients during rest and activities of daily living (ADL). For clinical comparison, percentages of SV >78 mL were determined. At rest, 2190 (VE) and 5772 (XVE) pump beats were registered and 4511 (VE) and 8713 (XVE) during ADL. Percentages of "SV >78 mL" at rest, respectively, were 42.5 +/- 3.5 and 78.2 +/- 4.7 (p < 0.01) and during ADL, respectively, 48.7 +/- 7.4 and 73.5 +/- 5.3 (p < 0.01). The Opti-Fill software shows a significant increase in percentage SV >78 mL and makes an important contribution to reducing the incidence of high peak pressures in the clinical setting.

Quantification of Pulsatility as a Function of Vascular Input Impedance: An In Vitro Study

The physiological benefits of pulsatility generated by ventricular assist device (VAD) support continue to be heavily debated as application of VAD support has been expanded to include destination and recovery therapies. In this study, the relationship between input impedance (Zart) and vascular pulsatility during continuous flow (CF) or pulsatile flow (PF) VAD support was investigated. Hemodynamic waveforms were recorded at baseline failure and with 50%, 75%, and 100% CF or PF VAD support for nine different Zart test conditions (combination of three different resistance and compliance settings) in a mock circulatory system simulating left ventricular failure. High-fidelity hemodynamic pressure and flow waveforms were recorded to calculate mean arterial pressure (MAP), Zart, energy equivalent pressure (EEP), and surplus hemodynamic energy (SHE) as metrics for quantifying vascular pulsatility. MAP and EEP were elevated with increasing resistance whereas SHE was reduced with increasing compliance. Vascular pulsatility was restored with increasing PF VAD support, but diminished by up to 90% with increasing CF VAD support. The nonpulsatile energy component (MAP) of the pressure waveform is dependent on resistance whereas the pulsatile energy component (SHE) is dependent on compliance. The impact of Zart and vascular pulsatility on patient recovery with VAD support warrants further investigation.

Improved Mechanical Reliability of the HeartMate XVE Left Ventricular Assist System

BACKGROUND

The HeartMate XVE left ventricular assist device is a valuable treatment option for patients with end-stage heart failure. During the past several years, the XVE has undergone a series of design enhancements to improve reliability. We compared the reliability of the two most recent design iterations of the XVE pump (stitch modification to the inflow valve assembly and new inflow valve housing redesign) to the earlier VE version.

METHODS

A retrospective evaluation of device reliability was performed for 268 devices implanted in 245 patients (VE: n = 167 devices, 147 patients, implant dates October 16, 1998, to December 19, 2003; XVE: n = 101 devices, 98 patients, implant dates August 1, 2002, to April 14, 2004).

RESULTS

Median duration of device support for the VE and XVE was 159 days (range, 0 to 1,206 days) and 229 days (range, 0 to 693 days), respectively (p = 0.495). Significantly fewer major device malfunctions occurred within the XVE group as compared with the VE group (6 versus 36, respectively; p = 0.0003). The number of major device malfunctions per patient-year of support for inflow valve dysfunction, bearing wear, and other failures for the VE and XVE were 0.2 versus 0.04 (p = 0.006), 0.16 versus 0.01 (p = 0.005), and 0.06 versus 0.04 (p = 1.000), respectively. The freedom from major device malfunction at 1 year was 76% +/- 6% for the VE and 97% +/- 2% for the XVE device (p < 0.001). The freedom from death as a result of major device malfunction at 1 year was 97% +/- 2% for the VE and 98% +/- 2% for the XVE (p = 0.698).

CONCLUSIONS

Design enhancements to the HeartMate XVE have significantly reduced the incidence of major device malfunctions compared with the earlier VE model because of a reduction in failure modes from bearing wear and inlet valve dysfunction. Further follow-up is necessary to establish the long-term durability of the most recent XVE pump version.

Improved durability of the HeartMate XVE left ventricular assist device provides safe mechanical support up to 1 year but is associated with high risk of device failure in the second year

BACKGROUND

Life-threatening device failure of the HeartMate VE due to biologic inflow valve incompetence or motor failure is a major drawback of long-term mechanical support when using this left ventricular assist device (LVAD). The new XVE model is the result of recent technical improvements. The aim of this study was to compare the clinical performance and durability of the new and earlier HeartMate versions.

METHODS

We analyzed the incidence of device failure and of other device-specific complications (infections, bleeding) in 9 VE and 17 XVE patients. Explanted pumps were examined and biologic valve damage classified according to a score ranging from 0 (no visible damage) to 3 (severe destruction).

RESULTS

Mean support time was 145 +/- 92 and 267 +/- 195 days in the VE and XVE groups, respectively (difference not significant [NS]). Survival was 89% (VE) vs 75% (XVE). The incidence of device failure requiring urgent heart transplantation or device replacement was 44% (VE) vs 31% (XVE) (NS). Device failure occurred significantly later in the XVE group (200 +/- 34 vs 487 +/- 53 days, p < 0.01). Causes of device failure were inflow valve incompetence (n = 6) and motor failure (n = 3). Acute device failure caused 1 death in the XVE group. One XVE patient has been on mechanical support for > 483 days. Macroscopic inflow valve damage score after explantation of the devices was 2.2 +/- 1.1 in the VE group and 2.0 +/- 0.8 in the XVE group (NS).

CONCLUSIONS

The novel HeartMate XVE offers greater durability and provides reliable mechanical support in the first year. However, there is a high risk of life-threatening device failure in the second year. Further technical refinements are necessary to meet the challenges of safe long-term circulatory assistance.

Ascending Aorta Outflow Graft Location and Pulsatile Ventricular Assist Provide Optimal Hemodynamic Support in an Adult Mock Circulation

Although continuous flow (CF) and pulsatile flow (PF) ventricular assist devices (VADs) are being clinically used, their effects on aortic blood flow, as a measure of overall blood distribution, remain unclear. In acute VAD support animal experiments, our group has described a zone of turbulent mixing in the aortic arch. The objective of this study was to confirm this finding in the controlled setting of an adult mock circulation, simulating ventricular pathophysiologic states (normal and failing ventricle). CF and PF flow VADs were connected to ventricular apical inflow and ascending aorta (AA) or descending aorta (DA) outflow cannulae. Cardiovascular pressure and flow waveforms were recorded at varying levels of VAD bypass resulting in four test conditions: (i) CF-AA; (ii) CF-DA; (iii) PF-AA; and (iv) PF-DA. Confirming the animal data, no differences in mean aortic flow between CF and PF VADs were found, and significantly lower mean aortic arch flow with DA cannulation was noted. Mean aortic root flow decreased with increasing VAD bypass flow. As in the animal studies, despite similar mean flow rates, significant differences in waveform morphology were observed for AA and DA outflow graft locations and varying levels of VAD bypass. At 100% VAD support in the failing heart, PF restored waveform pulsatility to normal baseline while CF resulted in little pulsatility. These results confirm our earlier findings in the animal model, suggesting that outflow graft location may have a significant effect on aortic blood flow distribution. The long-term implications of these findings are being examined in ongoing studies.

Hemodynamic and Pressure–Volume Responses to Continuous and Pulsatile Ventricular Assist in an Adult Mock Circulation

This study investigated the hemodynamic and left ventricular (LV) pressure-volume loop responses to continuous versus pulsatile assist techniques at 50% and 100% bypass flow rates during simulated ventricular pathophysiologic states (normal, failing, recovery) with Starling response behavior in an adult mock circulation. The rationale for this approach was the desire to conduct a preliminary investigation in a well controlled environment that cannot be as easily produced in an animal model or clinical setting. Continuous and pulsatile flow ventricular assist devices (VADs) were connected to ventricular apical and aortic root return cannulae. The mock circulation was instrumented with a pressure-volume conductance catheter for simultaneous measurement of aortic root pressure and LV pressure and volume; a left atrial pressure catheter; a distal aortic pressure catheter; and aortic root, aortic distal, VAD output, and coronary flow probes. Filling pressures (mean left atrial and LV end diastolic) were reduced with each assist technique; continuous assist reduced filling pressures by 50% more than pulsatile. This reduction, however, was at the expense of a higher mean distal aortic pressure and lower diastolic to systolic coronary artery flow ratio. At full bypass flow (100%) for both assist devices, there was a pronounced effect on hemodynamic parameters, whereas the lesser bypass flow (50%) had only a slight influence. Hemodynamic responses to continuous and pulsatile assist during simulated heart failure differed from normal and recovery states. These findings suggest the potential for differences in endocardial perfusion between assist techniques that may warrant further investigation in an in vivo model, the need for controlling the amount of bypass flow, and the importance in considering the choice of in vivo model.

Ventricular assist devices

PURPOSE OF REVIEW

Recent advances in technology as well as new indications for implantation have appeared in the field of ventricular assist devices. Progress has also been made in the understanding of the underlying mechanisms of myocardial recovery after ventricular assist device support.

RECENT FINDINGS

Technological progress includes the development of fully implantable pulsatile and continuous flow pumps, either axial flow or centrifugal, for left ventricular and total heart assistance. Among the new indications for ventricular assist device support, the most important is the use of the device as permanent treatment for end-stage cardiac failure patients. Increased knowledge has been acquired regarding the effects of mechanical assistance and of unloading of the heart on haemodynamics, as well as on the cellular, molecular and electrophysiological characteristics of the failing heart. All these findings suggest that depressed myocardial function can sometimes recover with ventricular assist device therapy. Ventricular assist device support, however, still carries a high rate of complications: the device itself can fail, bleeding and thromboembolism are common, immunity is disturbed and the incidence of infection remains high.

SUMMARY

In patients with end-stage heart failure, ventricular assist devices can be used as a bridge to transplantation or to recovery, but they are now also considered as a long-term myocardial replacement therapy. Which device is the most appropriate for each indication, however, remains to be defined. Even if the underlying mechanisms of myocardial recovery are progressively clarified, the use of ventricular assist devices as a bridge to recovery still has limited clinical success. Clinical trials with the fully implantable devices are in their early stages, but these pumps appear promising in terms of efficacy, reliability and complication rate, as well as being easy to implant. Because more patients will benefit from ventricular assist device placement in the future, anaesthesiologists must be prepared to manage patients undergoing ventricular assist device placement or presenting for noncardiac surgery while under ventricular assist device support.

The left ventricular assist device

Of the 400,000 people in the United States who develop end-stage heart failure each year, 60,000 are unresponsive to medical therapy and 2,500 undergo heart transplantation. Surgically implanted pumps, called LVADs, are extending many lives.

Hospital discharge for the ventricular assist device patient: historical perspective and description of a successful program

Patients who are supported with an implantable ventricular assist device (VAD) as a bridge to cardiac transplantation are potential candidates for hospital discharge. Hospital discharge rates in reported clinical series vary from 27% to 60%. Many of the patients included in these series, however, were subjects of premarket approval clinical trials and as such, are bound by rigid eligibility criteria for discharge. According to a voluntary registry maintained by Thermo Cardiosystems, Inc., the postmarket discharge rate in patients supported with the HeartMate VE LVAS is approximately 25%, a number that is artificially low due to incomplete reporting. The postmarket discharge rate at the busiest Thermo Cardiosystems HeartMate VE LVAS centers is 53%. Clearly, discharge rates need to increase if the VAD is ever to be considered a viable destination therapy for end-stage heart failure. The discharge program instituted at The University of Iowa incorporates patient and family training as well as community services education. Between January 1999 and April 2001, fourteen patients received VAD support as a bridge to transplantation with the HeartMate VE LVAS. Thirteen patients (93%) were discharged from the hospital. Eight of the 13 patients have been transplanted after having spent 65.8% +/- 31.4% of their period of blood pump support as an outpatient. Eight of 13 patients (62%) required a total of 20 unplanned repeat hospitalizations. A well defined, aggressively implemented discharge program can adequately prepare the VAD patient for the transition from hospital to home.

Recent progress on the vibrating flow pump as a totally implantable ventricular assist device

This study describes the present state of progress in the development of the vibrating flow pump (VFP) ventricular assist system. We have proceeded with development aiming at a totally implantable ventricular assist system with smaller size and lighter weight appropriate for Asians like the Japanese by increasing the drive frequency. An actuator is important for the development of the miniature sized and lightweight artificial heart. We applied a linear motor for the mechanical part at first. The step motor was applied after that. This form may be best if we want the lightweight small sized motor for an actuator. The cross slider form is applied at present. It succeeded in the miniaturization compared with the linear motor. In the VFP-type ventricular assist system, the blood contact parts are a central vibration tube with inflow and outflow chambers. We designed round diaphragms to prevent thrombus formation. In addition, we developed an energy transmission system for total implantation. The VFP creates a high frequency oscillated blood flow. It has a unique flow pattern. Brain blood flow increased although the total flow of the circulation did not change in the frequency of 25 to 30 Hz. The quantitative evaluation of the autonomic nerve function during the left heart assistance with an oscillated blood flow was carried out by spectral analysis. Some influences on an autonomic nerve were observed by the VFP left heart assistance. We will continue development research with the aim of clinical application.