Left Ventricular Assist Device Operation Research Articles

The Relation Between Viscous Energy Dissipation and Pulsation for Aortic Hemodynamics Driven by a Left Ventricular Assist Device.

Left ventricular assist device (LVAD) provides mechanical circulatory support for patients with advanced heart failure. Treatment using LVAD is commonly associated with complications such as stroke and gastro-intestinal bleeding. These complications are intimately related to the state of hemodynamics in the aorta, driven by a jet flow from the LVAD outflow graft that impinges into the aorta wall. Here we conduct a systematic analyses of hemodynamics driven by an LVAD with a specific focus on viscous energy transport and dissipation. We conduct a complementary set of analysis using idealized cylindrical tubes with diameter equivalent to common carotid artery and aorta, and a patient-specific model of 27 different LVAD configurations. Results from our analysis demonstrate how energy dissipation is governed by key parameters such as frequency and pulsation, wall elasticity, and LVAD outflow graft surgical anastomosis. We find that frequency, pulsation, and surgical angles have a dominant effect, while wall elasticity has a weaker effect, in determining the state of energy dissipation. For the patient-specific scenario, we also find that energy dissipation is higher in the aortic arch and lower in the abdominal aorta, when compared to the baseline flow without an LVAD. This further illustrates the key hemodynamic role played by the LVAD outflow jet impingement, and subsequent aortic hemodynamics during LVAD operation.

CARD33: Analyzing Off-Design Point Operation for LVADs Using Flexible Rotor Blades

Introduction: The use of left ventricular assist devices (LVADs) has exhibited clinical success in both destination and bridge-to-transplant therapy for patients suffering from end stage heart failure (HF). However, the operating speeds of the LVAD are often tuned to accommodate patient variability which might lead to operating the LVAD at off design points. This could cause a potential loss in efficiency and an increased risk of thrombosis/hemolysis. The flexible rotor blades will deform in response to the complex inflow conditions, and with an appropriate geometry and flexibility generate a flatter performance curve. This would not only improve the preload sensitivity of the LVAD but also offer a wider range of operation with optimal efficiencies. In this study, we investigate the hydraulic performance of varied impeller geometries for a centrifugal LVAD with flexible rotor blades at pediatric and adult pressure/flow conditions. Methods: The flexible rotor blades are manufactured by casting polyurethane resin (Easton, PA) of shore hardness 60A in a four-piece mold. These mold pieces were 3D printed using a J850 Pro (Stratasys Ltd, MN, USA). The inlet angle of the blades was set at 20o and the outlet angle was varied from 10o to 70o. The blade thickness was 1.2 mm and the blade height was 7 mm. These flexible rotor blades were evaluated on a hydrodynamic test setup with 40%wt glycerin solution as the working fluid. This setup consisted of a motor to drive the rotor, a flow probe, pressure transducers upstream and downstream of the rotor and a pinch valve to vary the downstream resistance. The mounted rotor is driven at 3000 RPM, and the flow produced by the rotor is measured as the pinch valve incrementally increases downstream resistance. The hydrodynamic results were then scaled to achieve pediatric and adult pressure/flow conditions of 70mmHg-2L/min and 90mmHg-5L/min respectively. To analyze the off-design point operation, the gradient of the pressure/flow curve at two operating conditions was calculated using the equation in Figure 1. Results: For a flexible rotor blade with an outlet angle of 30o, the operating speed for pediatric and adult pressure/flow conditions was found to be 2550 RPM and 3230 RPM respectively. The gradient at the pediatric and adult operating conditions was – 6.14 mmHg/ (L/min)-1 and – 11.12 mmHg/ (L/min)-1. Results for the remaining flexible rotor blades is currently being generated and will be presented at the conference. Conclusion: We successfully analyzed the hydraulic performance of a centrifugal LVAD with flexible rotor blades. The flexible rotor blades proposed in this study will be a positive step towards improving the off-design point operation of LVADs and thereby, reducing the blood damage associated with it.Figure 1. Equation used to calculate the gradient of the pressure/flow curve at two operating conditionsFigure 2. Hydrodynamic Test Setup SchematicFigure 3. Hydrodynamic Performance Results for Flexible Rotor Blade with 30o Outlet Angle

Impact of preoperative atrial fibrillation in patients with left ventricular assist device: A systematic review and meta-analysis.

Atrial fibrillation (AF) is a common finding in patients evaluated for left ventricular assist device (LVAD). There is conflicting data regarding the mortality risk as well as the thromboembolic risk in patients with preoperative AF who undergo LVAD implantation. We examined these risks by performing a meta-analysis. We performed a literature search of Pubmed, EMBASE, SCOPUS, and Cochrane from inception to February 2018. The eligible studies were used to compare mortality rate and thromboembolic risk between AF and Non-AF (NAF) groups after LVAD implantation. We obtained 391 articles from our search strategy. Seven retrospective studies were included and accounted for 5823 LVAD patients (AF 1589; NAF 4234). The median follow-up duration ranged from 7-24 months. The pooled analysis revealed a significantly increased risk of mortality in preoperative AF patients who underwent LVAD operation compared to those with NAF (Risk Ratio [RR] 1.16, 95% CI 1.05-1.28, I2 = 0%). Five studies reported thromboembolism events involving 1359 preoperative AF and 3893 NAF patients. The pooled analysis did not show a statistically significant association between risk of thromboembolic event and preoperative AF (Risk Ratio [RR] 1.08, 95% CI 0.86-1.36, I2 = 76.2%). Our study shows that preoperative AF may be associated with a higher mortality rate. This study is limited by the fact that the data are pooled from retrospective studies. Further prospective studies are warranted in order to validate these results.

Quantification of Pulsed Operation of Rotary Left Ventricular Assist Devices with Wave Intensity Analysis.

The current generation of left ventricular assist devices (LVADs) provides continuous flow and has the capacity to reduce aortic pulsatility, which may be related to a range of complications associated with these devices. Pulsed LVAD operation using speed modulation presents a mechanism to restore aortic pulsatility and potentially mitigate complications. We sought to investigate the interaction of axial and centrifugal LVADs with the LV and quantify the effects of continuous and pulsed LVAD operations on LV generated wave patterns under different physiologic conditions using wave intensity analysis (WIA) method. The axial LVAD created greater wave intensity associated with LV relaxation. In both LVADs, there were only minor and variable differences between the continuous and pulsed operations. The response to physiologic stress was preserved with LVAD implantation as wave intensity increased marginally with volume loading and significantly with infusion of norepinephrine. Our findings and a new approach of investigating aortic wave patterns based on WIA are expected to provide useful clinical insights to determine the ideal operation of LVADs.

Impact of Residual Valve Disease on Survival After Implantation of Left Ventricular Assist Devices

This retrospective single-institutional study investigated the effect of significant valvular regurgitation after left ventricular assist device (LVAD) implantation and the role of concomitant valve surgery during LVAD surgery. All patients receiving an LVAD (HeartMate II; Abbott, Abbott Park, IL) during 2008 and 2015 were enrolled. The cohort was divided into two groups based on the valve status at the end of the operation: patients without significant valvular regurgitation (no-VR) and those with significant valvular regurgitation (residual-VR). An LVAD was implanted in 270 patients. Of these, 233 had significant preoperative valve disease(s), and 180 (66.6%) received concomitant valve interventions (35 aortic, 90 mitral, 138 tricuspid). At the completion of the LVAD operation, 204 had no-VR and 66 had residual-VR. Short-term outcomes were similar in two groups, except for longer cardiopulmonary bypass time in the no-VR group (149 minutes) than in the residual-VR group (132 minutes, p= 0.038). During follow-up, mean pulmonary artery pressure (24 mm Hg in no-VR vs 27 mm Hg in residual-VR; p= 0.018) and pulmonary vascular resistance (1.8 Wood units in no-VR vs 2.7 Wood units in residual-VR, p= 0.008) significantly improved in no-VR group compared with the residual-VR group. Survival and heart failure admission-free rates at 1 year were significantly superior in the no-VR group compared with the residual-VR group (1-year survival: 72% ± 3% No-VR vs 55% ± 6% residual-VR; log-rank p= 0.026; admission-free survival: 91% ± 2% no-VR vs 74% ± 7% residual-VR; log-rank p= 0.026). Concomitant valve intervention was not an independent predictor of in-hospital death and morbidity. Absence of valvular lesion after LVAD implantation was associated with improved midterm hemodynamics and survival in the current study. Aggressive surgical intervention for significant valve disease(s) at the time of LVAD implant may be beneficial to selected patients.

Left Ventricular Assist Device Design Reduces von Willebrand Factor Degradation: A Comparative Study Between the HeartMate II and the EVAHEART Left Ventricular Assist System

Supraphysiologic shear stress from continuous-flow left ventricular assist devices (LVADs) accelerates von Willebrand factor (vWF) degradation and predisposes patients to nonsurgical bleeding. It is unknown whether unique design characteristics of LVADs differentially affect vWF degradation. We tested the hypothesis that the centrifugal-flow EVAHEART (Evaheart, Houston, TX) left ventricular assist system (LVAS), which was designed to minimize shear stress (low operational revolutions per minute [rpm], larger flow gaps, low shear stress, flat H-Q curve), reduced vWF degradation versus the axial-flow HeartMate II (Thoratec, Pleasanton, CA) LVAD. Whole human blood was obtained from volunteer donors (n= 22). Blood was circulated for 12 hours in mock circulatory loops through a HeartMate II (n= 10; 11,400 rpm, 6.3 ± 0.8 L/min, 76 ± 2 mm Hg) or an EVAHEART LVAS (n= 12; 2,300 rpm, 5.7 ± 0.1 L/min, 80± 1 mm Hg). vWF degradation was characterized with electrophoresis and immunoblotting for large vWF multimers and 11 vWF degradation fragments. The HeartMate II eliminated large vWF multimers and significantly (p < 0.05) increased 10 of 11 vWF degradation fragments at 6 and 12 hours. The increase was approximately 2.0-fold at 6 hours and 2.2-fold at 12 hours. In contrast, the EVAHEART LVAS modestly reduced large vWF multimers and significantly increased 5 of 11 and 8 of 11 vWF degradation fragments at 6 and 12 hours, respectively. The increase was approximately 1.5-fold at 6 hours and 1.7-fold at 12 hours. The EVAHEART LVAS caused significantly less degradation (p < 0.01) than the HeartMate II of the 140 kDa vWF fragment (cleavage product of ADAMTS-13, the vWF protease). The EVAHEART LVAS caused significantly less vWF degradation than the HeartMate II in a mock circulatory loop with whole human blood. LVAD design features may minimize vWF degradation. These data may inform the design and operation of next-generation LVADs to minimize blood trauma.

Acoustic Characterization of Axial Flow Left Ventricular Assist Device Operation In Vitro and In Vivo.

The use of left ventricular assist devices (LVADs), implantable pumps used to supplement cardiac output, has become an increasingly common and effective treatment for advanced heart failure. Although modern continuous-flow LVADs improve quality of life and survival more than medical management of heart failure, device malfunction remains a common concern. Improved noninvasive methods for assessment of LVAD function are needed to detect device complications. An electronic stethoscope was used to record sounds from the HeartMate II axial flow pump in vitro and in vivo. The data were then uploaded to a computer and analyzed using two types of acoustic analysis software. Left ventricular assist device acoustics were quantified and were related to pump speed, acoustic environment, and inflow and outflow graft patency. Peak frequency values measured in vivo were found to correlate strongly with both predicted values and in vitro measurements (r > 0.999). Plots of the area under the acoustic spectrum curve, obtained by integrating over 50 Hz increments, showed strong correlations between in vivo and in vitro measurements (r > 0.966). Device thrombosis was found to be associated with reduced LVAD acoustic amplitude in two patients who underwent surgical device exchange.

A clinical method for mapping and quantifying blood stasis in the left ventricle

In patients at risk of intraventrcular thrombosis, the benefits of chronic anticoagulation therapy need to be balanced with the pro-hemorrhagic effects of therapy. Blood stasis in the cardiac chambers is a recognized risk factor for intracardiac thrombosis and potential cardiogenic embolic events. In this work, we present a novel flow image-based method to assess the location and extent of intraventricular stasis regions inside the left ventricle (LV) by digital processing flow-velocity images obtained either by phase-contrast magnetic resonance (PCMR) or 2D color-Doppler velocimetry (echo-CDV). This approach is based on quantifying the distribution of the blood Residence Time (TR) from time-resolved blood velocity fields in the LV. We tested the new method in illustrative examples of normal hearts, patients with dilated cardiomyopathy and one patient before and after the implantation of a left ventricular assist device (LVAD). The method allowed us to assess in-vivo the location and extent of the stasis regions in the LV. Original metrics were developed to integrate flow properties into simple scalars suitable for a robust and personalized assessment of the risk of thrombosis. From a clinical perspective, this work introduces the new paradigm that quantitative flow dynamics can provide the basis to obtain subclinical markers of intraventricular thrombosis risk. The early prediction of LV blood stasis may result in decrease strokes by appropriate use of anticoagulant therapy for the purpose of primary and secondary prevention. It may also have a significant impact on LVAD device design and operation set-up.

Heart monitoring using left ventricle impedance and ventricular electrocardiography in left ventricular assist device patients

BackgroundPatients who develop critical arrhythmia during left ventricular assist device (LVAD) perfusion have a low survival rate. For diagnosis of unexpected heart abnormalities, new heart-monitoring methods are required for patients supported by LVAD perfusion. Ventricular electrocardiography using electrodes implanted in the ventricle to detect heart contractions is unsuitable if the heart is abnormal. Left ventricular impedance (LVI) is useful for monitoring heart movement but does not show abnormal action potential in the heart muscle.ObjectivesTo detect detailed abnormal heart conditions, we obtained ventricular electrocardiograms (v-ECGs) and LVI simultaneously in porcine models connected to LVADs.MethodsIn the porcine models, electrodes were set on the heart apex and ascending aorta for real-time measurements of v-ECGs and LVI. As the carrier current frequency of the LVI was adjusted to 30 kHz, it was easily derived from the original v-ECG signal by using a high-pass filter (cutoff: 10 kHz). In addition, v-ECGs with a frequency band of 0.1 – 120 Hz were easily derived using a low-pass filter. Simultaneous v-ECG and LVI data were compared to detect heart volume changes during the Q-T period when the heart contracted. A new real-time algorithm for comparison of v-ECGs and LVI determined whether the porcine heartbeats were normal or abnormal. Several abnormal heartbeats were detected using the LVADs operating in asynchronous mode, most of which were premature ventricle contractions (PVCs). To evaluate the accuracy of the new method, the results obtained were compared to normal ECG data and cardiac output measured simultaneously using commercial devices.ResultsThe new method provided more accurate detection of abnormal heart movements. This method can be used for various heart diseases, even those in which the cardiac output is heavily affected by LVAD operation.

Pulsatile mode of operation of left ventricular assist devices and pulmonary haemodynamics

To determine the effect of differing modes of left ventricular assist device (LVAD) operation: synchronous, independent (asynchronous or pseudosynchronous) or counter pulsation (antisynchronous), on left atrial pressure, pulmonary artery pressure, pulmonary blood flow and right ventricular work load, utilizing a previously published electrical analogy of the systemic and pulmonary circulation and the heart. A previously published electrical analogy of the systemic and pulmonary circulation was utilized. The Simulation Package with Integrated Circuit Emphasis (LTSPICE IV) was utilized. Three LVAD operation mode scenarios were analysed: synchronous, counter pulsation and independent pulsatile. The root mean square of the pulmonary artery pressure (PAP), left atrial pressure (LAP) and pulmonary blood flow (PBF) were calculated, as was the right ventricular work load. Counter pulsation LVAD operation resulted in the lowest LAP, PAP, right ventricular work load and the highest pulmonary blood flow. Independent pulsation resulted in the highest LAP, PAP and the lowest pulmonary blood flow. This technique actually increased RV work load. If an LVAD is to be operated in a pulsatile mode, the counter pulsation mode reduces pulmonary artery and left atrial pressure and increases pulmonary blood flow and thus cardiac output. This is in addition to the reduced right ventricular energetic requirement, a finding previously described. Clinical validation of our findings is necessary.

퍼지 subtractive 클러스터링 기법을 이용한 좌심실보조장치 모델링 및 흡입현상 검출

좌심실보조장치의 모델과 안전한 장치 구동을 위한 흡입현상 검출을 위한 방법을 제안한다. 좌심실보조장치인 축류혈액펌프는 심장에 문제가 있는 환자를 보조하기 위하여 사용되어 왔다. 축류혈액펌프는 비맥동성 펌프이며, 맥동성 펌프에 비하여 작은 크기와 효율성과 같은 장점이 있으나, 안전한 펌프 운전 조건을 결정하는 데 어려움이 있다. 축류혈액펌프는 정상상태와 흡입상태와 같은 상이한 펌프 동작 상태를 가지며, 이는 좌심실에서 흡입현상 발생여부에 좌우된다. 퍼지 subtractive 클러스터링 기법을 이용하여, 이와 같은 동작 특성을 가지는 축류혈액펌프 모델을 개발하며, 개발한 펌프 모델을 이용하여 흡입현상 발생 전후의 펌프 혈류량을 추정한다. 또한 퍼지 subtractive 클러스터링 기법을 이용하여 좌심실에서 흡입현상 발생여부를 감지할 수 있는 흡입현상 검출 모델을 개발한다. A method to model left ventricular assist device (LVAD) and detect suction occurrence for safe LVAD operation is presented. An axial flow blood pump as a LVAD has been used to assist patient with heart problems. While an axial flow blood pump, a kind of a non-pulsatile pump, has relative advantages of small size and efficiency compared to pulsatile devices, it has a difficulty in determining a safe pump operating condition. It can show different pump operating statuses such as a normal status and a suction status whether suction occurs in left ventricle or not. A fuzzy subtractive clustering method is used to determine a model of the axial flow blood pump with this pump operating characteristic and the developed pump model can provide blood flow estimates before and after suction occurrence in left ventricle. Also, a fuzzy subtractive clustering method is utilized to develop a suction detection model which can identify whether suction occurs in left ventricle or not.

Synchrony Relationships between the Left Ventricle and a left Ventricular Assist Device: An Experimental Study in Pigs

Synchronization between the left ventricle and a left ventricular assist device (LVAD) may be important for ventricular unloading and coronary perfusion. We assessed the synchrony between cardiac and LVAD cycles by increasing delays in steps of 100 msec throughout the cycle, under conditions of total and partial left ventricular support. We studied 7 healthy minipigs weighing 30-40 kg. A 60-cc Berlin Heart Excor LVAD was implanted and connected to a BCM 1200 console, making it possible to synchronize the LVAD systole and the EKG signal with a prefixed delay. We recorded hemodynamic parameters (including aortic, pulmonary, and left ventricular pressure) and LVAD flow for each delay. Intraventricular pressure during LVAD systole was minimized with delays of around 40-80% of one cycle. In addition, total flow was higher under these conditions. This study shows that the synchronous mode of LVAD operation is feasible. Moreover, a delay in device contraction until the second half of the cardiac cycle optimizes ventricular unloading and may eventually improve myocardial recovery.

Effects of Gamma Radiation on a Ventricular Assist Device and Its Percutaneous Lead Components

A patient supported by a left ventricular assist device (LVAD) presented with an abdominal tumor requiring consolidative radiation therapy. To assess the effects of radiation therapy on the operation of the ventricular assist device (VAD) system and assure that the treatment would be safe for the patient with regard to the operation of the VAD system, sample equipment was irradiated and then tested for functionality. Changes in the mechanical properties of components of the percutaneous lead were measured. After testing, it was concluded that radiation therapy would not impede the operation of the pump or produce deleterious alterations of mechanical properties of the various system components. The patient underwent radiation therapy with a total dose of 2,000 cGy without evident complications. There were no observed effects on the LVAD operation nor any indication of alarms or malfunctions. Subsequently, the patient recovered adequate cardiac function for explant of the LVAD and the recovered components were also analyzed confirming the absence of alterations in material properties that would endanger patient safety.

Development and Validation of a Computational Fluid Dynamics Methodology for Simulation of Pulsatile Left Ventricular Assist Devices

An unsteady computational fluid dynamic methodology was developed so that design analyses could be undertaken for devices such as the 50cc Penn State positive-displacement left ventricular assist device (LVAD). The piston motion observed in vitro was modeled, yielding the physiologic flow waveform observed during pulsatile experiments. Valve closure was modeled numerically by locally increasing fluid viscosity during the closed phase. Computational geometry contained Bjork-Shiley Monostrut mechanical heart valves in mitral and aortic positions. Cases for computational analysis included LVAD operation under steady-flow and pulsatile-flow conditions. Computations were validated by comparing simulation results with previously obtained in vitro particle image velocimetry (PIV) measurements. The steady portion of the analysis studied effects of mitral valve orientation, comparing the computational results with in vitro data obtained from mock circulatory loop experiments. The velocity field showed good qualitative agreement with the in vitro PIV data. The pulsatile flow simulations modeled the unsteady flow phenomena associated with a positive-displacement LVAD operating through several beat cycles. Flow velocity gradients allowed computation of the scalar wall strain rate, an important factor for determining hemodynamics of the device. Velocity magnitude contours compared well with PIV data throughout the cycle. Computational wall shear rates over the pulsatile cycle were found to be in the same range as wall shear rates observed in vitro.

Right ventricular blood flow during left ventricular support in an experimental porcine model

Right ventricular blood flow may be adversely affected during left ventricular assist device (LVAD) use leading to right ventricular (RV) ischemia and RV dysfunction. This study characterized normal RV blood flow responses to LVAD operation. Seven Yorkshire pigs weighing 74.4 +/- 3.4 kg underwent right coronary artery blood flow measurements with an ultrasonic flow probe and injection of radiolabeled microspheres. A Thoratec LVAD was used in either synchronous or asynchronous modes and RV loading was increased using a pulmonary artery snare. The RV blood flow was compared between three regions that differed in proximity to the right coronary artery: proximal segment, mid-RV, and distal. The right ventricular distal flow was 0.93 +/- 0.07 mL x min-1 x g-1 compared with 0.74 +/- 0.06 mL x min-1 x g-1 at right ventricular proximal flow during control measurements (p = 0.0001). This difference was maintained during LVAD operation in either synchronous or asynchronous modes and also during pulmonary artery constriction. Global RV flow is not adversely affected by LVAD use. A flow gradient occurs along the right coronary artery with the distal vascular bed having relatively less reserve, which may be more susceptible to ischemia in patients with preexisting coronary disease or RV distention during LVAD use.