Computational investigation of outflow graft variation impact on hemocompatibility profile in LVADs.

BACKGROUND: Hemocompatibility-related adverse events (HRAE) occur commonly in patients with left ventricular assist devices (LVADs) and add to morbidity and mortality. It is unclear whether the outflow graft orientation can impact flow conditions leading to HRAE. This study presents a simulation-based approach using exact patient anatomy from medical images to investigate the influence of outflow cannula orientation in modulating flow conditions leading to HRAEs. METHODS: A 3D model of a proximal aorta and outflow graft was reconstructed from a computed tomography (CT) scan of an LVAD patient and virtually modified to model multiple cannula orientations (n = 10) by varying polar (cranio-caudal) (n = 5) and off-set (anterior-posterior) (n = 2) angles. Time-dependent computational flow simulations were then performed for each anatomical orientation. Qualitative and quantitative hemodynamics metrics of thrombogenicity including time-averaged wall shear stress (TAWSS), oscillatory shear index (OSI), endothelial cell platelet activation potential (ECAP), particle residence time (PRT), and platelet activation potential (PLAP) were analyzed. RESULTS: Within the simulations performed, endothelial cell activation potential (ECAP) and particle residence time (PRT) were found to be lowest with a polar angle of 85°, regardless of offset angle. However, polar angles that produced parameters at levels least associated with thrombosis varied when the offset angle was changed from 0° to 12°. For offset angles of 0° and 12° respectively, flow shear was lowest at 65° and 75°, time averaged wall shear stress (TAWSS) was highest at 85° and 35°, and platelet activation potential (PLAP) was lowest at 65° and 45°. CONCLUSION: This study suggests that computational fluid dynamic modeling based on patient-specific anatomy can be a powerful analytical tool when identifying optimal positioning of an LVAD. Contrary to previous work, our findings suggest that there may be an "ideal" outflow cannula for each individual patient based on a CFD-based hemocompatibility profile.

Using aortic arch short axis views during transesophageal echocardiographic examination facilitates right ventricular assist device imaging.

Given the increased need for mechanical circulatory support and subsequent development of right ventricular assist devices (RVAD), appropriate imaging needs to be described to facilitate care in patients with cardiogenic shock and heart failure. We present three cases in which the upper esophageal aortic arch short axis (UE AA SAX) view on transesophageal echocardiography (TEE) was utilized to effectively image RVADs: to confirm normal positioning, to detect and guide repositioning, and to visualize malfunction. These cases support the importance of the UE AA SAX TEE view in RVAD outflow imaging and, when obtainable, should be included in routine RVAD assessment.

Circulatory Mechanotherapeutics: Moving with the Force.

PURPOSE OF REVIEW: This review describes the current state of advancements in mechanical circulatory support (MCS) devices with significantly improved hemodynamic performance and decreased adverse events. Novel considerations for future MCS designs that impart spiral flow regimes will be detailed. RECENT FINDINGS: Significant challenges in MCS device use have included size reduction, premature pump mechanical bearing failure, acquired bleeding disorders, and vascular complications related to high shear forces and jetting. Some of these problems have been improved upon, such as the use of magnetically levitated impellers and hydrodynamic bearings. The relative simplicity of continuous flow pumps has also enabled their miniaturization, portability, and reduced energy consumption. Recent studies by our group demonstrated that spiral forms of flow possess hemodynamically beneficial attributes at the MCS outflow cannula and aorta interface, reducing jet impact, organizing streamlines, and thereby improving endothelial function through wall shear stress modulation. Despite MCS design improvements, they are far from perfect. Induced spiral fluid modulation may help address the known flow-mediated disturbances in vascular mechanobiology.

Screening for Outflow Cannula Malfunction of Left Ventricular Assist Devices (LVADs) With the Use of Doppler Echocardiography: New LVAD-Specific Reference Values for Contemporary Devices.

BACKGROUND: Echocardiographic assessment of left ventricular assist devices (LVADs) is used as a screening tool to evaluate the integrity and mechanics of the pump and circuit. We aimed to 1) establish the normal range and upper reference limit of peak velocity of the outflow cannula for the modern era of LVADs and 2) assess the clinical performance of the currently cited and newly proposed reference limits in patients with continuous-flow LVADs as a screening tool for cannula malfunction. METHODS: LVAD outflow peak CW velocities were measured with the use of Doppler transthoracic echocardiography (TTE) in 57 patients with LVADs (44 with Heartmate II (HM2), 13 with Heartware (HW)). The average velocity and the upper and lower normal reference limits (defined as ±2 standard deviations from the mean) for each LVAD type was calculated. The upper reference limit was then used as a screening threshold for cannula malfunction. RESULTS: The average outflow cannula peak velocity for the normal HM2 cohort was 1.86 ± 0.44 m/s with upper and lower reference limits of 2.73 m/s and 0.98 m/s, respectively. The average outflow cannula peak velocity for the normal HW cohort was 2.36 ± 0.53 m/s with upper and lower reference limits of 3.42 m/s and 1.3 m/s, respectively, which was significantly higher than the HM2 cohort (P = .004). CONCLUSIONS: In both HM2 and HW LVADs, the average peak outflow velocity and reference limit for the normal population, as measured by Doppler TTE, was markedly higher than the currently used LVAD reference limits of 2 m/s and are significantly different between devices. Patients with peak outflow velocities above our upper reference limits should be evaluated for LVAD outflow cannula malfunction.

Outflow cannula alignment in continuous flow left ventricular devices is associated with stroke.

We sought to evaluate whether differences in left ventricular assist device (LVAD) canula alignment are associated with stroke. There is a paucity of clinical data on contribution of LVAD canulae alignment to strokes. We conducted a retrospective analysis of patients who underwent LVAD implantation at Houston Methodist hospital from 2011 to 2016 and included those who had undergone cardiac computed tomography (CT) with contrast. LVAD graft alignment using X-ray, echocardiography, and cardiac CT was evaluated. The primary outcome was stroke within 1 year of LVAD implantation. Of the 101 patients that underwent LVAD Implantation and cardiac CT scan during the study period, 78 met inclusion criteria. The primary outcome occurred in 12 (15.4%) patients with a median time to stroke of 77 days (interquartile range: 42-132 days). Of these, 10 patients had an ischemic and two had hemorrhagic strokes. The predominant device type was Heart Mate II (94.8%). Patients with LVAD outflow cannula to aortic angle lesser than 37.5° and those with outflow graft diameter of anastomosis less than 1.5 cm (assessed by cardiac CT) had significantly higher stroke risk (p < 0.001 and p = 0.01 respectively). In HMII patients, a lower LVAD speed at the time of CT scan was associated with stroke. Further studies are needed to identify optimal outflow graft configuration to mitigate stroke risk.

Outflow graft foreign body reaction and thrombosis in HeartMate 3 left ventricular assist device.

Outflow cannula occlusion is an infrequent complication occurring among recipients of continuous flow left ventricular assist devices. Hereby, we present a case of a 66-year-old man with a HeartMate 3 left ventricular assist device that presented 18 months after implantation and during his admission, evidence of poor left ventricular unloading and forward flow uncovered intrinsic and extrinsic outflow cannula obstruction. The patient's course was complicated by cardiogenic shock, diffuse alveolar hemorrhage and multiorgan failure. Post-mortem examination suggested foreign body reaction as a potential contributor of outflow graft obstruction at the level of the bend relief.

Conventional and alternative sites for left ventricular assist device inflow and outflow cannula placement.

Left ventricular assist device (LVAD) therapy is a well accepted and effective strategy to treat advanced heart failure. The miniaturized third-generation centrifugal pumps HeartWare HVAD (Medtronic, Dublin, Ireland) and HeartMate 3 (Abbott, Illinois, USA) are the two most commonly implanted systems in the contemporary era. Their design has allowed clinicians to pioneer several alternate and less invasive implantation techniques to tackle a broad spectrum of clinical scenarios. A brief review and discussion of alternative surgical techniques for both inflow and outflow cannula insertion, in the contemporary LVAD surgery era, are herein reported.

Novel echocardiographic parameters of aortic insufficiency in continuous-flow left ventricular assist devices and clinical outcome.

BACKGROUND: The aim of this study was to evaluate the prognostic performance of novel echocardiographic (transthoracic echocardiography, or TTE) parameters for grading aortic insufficiency (AI) severity in patients with continuous-flow left ventricular assist devices (CF-LVADs). The development of AI after CF-LVAD implantation is common, although the clinical significance remains unclear. We previously described novel TTE parameters that outperformed traditional TTE parameters in grading AI severity in these patients. METHODS: CF-LVAD patients with varying degrees of AI (N = 57) underwent Doppler TTE of the LVAD outflow cannula. Patients had AI severity graded by the novel parameters (systolic/diastolic velocity ratio and the diastolic acceleration of the LVAD outflow cannula) and the traditional vena contracta. The prognostic performance of novel and traditional AI parameters was determined by comparing rates of congestive heart failure re-admission, need for aortic valve intervention, urgent transplantation and death (composite end-points) for each parameter. RESULTS: Grading AI severity using novel AI parameters led to reclassification of 32% of patients from trace/mild AI to moderate or greater AI (N = 18). Using traditional AI parameters, there was no difference in the occurrence of the composite end-point between the moderate or greater group and the trace/mild group (1.50 vs 1.18 events/person, p = 0.46). With the novel AI parameters, there were significantly more events in the patients with moderate or greater AI compared to those with trace/mild AI (1.57 vs 0.13 events/person, p = 0.002). Novel parameters also better predicted the need for aortic valve intervention, urgent transplantation or death than traditional methods (p = 0.024 vs p = 0.343). CONCLUSIONS: In patients with CF-LVADs, traditional parameters tend to underestimate AI severity and future cardiac events. Novel AI TTE parameters are better able to discriminate AI severity and predict clinically meaningful outcomes.

Flow visualization in the outflow cannula of an axial blood pump.

The properties of blood flow in the outflow cannula of an axial blood pump play a critical role in potential thrombus formation and vascular injury. In this study, an in vitro flow visualization technique using particle image velocimetry (PIV) was applied to investigate the flow characteristics in the outflow cannula of a FW-2 model axial pump. The two-dimensional (2-D) flow field in the axial central section and the three-dimensional (3-D) flow field in the whole outflow cannula were examined with the PIV system. Tests were carried out with a blood-mimic working fluid in the axial pump at a rotational speed of 8500 ± 20 rpm with a flow rate of 5 L/min. The velocity distribution in the outflow cannula was analyzed to evaluate the flow characteristics. There was no backflow or stagnant flow in the tested area, while the flow velocity rapidly increased outside the boundary layer. A spiral flow was observed near the boundary layer, but this was worn off within the tested area. Based on the results, hemolysis and thrombus formation in the cannula, and injury to aortic endothelium are unlikely to occur due to spiral flow.

Surgical technique influences HeartMate II left ventricular assist device thrombosis.

BACKGROUND: Thrombosis of the HeartMate II (HM2 [Thoratec Corporation, Pleasanton, CA]) is a potentially devastating complication. While attention has been focused on anticoagulation strategies to prevent this complication, the impact of surgical technique has not been assessed. METHODS: Patients undergoing HM2 implantation at two institutions were reviewed. Pump thrombosis (PT) was defined as a clinical syndrome that included more than 30% elevation in pump power, more than 30% elevation in lactate dehydrogenase, and greater than 20% decrease in hemoglobin with the presence of thrombus in the HM2 stator or rotor, or both, at explant or autopsy. A blinded clinician reviewed dimensions and angles of the HM2 obtained from chest x-ray films. Patients demonstrating PT were compared with patients having normal function. RESULTS: Of the 49 patients reviewed, 11 (22.4%) displayed evidence of PT at a median of 42 days after HM2 implantation. Patient with PT had greater acute angulation of the HM2 inflow cannula immediately postoperatively (48.2 ± 6.8 versus 65.4 ± 9.2 degrees, p < 0.001) and after 30 days (50.1 ± 8.0 versus 65.1 ± 9.9 degrees, p < 0.001). Pump pocket depth was lower in the PT group immediately after HM2 implantation (107.0 ± 41.9 versus 144.3 ± 20.3 cm, p < 0.001) and after 30 days (86.0 ± 39.1 versus 113.1 ± 25.4 cm, p = 0.02). Patients with evidence of PT did not have a decrease in end-diastolic diameter (76 ± 9 versus 70 ± 15 mm, p = 0.24) whereas patients in the normal function group had effective remodeling of the left ventricle (70 ± 10 versus 56 ± 12 mm, p = 0.01). CONCLUSIONS: Meticulous surgical technique, which necessitates creating an adequately sized pump pocket and appropriately directing the inflow cannula at the time of operation, may reduce the risk of PT.