Effect of Outflow Graft Size on Flow in the Aortic Arch and Cerebral Blood Flow in Continuous Flow Pumps: Possible Relevance to Strokes

Outcomes After Heartmate 3 Left Ventricular Assist Device Implantation Using a 10 mm Outflow Graft

The presence of adhesions and patent bypass grafts may create challenges for standard 14 mm outflow graft placement during left ventricular assist device implantation. We retrospectively describe our experience using a 10 mm Bioline Fusion graft (Getinge, Goteborg, Sweden) as the outflow graft in patients undergoing primary Heartmate 3 (Abbott, Abbott Park, IL) implantation. One hundred one patients underwent Heartmate 3 left ventricular assist device implantation, 80% via a thoracotomy approach, with the standard 14 mm outflow graft (78) or a 10 mm Bioline Fusion outflow graft (23). Initial postoperative rotor speed-to-flow ratio (the revolutions per minutes (RPMs) required to achieve a given flow) was significantly higher in 10 mm graft patients (1,472 vs. 1,283 RPM/L/min; p = 0.03), suggesting elevated resistance in the smaller graft. Furthermore, the initial postoperative vasoactive-inotrope score was higher in the 10 mm graft patients (24.1 vs. 17.6; p = 0.022). Postoperative outcomes were similar between groups. In conclusion, the use of a 10 mm graft was associated with higher RPMs needed to generate a given flow and a higher vasoactive-inotrope score, but these differences were not associated with increased right ventricular failure or mortality.

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.

Flow assessment as a function of pump timing of tubular pulsatile pump for use as a ventricular assist device in a left heart simulator

INTRODUCTION

Although mechanical circulatory support saved many lives during the last decade, clinical observations have shown that the continuous flow pumps are associated with a much higher incidence of gastrointestinal bleeding and kidney problems, among others, compared with the earlier generation pulsatile pumps. However, the presence of several moving mechanical components made pulsatile pumps less durable, bulky, and prone to malfunction, ultimately leading to bias in favor of continuous flow designs.

OBJECTIVE

The aim of the current work is to create a prototype tubular pulsatile pump and to test the timing of the pump in a left heart simulator.

METHODS

A left heart simulator to mimic pumping from a failing heart was created. This was used to experimentally test the output of a prototype ventricular assist device relative to a failing heart in the form of flow and pressure. The effect of pulsation timing was quantified.

RESULTS

A failing heart was simulated with an average flow rate of 1.1 L/min and a systolic pressure of 47 mmHg. With the pump, the flow rate increases to 4.8 L/min and a systolic pressure of 110mmHg, in a copulsation mode, while activating for 300-400 ms. If the activation time is reduced, or increased, the pump becomes less effective. Load on the heart is reduced when the pump operates in a counterpulsation mode.

CONCLUSION

A pulsatile pump, like the one proposed, provides adequate output for mechanical circulatory support, while minimizing the number of moving parts that could otherwise lead to tribological wear.

Graft Resistance Difference after HVAD to HeartMate 3 Left Ventricular Assist Device Exchange

BACKGROUND

A likely consequence of discontinued distribution and sale of the Medtronic HVAD™ System (HVAD) will be an increase in replacement with the Abbott HeartMate 3™ Left Ventricular Assist Device (HeartMate 3) when device exchange is necessary. If part or all if the HVAD 10 mm-diameter outflow graft is retained during replacement, the HeartMate 3 will have to run at a higher speed than it would with its 14 mm-diameter graft.

METHODS

A steady-state, in vitro study was run with 250 mm-long samples of HVAD, HeartMate 3, and half-HVAD/half-HeartMate 3 grafts and, additionally, 125 and 375 mm-long samples of HVAD graft. Flows of 3.0, 3.9, 4.3, 4.7, and 6.0 L/min were applied to encompass expected clinical conditions.

RESULTS

At typical and high flow rates of 4.3 and 6.0 L/min, HeartMate 3 rotor speeds with the full HVAD graft had to be increased relative to those with the HeartMate 3 graft from 5350 to 5700 and 6350 to 6900 rpm, respectively, with power consumption increases from 3.7 to 4.3 (16%) and 5.5 to 6.8 W (24%), respectively.

CONCLUSIONS

The study did not elucidate a severe consequence of utilizing remnant HVAD graft during pump exchange, but the incremental risks of a higher rotor speed, disadvantage to the patient in battery runtime, and the general benefit of complete conversion to the HeartMate 3 graft should be balanced against other procedural considerations. Complete graft replacement during HVAD-to-HeartMate 3 conversion remains the preferred approach from an engineering point of view.

Simplified, less-invasive left ventricular assist device implantation in patients with post-coronary artery bypass grafting

Less-invasive left ventricular assist device implantation is becoming the technique of choice for implementation of new-generation heart pumps. The less-invasive technique is particularly useful for patients with a history of cardiac surgical procedures, such as coronary artery bypass grafting. Yet, in redo cases, severe pericardial adhesions may force experienced operators to opt for alternative outflow graft placement, such as in the descending aorta. We propose a surgical technique in which the left ventricular device is implanted through a limited thoracic incision, with the outflow graft anastomosed to the ascending aorta, avoiding collision with coronary bypass grafts. The key element of the proposed technique is a tunnel connecting 2 pleural cavities, in which the outflow graft is placed, entirely bypassing the pericardium.

Proximal and Distal Occlusion of Complex Cerebral Aneurysms-Implications of Flow Modeling by Fluid-Structure Interaction Analysis

BACKGROUND

In complex cerebral aneurysms, adequate treatment by complete occlusion is not always possible. Partial occlusion by either proximal or distal occlusion is an alternative. However, the hemodynamic consequences of these partial occlusion options are often not easily predictable.

OBJECTIVE

To assess the feasibility of fluid-structure interaction (FSI) analysis to investigate the hemodynamic changes after partial occlusion in cerebral aneurysms.

METHODS

Two patients were analyzed. One was treated by proximal occlusion and 1 by distal occlusion. In both, flow replacement bypass surgery was performed. Three-dimensional models were constructed from magnetic resonance angiography (MRA) scans and used for FSI analysis. A comparative study was done for pre- and postoperative conditions. Postoperative thrombosis was modeled and analyzed for the distal occlusion. FSI results were compared to postoperative angiograms and computed tomography (CT)-scans.

RESULTS

Proximal occlusion resulted in reduction of velocity, wall shear stresses, and disappearance of helical flow patterns in the complete aneurysm. Distal occlusion showed a decrease of velocity and wall shear stress in the dome of the aneurysm. Results were validated against postoperative CT-scans and angiograms at 1-, 7-, and 9-mo follow-up. Addition of thrombus to the distal occlusion model showed no change in velocities and luminal pressure but resulted in decrease in wall tension.

CONCLUSION

This pilot study showed hemodynamic changes in 2 patients with proximal and distal occlusion of complex cerebral aneurysms. The FSI results were in line with the follow-up CT scans and angiograms and indicate the potential of FSI as a tool in patient-specific surgical interventions.

Prevention and Treatment of Thrombotic and Hemorrhagic Complications in Patients Supported by Continuous-Flow Left Ventricular Assist Devices

PURPOSE OF REVIEW

The purpose of this review is to describe the current knowledge in prevention and treatment of thrombotic (pump thrombosis and ischemic stroke) and bleeding (gastrointestinal and hemorrhagic stroke) complications in patients supported by continuous-flow left ventricular assist devices (CF-LVAD).

RECENT FINDINGS

Left ventricular assist devices (LVADs) are now widely used for the management of end-stage heart failure. Unfortunately, in spite of the indisputable positive impact LVADs have on patients, the frequency and severity of complications are limitations of this therapy. Stroke, pump thrombosis, and gastrointestinal bleeding are among the most serious and frequent complications in these patients. The balance between hemorrhagic and thrombotic complications in patients supported with CF-LVAD is difficult as most patients do not necessarily fit a "bleeder" or a "clotter" profile but rather move from one side to the other of the thrombotic/bleeding spectrum. Further research is necessary to better understand the risk factors and mechanisms involved in the development of these complications.