A continuous LVAD flow through the aortic valve was modeled as if the LVAD was placed inside the left ventricle. This hypothetical model brings out the difference between pulsatile and continuous flow. Figure 5 is similar to Figure 4. The flows are similar, with lower velocities.
The flow patterns for a 10 and a 14 mm graft at an angle of 30° and 45° are shown in Figure 6. Both flows exhibited a striking jet effect, although the peak velocities and hence the sharpness of the jet was understandably more for a 10 mm graft. Flow through the neck vessels was not affected significantly (Table 1). Importantly, there was more stasis in the arch as shown by the particle count (Figure 7).
The effect of aortic regurgitation on arch flow was interesting. Because of a large pressure differential across the valve, a small 2 mm diameter opening in the aortic valve created 20% regurgitation. Normally there is a stationary column of blood above the aortic valve and below the out flow graft to “support” the jet of flow from the VAD.
This standing column below the jet is disturbed by aortic regurgitation and the jet is deflected downwards causing dramatic changes in the blood flow pattern in the arch (Figure 8) resulting in decreased cerebral perfusion and this is more pronounced as the angle of graft insertion approaches 90°. This has obvious clinical implications.
One of the obvious differences between Heartmate II and HVAD is the diameter of the outflow graft and there has been no study looking at its role in the causation of strokes. Although the jet effect of the outflow graft has been noted in several studies,16,17 the relationship between graft diameter and stasis and eddies in the arch and cerebral blood flow have not been investigated in detail.
Under normal pulsatile conditions or continuous flow through the aortic valve as if LVAD is placed in the ventricle, blood flow in the aortic arch and cerebral vessels is fully developed with a striking absence of stasis in the aortic arch. On the basis of our simulations, it is beneficial for future VAD designs, to use this concept of flow originating close to the aortic valve through a large orifice.
The flow into the innominate and carotids depend on the direction and the width of the jet from the graft. A 14 mm graft of Heartmate II, while sharing the same flow characteristics as HVAD, has a more diffused jet and is not as sensitive to graft attack angle. Flow in the innominate is significantly reduced at certain angles with a 10 mm graft, leading to the hypothesis that hypoperfusion may be one of the important contributors to the difference in the stroke rates observed.
One of the surprising findings in our study was the impact of aortic incompetence on cerebral blood flow prompting the possibility of it being a risk factor. Depending on the graft angle, aortic incompetency may result in a disproportionate drop in cerebral blood flow. This finding may have relevance in regular CPB as well, as aortic incompetence happens quite often under bypass conditions if the aorta is not clamped.
Progressive drop in cognitive function has been observed in LVADS in almost one in four patients but has been attributed to old age.18 As per our study, cerebral hypoperfusion may well be an important cause.
The “normal” circle of Willis shows a lot of variations and is complete in less than 30% of individuals19 and an absence of the posterior communicating artery observed commonly by middle age.20 An absent posterior communicating artery is an important contributor to stroke if the innominate is occluded.21 Our finding of a drop in blood flow in the innominate artery assumes significance in the light of these findings.
Several publications have highlighted the preponderance of strokes involving the right cerebral hemisphere, both in LVADS and in CPB.22 Our findings suggest that hypoperfusion and stasis in the innominate could equally be responsible apart from other factors.
Interestingly, strokes during CPB have received a lot of attention using CFDs studies.23 Kaufmann et al.24 have shown that jet effect through a narrow aortic cannula, 24 or 26 F,(8–8.5 mm) has an impact on the flow through neck vessels. Although we agree with the views expressed by Kaufmann et al.25 that cerebral autoregulation plays an important role in rotary blood pump and LVADs, it should be borne in mind that about 20% of the patients even in CPB have impaired autoregulation.26
The site, angle of insertion, and the degree of beveling of an outflow graft are of course highly variable and are never measured precisely during surgery. Also, the findings of this study are valid for this particular arch anatomy which is very variable as is the branching pattern of the neck vessels. Nevertheless and precisely for this reason, this study serves to highlight the importance of these factors in flow into cerebral vessels when arterial return is through a narrow graft. Also the choice of 30° and 45° was merely to highlight the importance of attack angle.
We are aware of the shortcomings of this study.
Arterial return through a narrow graft has important jet effects and results in significant flow perturbations in the aortic arch and cerebral vessels and stasis. A 10 mm graft is more sensitive to the angle of insertion than a 14 mm graft. Under some conditions, serious hypoperfusion of the innominate artery is possible. Aortic incompetence results in a significant decrease of cerebral blood flow. In some vulnerable situations, anatomic or physiological, these factors can result in a stroke. This study offers some insights into the possible reasons for the observed differences in stroke rate between Heartmate II and HVAD.
1. Pagani FD, Milano CA, Tatooles AJ, et al. HeartWare HVAD for the treatment of patients with advanced heart failure ineligible for cardiac transplantation: Results of the ENDURANCE destination therapy trial. J Hear Lung Transplant. 2015.34: S9.
2. Stulak JM, Davis ME, Haglund N, et al. Adverse events in contemporary continuous-flow left ventricular assist devices: A multi-institutional comparison shows significant differences. J Thorac Cardiovasc Surg. 2016.151: 177–189.
3. Lalonde SD, Alba AC, Rigobon A, et al. Clinical differences between continuous flow ventricular assist devices: A comparison between HeartMate II and HeartWare HVAD. J Card Surg. 2013.28: 604–610.
4. Theory and Modeling Guide Volume III : ADINA CFD & FSI. 2012.III.
5. Reymond P, Merenda F, Perren F, Rüfenacht D, Stergiopulos N. Validation of a one-dimensional model of the systemic arterial tree. Am J Physiol Heart Circ Physiol. 2009.297: H208–H222.
6. Osorio AF, Osorio R, Ceballos A, et al. Computational fluid dynamics analysis of surgical adjustment of left ventricular assist device
implantation to minimise stroke
risk. Comput Methods Biomech Biomed Engin. 2013.16: 622–638.
7. Weber TF, Müller T, Biesdorf A, et al. True four-dimensional analysis of thoracic aortic displacement and distension using model-based segmentation of computed tomography angiography. Int J Cardiovasc Imaging. 2014.30: 185–194.
8. Raghavan ML, Ma B, Fillinger MF. Non-invasive determination of zero-pressure geometry of arterial aneurysms. Ann Biomed Eng. 2006.34: 1414–1419.
9. Farag MB, Karmonik C, Rengier F, et al. Review of recent results using computational fluid dynamics simulations in patients receiving mechanical assist devices for end-stage heart failure. Methodist Debakey Cardiovasc J. 2014.10: 185–189.
10. Karmonik C, Bismuth J, Shah DJ, Davies MG, Purdy D, Lumsden AB. Computational study of haemodynamic effects of entry- and exit-tear coverage in a DeBakey type III aortic dissection: technical report. Eur J Vasc Endovasc Surg. 2011.42: 172–177.
11. Karmonik C, Anderson J, Partovi S, et al. Quantitative description of hemodynamics alterations in the aorta after LVAD implantation: A computational fluid dynamics study. Radiat Oncol. 2007.69: 8–9.
12. Kim HJ, Vignon-Clementel IE, Figueroa CA, et al. On coupling a lumped parameter heart model and a three-dimensional finite element aorta model. Ann Biomed Eng. 2009.37: 2153–2169.
13. Kar B, Delgado RM 3rd, Frazier OH, et al. The effect of LVAD aortic outflow-graft placement on hemodynamics and flow: Implantation technique and computer flow modeling. Tex Heart Inst J. 2005.32: 294–298.
14. Travis AR, Giridharan GA, Pantalos GM, et al. Vascular pulsatility in patients with a pulsatile- or continuous-flow ventricular assist device. J Thorac Cardiovasc Surg. 2007.133: 517–524.
15. Liu X, Pu F, Fan Y, Deng X, Li D, Li S. A numerical study on the flow of blood and the transport of LDL in the human aorta: The physiological significance of the helical flow in the aortic arch. Am J Physiol Heart Circ Physiol. 2009.297: H163–H170.
16. Kaufmann TA, Hormes M, Laumen M, et al. Flow distribution during cardiopulmonary bypass in dependency on the outflow cannula positioning. Artif Organs. 2009.33: 988–992.
17. Inci G, Sorgüven E. Effect of LVAD outlet graft anastomosis angle on the aortic valve, wall, and flow. ASAIO J. 2012.58: 373–381.
18. Fendler TJ, Spertus JA, Gosch KL, et al. Incidence and predictors of cognitive decline in patients with left ventricular assist devices. Circ Cardiovasc Qual Outcomes. 2015.8: 285–291.
19. Saikia B, Handique A, Phukan P, Lynser D, Jamil M. Study of anomalies in the circle of Willis using magnetic resonance angiography in north eastern India. J Anat Soc India. 2014.63: 67–73.
20. Suemoto C, Grinberg L, Aparecida MSS, Jacob Filho W, Pasqualucci C. Anatomical variations of circle of Willis in an autopsy study in the city of São Paulo. Braz J Morphol Sci. 2008.25: 157–214.
21. Hendrikse J, Hartkamp MJ, Hillen B, Mali WP, van der Grond J. Collateral ability of the circle of Willis in patients with unilateral internal carotid artery occlusion: Border zone infarcts and clinical symptoms. Stroke
. 2001.32: 2768–2773.
22. Kaufmann TA, Schmitz-Rode T, Steinseifer U. Implementation of cerebral autoregulation into computational fluid dynamics studies of cardiopulmonary bypass. Artif Organs. 2012.36: 754–758.
23. Kato TS, Ota T, Schulze PC, et al. Asymmetric pattern of cerebrovascular lesions in patients after left ventricular assist device
. 2012.43: 872–874.
24. Kaufmann TA, Neidlin M, Büsen M, Sonntag SJ, Steinseifer U. Implementation of intrinsic lumped parameter modeling into computational fluid dynamics studies of cardiopulmonary bypass. J Biomech. 2014.47: 729–735.
25. Kaufmann T, Schmitz-Rode T, Moritz A, Steinseifer U. Effect of outflow cannula placement and pulsatility of blood pumps on cerebral blood flow and wall shear stress during cardiac assist. Blucher Mech Eng Proc. 2012.1: 822–835.
26. Ono M, Joshi B, Brady K, et al. Risks for impaired cerebral autoregulation during cardiopulmonary bypass and postoperative stroke
. Br J Anaesth. 2012.109: 391–398.