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EDITORIAL COMMENTARY
Microparticles and left ventricular assist device complications: A causal association? Keyur B. Shah, MD, and Michael C. Kontos, MD From the Division of Cardiology, Virginia Commonwealth University, Richmond, Virginia.
Although left ventricular assist device (LVAD) use continues to rise, recent publications reporting increasing rates of pump thrombosis ground us in the reality of deviceassociated complications. As we scramble to explain the clinical and rheological reasons for these trends, published efforts thus far have focused on identifying clinical predictors of LVAD complications. Pump thrombosis has been associated with conditions resulting in lower LVAD flow (lower speed operation and high systemic blood pressure), echocardiographic indicators of inadequate decompression (abnormal response to speed ramping, increased LV diameter, and frequent aortic valve opening), cannula malposition, rising lactate dehydrogenase (LDH) levels, infections, and lapses in anti-coagulation.1–5 Mechanistic explanations, however, are lagging behind. In the present study, Nascimbene et al6 analyzed the circulating concentrations of membrane microparticles (MPs), a biomarker with the mechanistic potential to predict complications in the “pre-disease” state for vascular, inflammatory, and pro-thrombotic conditions. MPs are membrane-derived vesicles released from a wide variety of cell types, including erythrocytes, endothelial cells, platelets, vascular smooth muscle cells, and cardiomyocytes. The vesicles form by outward blebbing of the outer membrane when the cells are stressed or activated by inflammation, shear forces, or various signaling pathways (i.e., mediators of the coagulation cascade). During MP formation, phosphatidylserine (PS), an aminophospholipid typically found on the inner leaflet of the plasma membrane of healthy cells, becomes externalized.7 Although MPs were initially thought to be biologically inert “cellular debris,” they are now known have regulatory and signaling roles for increasing inflammation, oxidative stress, and angiogenesis. In addition, the negatively charged PS particles covering the surfaces of these molecules have a pro-coagulant effect through interactions with positively charges clotting factors. MPs derived from platelets are in fact exponentially more pro-coagulant than activated
platelets. In addition, erythrocyte-derived MPs have been associated with thrombin generation.8,9 The specific cellular signaling pathways through which MPs exert their effects are yet to be clearly defined; however, elevated concentrations have been associated with hypercoagulable conditions, including systemic lupus erythematosus, essential thrombocythemia, and patients suffering thrombotic events. MPs predict adverse outcomes, including pulmonary hypertension, atherosclerotic disease, and heart failure.10 Investigators have observed elevated leukocyte, platelet, and endothelial-derived MPs in continuous-flow LVAD patients, with elevated PS-positive expressing MPs in both centrifugal-flow and axial-flow pumps.11–13 One could postulate these are related to impeller-related stress on blood components or flow characteristics that alter the natural shear forces on the vascular endothelium. In the present study, the investigators performed serial measurements of PS-positive MP concentrations in patients after implantation of an axial continuous-flow LVAD. They confirmed that MP levels are elevated in patients with congestive heart failure and remain elevated after LVAD implantation. They also discovered that patients who subsequently suffered adverse events (all but one related to bleeding or thrombosis) had significantly higher MPs levels before hospital discharge for their LVAD implantation. The authors deserve credit for their efforts to better understand the mechanistic underpinnings of LVAD-related complications; however, the present study design leaves many unanswered questions about the role of MPs in LVAD pathobiology. Rather than study the association of MPs to a single type of complication, for example thrombosis, the authors instead developed a composite end point consisting of heterogeneous outcomes (arrhythmias, bleeding, thrombosis, and myocardial infarction). With the current study design, one must make an undesirable leap of logic to associate the biomarker with complications of arising from various pathophysiologies or simply assume that the rise in
1053-2498/$ - see front matter r 2014 International Society for Heart and Lung Transplantation. All rights reserved. http://dx.doi.org/10.1016/j.healun.2014.02.031
Shah and Kontos
LVADs and Microparticles
MPs is an epiphenomenon associated with an adverse outcome. The present data do not differentiate whether MPs play a causal role in the progression to subsequent complications or simply are nonspecific bystanders that signal a general inflammatory or altered physiological state. A comprehensive simultaneous comparison that includes other inflammatory markers at various time points may help elucidate a mechanistic order to the increase in MP concentrations. Furthermore, the type of MPs and biological activity of the molecules were not reported, and future studies should better identify the MP cell lines of origin and quantitative functional capacity before and after LVAD implantation. Defining the role of MPs as a clinical biomarker for prediction and treatment is only in the early stages, and the finding in this report should be interpreted as such. As acknowledged by the investigators, this was a pilot study that established the feasibility of conducting these measurements for clinical research in a larger confirmatory study. Owing to the limited number of only 17 patients with available laboratory data for analysis, an unduly increased MP concentration in the small sampled “event” group could have resulted in a false-positive (type I error) conclusion that the MPs were elevated in patients with events. Furthermore, with the small sample size, the dichotomization of the continuous variable (LDH) may have masked the prognostic utility of previously established markers that have previously detected events. Lastly, a comparison of baseline (pre-device) MPs levels in those with events and non-events would be desirable, because developing a marker that predicts risk before the LVAD is inserted would allow for even earlier risk stratification and preventive planning. For successful adoption into clinical practice, novel cardiac biomarker development, such as seen in the current study, must embody several characteristics.14 The first is to be able to accurately measure the marker with good reproducibility and low intra-patient variability. Next, the marker should be able to accurately separate individuals into those with and without the outcome of interest using end points that are clinically significant. Retrospective selection of diagnostic cutoff values should be confirmed in larger prospective studies. Not infrequently, the initial study of a new marker is often dramatically “positive,” particularly if the number of patients is small, as was seen in the current study, with subsequent studies showing lower, and in some cases, no association with the selected outcome. A more difficult step in evaluating a new biomarker is demonstrating that it is additive to known risk factors and other established markers that are used in clinical practice, improving diagnosis, prognosis, and/or risk stratification. The small numbers of patients in the current study limit the ability to perform a multivariate analysis. Finally, probably the highest hurdle for a new marker to achieve is the ability to guide therapy, either to identify the most appropriate treatment or to follow treatment response.
469 In conjunction with this, its use in clinical management should improve clinical outcomes. With this in mind, the use of MPs for predicting LVAD complications shows promise, but is still very early on the journey for routine use for identifying high-risk patients. Future, adequately powered studies of MPs with focused end points and more comprehensive laboratory analysis will better define the additive utility of this novel biomarker to the prognostic armamentarium and its mechanistic role in LVAD complications.
Disclosure statement Dr Shah has received institutional grants from Thoratec Corp and Medtronic Inc. Dr Kontos does not have a financial relationship with a commercial entity that has an interest in the subject of the presented manuscript or other conflicts of interest to disclose.
References 1. Najjar SS, Slaughter MS, Pagani FD, et al. An analysis of pump thrombus events in patients in the HeartWare ADVANCE bridge to transplant and continued access protocol trial. J Heart Lung Transplant 2014;33:23-34. 2. Uriel N, Han J, Morrison KA, et al. Device thrombosis in HeartMate II continuous-flow left ventricular assist devices: a multifactorial phenomenon. J Heart Lung Transplant 2014;33:51-9. 3. Stulak JM, Lee D, Haft JW, et al. Gastrointestinal bleeding and subsequent risk of thromboembolic events during support with a left ventricular assist device. J Heart Lung Transplant 2014;33:60-4. 4. Cowger JA, Romano MA, Shah P, et al. Hemolysis: a harbinger of adverse outcome after left ventricular assist device implant. J Heart Lung Transplant 2014;33:35-43. 5. Kirklin JK, Naftel DC, Kormos RL, et al. Interagency Registry for Mechanically Assisted Circulatory Support (INTERMACS) analysis of pump thrombosis in the HeartMate II left ventricular assist device. J Heart Lung Transplant 2014;33:12-22. 6. Nascimbene A, Hernandez R, George JK, et al. Association between cell-derived microparticles and adverse events in patients with nonpulsatile left ventricular assist devices. J Heart Lung Transplant 2014;33:470-7. 7. Burger D, Schock S, Thompson CS, Montezano AC, Hakim AM, Touyz RM. Microparticles: biomarkers and beyond. Clin Sci 2013;124:423-41. 8. Owens AP 3rd, Mackman N. Microparticles in hemostasis and thrombosis. Circ Res 2011;108:1284-97. 9. Van Beers EJ, Schaap MC, Berckmans RJ, et al. Circulating erythrocyte-derived microparticles are associated with coagulation activation in sickle cell disease. Haematologica 2009;94:1513-9. 10. Nozaki T, Sugiyama S, Sugamura K, et al. Prognostic value of endothelial microparticles in patients with heart failure. Eur J Heart Fail 2010;12:1223-8. 11. Birschmann I, Dittrich M, Eller T, et al. Ambient hemolysis and activation of coagulation is different between HeartMate II and HeartWare left ventricular assist devices. The Journal of Heart and Lung Transplantation 2014;33:80-7. 12. Diehl P, Aleker M, Helbing T, et al. Enhanced microparticles in ventricular assist device patients predict platelet, leukocyte and endothelial cell activation. Interact Cardiovasc Thorac Surg 2010; 11:133-7. 13. Pitha J, Dorazilova Z, Melenovsky V, et al. The impact of left ventricle assist device on circulating endothelial microparticles—pilot study. Neuro Endocrinol Lett 2012;33(Suppl 2):68-72. 14. Hlatky MA, Greenland P, Arnett DK, et al. Criteria for evaluation of novel markers of cardiovascular risk: a scientific statement from the American Heart Association. Circulation 2009;119:2408-16.
EDITORIAL COMMENTARY
Microparticles and left ventricular assist device complications: A causal association? Keyur B. Shah, MD, and Michael C. Kontos, MD From the Division of Cardiology, Virginia Commonwealth University, Richmond, Virginia.
Although left ventricular assist device (LVAD) use continues to rise, recent publications reporting increasing rates of pump thrombosis ground us in the reality of deviceassociated complications. As we scramble to explain the clinical and rheological reasons for these trends, published efforts thus far have focused on identifying clinical predictors of LVAD complications. Pump thrombosis has been associated with conditions resulting in lower LVAD flow (lower speed operation and high systemic blood pressure), echocardiographic indicators of inadequate decompression (abnormal response to speed ramping, increased LV diameter, and frequent aortic valve opening), cannula malposition, rising lactate dehydrogenase (LDH) levels, infections, and lapses in anti-coagulation.1–5 Mechanistic explanations, however, are lagging behind. In the present study, Nascimbene et al6 analyzed the circulating concentrations of membrane microparticles (MPs), a biomarker with the mechanistic potential to predict complications in the “pre-disease” state for vascular, inflammatory, and pro-thrombotic conditions. MPs are membrane-derived vesicles released from a wide variety of cell types, including erythrocytes, endothelial cells, platelets, vascular smooth muscle cells, and cardiomyocytes. The vesicles form by outward blebbing of the outer membrane when the cells are stressed or activated by inflammation, shear forces, or various signaling pathways (i.e., mediators of the coagulation cascade). During MP formation, phosphatidylserine (PS), an aminophospholipid typically found on the inner leaflet of the plasma membrane of healthy cells, becomes externalized.7 Although MPs were initially thought to be biologically inert “cellular debris,” they are now known have regulatory and signaling roles for increasing inflammation, oxidative stress, and angiogenesis. In addition, the negatively charged PS particles covering the surfaces of these molecules have a pro-coagulant effect through interactions with positively charges clotting factors. MPs derived from platelets are in fact exponentially more pro-coagulant than activated
platelets. In addition, erythrocyte-derived MPs have been associated with thrombin generation.8,9 The specific cellular signaling pathways through which MPs exert their effects are yet to be clearly defined; however, elevated concentrations have been associated with hypercoagulable conditions, including systemic lupus erythematosus, essential thrombocythemia, and patients suffering thrombotic events. MPs predict adverse outcomes, including pulmonary hypertension, atherosclerotic disease, and heart failure.10 Investigators have observed elevated leukocyte, platelet, and endothelial-derived MPs in continuous-flow LVAD patients, with elevated PS-positive expressing MPs in both centrifugal-flow and axial-flow pumps.11–13 One could postulate these are related to impeller-related stress on blood components or flow characteristics that alter the natural shear forces on the vascular endothelium. In the present study, the investigators performed serial measurements of PS-positive MP concentrations in patients after implantation of an axial continuous-flow LVAD. They confirmed that MP levels are elevated in patients with congestive heart failure and remain elevated after LVAD implantation. They also discovered that patients who subsequently suffered adverse events (all but one related to bleeding or thrombosis) had significantly higher MPs levels before hospital discharge for their LVAD implantation. The authors deserve credit for their efforts to better understand the mechanistic underpinnings of LVAD-related complications; however, the present study design leaves many unanswered questions about the role of MPs in LVAD pathobiology. Rather than study the association of MPs to a single type of complication, for example thrombosis, the authors instead developed a composite end point consisting of heterogeneous outcomes (arrhythmias, bleeding, thrombosis, and myocardial infarction). With the current study design, one must make an undesirable leap of logic to associate the biomarker with complications of arising from various pathophysiologies or simply assume that the rise in
1053-2498/$ - see front matter r 2014 International Society for Heart and Lung Transplantation. All rights reserved. http://dx.doi.org/10.1016/j.healun.2014.02.031
Shah and Kontos
LVADs and Microparticles
MPs is an epiphenomenon associated with an adverse outcome. The present data do not differentiate whether MPs play a causal role in the progression to subsequent complications or simply are nonspecific bystanders that signal a general inflammatory or altered physiological state. A comprehensive simultaneous comparison that includes other inflammatory markers at various time points may help elucidate a mechanistic order to the increase in MP concentrations. Furthermore, the type of MPs and biological activity of the molecules were not reported, and future studies should better identify the MP cell lines of origin and quantitative functional capacity before and after LVAD implantation. Defining the role of MPs as a clinical biomarker for prediction and treatment is only in the early stages, and the finding in this report should be interpreted as such. As acknowledged by the investigators, this was a pilot study that established the feasibility of conducting these measurements for clinical research in a larger confirmatory study. Owing to the limited number of only 17 patients with available laboratory data for analysis, an unduly increased MP concentration in the small sampled “event” group could have resulted in a false-positive (type I error) conclusion that the MPs were elevated in patients with events. Furthermore, with the small sample size, the dichotomization of the continuous variable (LDH) may have masked the prognostic utility of previously established markers that have previously detected events. Lastly, a comparison of baseline (pre-device) MPs levels in those with events and non-events would be desirable, because developing a marker that predicts risk before the LVAD is inserted would allow for even earlier risk stratification and preventive planning. For successful adoption into clinical practice, novel cardiac biomarker development, such as seen in the current study, must embody several characteristics.14 The first is to be able to accurately measure the marker with good reproducibility and low intra-patient variability. Next, the marker should be able to accurately separate individuals into those with and without the outcome of interest using end points that are clinically significant. Retrospective selection of diagnostic cutoff values should be confirmed in larger prospective studies. Not infrequently, the initial study of a new marker is often dramatically “positive,” particularly if the number of patients is small, as was seen in the current study, with subsequent studies showing lower, and in some cases, no association with the selected outcome. A more difficult step in evaluating a new biomarker is demonstrating that it is additive to known risk factors and other established markers that are used in clinical practice, improving diagnosis, prognosis, and/or risk stratification. The small numbers of patients in the current study limit the ability to perform a multivariate analysis. Finally, probably the highest hurdle for a new marker to achieve is the ability to guide therapy, either to identify the most appropriate treatment or to follow treatment response.
469 In conjunction with this, its use in clinical management should improve clinical outcomes. With this in mind, the use of MPs for predicting LVAD complications shows promise, but is still very early on the journey for routine use for identifying high-risk patients. Future, adequately powered studies of MPs with focused end points and more comprehensive laboratory analysis will better define the additive utility of this novel biomarker to the prognostic armamentarium and its mechanistic role in LVAD complications.
Disclosure statement Dr Shah has received institutional grants from Thoratec Corp and Medtronic Inc. Dr Kontos does not have a financial relationship with a commercial entity that has an interest in the subject of the presented manuscript or other conflicts of interest to disclose.
References 1. Najjar SS, Slaughter MS, Pagani FD, et al. An analysis of pump thrombus events in patients in the HeartWare ADVANCE bridge to transplant and continued access protocol trial. J Heart Lung Transplant 2014;33:23-34. 2. Uriel N, Han J, Morrison KA, et al. Device thrombosis in HeartMate II continuous-flow left ventricular assist devices: a multifactorial phenomenon. J Heart Lung Transplant 2014;33:51-9. 3. Stulak JM, Lee D, Haft JW, et al. Gastrointestinal bleeding and subsequent risk of thromboembolic events during support with a left ventricular assist device. J Heart Lung Transplant 2014;33:60-4. 4. Cowger JA, Romano MA, Shah P, et al. Hemolysis: a harbinger of adverse outcome after left ventricular assist device implant. J Heart Lung Transplant 2014;33:35-43. 5. Kirklin JK, Naftel DC, Kormos RL, et al. Interagency Registry for Mechanically Assisted Circulatory Support (INTERMACS) analysis of pump thrombosis in the HeartMate II left ventricular assist device. J Heart Lung Transplant 2014;33:12-22. 6. Nascimbene A, Hernandez R, George JK, et al. Association between cell-derived microparticles and adverse events in patients with nonpulsatile left ventricular assist devices. J Heart Lung Transplant 2014;33:470-7. 7. Burger D, Schock S, Thompson CS, Montezano AC, Hakim AM, Touyz RM. Microparticles: biomarkers and beyond. Clin Sci 2013;124:423-41. 8. Owens AP 3rd, Mackman N. Microparticles in hemostasis and thrombosis. Circ Res 2011;108:1284-97. 9. Van Beers EJ, Schaap MC, Berckmans RJ, et al. Circulating erythrocyte-derived microparticles are associated with coagulation activation in sickle cell disease. Haematologica 2009;94:1513-9. 10. Nozaki T, Sugiyama S, Sugamura K, et al. Prognostic value of endothelial microparticles in patients with heart failure. Eur J Heart Fail 2010;12:1223-8. 11. Birschmann I, Dittrich M, Eller T, et al. Ambient hemolysis and activation of coagulation is different between HeartMate II and HeartWare left ventricular assist devices. The Journal of Heart and Lung Transplantation 2014;33:80-7. 12. Diehl P, Aleker M, Helbing T, et al. Enhanced microparticles in ventricular assist device patients predict platelet, leukocyte and endothelial cell activation. Interact Cardiovasc Thorac Surg 2010; 11:133-7. 13. Pitha J, Dorazilova Z, Melenovsky V, et al. The impact of left ventricle assist device on circulating endothelial microparticles—pilot study. Neuro Endocrinol Lett 2012;33(Suppl 2):68-72. 14. Hlatky MA, Greenland P, Arnett DK, et al. Criteria for evaluation of novel markers of cardiovascular risk: a scientific statement from the American Heart Association. Circulation 2009;119:2408-16.
