doi: 10.1161/CIRCULATIONAHA.106.677898
(Circulation. 2007;116:I-8 – I-15.)
© 2007 American Heart Association, Inc.
Cardiac Transplantation and Surgery for Heart Failure |
Hemodynamic and Exercise Performance With Pulsatile and Continuous-Flow Left Ventricular Assist Devices
From the Section of Cardiac Surgery (J.H., F.D.P.) and the Division of Cardiovascular Medicine (W.A., D.B.D., K.D.A., T.M.K.), University of Michigan Health System, Ann Arbor, Mich; and Thoratec Corp (D.J.F.), Pleasanton, Calif.
Correspondence to Francis D. Pagani, MD, PhD, Associate Professor of Surgery, University of Michigan Health System, Cardiovascular Center, Rm 5161, 1500 East Medical Center Drive, SPC 5864, Ann Arbor, MI 48109-5864. E-mail fpagani{at}umich.edu
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Abstract |
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Background— Continuous-flow rotary pumps with axial design are increasingly used for left ventricular assist support. The efficacy of this design compared with pulsatile, volume displacement pumps, with respect to characteristics of left ventricular unloading, and exercise performance remains largely unstudied.
Methods and Results— Thirty-four patients undergoing implantation with a pulsatile, volume displacement pump operating in a full-to-empty cycle (HeartMate XVE; Thoratec Inc, Pleasanton, Calif; n=16) or continuous-flow rotary pump with an axial design operating at a fixed rotor speed (HeartMate II; Thoratec Inc; n=18) were evaluated with right heart catheterization and echocardiography preoperatively and at 3 months postoperatively and cardiopulmonary exercise testing 3 months postoperatively. Support with either the XVE or II resulted in significant (P<0.05) increases in cardiac output and reduction in mean pulmonary artery and pulmonary wedge pressures. Exercise capacity at 3 months was similar between groups (% predicted peak 
O2–XVE: 46.8±10.2 versus II: 49.1±13.6). Echocardiography at 3 months demonstrated a significantly (P<0.05) greater reduction in left ventricular end-diastolic volume (–49±16% versus –35±20%), left ventricular end-systolic volume (–59±20 versus –37±21%), and percent mitral valve regurgitant volume (–99±2% versus –52±56%) for the XVE compared with II, respectively.
Conclusions— The HeartMate XVE or II provided equivalent degrees of hemodynamic support and exercise capacity. The XVE was associated with greater left ventricular volume unloading. Characteristics of left ventricular pressure and volume unloading between these pump designs and mode of operation do not influence early exercise performance.
Key Words: exercise • heart failure • hemodynamics • surgery • transplantation
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Introduction |
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Left ventricular assist device (LVAD) therapy is an established treatment modality for patients with advanced heart failure.1–3 The majority of patients are supported by pulsatile, volume displacement devices such as the HeartMate (HM) XVE (Thoratec Inc, Pleasanton, Calif) or Novacor (WorldHeart, Oakland, Calif) that are operated in a full-to-empty mode that automatically adjusts beat rate to changes in left ventricular (LV) preload.4 These devices provide excellent hemodynamic support and improve survival to transplantation but have important constraints, including body size requirements for implantation and limitations in long-term mechanical durability.3,4 New LVAD designs incorporating continuous-flow rotary pump technology with axial configuration are currently in human evaluation.5–7 These devices have the advantage of a small pump size by elimination of the reservoir chamber needed for pulsatile pumps and potential for greater long-term mechanical reliability.
Continuous-flow rotary pumps are currently designed to operate at fixed rotor speeds that do not automatically adjust to changes in LV preload.5,8 The hydrodynamic performance of rotary pumps with an axial design is directly proportional to rotor speed and inversely proportional to pressure differences across the inlet and outlet orifices of the pump.5 Therefore, rotary pumps with an axial design have a potential capacity to adjust flow to increases or decreases in LV preload.9–11 The fixed operating mode of rotary pumps is different from pulsatile, volume displacement pumps that operate in a full-to-empty cycle and adjust beat rate automatically to accommodate changes in LV preload. Algorithms for demand-responsive control of rotary pumps have been developed to overcome this perceived limitation in pump design.12 Previous studies have suggested that rotary pumps with an axial design provide similar degrees of pressure unloading but less volume unloading of the LV as compared with pulsatile, volume displacement pumps under resting conditions.13,14 These previous observations suggest that rotary pumps with an axial design operating at a fixed rotor speed may not adequately adjust to periods of increased LV preload during exercise. These observations may have important implications for exercise performance and LV remodeling with respect to device design. We investigated the effects of LVAD support with either a pulsatile, volume displacement pump or continuous-flow rotary pump with an axial design on hemodynamic and exercise performance.
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Methods |
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Patients
From March 24, 2004, to June 28, 2006, 56 consecutive patients underwent LVAD implantation. Patients eligible for study included those implanted with the HM XVE (pulsatile, volume displacement device operated in a full-to-empty cycle; Figure) or HM II (continuous-flow rotary pump with an axial design operated at a fixed rotor speed; Figure) and able to undergo complete preoperative assessment with echocardiography and right heart catheterization, and remaining alive and well on device support at 3 months after device implantation for evaluation with right heart catheterization, echocardiography, and cardiopulmonary exercise testing. The decision for type of device implanted was based on presenting clinical characteristics, including acuity of illness, presence of preoperative extracorporeal circulatory support, need for biventricular assistance, device availability, and patient size. Of these 56 patients, 22 (39%) were excluded from the study: 3 for ineligible device type (Thoratec IVAD; Thoratec, Inc); 15 (26.7%) with extracorporeal mechanical circulatory support that precluded echocardiographic assessment of native LV function under full loading conditions (extracorporeal membrane oxygenation [5]; Abiomed BVS 5000, Abiomed, Danvers, Mass [4]; TandemHeart pVAD, CardiacAssist, Pittsburgh, Pa [6]); one (1.8%) patient (HM II) died on LVAD support before 3-month follow up; and 3 (5.4%) patients (one HM XVE; 2 HM II) underwent transplantation before 3-month follow up. Thirty-four (61%) patients were eligible for the study and completed 3-month follow up (n=16, HM XVE; n=18, HMII). Implantation of both devices was performed concurrently throughout the study. Patients underwent preoperative evaluation with right heart catheterization (all within 48 hours of operation) and echocardiography (all
30 days from operation). Postoperative evaluation with right heart catheterization, echocardiography, and cardiopulmonary exercise testing was performed at 3±0.5 months after implantation. These data were part of a routine management protocol at 1, 3, 6, and 12 months to assess device function, pulmonary vascular resistance, and myocardial recovery. Thirty-two of 34 patients (94%) were on inotrope therapy at the time of preoperative evaluation (2 patients not on inotrope therapy for intractable arrhythmias; one HM XVE and one HM II). No patient was on inotrope therapy at the 3-month evaluation.
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Data were prospectively entered into a database maintained within the Section of Cardiac Surgery. Data collection, analysis, and reporting were approved by the University of Michigan Institutional Review Board. Patient consent for data collection and reporting was performed through a signed informed consent process.
Left Ventricular Assist Systems
The HM XVE is an implantable, pulsatile volume displacement LVAD designed for long-term circulatory support (Figure).2,3,15,16 The HM XVE consists of a flexible polyurethane diaphragm within a ridged outer titanium alloy housing. The inflow and outflow conduits of the HM XVE each contain a 25-mm porcine valve within a titanium cage to ensure unidirectional blood flow. Diaphragm movement and blood ejection depend on an electric motor positioned below the diaphragm (Figure). The HM XVE is typically operated in a full-to-empty cycle. Beat rate is regulated by an external computer controller from information provided by Hall sensors on filling volume within the pump. Beat rate automatically increases or decreases to changes in LV preload and filling rate of the pump chamber. Maximum device output is approximately 10 L/minute at a beat rate of 120 beats/minute.
The HM II (Thoratec, Inc) is a continuous-flow rotary pump with an axial design (Figure).5,8–10 The system consists of an internal blood pump with a percutaneous lead that connects the pump to an external system driver and power source. The pump has an operating RPM range of 6000 to 15 000 and can generate up to 10 L/minute of flow at an approximate pressure of 100 mm Hg. The axial flow design and absence of a reservoir chamber eliminates the need for venting currently required for pulsatile, volume displacement pumps, thus reducing the size of the percutaneous drive lead and eliminating the need for internal one-way valves.
Study Protocol
Measurements (Swan-Ganz catheter; Baxter Healthcare, Irvine, Calif) of pre- and postoperative hemodynamics were performed at rest in a supine position with full device support. The HM XVE was set in automatic mode and the HM II was set at a fixed RPM speed. RPM speed was optimized by echocardiography at the time of LVAD implantation, before discharge from the implant hospitalization, and as needed for clinical events (ie, detection of LV collapse). Pump speed (RPMs) was set at a threshold to achieve complete aortic valve closure during ventricular systole and optimize LV septal position (prevention of leftward septal shift causing right heart failure). The RPM settings immediately after LVAD implantation, at discharge from the implant hospitalization, and at 3-month follow up were 9376±468, 9552±357, and 9482±324, respectively (not significantly different). The RPM settings were altered by 400 RPMs in 2 patients after discharge because of repeated LV collapse (RPM speed was reduced from 10 000 to 9600 in both cases). In 4 patients, RPM settings were altered by only 200 RPMs (one increased; 3 decreased). Device settings were not altered during or immediately before hemodynamic assessment at 3 months. Echocardiography at 3 months demonstrated only intermittent aortic valve opening in 3 patients (17%). In all other patients, no aortic valve opening during the cardiac cycle was noted. All patients were on standard heart failure therapy titrated as clinically appropriate. Seventeen of 18 (94%) patients in the HMII group and 15 of 16 (93%) patients in the XVE group were on ß-blocker therapy at the time of exercise testing (2 patients withheld because of intolerance). Defibrillator or biventricular pacing settings were not altered during exercise testing. All patients were ambulatory at home at the 3-month evaluation. A structured rehabilitation program was prescribed for each patient during their implant hospitalization.
Echocardiographic Measurements
Transthoracic echocardiograms were recorded using commercial equipment from standard parasternal and apical views. Echocardiograms were performed 30 days before LVAD implantation and repeated at 3 months postoperatively. Echocardiographic images were acquired digitally and transferred to a digital file server where they were retrieved for online electronic measurement using a commercial archiving system (Prosolv, Inc, Indianapolis, Ind). Measurements were made in triplicate and averaged. Measurements were made by a single echocardiographer blinded to the type of LVAD device but not to the patient’s status with respect to pre- or post-LVAD implantation. It was not possible to blind the echocardiographer to the pre- versus postimplant status of the patient because of the presence of the large-bore LV apical cannula.
The LV internal dimension at end-diastole and end-systole was measured from a parasternal long axis view from which severity of aortic insufficiency was also assessed. The LV end-diastolic volume and systolic volumes were quantified using the rule of discs (Simpson’s rule) from the apical 4-chamber view.17,18 Left atrial area was determined from the 4-chamber view at its greatest apparent volume. For both parasternal and apical views, end-diastole was defined as the largest chamber dimension in close proximity to the QRS and end-systole as the smallest ventricular dimension or volume identified within the cardiac cycle. Stroke volume was calculated as the difference between the end-diastolic and end-systolic volumes determined from the apical 4-chamber view. Eccentricity index was defined as LV length from the apical 4-chamber view divided by LV end-diastolic dimension from the parasternal long axis view. Mitral regurgitation severity was quantified qualitatively as minimal, mild, moderate, or severe by visual estimation of area from multiple views and further quantified as the mitral regurgitation area divided by left atrial area in the apical 4-chamber view.17,18
Exercise Testing
Patients underwent a modified Naughton protocol with 2-minute stages. Measurements during exercise included heart rate, blood pressure, respiratory rate, oxygen consumption, carbon dioxide production, respiratory exchange ratio, exercise duration, and device flow. The HM XVE was operated in automatic mode and the HM II was operated at a fixed RPM setting during exercise. Exercise was terminated when patients experienced angina, dyspnea, hypotension, or arrhythmias.
Statistical Analysis
Comparisons of baseline demographics between patient groups were performed using Fisher’s exact test or independent samples t tests as appropriate for dichotomous or continuous variables, respectively. Comparison of hemodynamic, laboratory, and echocardiographic data were analyzed using a 2-way analysis of variance to compare parameters with their baseline values over time and to compare time-matched baseline parameters. When statistically significant differences were identified by analysis of variance for group effects or group–time interactions, Tukey’s range test for multiple comparisons was used to compare means at individual time points. The absolute change in hemodynamic, laboratory, and echocardiographic variables between baseline and 3 months was calculated for each group and the mean values compared with an independent samples t test. Data are presented as the mean±SD. Statistical significance was defined at P<0.05.
The authors had full access to the data and were responsible for its integrity. All authors read and agreed to the article as written.
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Results |
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A total of 34 patients (n=16, XVE; n=18, HM II) met criteria for evaluation. Median duration of support (HM XVE=5.6 months; HM II=5.1 months) and length of postoperative hospital stay after LVAD implantation (HM XVE=30 days; HMII=26 days) were not significantly different between groups. Twelve of 16 (75%) patients in the HM XVE group and 10 of 18 (56%) patients in the HM II group underwent transplantation after 3 months. Two of 16 patients (12.5%) in the HM XVE group and 8 of 18 patients (44%) in the HM II group remain alive with ongoing LVAD support. Two of 16 patients (12.5%) in the HM XVE group died while on LVAD support after 3 months (18.5 and 6 months, respectively).
There were no significant differences in preoperative patient characteristics (Table 1) and hemodynamics between groups (Table 2). Patients in the HM XVE group had a significant decrease in heart rate, pulmonary wedge pressure, mean pulmonary artery pressure, and central venous pressure, and a significant increase in systolic arterial pressure and cardiac index at 3 months compared with baseline preoperative hemodynamics. Patients in the HM II group had a significant decrease in pulmonary wedge pressure, mean pulmonary artery pressure, and right ventricular stroke work index and a significant increase in diastolic and mean arterial pressure and cardiac index at 3 months compared with preoperative hemodynamics. The increase in diastolic arterial pressure from baseline to 3 months in the HM II group was significantly greater compared with the increase in diastolic arterial pressure in the HM XVE. The increase in systolic arterial pressure in the HM XVE from baseline to 3 months was significantly greater compared with the HM II. Patients in the HM XVE group had a significantly higher preoperative brain natriuretic peptide level compared with the HM II group (Table 3). Patients in the HM XVE group and the HM II group had a significant increase in serum sodium and mixed venous oxygen saturation and significant decrease in brain natriuretic peptide level at 3 months compared with preoperative baseline. Patients in the HM II group had a significantly greater increase in the international normalized ratio compared with the XVE group (need for warfarin). Patients in the HM XVE group had a significantly greater decrease in total bilirubin from baseline to 3 months compared with the HM II group.
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Two of 16 (12.5%) patients in the HM XVE group and 7 of 18 (38%) patients in the HM II group underwent tricuspid repair at the time of LVAD implantation (P=0.087). No patient underwent a mitral valve procedure. There were no significant differences in baseline preoperative echocardiographic assessment of LV size, function, or degree of mitral and tricuspid insufficiency between groups (Table 4). There was a significant decrease in LV size, left atrial area, degree of mitral and tricuspid insufficiency, and increase in LV ejection fraction and sphericity index in the HM XVE group at 3 months. At 3 months, there was a significant decrease in LV size, left atrial area, and degree of mitral and tricuspid insufficiency in the HM II group. There was no significant change in LV ejection fraction or improvement in sphericity index in the HM II group at 3 months. The decrease in LV size, reduction in degree of mitral insufficiency, and increase in LV ejection fraction was significantly greater for the HM XVE group compared with the HM II.
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Exercise performance assessed by peak exercise oxygen consumption (O2), duration of exercise, and respiratory exchange ratio was not significantly different between groups at 3 months (Table 5). Preexercise pump flow was not significantly different between groups. Pump flow at peak exercise was significantly increased in each group, but the increase in the HM XVE group was significantly greater than the increase observed in the HM II group.
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Discussion |
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Exercise performance at 3 months after LVAD implantation with a pulsatile, volume displacement pump was not significantly different than exercise performance achieved with a continuous-flow rotary pump with axial design. No significant differences in peak
O2 were noted despite significant observed differences in LV volume unloading between the different LVAD designs. These data suggest that major differences in LVAD design do not significantly influence exercise performance in the clinical setting of this study. Each LVAD design achieved a similar degree of pressure unloading in the resting state. A major difference in device performance was noted in the degree of LV volume unloading. These observations are consistent with previously reported studies comparing LV pressure and volume unloading characteristics with pulsatile displacement devices and continuous-flow rotary pumps.12,13 This study importantly extends the findings of these previous observations by demonstrating equivalent degrees of exercise performance with each device design.
An important consideration in this study is whether the timing of the assessment of exercise performance was adequate to obtain sufficient exercise capacity to detect differences in pump design. Jaski et al evaluated exercise performance in patients 1 to 3 months after LVAD implantation (HM IP 1000 LVAD or HM VE LVAD; Thoratec, Inc).19 These authors demonstrated a peak O2 of 14.5±3.9 mL/kg/min after LVAD implantation. The authors observed no significant correlation between time after LVAD implantation and peak O2 and no differences in peak O2 between the different LVAD designs (both devices pumps are pulsatile, volume displacement pumps but differ in mechanism of device actuation; HM IP-pneumatic; HM VE-electrical). de Jonge et al studied exercise performance in patients 8 and 12 weeks after LVAD implantation.20 The authors noted significant improvement in peak O2 from 8 to 12 weeks after LVAD implantation (21.3±3.8 to 24.2±4.8 mL/Kg/min, respectively). Mathews et al observed a significant improvement in peak O2 from 6 to 18 weeks after LVAD implantation but no significant difference from 18 to 32 weeks after LVAD implantation.21 The observations in these studies have significant relevance to findings in this present study. The average peak O2 for all patients in the present study was 15.5±4.4 mL/kg/min at 3 months and was consistent with the findings of Jaski et al (14.5±3.9 mL/kg/min),19 but appeared significantly less than the peak O2 obtained in the study by de Jonge et al at 3 months (24.2±4.8 mL/kg/min).20 Age and sex has a significant influence on peak O2,22 and the mean age of patients in the study by Jaski et al19 (48 years) was similar to the mean age of all patients in the present study (51 years) but appreciably older than patients in the study by de Jonge et al (37 years).20 Furthermore, the percentage of male patients was 100% in the study by de Jonge,20 94% in the study by Jaski,19 and 88% in the present study. Another important finding is that these studies do not appear to support a significant improvement in peak O2 beyond 3 to 4 months after LVAD implantation. These observations are important to exclude the possibility that low levels of exercise performance attributable to deconditioning and muscle wasting22 at 3 months after LVAD implantation in the present study limited levels of exercise performance that may have had the potential to unmask differences in pump performances between the different device designs. Studying patients at 3 months after LVAD implantation should have permitted clinically relevant levels of exercise performance needed to identify differences in exercise performance related to pump design. Ideally, serial assessments of exercise capacity would have been an important addition to the evaluation of exercise capacity between the pump designs to eliminate the possibility of deconditioning influencing interpretation of the results of this present study. However, patients evaluated in this study were largely a bridge to a transplant cohort and there was significant attrition of patients to transplantation after 3 months of LVAD support that prevented meaningful serial exercise evaluation. In patients with LVAD support, right ventricular function and postoperative changes in pulmonary vascular resistance are additional factors influencing exercise performance.23 Thus, persistent limitations in right ventricular output may be a limiting factor in achieving optimal exercise performance in the present study and not necessarily differences in LVAD designs. It is also unlikely that significant differences in physical conditioning existed between groups that may have influenced exercise capacity. Similar durations of heart failure symptoms, preoperative length of hospital and intensive care unit stay, preoperative serum prealbumin, and duration of postoperative recovery assessed by postoperative length of hospital stay were observed with each group. Both groups of patients received ß-blocker therapy that may have had an adverse affect on maximal exercise capacity. Each group achieved similar heart rate responses during exercise suggesting equivalent degrees of medical therapy with ß-blockers.
There are several important limitations of this study. First, the size of the study cohort may have limited the detection of small but significant differences in peak O2 between groups. Importantly, differences in baseline values between groups may have accounted for changes observed from baseline to 3 month follow up (ie, total bilirubin). Furthermore, the ability to determine how much of the augmented cardiac output during exercise was attributed to the LVAD or native cardiac function was not possible in the current study. This was attributed to the lack of an accurate and direct assessment of device flow with the HM II.5,9,10 HM II provides only an estimated pump flow based on device power consumption, whereas the HM XVE provides direct measurement of device output.5,9,10 Thus, differences in the observed respective pump outputs during exercise (see Table 5) may not be representative of true or actual differences, but does support the hydrodynamic capabilities of rotary pumps. The contribution of device output to the total increase in cardiac output during exercise may have been similar or proportionally different for each pump design. Measuring cardiac output during exercise with thermodilution technique may have yielded information on total cardiac output but no information on proportional output between device and the native heart. In the study of Klotz and colleagues,14 these authors compared the DeBakey (MicroMed, Inc, Houston, Texas) continuous-flow rotary pump with axial design with the HM VE and Novacor pulsatile pumps. The DeBakey device measures flow directly by Doppler that enables accurate assessment of device output. The authors observed significantly less pump output with a continuous-flow rotary pump compared with a pulsatile, volume displacement pump under resting conditions.
Prolonged LVAD support may result in favorable changes in the cellular biology of the myocardium and recovery of LV function permitting device explant with sustained improvements in contractility.24–28 It is unknown to what extent the differences observed in LV volume unloading between the continuous-flow rotary pumps and pulsatile pumps have on the potential for LV recovery. Despite less LV unloading and less LV mass regression with continuous-flow rotary pumps compared with a pulsatile pumps, Thohan et al observed similar changes in the degree of cellular recovery between the different pump designs.13 Possibly, the differences in pump design may have little or no potential influence on the degree of LV recovery. Current biases are such that maximal unloading of the LV is probably thought to be of significant importance; however, muscle atrophy from continued, complete unloading may have an adverse effect on LV recovery. Therefore, partial loading of the LV, consistent with conditions under continuous-flow rotary pumps with axial flow, may be preferable. It is also conceivable that more volume unloading could have been obtained at higher RPM speeds than set in the current study. However, little or no data exist on the methods or end points for defining optimal LV unloading with continuous-flow rotary pumps.
In summary, pulsatile, volume displacement pumps operating in a full-to-empty mode achieve a greater degree of LV volume unloading compared with continuous-flow rotary pumps with an axial design operating at a fixed motor speed. However, despite a greater degree of LV volume unloading observed with pulsatile, volume displacement pumps, pressure unloading and exercise performance at 3 months after LVAD implantation is similar between the 2 LVAD designs.
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Acknowledgments |
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Disclosures
F.D.P. and J.H. are the recipients of sponsored research funding from Thoratec Corporation, Pleasanton, Calif. D.J.F. is an employee and stockholder of Thoratec Corporation, Pleasanton, Calif. D.B.D. receives honoraria for teaching from Thoratec Corporation, Pleasanton, Calif.
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Footnotes |
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Presented at the American Heart Association Scientific Sessions, Chicago, Ill, November 12–15, 2006.
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References |
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