Bridge-to-Bridge Left Ventricular Assist Device Implantation Strategy vs. Primary Left Ventricular Assist Device Implantation Strategy

Abstract

Background: The bridge-to-bridge (BTB) strategy, a conversion to durable left ventricular assist device (LVAD) after stabilization using surgical temporary mechanical circulatory supports for a couple of months, is not uncommon in Japan. However, its effect on clinical outcomes in comparison with a primary durable LVAD implantation strategy remains unknown.

Methods and Results: Data of 837 consecutive patients (median age 45, 73% males) who underwent durable LVAD implantation as BTB (n=168) or primary implant (n=669) between April 2011 and April 2019 were retrospectively reviewed from the prospective multicenter Japanese mechanically assisted circulatory support registry. The BTB group was younger and had comparable end-organ function, better hemodynamic profile, and longer operative time compared with the primary implant group at baseline. The 3-year survival was 80% vs. 87% (P=0.007) for the BTB and primary implant groups respectively, with greater observed rates of stroke and infection as the predominant causes of death. The BTB strategy was independently associated with increased 3-year mortality (hazard ratio 2.69 [1.43–5.07], P=0.002) in addition to other significant risk factors.

Conclusions: The BTB cohort had comparable baseline characteristics to the primary implant cohort at the time of durable LVAD conversion, but had lower 3-year survival. Detailed analysis clarifying the causality of this finding should improve outcomes with the BTB strategy.

Given the improvement of mechanical assist device technology and increasingly sophisticated perioperative management, the mortality and morbidity following durable left ventricular assist device (LVAD) implantation have greatly improved, particularly among those with stable preoperative hemodynamics dependent on continuous inotropes who typically are classified as INTERMACS profile 3.^1,2 However, optimal therapeutic strategies for those with clinical deterioration assigned to INTERMACS profile 1 remain controversial.³

Based on the guidelines from Japanese academic societies, which do not recommend durable LVAD implantation for those with INTERMACS profile 1,⁴ the paracorporeal pulsatile LVAD implantation (Nipro VAD or AB5000), a counterpart of the Thoratec PVAD (Thoratec, Pleasanton, CA, USA), has long been used as a strategy for bridge-to-decision or -candidacy, followed by conversion to durable LVAD as a bridge-to-transplantation (i.e., secondary device) if applicable. Recently, an extracorporeal continuous-flow centrifugal pump (RotaFlow, Gyro pump, or Mera pump), which is a counterpart of CentriMag (Levitronix LLC, Waltham, MA, USA), has replaced almost all paracorporeal pulsatile LVADs, because of having fewer adverse events. In addition, central extracorporeal membranous oxygenation (ECMO) systems with extracorporeal centrifugal pumps have also become popular, especially in the presence of right ventricular dysfunction. This strategy that utilizes temporary mechanical support before durable LVAD is termed “bridge to bridge” (BTB).⁵

The BTB strategy might be promising, based on data from several small-center studies,^6–8 but its survival benefit has not been demonstrated in large-scale studies. The recently published second report of the Japanese registry for Mechanical Assisted Circulatory Support (J-MACS) observed a relatively worse outcome of the BTB strategy compared with the primary LVAD implantation strategy.⁹ J-MACS is the Japanese counterpart of INTERMACS, and has collected all data of reimbursed durable LVADs in a nationwide and mandatory manner. To date, detailed mechanisms and baseline characteristics affecting the outcome differences between the 2 strategies need further investigation. Given that BTB should be an essential strategy, at least for some critically sick patients, such an analysis has great implications for establishing a more sophisticated BTB strategy.

In this study of J-MACS registry data, we compared baseline characteristics and clinical outcomes between the BTB and primary LVAD implant strategies to investigate the clinical implications of the BTB strategy.

Methods

Patient Selection

Using the J-MACS database,⁴ we collected data on consecutive patients who underwent durable LVAD implantation as BTB (i.e., secondary device) or the primary device at 49 Japanese institutions between April 2011 and April 2019. Because current Japanese insurance policy only allows durable LVAD implantation in patients who are already listed or being listed on the Japanese Organ Transplant registry, all participants were under a bridge-to-transplant strategy. All participants provided informed consent at each institute before LVAD implantation. The use of the J-MACS registry was approved beforehand (reference no. 1902, April 9, 2019 by J-MACS Committee).

Follow-up Protocol

In the BTB group, patients were bridged from paracorporeal LVAD or extracorporeal centrifugal pump as LVAD or central ECMO (first devices; called surgical temporary mechanical circulatory support [MCS]) to durable LVADs (second devices). We defined the day of the device bridge as day 0. In the same manner, the day of durable LVAD implantation was defined as day 0 in the primary implant group.

Patients were managed according to established standard of care guidelines,¹⁰ including appropriate dosing of aspirin and warfarin targeted to established goals of international normalized ratio (INR) for each device. LVAD speed adjustments were made specific to the patient’s clinical condition.

Variables Evaluated

The latest data of baseline characteristics, and echocardiographic, hemodynamic, and laboratory tests obtained within 1 month before durable LVAD implantation were collected in addition to demographic data. These data were also collected 1 month following durable LVAD implantation. The primary endpoint was all-cause death during the 3-year study follow-up. Date of death and cause were collected. Dates of major adverse events, with definitions detailed in a prior publication,⁴ were also collected.

Statistical Analysis

Statistical analyses were performed with SPSS Statistics 23 (SPSS Inc., Armonk, IL, USA). Two-sided P values <0.05 were considered significant. Continuous variables are expressed as median and interquartile and compared between groups using the Mann-Whitney U test. Categorical variables are expressed as numbers and percentages and compared between groups using Fisher’s exact test.

The primary endpoint was all-cause death at 3 years following durable LVAD implantation. Kaplan-Meier analyses were performed to assess the 3-year survival, which was stratified and compared between the BTB and primary implant strategies using a log-rank test. Cox proportional hazard ratio regression analyses were performed to investigate the effect of individual baseline characteristics, including BTB strategy vs. primary implant strategy, on 3-year mortality. Variables significant in the univariable analyses were enrolled in the multivariable analyses with a step-wise method.

Results

Baseline Characteristics

Among a total of 837 patients registered in J-MACS,⁴ 168 patients underwent durable LVAD implantation as BTB, and 669 patients as a primary implant (Table 1). Median age was 45 years old and 27% were female.

Table 1. Baseline Characteristics

	Total (n=837)	BTB (n=168)	Primary LVAD (n=669)	P value
Demographics
Age, years	45 (34, 55)	40 (29, 47)	43 (34, 54)	0.002*
Female sex	226 (27%)	52 (31%)	174 (26%)	0.12
Body surface area, m2	1.65 (1.50, 1.76)	1.60 (1.49, 1.73)	1.66 (1.52, 1.75)	0.27
Device type				0.025*
DuraHeart	47 (6%)	14 (8%)	33 (5%)
EVAHEART	91 (11%)	11 (6%)	80 (12%)
HeartMate II	531 (63%)	99 (59%)	432 (65%)
HeartWare	13 (2%)	4 (2%)	9 (1%)
Jarvik 2000	155 (19%)	40 (24%)	115 (17%)
Comorbidity
Ischemic etiology	107 (13%)	46 (27%)	61 (9%)	<0.001*
Diabetes mellitus	154 (18%)	29 (17%)	125 (19%)	0.38
Chronic pulmonary obstructive disease	10 (1%)	4 (2%)	6 (1%)	0.12
History of stroke	68 (8%)	38 (23%)	30 (5%)	<0.001*
Echocardiography
Left ventricular end-diastolic diameter, cm	6.8 (5.9, 7.7)	5.7 (4.9, 6.8)	7.1 (6.2, 7.8)	<0.001*
Left ventricular ejection fraction <20%	472 (56%)	93 (55%)	379 (57%)	0.33
Aortic regurgitation moderate or greater	40 (5%)	8 (5%)	32 (5%)	0.51
Mitral regurgitation moderate or greater	438 (52%)	26 (15%)	412 (62%)	<0.001*
Tricuspid regurgitation moderate or greater	299 (36%)	25 (15%)	274 (41%)	<0.001*
Laboratory data
Hemoglobin, g/dL	11.4 (10.0, 12.8)	10.3 (9.0, 11.3)	12.0 (10.9, 13.2)	0.24
Platelets, 104/μL	19.0 (14.5, 24.7)	23.9 (19.4, 29.7)	18.6 (14.7, 23.5)	<0.001*
Albumin, g/dL	3.7 (3.2, 4.0)	3.6 (3.1, 3.9)	3.7 (3.3, 4.1)	0.002*
Total bilirubin, mg/dL	0.9 (0.6, 1.5)	0.8 (0.6, 1.1)	0.9 (0.7, 1.4)	0.002*
Creatinine, mg/dL	0.94 (0.70, 1.20)	0.74 (0.59, 0.90)	0.98 (0.78, 1.18)	<0.001*
Sodium, mEq/L	137 (134, 139)	139 (137, 140)	136 (134, 138)	<0.001*
Potassium, mEq/L	4.3 (4.0, 4.6)	4.2 (4.1, 4.5)	4.3 (4.0, 4.6)	0.10
C-reactive protein, mg/L	0.4 (0.1, 1.7)	0.4 (0.2, 1.3)	0.3 (0.1, 1.1)	<0.001*
Logarithm of B-type natriuretic peptide	2.67 (2.37, 2.94)	2.29 (2.14, 2.54)	2.71 (2.46, 2.96)	<0.001*
Hemodynamics
Heart rate, beats/min	80 (71, 94)	89 (80, 98)	80 (71, 91)	<0.001*
Systolic blood pressure, mmHg	88 (81, 97)	90 (84, 102)	88 (81, 97)	0.95
Diastolic blood pressure, mmHg	58 (51, 64)	61 (59, 68)	56 (50, 62)	<0.001*
Right atrial pressure, mmHg	21 (13, 28)	10 (8, 14)	23 (15, 28)	<0.001*
Pulmonary capillary wedge pressure, mmHg	21 (14, 27)	12 (9, 16)	22 (16, 28)	<0.001*
Cardiac index, L/min/m2	2.07 (1.74, 2.49)	2.61 (2.35, 3.02)	1.99 (1.69, 2.36)	<0.001*

Variables were compared between groups using the Mann-Whitney U test or Fischer’s exact test. *P<0.05. BTB, bridge to bridge; LVAD, left ventricular assist device.

BTB patients were younger, more likely to have ischemic cardiomyopathy and comparable baseline end-organ function to the primary implant patients. BTB patients also had more optimized hemodynamics under the pre-implant surgical temporary MCS support compared with the primary implant group. Prothrombin time with INR measured at 1, 3, 6, and 12 months following LVAD implantation did not significantly differ between the 2 groups (P>0.05 for all; Supplementary Table).

Perioperative Data

Both total operative time and cardiopulmonary bypass time were significantly longer in the BTB group compared with the primary implant group (P<0.05 for both; Table 2). Both the volume of perioperative blood transfusion and the duration of the index hospitalization were greater in the BTB group compared with the primary implant group (P<0.05 for both).

Table 2. Perioperative Data

	Total (n=837)	BTB (n=168)	Primary LVAD (n=669)	P value
Total operation time, min	437 (333, 572)	572 (445, 705)	410 (324, 534)	<0.001*
Cardiopulmonary bypass time, min	134 (98, 172)	146 (110, 195)	123 (92, 159)	<0.001*
Blood infusion, mL	1,960 (973, 3,640)	3,700 (2,370, 5,960)	1,680 (930, 2,840)	<0.001*
Duration of index hospitalization, days	91 (61, 135)	102 (75, 139)	90 (63, 124)	0.015*

Variables were compared between groups using the Mann-Whitney U test. *P<0.05. BTB, bridge to bridge; LVAD, left ventricular assist device.

1-Month Outcomes

At 1 month following durable LVAD implantation, the BTB group overall had smaller left ventricular dimensions with less mitral regurgitation (P<0.05 for both; Table 3). End-organ function was comparable between groups. Right atrial pressure was significantly higher in the BTB group (P=0.015).

Table 3. Clinical Data at 1 Month Following Durable LVAD Implantation

	Total (n=837)	BTB (n=168)	Primary LVAD (n=669)	P value
Echocardiography (n=570)
Left ventricular end-diastolic diameter, cm	5.7 (4.8, 6.6)	4.4 (4.2, 5.5)	5.9 (5.0, 6.8)	0.001*
Left ventricular ejection fraction <20%	310/548 (57%)	63/107 (59%)	247/441 (56%)	0.34
Aortic regurgitation moderate or greater	27/567 (5%)	5/109 (5%)	22/458 (5%)	0.58
Mitral regurgitation moderate or greater	68/563 (12%)	7/108 (6%)	61/455 (13%)	0.029*
Tricuspid regurgitation moderate or greater	101/564 (18%)	13/110 (12%)	88/454 (19%)	0.039*
Laboratory data (n=812)
Hemoglobin, g/dL	10.4 (9.6, 11.4)	9.8 (9.3, 10.5)	10.4 (9.5, 11.3)	0.26
Platelets, 104/μL	27.2 (20.7, 33.7)	29.6 (23.7, 30.8)	29.7 (23.4, 35.7)	0.23
Albumin, g/dL	3.3 (2.9, 3.6)	3.3 (3.2, 3.8)	3.3 (2.9, 3.6)	0.017*
Total bilirubin, mg/dL	0.8 (0.6, 1.1)	0.8 (0.6, 1.4)	0.7 (0.6, 1.0)	0.52
Creatinine, mg/dL	0.70 (0.55, 0.88)	0.67 (0.61, 0.76)	0.71 (0.57, 0.87)	0.016*
Sodium, mEq/L	138 (135, 140)	138 (135, 141)	138 (136, 141)	0.054
Potassium, mEq/L	4.2 (3.9, 4.5)	4.0 (3.8, 4.3)	4.2 (3.9, 4.5)	0.29
C-reactive protein, mg/L	1.5 (0.7, 3.1)	1.3 (0.8, 3.9)	1.4 (0.6, 2.6)	0.51
Logarithm of B-type natriuretic peptide	2.24 (2.01, 2.51)	2.24 (2.09, 2.41)	2.24 (2.04, 2.49)	0.053
Hemodynamics (n=220)
Right atrial pressure, mmHg	8 (5, 12)	12 (7, 13)	6 (4, 11)	0.015*
Pulmonary capillary wedge pressure, mmHg	11 (8, 14)	14 (9, 13)	10 (8, 14)	0.15
Cardiac index, L/min/m2	2.53 (2.19, 2.84)	2.47 (2.37, 3.02)	2.54 (2.16, 2.89)	0.70

Variables were compared between groups using the Mann-Whitney U test or Fischer’s exact test. *P<0.05. BTB, bridge to bridge; LVAD, left ventricular assist device.

3-Year Outcomes

During the 3-year observational period, the most predominant adverse events observed were stroke and infection (Figure 1). There were no statistically significant differences in the incidence of these major adverse events between the BTB and primary implant groups (P>0.05 for all).

Figure 1.

Individual event rate comparison. Variables were compared between the BTB and primary implant groups using Mann-Whitney U test. BTB, bridge to bridge; RVF, right ventricular failure.

In total, 97 patients died during the 3-year observational period (28 BTB and 69 primary implants); 3-year survival was significantly lower in the BTB group compared with the primary implant group (80% vs. 87%, P=0.007; Figure 2). In the BTB group, the most common adjudicated cause of death was stroke (61%) followed by infection (14%) and right ventricular failure (10%). Of note, fatal strokes developed at 168 (42, 245) days following the bridge. In the primary implant group, the prevalence of stroke was 38%, followed by infection (19%) and other various comorbidities as causes of death (Figure 3).

Figure 2.

The 3-year survival following durable LVAD implantation. *P<0.05 by log-rank test between the BTB and primary implant groups. BTB, bridge to bridge; LVAD, left ventricular assist device.

Figure 3.

Individual causes of death. MOF, multiple organ failure; RF, renal failure; RVF, right ventricular failure; Vf, ventricular fibrillation; VT, ventricular tachycardia.

Predictors of 3-Year Mortality

Univariable and multivariable analyses demonstrated that the BTB strategy was an independent predictor of 3-year mortality (hazard ratio 2.69 [1.43–5.07], P=0.002). Other predictors, including older age, lower body surface area, lower albumin, and higher creatinine levels, were also associated with an increased risk of death (P<0.05 for all; Table 4).

Table 4. Baseline Characteristics Associated With 3-Year Mortality Following Durable LVAD Implantation

	Univariable analyses		Multivariable analyses
	Hazard ratio (95% CI)	P value	Hazard ratio (95% CI)	P value
Age, years	1.03 (1.02–1.05)	<0.001*	1.03 (1.00–1.05)	0.024*
Female sex	1.62 (1.08–2.43)	0.019*		NA
Body surface area, m2	0.28 (0.10–0.78)	0.014*	0.03 (0.01–0.13)	<0.001*
Ischemic etiology	1.70 (1.04–2.78)	0.033*		NA
Diabetes mellitus	1.02 (1.01–1.04)	0.013*		NA
Chronic pulmonary obstructive disease	2.55 (0.81–8.03)	0.11
History of stroke	0.52 (0.19–1.42)	0.20
Left ventricular end-diastolic diameter, cm	0.85 (0.73–0.98)	0.023*		NA
Left ventricular ejection fraction <20%	0.73 (0.49–1.08)	0.12
Mitral regurgitation moderate or greater	1.00 (0.66–1.50)	0.98
Tricuspid regurgitation moderate or greater	1.23 (0.82–1.84)	0.33
Hemoglobin, g/dL	0.83 (0.75–0.92)	<0.001*		NA
Platelets, 104/μL	0.99 (0.98–1.01)	0.35
Albumin, g/dL	0.44 (0.31–0.62)	<0.001*	0.55 (0.34–0.88)	0.012*
Total bilirubin, mg/dL	1.06 (0.92–1.23)	0.40
Creatinine, mg/dL	1.90 (1.33–2.70)	<0.001*	4.12 (2.29–7.41)	<0.001*
Sodium, mEq/L	1.00 (0.96–1.05)	0.88
Potassium, mEq/L	0.91 (0.63–1.32)	0.63
C-reactive protein, mg/L	1.03 (1.01–1.05)	0.014*		NA
Logarithm of B-type natriuretic peptide	1.35 (0.77–2.38)	0.30
Heart rate, beats/min	1.01 (1.00–1.02)	0.061
Systolic blood pressure, mmHg	0.99 (0.97–1.01)	0.20
Diastolic blood pressure, mmHg	1.02 (1.01–1.04)	0.013*		NA
Right atrial pressure, mmHg	1.00 (0.98–1.03)	0.93
Pulmonary capillary wedge pressure, mmHg	0.99 (0.97–1.02)	0.41
Cardiac index, L/min/m2	1.66 (1.18–2.33)	0.003*		NA
BTB vs. primary LVAD	1.85 (1.18–2.91)	0.007*	2.69 (1.43–5.07)	0.002*

*P<0.05 by Cox proportional hazard ratio regression analysis. BTB, bridge to bridge; CI, confidence interval; LVAD, left ventricular assist device; NA, not applicable.

Among the perioperative variables, the operation time per 60 min was independently associated with the 3-year mortality following durable LVAD implantation with an adjusted hazard ratio of 1.14 (95% confidence interval 1.07–1.21, P<0.001).

Discussion

We compared the perioperative characteristics between a BTB cohort (bridged from surgical temporary MCS to durable LVAD) and a primary implant cohort using data from a prospective multicenter J-MACS registry.⁴ The major findings were as follows. (1) Among all durable LVADs implanted in Japan, 20% were implanted as BTB. (2) The BTB cohort was younger and more often had ischemic cardiomyopathy, as well as more optimized baseline hemodynamics at the time of durable LVAD implantation. (3) Baseline end-organ function was comparable between the cohorts at the time of durable LVAD implantation. (4) The BTB cohort had longer operative time and required more postoperative blood transfusions. (5) The 3-year survival was lower in the BTB cohort predominantly because of a higher incidence of fatal stroke. (6) The BTB strategy was an independent risk factor for 3-year mortality.

Background Comparison

This is the first published report comparing baseline characteristics between BTB and primary implant cohorts undergoing durable LVAD implantation. It is reasonable to assume that BTB patients would more often have, ischemic heart disease given that the most common etiology of acute cardiogenic shock is acute coronary syndrome. The BTB cohort was younger than the primary implant cohort, possibly because older patients might have died, or end-organ functional recovery might have not been sufficient for heart transplantation listing following surgical temporary MCS implant.

Most BTB patients had an INTERMACS profile 1, but a small portion had profile 2 before the first surgery, although durable LVAD implantation was performed on condition that the first MCS therapy successfully restored hemodynamics resulting in normalization of end-organ function to a level comparable to that of the primary implant cohort.

Survival Comparison

Despite comparable background characteristics, the BTB cohort had an observed 10% lower survival at 3 years compared with the primary implant cohort following durable LVAD support. The BTB strategy was independently associated with increased risk of death in addition to other conventional risk factors including age, smaller body size, hypoalbuminemia, and renal impairment.^11–13 The predominant causes of death were stroke and infection in both cohorts.

As reported previously, the incidence of pump infection, probably due to more complex and longer surgery, would be higher in the BTB cohort than in the primary implant cohort, given the presence of pre-implanted surgical temporary MCS.⁹ Such device-related infection might increase the risk of systemic infection, which is known to subsequently dramatically increase the risk of stroke following LVAD implant.¹⁴ Consistently, most of the fatal strokes occurred within 1 year following the bridge (168 days on median). Central venous pressure at postoperative 1 month was higher in the BTB cohort, probably due to longer cardiopulmonary bypass time and more blood transfusions.¹⁵ Of note, a longer operation time was an independent risk factor of death following durable LVAD implantation. Subclinical right heart failure in the BTB cohort might stimulate chronic inflammation and thus increase the risk of stroke.¹⁶ Prior temporary MCS in the BTB cohort might lead to acquired von Willebrand disease, which increases the risk of hemorrhagic stroke.^17,18

Future Directions

Despite our findings, the BTB strategy still fills an unmet need, given that this is the only strategy to rescue INTERMACS profile 1 patients, who cannot be listed for heart transplantation immediately.⁹ Even when destination therapy is approved in Japan, direct implantation of durable LVADs in patients with INTERMACS profile 1 is challenging, given its poor postoperative prognosis.¹ Severe donor shortage in Japan makes the waiting period extremely long (>4 years) and the Japanese organ donation system does not allocate sicker patients in a fast-track listing. Therefore, we cannot help but implant durable LVADs in those who are listed for transplant after stabilization by temporary MCS to keep them hemodynamically safe and maintain a better quality of life during the long waiting period.

We believe that the BTB should remain an essential strategy in Japan, and that it has acceptable clinical outcomes considering the crash-and-burn starting line. Currently, we only apply the BTB strategy for patients who have potential eligibility for transplant. On the other hand, the BTB strategy for those who carry a non-modifiable ineligibility for transplant (e.g., age ≥65 years) might require further discussion. The innovation of more sophisticated surgical techniques that minimize operative time, infection risk, and blood transfusions might make the prognosis of the BTB strategy better, and thus it could be expanded to destination therapy.

Optimal timing of the BTB surgery and appropriate patient selection might also improve survival. Long-term MCS support increases the risk of post-bridge device-related infection and associated deaths. However, optimization of end-organ function and hemodynamics for successful bridge therapy would require a certain period of temporary MCS support. Reducing infection at the cannulation site of the temporary support device is a critically important issue for minimizing surgical complexity and suppressing the occurrence of postoperative infection. We should also consider a better way to cardiac reverse remodeling as a means for potential explantation of temporary MCS before bridging to a durable LVAD.¹⁹

Study Limitations

Given the nature of multicenter large-scale registries, we could not specify some important data elements, including the types of stroke and infections. Given the major purpose of the J-MACS registry is to collect data on durable LVADs, detailed data of temporary MCS were lacking. The detailed causality between the BTB strategy and death from fatal stroke or infection needs further investigation. Further analyses using blood biomarkers are warranted. The device type, including surgical temporary MCS and durable LVADs, might also have an effect on the outcome. We did not perform subanalyses of each device given the small subgroup numbers. Less invasive percutaneous LVADs could be used as a bridging tool to durable LVAD in the near future, although such a percutaneous bridge strategy is not defined as BTB. Clinical outcomes of such a strategy would be another future concern.

Conclusions

The BTB cohort had comparable baseline characteristics to the primary implant cohort at the time of durable LVAD implantation, but had lower 3-year survival. Further detailed analyses clarifying the causality of this finding may improve outcomes with the BTB strategy.

Acknowledgments

The authors acknowledge all participants of the J-MACS registry and all clinical institutions, surgeons, and medical staff who contributed to this project.

Source of Funding

None.

Disclosures

K.K., M.O., and Y. Sawa are members of Circulation Journal’s Editorial Team.

Data Availability

The deidentified participant data will not be shared.

IRB Information

The present study was approved by the J-MACS committee (reference no. 1902, April 9, 2019).

Supplementary Files

Please find supplementary file(s);

http://dx.doi.org/10.1253/circj.CJ-20-0840

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