The Development of Ventricular Assist Devices over the Past 30 Years: A Bibliometric Analysis

Abstract

Background: A large gap exists between the needs of patients with end-stage heart failure and the number of gold-standard heart transplants. Over the past 30 years, a revolutionary treatment strategy using ventricular assist devices (VADs) has rapidly developed and become widely used in clinical practice. However, few analyses have assessed the application and publication trends in the VAD field.

Method: We used the Web of Science core collection to identify VAD research published between 1992 and 2022. We performed analysis and data visualization with CiteSpace, Scimago Graphica, and VOSviewer.

Results: We identified 13,274 articles published in 1129 journals, describing work from 6351 institutions in 86 countries. Among them, the United States contributed the most to VAD research, and almost all the top ten authors and institutions contributing to VAD research were from the United States.

Conclusions: In the past 5 years, VAD research has focused on right heart failure, outcomes, effects and risk factors, societies of surgeons, and clinical guidelines. Because of the large number of patients with heart failure, we expect VAD development to peak in the next decade.

Main article text

Introduction

Heart failure (HF) is a serious and terminal stage of various heart diseases with high morbidity, mortality, and re-admission rates. As a disease with an increasing incidence rate, HF affects at least 38 million people worldwide [1]. In early stages of HF, positive results can be achieved with appropriate drug treatments such as diuretics, β-blockers, angiotensin receptor-neprilysin inhibitors, angiotensin-converting enzyme inhibitors, or angiotensin receptor blockers [2, 3]. However, some patients inevitably progress to end stage of HF. Surgery is currently the only option, because medical treatment does not have a satisfactory prognosis [3, 4]. For the treatment of end-stage HF, heart transplantation (HT) currently remains the gold standard, and has a 1-year survival rate of approximately 90% and a median survival of 12.5 years. Limited by the number of donor hearts, fewer than 5000 heart transplants worldwide are performed annually, and the substantial demand is far from being met [5].

With decades of exploration and development, ventricular assist device (VAD) technology has matured and is widely used to treat end-stage HF. VAD can be used as a bridge to transplant (BTT), bridge to recovery, or bridge to candidacy, as well as a destination therapy (DT) [6]. Over the past 10 years, VAD has markedly developed, and many research achievements have been reported. Traditionally, a review of research requires extensive manual reading, and summarizes only specific parts of the specialty. VAD researchers have difficulty in gaining a complete picture of existing research. Therefore, we used bibliometrics, a more effective and advanced method than manual review. This method can be used to conduct quantitative analysis of research by using mathematics and statistics, and to analyze the research status and knowledge structure of research in a field according to the time and space dimensions through a literature visualization network [7, 8]. Herein, we address the lack of bibliometric analysis to date in the field of VAD.

Methods

Sources of Data

After a discussion with all authors, the requirement for informed consent was waived. We retrieved and downloaded data from the Web of Science (WOS). We chose the core collection of WOS and set the time span from January 1, 1992, to November 9, 2022. The search term was as follows: (TS = ventricular AND assist AND device). We excluded conference summaries, unpublished articles, letters, meeting abstracts, conference proceedings, editorial materials, corrections, early access articles, notes, book chapters, news items, retractions, biographical items, addition corrections, data articles, and retracted publications. We obtained 13,274 articles, including 11,587 articles and 1687 reviews. All information regarding these articles, including the title, author, abstract, keywords, and cited references, was exported in plain text format.

Bibliometric Analysis and Visualization

We used CiteSpace (CiteSpace Home (podia.com)) to segment the data to obtain the annual number of publications worldwide [9]. Subsequently, we imported these data into Microsoft Excel 2019 (Microsoft Corporation, Redmond, Washington, WA), calculated the annual growth rate, and fitted a trend line model of the number of documents. We analyzed cooperation among countries, authors, and institutions according to co-authorship in VOSviewer 1.6.18 (VOSviewer–Visualizing scientific landscapes). We selected countries with more than five articles to establish cooperation networks, then entered these data into Scimago Graphica (Scimago Graphica) for geographical visualization. Finally, we used CiteSpace for keyword emergence analysis to present the trends in VAD research over the past 30 years and the current major research topics. In addition, after merging synonymous keywords and deleting meaningless keywords, we analyzed all keyword co-occurrences and constructed keyword density maps for the initial 20 years and the past 10 years in VOSviewer 1.6.18.

Results

VAD Publication Trends Over the Past 30 Years

After screening, we identified 13,274 VAD articles published over the past 30 years. After using CiteSpace to delete duplicates, we obtained 13,260 articles. The number of documents published generally increased each year. We fitted a polynomial trend line to show the relationship between the number of articles and the year. The trend line model fitting result was acceptable, with a determination coefficient R² = 0.9606 (Figure 1). The annual growth rate of publications was calculated separately. The years with an annual growth rate of more than 20% included 1994, 1996, 1999, 2003, 2010, 2012, and 2018. We divided the period from 1992 to 2021 into three decades and used the POWER function to calculate the average growth rates for each of these decades, which were 11.69%, 10.65%, and 7.13%, for 1992–2001, 2002–2011, and 2012–2021, respectively.

Figure 1

Numbers of Articles from 1992 to 2022.

Contributions and Cooperation Network of Countries

We counted the top ten countries with the greatest number of publications according to co-authorship in VOSviewer (Table 1). As a result of the steady growth and accumulation of VAD research, the average publication years in these countries were all in the last decade. The United States ranked first worldwide, with a total of 7478 (56.40%) articles, a number far exceeding those in other countries. The average number of citations in eight of the top ten countries was more than 20. Japan and China were the two exceptions, which had an average number of citations of 11.83 and 11.18, respectively. According to the weight of the number of publications, we constructed a cooperation network among countries (Figure 2; different colors represent different cooperation clusters). Cooperation and communication in VAD research were concentrated primarily in the United States and European countries.

Figure 2

World Map of the Intensity of Cooperation among Countries.

The size of the country label is set according to the weight of the number of documents. Different colors indicate different clusters.

Table 1

Top Ten Countries with the Most Articles.

Rank	Country	Number of articles (%)	Number of citations	Average citation per article	Average publication year
1	United States	7478 (56.40)	213,999	28.62	2013
2	Germany	1674 (12.62)	38,105	22.76	2013
3	Japan	972 (7.33)	11,503	11.83	2012
4	Italy	759 (5.72)	17,444	22.98	2016
5	United Kingdom	756 (5.70)	21,555	28.51	2015
6	Canada	545 (4.11)	14,493	26.59	2015
7	Australia	428 (3.23)	9542	22.29	2015
8	China	395 (2.98)	4416	11.18	2015
9	Netherlands	389 (2.93)	11,785	30.30	2015
10	Switzerland	359 (2.71)	9104	25.36	2015

Contributions and Cooperation Network of Authors and Institutions

We used co-authorship analysis to identify the most influential authors and institutions. In addition, authors and institutions with more than 40 articles were included in the cooperative network. In Figure 3, the size of the marks indicates the number of articles published. Different colors represent different research cooperation clusters. The authors and institutions with the most publications did not necessarily have the most citations. Therefore, we ranked the top ten authors and institutions according to citations and calculated the average number of citations per article and average publication years (Tables 2 and 3). Mehmet Oz from Bozok University, Turkey and the other nine authors published primarily in the past 10 years. Additionally, the nine authors and ten institutions that contributed the most to VAD research were all located in the United States.

Figure 3

Collaboration Networks.

Network maps based on VOSviewer analysis of (A) author collaboration and (B) institutional collaboration.

Table 2

Top Ten Authors with the Most Citations.

Rank	Author	Institution	Number of articles	Number of citations	Average citation per article	Average publication year
1	Francis D. Pagani	University of Michigan, USA	163	16,401	100.62	2016
2	Naka, Yoshifumi	Columbia University, USA	233	11,093	47.61	2016
3	Mark S. Slaughter	University of Louisville, USA	150	10,099	67.33	2015
4	D.J. Farrar	Abbott Laboratories, USA	66	10,044	152.18	2013
5	Joseph G. Rogers	Texas Heart Institute, USA	92	9057	98.45	2015
6	Mehmet Oz	Bozok University, Turkey	111	8875	79.96	2000
7	Stuart D. Russell	Johns Hopkins Medicine, USA	68	8770	128.97	2014
8	Robert L. Kormos	Abbott Vascular, USA	84	8688	103.43	2015
9	Carmelo Milano	Duke University, USA	89	8658	97.28	2015
10	James K. Kirklin	University of Alabama Birmingham, USA	116	8627	74.37	2017

Table 3

Top Ten Institutions with the Most Citations.

Rank	Institution	Country	Number of articles (%)	Number of citations	Average citation per article	Average publication year
1	Columbia University	USA	572 (4.31)	27,398	47.90	2014
2	Duke University	USA	372 (2.81)	19,689	52.93	2012
3	University of Pittsburgh	USA	353 (2.66)	19,738	55.92	2016
4	University of Michigan	USA	288 (2.17)	19,120	66.39	2015
5	University of Minnesota	USA	252 (1.90)	18,794	74.58	2013
6	Brigham and Women’s Hospital	USA	205 (1.55)	16,862	82.25	2015
7	The Texas Heart Institute	USA	191 (1.44)	13,805	72.28	2011
8	Mayo Clinic	USA	371 (2.80)	12,101	32.62	2016
9	University of Alabama at Birmingham	USA	213 (1.61)	11,289	53.00	2017
10	University of Washington	USA	213 (1.61)	10,971	51.51	2015

Contributions of Journals

We used citation analysis to identify 1128 journals that published articles on VAD, 523 of which had published only one article. We ranked the top ten journals according to the number of citations. We queried the most recent impact factor (IF) and Journal Citation Reports (JCR) categories in WOS, and calculated the average number of citations per article and average publication years (Table 4). Notably, the Journal of Heart and Lung Transplantation, the leading journal in the field of HT, ranked first, with the greatest number of citations, and ranked third in the number of articles. The top two journals with the highest number of articles, ASAIO Journal and Artificial Organs, are not medical journals but are associated with the engineering and biomedical fields. These two journals published articles primarily on technical innovation associated with VAD, and their average numbers of citations were relatively low, at 12.49 for ASAIO Journal and 13.89 for Artificial Organs.

Table 4

Top Ten Journals with the Most Citations.

Rank	Journal	Impact factor* (JCR rank†)	Number of articles (%)	Number of citations	Average citation per article	Average publication year
1	Journal of Heart and Lung Transplantation	8.9 (Q1)	818 (6.17)	37,974	46.42	2011
2	Circulation	37.8 (Q1)	228 (1.72)	25,175	110.42	2006
3	Annals of Thoracic Surgery	4.6 (Q1)	660 (4.98)	23,514	35.63	2007
4	Journal of the American College of Cardiology	24.0 (Q1)	152 (1.15)	16,512	108.63	2011
5	ASAIO Journal	4.2 (Q2)	1237 (9.33)	15,455	12.49	2013
6	Artificial Organs	2.4 (Q3)	988 (7.45)	13,720	13.89	2010
7	New England Journal of Medicine	158.5 (Q1)	17 (0.13)	13,648	802.82	2009
8	Journal of Thoracic and Cardiovascular Surgery	6.0 (Q1)	324 (2.44)	13,339	41.17	2009
9	European Journal of Cardio-Thoracic Surgery	3.4 (Q1)	291 (2.19)	7181	24.68	2012
10	Circulation-Heart Failure	9.7 (Q1)	171 (1.29)	6863	40.13	2016

*Impact factor for 2022 from Web of science, ^†JCR, journal Citation Report from Web of Science. The discipline category of JCR is cardiac & cardiovascular systems or transplantation.

Analysis of Keywords and Frequent Research Topics

We analyzed the co-occurrence of all keywords in VOSviewer. We selected the keywords appearing more than 100 times to construct network and density visualization maps for the initial 20 years and the past 10 years, respectively (Figures 4 and 5). VAD, an important mechanical circulatory support device, was always associated with HF and HT. However, the specific research focus changed. We identified the top 30 keywords with the strongest citation bursts in CiteSpace; these keywords were considered to indicate research frontiers or emerging trends over time (Figure 6). The keyword citation bursts after 2015, which have continued to date, were right heart failure (RHF), outcomes, impact and risk factors, societies, and guidelines, which were the most common VAD research topics.

Figure 4

Keywords Co-occurrence Networks.

Keyword co-occurrence analysis with VOSviewer of (A) the initial 20 years and (B) the past 10 years.

Figure 5

Density Visualization Maps.

Density map of keywords generated in VOSviewer for (A) the initial 20 years and (B) the past 10 years.

Figure 6

Top 30 Keywords with the Strongest Citation Bursts.

Discussion

General Information

VAD is becoming an indispensable option for patients with end-stage HF who cannot undergo HT, and the number of implants continues to increase. According to the 36th adult HT report from the International Society for Heart and Lung Transplantation (ISHLT), the number of adult HTs during 1992–2000, 2001–2009, and 2010–2018 was 37,794, 33,625, and 36,883, respectively. The number of HTs has scarcely increased since the beginning of the 21^st century [10]. And the development and updating of VAD occurred concurrently. Since the first VAD, Abiomed BVS 5000, was approved by the U.S. Food and Drug Administration for BTT in 1992, VAD research and implantation have expanded over the past 30 years [11]. The number of publications has grown over the past two decades, except in 2021 and 2022, because of the effects of the COVID-19 pandemic. A growing number of VADs have been used to treat patients. An Interagency Registry for Mechanically Assisted Circulatory Support (INTERMACS) report has indicated implantation of 28,447 VADs in the United States in the 10 years from 2011 to 2020. Notably, VAD use has gradually shifted from BTT to DT, the former of which has accounted for a substantial proportion of procedures since 1992. VADs are currently implanted primarily for DT [12]. Research also has shown that the short-term effects of DT with the most recent VAD type (HeartMate 3) are similar to those of HT [13].

At present, the use of VADs has been concentrated mainly in the United States, Europe, Japan, and other developed regions. Only several products have been listed and clinically promoted in some countries. The United States has contributed most of the research results on VAD, with numbers of articles and citations far exceeding those for other countries. In addition, almost all ten of the leading researchers and institutions in VAD research are in the United States. China is the only developing country among the top ten countries contributing to VAD research. Although VAD research in China started relatively late, with the growing emphasis on HF and HT, research process in this field has accelerated [14,15]. By the end of 2022, three devices had been verified and launched in China, and are being applied in clinical practice. Because of the large number of HF patients in China (nearly 12.1 million) [16], we expect VAD development to peak in the next 10 years. Challenges for China will include establishing a nationwide VAD registry and standardizing the postoperative management of patients. Moreover, China must join the global VAD registry and contribute more Chinese data to VAD research. Among the top ten countries, the average number of citations for Japan and China was far lower than those of the other eight countries. This finding suggests that, although these two countries have made substantial achievements in VAD, a research gap persists in the number of contributions between those countries and the other eight countries. In addition, the cooperation network map indicated that these two Asian countries lack international cooperation and must strengthen cooperation in the future.

Among the top ten authors, only Mehmet Oz from Bozok University, Turkey, was active in the early period (1992–2001). He might potentially not have continued to make further contributions to VAD research because his focus of work changed and shifted toward the development of education and television production. In addition, Mark S. Slaughter and Robert L. Kormos, two professors from Abbott, have provided the foundation of Abbott’s global leadership in VAD research. Unexpectedly, the top ten institutions were all in the United States; they have contributed 3030 articles (22.85%) to global VAD research. VAD is a by-product of medicine and engineering. As such, although ASAIO Journal and Artificial Organs are not medical journals, they were among the top ten most cited journals, and their numbers of articles ranked first and second. After clinicians propose products for diagnosis and treatment, engineers design and improve these products. Therefore, beyond clinical research, the exploration of VAD has inevitably led to many technical innovations and achievements in the field of medical engineering. Furthermore, the other eight rankings included leading journals in cardiovascular and cardiothoracic surgery, and HT research.

Analysis of Common VAD Research Topics

Some changes have emerged in the research focus on VAD as an alternative to HT for patients with end-stage HF over the past 30 years. In the past 20 years, researchers have focused primarily on VAD, HT, and management and prognosis after implantation, as well as the primary diseases of HF and the survival rates of patients with HF. In the past 10 years, in addition to these four main directions, researchers have begun to pay greater attention to left VAD (LVAD) and cardiac blood flow after implantation, and to focus on the prognosis and postoperative management of patients. RHF, outcomes, impact and risk factors, societies, and guidelines are all frequent research topics that have emerged and have continued to be focal points since 2015.

Right Heart Failure

Isolated LVAD currently accounts for 95% of VAD implantations [12]. LVAD significantly affects the hemodynamics of the right ventricle (RV), particularly when the pump flow does not match the right ventricular output. Therefore, RHF easily occurs in early stages after LVAD implantation, with an incidence of 10–40% [17]. RHF is the main cause of early death after LVAD implantation, thus posing a substantial challenge to postoperative management and significantly increasing the incidence of other adverse events. Patients with early RHF usually have poor long-term prognosis [18, 19]. Some studies have shown that pulmonary vasodilators can be used to decrease pulmonary vascular resistance and prevent RHF, but the specific effects must be demonstrated by additional clinical trials [20]. Although many researchers would like to explore the risk factors and predictors of post-LVAD RHF, such research is difficult to conduct. A meta-analysis has indicated that patients on ventilatory support or receiving continuous renal replacement therapy are at high risk of post-LVAD RHF, similarly to patients with slightly elevated international normalized ratios, high N-terminal pro-brain natriuretic peptide, or leukocytosis. High central venous pressure, a low right ventricular stroke work index, and an enlarged RV with concomitant low RV strain can also be used to identify patients at high risk of RHF [21]. Several models based on LVAD cohorts for predicting post-LVAD RHF have achieved good prediction in the modeling process but have not always achieved satisfactory performance in external validation [22–24]. Most of those studies have been conducted at single centers, and the diagnostic criteria and inspection methods for RHF are not agreed upon by each center. Consequently, much progress remains to be made in research on the prediction and treatment of post-LVAD RHF.

Outcomes

With the continuing iteration of VAD and improvement in post-VAD management, the outcomes of VAD implantation are improving. Despite an older and sicker patient cohort, survival from 2016 to 2020 at 1 and 2 years has continued to improve, at 82.8% and 74.1%, respectively [12]. In particular, since Abbott’s HeartMate 3 was approved by the U.S. Food and Drug Administration in 2018, it has achieved short-term effects comparable to those HT, with a 2-year survival rate of 83% [13, 25]. Moreover, the incidence of adverse events after VAD implantation has improved with respect to that in 2011 to 2015. Incidents such as stroke, gastrointestinal bleeding, infection, and pump thrombosis have significantly decreased. The most commonly reported reasons for readmission include major infection (13.5%), major bleeding (12.9%), fluid overload (5.1%), arrhythmias (5.1%), and neurologic dysfunction inclusive of stroke (4.8%) [12]. Nevertheless, these postoperative complications still hinder the use of VAD, and identifying and diagnosing these problems early, through laboratory, imaging, and even invasive examinations, is particularly important [21, 26]. At present, the median survival after adult HT is 12.5 years, and this value increases to 14.8 years among 1-year survivors. The main adverse events after transplantation are acute rejection, malignancy, cardiac allograft vasculopathy, and renal failure [5, 7]. VADs require further upgrades and postoperative management improvement to address the problems of adverse events to close the gap in long-term survival with respect to HT. For example, to prevent infection, researchers have been designing less invasive, completely implanted VADs [27, 28]. For gastrointestinal bleeding after VAD implantation, the risk can be decreased by using thalidomide, omega-3-fatty acids, octreotide, and danazol to optimize management [29]. Research on the bleeding mechanism is also in progress [30, 31].

Impact and Risk Factors

VAD can replace most or all heart functions, and the effects of implantation are influenced by many factors. In recent years, with the maturity of VAD and a substantial increase in clinical implantation, the risk factors for death and various adverse events have been widely studied to guide the accurate screening and postoperative management of recipients and further improve prognosis. Failure to follow the doctor’s instructions for postoperative care and examinations, failure to cease smoking and alcohol consumption, inability to move because of severe chronic obstructive pulmonary disease or symptomatic peripheral vascular disease, and inability to meet postoperative management needs because of poor living conditions are all risk factors confirmed to lead to poor prognosis [32, 33]. However, the effects of age and obesity on VAD implantation are controversial, and currently, the specific cutoff values for age or body mass index that would limit the surgery remain unclear [34, 35]. The most recent research has suggested that the risk of infection, gastrointestinal bleeding, pump thrombosis, and re-admission among patients with psychosocial risk will greatly increase [36]. In addition, central VAD volumes, baseline health condition, and socioeconomic factors can also affect death and adverse events after VAD implantation [37–39]. Clear causality and cut-off points must be determined through further in-depth study.

Societies and Guidelines

The International Society for Heart and Lung Transplantation Registry for Mechanically Assisted Circulatory Support (IMACS) was first conceived and founded by ISHLT in 1999. The United States and Europe established INTERMACS and the European Registry for Patients with Mechanical Circulatory Support (EUROMACS) in 2005 and 2009, respectively, and began to contribute to the global registry IMACS in 2016. Given the rapid growth of VAD, INTERMACS, EUROMACS, and IMACS published their first reports in 2008, 2015, and 2016, respectively [40–42]. In 2012, EUROMACS became an official committee of the European Association for Cardiothoracic Surgery. On January 1, 2018, the INTERMACS Database became part of the Society of Thoracic Surgeons National Database. Societies subsequently began to publish annual reports summarizing and analyzing the number of VAD implantations, patient characteristics, and follow-up results after implantation. In particular, these reports in recent years were highly cited articles in WOS and in the top 1% of articles in the field of VAD [12, 43]. More importantly, many relevant studies have been conducted by using these societies’ databases. The accumulation of experience in the area of VAD has promoted the establishment of consensus and guidelines [44–46]. More work is needed, however, because published evidence remains insufficient to formulate more definitive guidelines.

Limitations

Several limitations apply to our study. First, the analysis was restricted to research articles published exclusively in English, thus potentially leading to omission of relevant studies published in other languages. Second, we used the topic-searching method, and the study’s scope was limited to articles indexed within the WOS database; consequently, some relevant studies might have been missed. In addition, the lack of evaluation of publication quality might potentially have affected the study’s outcomes. Similarly, the analysis did not assess the broader influence of the publications; consequently, insights into the research’s significance could not be offered. Furthermore, we did not investigate the funding sources of the publications, which might have shed light on the financial support for VAD research.

Conclusion

Through an analysis of nearly 30 years of research in VAD by bibliometrics, we found that studies have been concentrated in developed countries, given that VAD is a cutting-edge procedure. The United States has made the greatest contribution and has closely cooperated with Europe. At present, Europe and the United States have gradually established a high-quality VAD registration system. Regarding frequent research topics in recent years, prevention, diagnosis, and treatment of post-LVAD RHF, the risk factors for death and adverse events, and outcomes of patients with VAD implantation have attracted increasing attention. In addition, societies and their databases have added value to VAD research by continually providing a higher level of evidence of improvements in VAD, as well as in the prognosis and management of patients. We expect that in the next decade, sufficient evidence in VAD research will be accumulated to develop authoritative, meaningful, and global guidelines.

Data Availability Statement

All authors confirm that the data supporting the findings of the study are available on reasonable request.

Ethics Statement

This study was approved by the Anzhen Hospital’s Ethics Committee (clinical ethics approval number 2021-28). The need for informed consent was waived.

Author Contributions

Zhou Liu conceived the study. Zhou Liu and Siyue Zheng performed data collection and analyzed the data. Zhou Liu, Siyue Zheng, Yazhe Zhang, and Maozhou Wang reviewed the literature and prepared the first draft of this manuscript. Wenjian Jiang, Ming Gong, and Hongjia Zhang critically reviewed and edited the manuscript, and approved the final version. The final version of this manuscript was reviewed and revised by all authors, and all authors approved its submission.

Conflict of Interest

The authors have no conflicts of interest to declare.

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