Analysis of the four 5-year intervals within the study period
Table 1 presents the number of articles published in the four 5-year intervals within the overall analysis period for the main contributors, as well as the proportions of the published articles that are in the top 1% and top 10%. We also examined the impact of these publications in terms of citations. Researchers from some countries were more likely than others to have authored publications in the top 1% and 10% most cited articles. A higher rate was seen for those from Canada and Australia than for those from the USA and Europe, and those from Asia were least likely to have authored articles in the top 1% and 10% of citations
In Web of Science and InCites, journals are classified into 254 categories. Figure 3 shows the major changes that have occurred in the categories for the 100 789 articles. Several categories (peripheral vascular disease, cardiac & cardiovascular systems] were stable and are not included in Fig. 3. Marked decreases between the periods 1997–2001 and 2012–2016 were noted in ‘pharmacology & pharmacy’ (34%), ‘physiology’ (22%), and ’transplantation’ (49%). Marked increases were noted in ‘public, environmental & occupational health’ (171%), ‘nutrition & dietetics’ (181%) and ‘oncology’ (265%). These changes reflect evolution in the fields of research relating to hypertension.
Analysis by journal was also performed: during the entire study period, 7042 articles were published in Hypertension, 5633 in Journal of Hypertension, 4214 in American Journal of Hypertension, and 2525 in Journal in Human Hypertension; 1044 articles were published in PLoS One (in the past 10 years, since its foundation); and 934 in American Journal of Physiology, Heart and Circulation section, 904 in Circulation and 860 in Journal of Cardiovascular Pharmacology. A complete list is provided in the Supplementary Data, http://links.lww.com/HJH/B104.
The 100 789 articles have been cited almost 2 556 000 times during the period 1997–2016 (a mean of about 25 citations per article). The most citations were in Hypertension (7042 articles cited 374 948 times), followed by Journal of Hypertension (5633 articles, 157 526 citations), Circulation (904 articles, 101 286 citations), American Journal of Hypertension (4214 articles, 96 209 citations), American Journal of Physiology (2729 articles, 80 263 citations), Lancet (312 articles, 69 166 citations), New England Journal of Medicine (160 articles, 62 720 citations), Journal of Human Hypertension (2525 articles, 43 083 citations), Kidney International (786 articles, 36 447 citations), and Journal of the American Medical Association (154 articles, 33 729 citations). A complete list is provided in the Supplementary Data, http://links.lww.com/HJH/B104.
Analysis of the top 1% of articles
To determine the most influential articles in the past decade, we analysed the 433 articles that, of all those published in the period 2007–2016, were classified as being among the 1% most frequently cited in the world (top 1%). In order to compare types of research in the European Union and USA, we analysed the 239 articles in the global top 1% that had authors from the European Union and the 250 articles in the top 1% that had authors from the USA. Exclusion of guidelines, nonsystematic reviews and articles not directly relating to hypertension resulted in the retention of 183 articles with European Union authors and 175 with US authors (of these, 52 articles had authors from both the European Union and USA). These articles were then classified by type of research (Table 2). Distribution across these types of research differed statistically between the US and European Union subsets (chi-squared test, P = 0.0095); the difference was greatest in epidemiological research, which was more prominent in the USA.
Analysis of European Union authorship
Table 3 shows the numbers of publications for the 12 European countries with at least 1000 articles published in the period 1997–2016. The first-ranked country based on the number of articles with authors from that country was the UK (19.5% of all articles with European Union authors), followed by Germany (18.3%), Italy (18.0%), France (12.8%), Netherlands (9.2%), Spain (8.6%) and Sweden (6.0%). When comparing the output in 1997–2006 with that in 2007–2017, all of these countries showed an increase, except for France and Finland, where output decreased.
Strongest increases in both the top 1% and top 10% impact indicators in 2012–2016 relative to the earlier 5-year periods were observed for Germany, Spain, Netherlands, Belgium and Poland (2–12 articles in top 1%). It should be noted, however, that the proportions of articles in the top 1% and top 10% must be interpreted with caution for countries with small numbers of articles.
European Union collaborations
Finally, we looked at collaborations between European Union countries in the final 5-year period. In order to analyse only articles with an important scientific impact, we extracted the 1363 articles that were among the 10% most cited articles and that had also been authored by at least one researcher in the European Union. Analysis was then performed using VosViewer software (Fig. 4).
The VosViewer results show the importance of the UK, Germany, Italy, Netherlands and France in the European Union collaboration network relating to the top 10% most cited articles. It positions USA (which is present because of co-authoring with European Union authors) at the centre, with UK very close. Countries are divided into three clusters: Italy, France, Poland, Spain, Belgium, Greece and Romania are grouped in the blue cluster. Germany, Netherlands, Denmark, Finland, Norway, Austria and Switzerland (not in European Union, but present because of co-authoring) are grouped in the green cluster. The UK has strong links with the USA, Sweden, Australia, Japan, China, Canada, Hungary and Czech Republic (red cluster).
This bibliometric analysis of the global research output in the field of hypertension shows an increase in the number of publications over the 20-year analysis period, although the rate of increase is less than that seen for all medical and health sciences articles during the same period.
We chose PubMed for the extraction as it is the largest database in the biomedical field and it provides a precise and specific search engine. PubMed also provides wider coverage of the biomedical field than Web of Science. However, not all journals indexed in PubMed are indexed in the Science Citation Index (used to compute impact factors), so PubMed articles published in journals without impact factors are not indexed in Web of Science and InCites. Results depend not only on the database used but are also strongly dependent on the search strategy. Despite using an iterative process to develop the strategy, a few articles not directly related to hypertension were identified in the search, mainly in the fields of congestive heart failure and diabetes. However, the influence of these articles is likely to be limited because of the large number of articles overall and because analyses were performed for several consecutive periods (so it is likely that there would be a similar influence in all periods).
The growth in the number of publications from China has been impressive in all fields, including hypertension. This has not been accompanied by an increase in the proportion of articles from China that are in the top 1% and top 10% most frequently cited articles about hypertension. It is important to note, however, that citation frequency does not necessarily reflect the intrinsic scientific value of an article. The likelihood of citation may be reduced because the names of the Chinese researchers are not yet recognized by the English-speaking community. Australia and Canada (predominantly English-speaking) are two countries outside the USA and the European Union that provide an above-average contribution.
Differences between European countries were observed in all four 5-year periods. These differences can be interpreted with reference to the gross domestic product (GDP) at purchasing power parity exchange rate (PPP). It would be particularly interesting, for example, to conduct an in-depth analysis of the research and care systems of the Netherlands (GDP at PPP: US$915.2 billion) because researchers from this country produce a high absolute number of publications and a high proportion of articles in the top 1% and top 10% categories, with sustained growth in the absolute numbers and an increase in the proportions in the top 1% and top 10% in 2012–2016 relative to 1997–2006. The Netherlands aside, the largest nations of the European Union are the most productive, led by the UK (GDP at PPP: US$2880 billion), Germany (US$4150 billion), and Italy (US$2307 billion), whereas France's performance is declining (US$2826 billion). Among the countries that joined the European Union more recently, contributions are increasing from Spain (GDP at PPP: US$1769 billion) and Poland (US$1111 billion).
Hypertension studies are published in a wide range of journals covering many specialties. Observed changes in the numbers of hypertension articles published may reflect developments in specific research fields. For example, better management of immunosuppression might explain the decrease in the number of articles published about transplantation, or hypertension as an adverse effect of new anticancer drugs might explain the rise in the number of articles relating to hypertension published in oncology journals. Not surprisingly, given that few new chemical entities with antihypertensive potential have been discovered in recent years, there have been few pharmacology articles published; by contrast, new lines of research are increasing in areas, such as genetics, resulting in increased numbers of publications.
Instead of looking at the impact factors of journals in which hypertension research is published, we considered citation frequency. Although impact factors are used in the evaluation of research careers and in decisions regarding funding research, these factors are journal-level metrics and so change with time and with journal publication policies – which often have an economic basis. By contrast, if an article has been cited frequently, this suggests that the findings reported have been useful to other researchers for initiating, performing, or interpreting their own research. It must be acknowledged that the citation frequency does not provide any information on the internal validity of the results – an article might be cited in a negative as well as a positive context (e.g. to refute the results it reports) – but even such negative citations indicate that the results have been of interest to other researchers. By contrast, it is clear that the many articles that are never or rarely cited have not generated widespread interest, and are therefore, unlikely to have contributed significantly to the wider body of scientific knowledge.
We focused on the 1% and 10% of articles that had been most frequently cited in the period 2007–2016. As the results of an automatic extraction depend on the search strategy used, we manually checked the top 1% of citations to allow exclusion of guidelines, expert consensus statements and opinion-based general reviews, as well as articles reporting animal studies and articles whose main focus were identified, after careful reading, as congestive heart failure, renal insufficiency or diabetes. Studies were classified as clinical investigations when the patient cohorts were from hospitals, and as epidemiology when performed in the general population. Clinical investigations included some controlled randomized studies involving small numbers of patients, mainly on biomarkers or surrogate endpoints.
Differences between the USA and the European Union were identified: while various types of research were represented in both regions, there were more articles on epidemiology and experimental research authored by US investigators, whereas articles on clinical investigations, genetics and meta-analyses were more numerous in research published by European Union authors.
Research is becoming increasingly collaborative, so it is important to analyse links between investigators in different countries. Use of VosViewer to visualize the main collaborations identified countries whose predominant language is English as being in a central position; these countries were also the most productive in terms of publications relating to hypertension. This cluster visualization may help to stimulate more collaborations between countries, either to reinforce existing ones because of their productivity, or to encourage new ones.
In conclusion, about 80% of the burden of hypertension occurs in low- and middle-income economies, so it will be important to undertake research specific to these countries in the near future. The problems that will be encountered in hypertension management and research in these regions are likely to be very similar to those that have been seen in high-income countries over the past 50 years. Worldwide monitoring of publications in hypertension might be useful to all.
The authors thank Oxford PharmaGenesis, Oxford, UK for assistance with the editing and formatting of this article.
Part of this work was presented at the 28th European Meeting on Hypertension and Cardiovascular Protection, Barcelona, Spain in June 2018
Contributions: J.M. and P.D. developed the study concept, P.D. conducted the extractions and analyses, J.M. checked the content of all top 1% publications. J.M. and P.D. interpreted the results and prepared the manuscript for publication.
Conflicts of interest
There are no conflicts of interest.
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