Standing on the shoulders of Giants: a citation analysis of the paediatric congenital heart disease literature

Abstract Objective: The citation history of a published article reflects its impact on the literature over time. We conducted a comprehensive bibliometric analysis to identify the most cited papers on CHD in children. Methods: One-hundred and ninety journals listed in Journal Citation Reports were accessed via Web of Science. Publications with 250 or more citations were identified from Science Citation Index Expanded (1900–2020), and those relating to structural CHD in children were reviewed. Articles were ranked by citation count and the 100 most cited were analysed. Results: The number of citations ranged from 2522 to 309 (median 431, IQR 356–518), with 35 published since 2000. All were written in English, most originated from the United States (74%), and were published in cardiovascular journals, with Circulation (28%) the most frequent. There were 86 original research articles, including 50 case series, 14 cohort studies, and 10 clinical trials. The most cited paper was by Hoffman JI and Kaplan S on the incidence of CHD. Thirteen authors had 4 or more publications in the top 100, all of whom had worked in Boston, Philadelphia, San Francisco, or Dallas, and the most prolific author was Newburger JW (9 articles). Conclusions: Citation analysis provides a historical perspective on scientific progress by assessing the impact of individual articles. Our study highlights the dominant position of US-based researchers and journals in this field. Most of the highly cited articles remain case series, with few randomised controlled trials in CHD appearing in recent years.

libraries, including the title, journal, year, authors, centre and country of origin, language, type of study, level of evidence, 4 field of study, topic, and funding source.

Results
One hundred and ninety journals listed in one or more categories of the Journal Citation Reports were included in the search (see supplement) and 134 articles with 250 or more citations which met the criteria were identified. Articles were ranked according to their citation count and the 100 most cited papers, published in 24 journals, were included in the analysis ( Table 1).
The number of citations ranged from 2522 to 309 (median 431, IQR 356-518) and articles were published between 1939 and 2014, with the most cited paper originating from 2002. The most represented decade was the 2000s (28) followed by the 1990s (20), with 35 published since 2000 (Fig 1). All the top 100 papers were published in the English language and the most frequent journals were Circulation (28), Journal of the American College of Cardiology (10), New England Journal of Medicine (8), and Journal of Pediatrics and Journal of Thoracic and Cardiovascular Surgery (both 7); there were seven papers in non-clinical scientific journals, including four in Nature. Most articles originated from the United States (74), followed by Canada (10), United Kingdom (7), Japan (5), France (4), Germany, Italy, and Sweden (2 each), and one each from Austria, the Netherlands, and New Zealand, including seven international collaborations.
There were 542 individuals listed as authors, of whom 81 appeared more than once. Thirteen authors had four or more publications included in the list, as shown in Table 2; all these authors were based at four leading US centres for most or all their publications: Boston Children's Hospital and Harvard University; Children's Hospital of Philadelphia and University of Pennsylvania; University of California San Francisco; and/or University of Texas Southwestern Medical Center. The leading paediatric cardiologists were Dr Jane W Newburger (9) and Dr Gil Wernovsky (8), and the leading surgeons were Dr Richard A Jonas (6), Dr Thomas L Spray, and Dr J William Gaynor (5 each). Fifty-nine papers reported specific funding, with the US National Institutes of Health cited as the sole or a contributing funder in 28 articles.
CHD overall was the most considered disease group (46 articles), followed by the spectrum of single-ventricle heart conditions (11), transposition (8), septal defects (7), and patent arterial duct (6). The most common subject was surgical outcomes (19), followed by pathology (18), surgical technique (13), epidemiology (12), and catheter intervention (9) (Table 3). There were 86 original research articles, including 50 case series, 14 cohort studies, and 10 clinical trials (Table 4); one additional case series was published as a conference abstract. The level of evidence of articles by decade of publication is shown in Figure 2, with level 4 evidence predominant before the 1980s and level 3 evidence most prevalent in more recent decades; all bar one of the most cited level 1 evidence has been published in the last 30 years.

Discussion
The number of times that a published article is cited in the literature reflects its influence and impact over time. 1 Citation counts are being used increasingly as a tool to assess, rank, and compare individuals, institutions, and journals through metrics such as h-index and impact factor. 5 These measures assume that influential researchers and important publications are cited proportionately more often than those of lesser value, providing a measure of impact on the scientific field over time. There is marked variability in the rate of accumulating citations amongst highly cited papers with some peaking early before falling away, whilst others have a long rise to prominence. 2 In this study, we identified the most cited papers on CHD in children, spanning 75 years, from Robert Gross and John Hubbard's landmark 1939 study on surgical ligation of a patent arterial duct in Boston, MA, 6 to Ariane Marelli and colleagues' 2014 paper on the lifetime prevalence of CHD in Quebec, Canada. 7 Whilst many historical classics remain relevant today, one in three of the most highly cited articles was published in the last 20 years.

Types of highly cited study
Much of modern clinical practice in paediatric cardiology and cardiac surgery is based on expert opinion and institutional case series, 8  Twelve of the top 100 articles were epidemiology studies, including 5 of the top 20 with over 5700 citations between them. Incidence, prevalence, and natural history are often quoted by authors in their introduction as a way of setting the scene, leading to high numbers of citations for those studies that break through and become the standard reference.
Only nine articles reported the outcomes of randomised controlled trials and five of these were on the early and late findings from the Boston Circulatory Arrest trial, [9][10][11][12][13] totalling 2294 citations and involving 8 of the 12 authors appearing in Table 2. The other highly cited randomised controlled trials were the Pediatric Heart Network's multicentre Single-Ventricle Reconstruction trial (463 citations), 14 a national trial of indomethacin in pre-mature infants with a patent arterial duct (427), 15 the multicentre PRIMACORP study of prophylactic milrinone after surgery (404), 16 and the first clinical application of remote ischemic pre-conditioning for myocardial protection (387). 17 The low numbers of randomised controlled trials on the list may reflect the lack of late-phase, practice changing trials in paediatric cardiology and cardiac surgery, 18,19 or that clinical trials address specific research questions, and therefore the findings have a narrower appeal. However, all bar one of these trials were published in the last 30 years yet have garnered sufficient citations to enter the top 100 most cited articles in the field.
Over recent years, clinical guidelines endorsed by national or international bodies have emerged as a key resource for patient management. Four scientific statements from the American Heart Association were included on the list, and it is likely that the number of such articles amongst the most cited will increase in the coming decade.

Origins of highly cited studies
We found that the most cited articles on structural CHD in children were all published in English and dominated by clinical       The observation that all authors featuring four or more times had worked at one of four US institutions for most or all their impactful publications reinforces the benefits of "team science." These groups established and sustained successful and collaborative research programmes, where success breeds success, although multiple highly cited papers originating from one clinical trial contributed much to this achievement. However, it also raises the possibility of a publication bias in which work from leading US institutions is favoured by leading US journals.

The most highly cited papers
At the time of our search, three papers had generated over 1000 citations. Julien Hoffman and Samuel Kaplan's article on the incidence of CHD was by far the most cited with 2,522 citations. 22 Published in 2002, it is likely to retain this position for many years, although the systematic review and meta-analysis on worldwide prevalence by Van der Linde and colleagues from 2011 is rising fast, already in fourth place with 990 citations. 23 The second most cited paper was a surgical classic from 1971, Fontan and Baudet's original description of ventricularisation of the right atrium to direct inferior vena caval blood to the left lung, receiving 1631 citations 24 ; the evolution and outcomes of the Fontan  procedure have remained the focus of intense research over the last five decades. In third place was Philip Hagen and colleagues' 1984 autopsy study on the incidence and size of patent foramen ovale by age in children and adults and has grossed over 1500 citations. 25

Strengths and limitations
This study provides a unique insight into the most influential peerreviewed publications that have shaped the development of our specialty over the last 100 years, providing a reference list that will be of interest to students, clinicians, and researchers. Our comprehensive search of a wide range of journals listed on the expanded Science Citation Index is likely to have missed few if any of the most cited papers. However, our findings are limited by our use of only one bibliometric tool to determine citation counts; other databases, such as Scopus or Google Scholar, use different data sources and algorithms which may impact on the number of citations detected and therefore which papers appear in the top 100 26 although as both index fewer articles published prior to the 1990s, they are less suited to historical analyses. Similarly, metrics related to other sources of information, such as number of times accessed online and twitter engagements, were not assessed but are much more relevant to recently published works. Citation analysis demonstrates the overall impact of a paper since publication but is not adjusted to account for its current influence or its quality; infamous papers may receive as much attention as celebrated papers. Like any bibliometric analysis, our findings are a snapshot in time, a now historical view on the most cited articles in the field.  28 have been superseded by updated reviews 29 and are therefore likely to plateau. As a time-dependent process, other groundbreaking but recently published articles may have had too little time to accrue sufficient citations to make our list.

Conclusions
This study identified the most influential papers on CHD in children, providing a historical perspective on scientific progress over the last century. Our findings demonstrate that highly cited papers are exclusively published in English, and that researchers and journals based in the United States hold a dominant position in this field. Most of the highly cited articles remain case series, with few randomised controlled trials in CHD appearing in recent years.