Visual analysis of the current status and hotspots of quality of life research in patients with chronic heart failure based on CiteSpace

Qi Zhang^1#, Nan Gan^1#, Pei-Pei Zou², De-Jing Yuan^3*, Qing Wu^3*

¹School of Nursing, Suzhou Medical College, Soochow University, Suzhou 215006, China.

²Department of Nursing, Yangcheng Lake Street Community Health Service Center, Suzhou 215006, China.

³Department of Cardiology, First Affiliated Hospital of Soochow University, Suzhou 215006, China.

^#These authors contributed equally to this work and are co-first authors for this paper.

^*Correspondence to: De-Jing Yuan, Qing Wu. Department of Cardiology, The First Affiliated Hospital of Soochow University, No. 899, Pinghai Road, Gusu District, Suzhou 215006, China. E-mail: ydjyzu@126.com; qwu@suda.edu.cn.

Background

Heart failure (HF) is a severe manifestation or end-stage consequence of various heart diseases, with high mortality and rehospitalization rates. The prevalence of HF in adults in developed countries is 1.0%–2.0% [1]. The prevalence of HF in China is increasing due to the aging population, the increasing incidence of chronic diseases such as coronary heart disease (CHD), hypertension, and diabetes, and the improvement of medical standards that prolong the survival of patients with cardiovascular disease. Wang reported that the agestandardized prevalence of HF among urban patients in China was 0.57% for those aged 25–64 years, 3.86% for those aged 65–79 years, and 7.55% for those aged ≥ 80 years [2]. Cardiovascular diseases remain the leading cause of death globally, accounting for approximately 32% of all deaths worldwide [3], which brings a heavy burden to society and families.

Although great progress has been made in the clinical diagnosis and treatment of HF, the quality of life in elderly patients with HF is significantly lower than that in other elderly patients with chronic diseases due to the long course of treatment, many complications, and frequent acute exacerbations [4]. Quality of life is one of the important indicators to evaluate the health outcomes of patients with HF [5]. Chinese scholars began to study the relationship between HF and quality of life in the 1990s. Studies have found that the quality of life in patients with HF outside the hospital is poor, and more than 50% of patients with HF have serious complications within 30 days of discharge, leading to a decline in quality of life [6]. There have been many reviews on the quality of life of patients with CHD, and the specific methods to improve the quality of life of patients with HF have been systematically summarized, such as drug management, exercise management, psychological intervention, remote monitoring, and so on. However, such reviews mainly rely on the induction and integration of existing interventions, making it difficult to systematically identify research hotspots, thematic evolution rules, cooperation networks, and frontier trends in this field.

CiteSpace (6.4.R1) is a data analysis tool for bibliometric analysis and visualization, widely used to summarize hotspots and trends across various research directions in the medical field. However, there are no reports on the quality of life in HF. Thus, this study visually analyzed literature in the field of quality of life in chronic heart failure (CHF) in China National Knowledge Infrastructure (CNKI) and Web of Science (core collection) databases, aiming to explore the research status, hotspots, and development trends in this field so as to provide a basis for formulating targeted interventions, prevent adverse events and improve the prognosis of patients with HF, and finally reduce the social and economic burden.

Methods

Data collection and search strategy

We searched the China National Knowledge Infrastructure (CNKI) AND Web of Science (WOS) Core Collection database by computer. The Chinese search format was (chronic heart failure) AND (quality of life), and the language was Chinese; and the English search formula was: TI=chronic heart failure, TS= “quality of life” OR “life quality”, and the language was English. The search time limit was from the establishment of the database to November 1, 2024. Two researchers independently searched and screened the literature. The inclusion criteria were: 1) published literature related to the quality of life of patients with CHF; 2) the source of literature was journal articles or dissertations. The exclusion criteria were as follows: 1) conference papers, newspapers, and other non-academic reports, and reports of other disciplines that were obviously inconsistent with the content of this study; 2) literature with incomplete information, repeated publication of the same research content, and obvious errors. According to the inclusion and exclusion criteria of the literature, 4,743 pieces of literature were finally included, including 4,185 pieces of Chinese literature and 558 pieces of English literature.

Analysis tool

We used CiteSpace6.4.R1 software to visually analyze the literature related to the quality of life in patients with CHF. Firstly, we imported the retrieved files into NoteExpress software for deduplication and screening. The processed data were imported into CiteSpace. Then we set the relevant parameters: the time span was from January 1996 to November 2024, and the time slice was set to 1 year. After that, we set the selection criteria to a TOPN threshold of 50, selected the Pathfinder method as the shearing method, and set the rest as default. Finally, the knowledge map of the quality of life in patients with CHF was drawn and analyzed.

Results

Analysis of publication characteristics

The results show that the focus and research on the quality of life in patients with CHF began in 1996. The number of Chinese literature published increased from 2 in 1996 to 435 in 2020, showing a gradual upward trend. The number of English-language articles published had a relatively stable change, both below 50, and the highest point was in 2020 (46) (Figure 1).

Figure 1 The number of articles published in English and Chinese on quality of life in patients with chronic heart failure from 1996 to 2024.

This figure illustrates the linear trend of the number of published articles in the field of chronic heart failure quality of life in both Chinese and English from 1996 to 2024. The blue solid line is the linear change trend of the number of publications in the CNKI database, and the yellow solid line is the linear change trend of the number of publications in the Web of Science database.

Analysis of authors

The number of authors’ publications is represented by the size of nodes; the larger the node, the more publications. The line between nodes represents the cooperative relationship between authors; the line width indicates the cooperation strength, and the color represents the cooperation time. The results of Chinese literature analysis are shown in Figure 2A, which shows that there are 201 nodes and 148 lines in the author’s research map, with a density of 0.0074. The top three authors with the highest number of publications are Zhang Yan (15 ), Wang Ying (7), and Xu Hong (6). The analysis results of English literature are shown in Figure 2B, which shows that there are 298 nodes and 343 lines in the author’s research map, with a density of 0.0078. The German scholars Anker, Stefan D ranked first in the number of publications, with 11 publications. There is closer collaboration among authors other than those from China.

Analysis of institutions

In CiteSpace software, when “Institutions” is selected as the Node Type, each node represents a research institution, while each line represents a cooperation between the two institutions, and the network density reflects the strength of cooperation between these institutions. The analysis results of Chinese literature are shown in Figure 3A. There are 166 nodes in the institutional research map, 36 lines, and the density is 0.0026. The top three institutions in the number of Chinese literature publications were Guangzhou University of Chinese Medicine (47), Shandong University of Chinese Medicine (44), and Beijing University of Chinese Medicine (37). The results of visual analysis of the publishing institutions of English literature are shown in Figure 3B. There were 236 nodes, 334 lines, and the density was 0.012. Berlin Institute of Health (17), Charite Universitatsmedizin Berlin (16), and Free University of Berlin (15) had more than 15 articles, all of which are German research institutions with very close cooperation between institutions.

Analysis of keywords

Co-occurrence analysis of keywords. The frequency of keywords reflects the hot spots in the research field. In the graph, the more times the keywords appear, the larger the font and node size. To ensure the accuracy of the analysis, we normalized the keywords by incorporating synonyms, acronyms, as well as single or plural forms, and removed meaningless and irrelevant keywords. On this basis, we conducted a keyword co-occurrence analysis and extracted the top 10 keywords with the highest frequency in the co-occurrence analysis of Chinese and English literature, as shown in Table 1. The keyword co-occurrence map of Chinese literature is shown in Figure 4A. The keywords with large size in Figure 4A were “quality of life,” “chronic heart failure,” “cardiac function,” “elderly patients,” and “nursing intervention”. The keyword co-occurrence map of English literature is shown in Figure 4B. The keywords with large size in Figure 4B were “chronic heart failure,” “quality of life,” “outcome,” “management,” and “exercise tolerance”.

Cluster analysis of keywords. Cluster analysis of keywords can effectively grasp the research hotspots in this field and improve the accuracy of summarizing the research field. The research results of Chinese literature are shown in Figure 5A. The cluster module value (Q) is 0.8664, and the average silhouette value (S) is 0.9767. Q > 0.3, S > 0.5, indicating that the clustering effect of this study was significant and reliable. The top 8 clusters of Chinese literature were #0 heart failure, #1 rehabilitation exercise, #2 depression, #3 clinical observation, #4 community, #5 efficacy, #6 health education, and #7 clinical research. The keyword cluster analysis of English literature is shown in Figure 5B. The cluster module value (Q) is 0.7341, and the average contour value (S) is 0.8907. The top 8 clusters of English literature were #0 left ventricular function, #1 education program, #2 support education, #3 placebo-controlled double-blind study, #4 low-volume high-intensity interval training, #5 vagal neuromodulation, #6 resistance training, #7 adaptive servo-ventilation therapy.

Timeline analysis of keywords. We used the results of keyword clustering to make a keyword timeline map. In this map, the horizontal thick line segment indicates the distribution of each cluster along the time axis, and a longer line segment means that the cluster spans a wider time range. The map formed a total of the first 10 clusters numbered #0 to #9, where the smaller the number of clusters contained , the greater the number of literature. In the cluster analysis of keywords in Chinese literature, the top three clusters with timelines extending to the past two years were heart failure, rehabilitation exercise, and depression. In the cluster analysis of keywords in English literature, the top three clusters with timelines extending to the last 2 years were left ventricular function, support education, and low-volume high-intensity interval training. The keyword clustering timelines in the field of quality of life of patients with CHF in Chinese and English literature are shown in Figures 6A and 6B.

Figure 2 Author co-occurrence maps of Chinese- and English-language publications.

(A) Author co-occurrence map of Chinese-language publications. (B) Author co-occurrence map of English-language publications. Each circular node represents an author, and the node size is proportional to the number of publications. The colored rings indicate the publication period, ranging from cool colors for earlier years to warm colors for more recent years. The links between nodes represent collaborative relationships between authors. Link width indicates the strength of collaboration, while link color indicates the time of collaboration.

Figure 3 Co-occurrence map of Chinese- and English-language literature institutions.

(A) Co-occurrence map of Chinese literature institutions. (B) Co-occurrence map of English literature institutions. Each circular node represents an institution; the size of the node is proportional to the published articles, and the colored ring of the node represents the published time, ranging from cool (early) to warm (recent). Each line between nodes represents a cooperation between two institutions.

Figure 4 Co-occurrence map of keywords in Chinese- and English-language literature.

(A) Co-occurrence map of keywords in Chinese literature. (B) Co-occurrence map of keywords in English literature. Each circular node represents a co-occurring keyword, and the size of the node is proportional to the number of keyword appearances. The colored ring of each node represents the publication year, ranging from cold (early) to warm (recent). The lines between nodes represent two keywords that appear in the same article.

Table 1 High-frequency keywords and centrality

CNKI, China National Knowledge Infrastructure; WOS, Web of Science.

Figure 5 Clustering map of keywords in Chinese- and English-language literature.

(A) Clustering map of keywords in Chinese literature. (B) Clustering map of keywords in English literature. Each circular node represents a keyword. Shapes in different colors represent different clusters, and colors indicate publication year, ranging from cold (early) to warm (recent). Cluster numbers indicate how often keywords appear in the literature, with smaller numbers indicating higher frequencies. In order to grasp the research hotspots in this field and improve the accuracy of summary research field.

Figure 6 Clustering time map of keywords in Chinese- and English-language literature.

(A) Clustering time map of keywords in Chinese literature. (B) Clustering time map of keywords in English literature. Thick horizontal segments indicate the distribution of each cluster along the time axis; longer segments mean that the cluster spans a wider time range, and line colors indicate publication years, ranging from cold (early) to warm (recent). Each circular node on the line segment represents a keyword, and the keywords in the same cluster are arranged on the same horizontal line in chronological order. The arc starting from the node represents the span of time and intuitively presents the emergence, persistence, and extinction process of keywords in this cluster.

Emergence analysis of keywords. Based on the knowledge graph of keywords, we further analyzed the emergence of keywords, that is, the change in the frequency of keywords in a specific time period, reflecting the research frontier and development trend in this field. The 20 most emergent ones are shown in Figures 7A and 7B. The emergent words in the field of quality of life in patients with CHF in Chinese literature included “congestive,” “chronic heart failure,” “social support,” “nursing intervention,” “community,” etc. The keyword with the longest duration was “congestive” (2004–2014), and the keyword with the highest emergence intensity was “chronic heart failure” (27.42), followed by nursing intervention (17.69) at present. Keywords such as “cardiac rehabilitation,” “dapagliflozin,” and “compliance behavior” showed a strong emerging trend from around 2020, and are expected to continue to exert an influence on research in this field in the future. We also analyzed the evolution of emergent words in English literature. In the early stage, the main focus was “left ventricular dysfunction,” “congestive heart failure,” and “exercise tolerance”. In recent years, the research content has been enriched, such as “reduced ejection fraction,” “cardiac function,” and “city cardiomyopathy questionnaire”. The analysis of the 20 keywords with the strongest emergence in the English literature can be summarized as the following characteristics: 1) The subjects of the study were mainly elderly patients and european society; 2) The study instrument was the city cardiomyopathy questionnaire; 3) study types were double blind and questionnaire; 4) The research contents involved left ventricular dysfunction, exercise tolerance, cardiac rehabilitation, etc. 5) social support is a positive nursing intervention.

Figure 7 Emergence map of keywords in Chinese- and English-language literature.

(A) Emergence map of keywords in Chinese literature. (B) Emergence map of keywords in English literature. The green line represents the time period from 1996 to 2024, and the red line represents the time span of the keyword burst. It reflects the research frontier and development trend in this field.

Discussion

Analysis of research status of quality of life in patients with CHF

The concept of “quality of life” was first introduced by American economist J.K. Galbraith in 1958. He noted that people’s quality of life is the degree of satisfaction that is derived from both material and spiritual dimensions [7]. The World Health Organization’s Quality of Life Study Group defines quality of life as the subjective experience of individuals in different cultures and value systems with respect to their goals, expectations, standards, and concerns about living conditions [8]. Health-related quality of life is a complex, multidimensional construct that reflects the relationship between an individual’s quality of life and their health or disease status [9].

Analysis of the number of publications. From the analysis of publication data, it is evident that the number of publications in English remains relatively low, with an annual total of less than 50. The overall trend has been generally stable. The number of publications in China shows an upward trend, and the whole process can be divided into three stages. The first stage was from 1996 to 2003, during which a total of 19 pieces of literature were published. This period represents the enlightenment in exploring the quality of life for patients with CHF. Although research production during this time was somewhat fragmented, it still established a foundational basis for subsequent investigations in this field. The second stage was from 2004 to 2020 and was characterized as a period of rapid development in research concerning quality of life in patients with CHF. The number of published studies increased year by year. Notably, there was a significant surge in publications from 2016 to 2020, reaching a peak in 2020 with 435 pieces of literature published. At the same time, English publications also reached their peak at that time with 46 pieces of literature. This increase likely reflects heightened attention and interest, which we speculate may be related to the requirement of the “Healthy China 2030 Plan” to reduce premature mortality caused by chronic diseases and effectively control the burden of chronic diseases [10, 11]. From 2021 to the present, it is the third stage of research development on the quality of life in patients with CHF. The number of publications on the quality of life in patients with CHF worldwide shows a downward trend year by year. This trend may be related to the following factors: the global outbreak of COVID-19 and, after years of development in this field, basic research becoming saturated, shrinking the space for innovation. In addition, research hotspots have gradually differentiated into more subdivided sub-directions such as digital health and palliative care, and some literature no longer uses “quality of life” as the core keyword.

Lack of cooperation between institutions. There is relatively little cooperation among Chinese authors, whereas English-language authors maintain a relatively close cooperative relationship. An analysis of Chinese publishing institutions reveals that Guangzhou University of Traditional Chinese Medicine has the highest publication output. While cooperation within and among affiliated institutions is frequent, each research institution carries out research projects in its own region and has little cooperation with outside institutions. This phenomenon is consistent with the “principle of minimum effort,” which states that people tend to solve problems with minimal input [12]. However, to pursue the principle of saving time and effort, cooperation only within the region will lead to the unbalanced development of each region. In view of the current situation in China, communication and cooperation between scientific research institutions and scientific research teams should be strengthened to promote the comprehensive development of the research field on the quality of life in patients with CHF. The top three English publications are all from Berlin research institutions, and cooperation among institutions is very close.

Research hotspots of quality of life in patients with CHF

Explore the influencing factors of quality of life and provide support for unmet needs. The co-occurrence analysis of our keywords revealed “prognosis,” “clinical efficacy,” and “outcome” as prominent terms, indicating that the research primarily emphasized treatment effects and prognostic outcomes in patients with CHF. The keywords “elderly,” “coronary heart disease,” and “depression” indicate that advanced age, complicated chronic diseases, and negative emotions have become research hotspots regarding the quality of life in patients with CHF. Cardiac function serves as a standard metric for assessing therapeutic efficacy and prognosis in patients with CHF [13], and a study found that NYHA classification is an independent factor affecting the quality of life in patients with CHF [14]. The higher the NYHA classification of patients, the worse the prognosis of the disease and the lower the quality of life score; therefore, researchers pay more attention to the prognosis and treatment effect of the disease. Elderly patients are the main focus group, and Kim found that age is an independent factor affecting the quality of life in patients with CHF [15]. With the increase of age, the cardiovascular structure and function of patients are degraded [16], and the quality of life is gradually reduced. With the aggravation of aging in China [17], it is urgent to improve the quality of life of elderly patients with HF. HF is rarely isolated, and more than 80% of patients have two or more chronic diseases [18]. The course of coronary heart disease combined with chronic heart failure is protracted and challenging to manage; compounded by patients’ poor health awareness and improper behavior, diseases always flare up and worsen, and finally reduce the quality of life of patients [19]. Therefore, in the process of treatment and care, healthcare providers should pay attention to patients with HF and multiple chronic diseases. In addition, researchers have begun to pay attention to the patients’ negative emotions. Due to the prolonged and recurrent nature of the disease, patients with CHF are susceptible to anxiety, depression, and other negative emotions [20]. The level of depression in patients with CHF is strongly correlated with their quality of life [21], while anxiety, depression, and similar negative emotions exert a substantial adverse effect on it [22]. In summary, within the domain of quality of life in patients with CHF, it is of positive clinical significance to study how to improve the clinical efficacy of patients and provide personalized support to elderly patients with other chronic diseases to help them relieve negative emotions and improve their quality of life.

Implement effective intervention measures to improve the quality of life of patients. Among the co-occurrence results, “nursing stem,” “comfort care,” “management,” and “exercise” highlighted a focus on various interventions and management measures. The emergent results showed that the intensity of nursing intervention was 17.69, ranking second only to CHF itself (27.42). Traditional Chinese Medicine (TCM) interventions, including Baduanjin exercises and acupoint application, have maintained their prominence since 2022 and continue to exert a significant influence in research on the quality of life in patients with CHF. Patients with chronic heart failure have low self-care ability and low medication compliance after discharge, which leads to the recurrence of their condition [23]. A systematic review shows that nursing interventions can effectively improve the self-care ability of patients with chronic heart failure. [24]. Currently, intervention research on the quality of life of patients with CHF is still in the exploratory stage, mainly including continuous nursing, exercise or TCM, and various targeted interventions based on theories and Internet technology development. Studies have found that these interventions can not only improve the quality of life of patients but also significantly improve patient compliance, cardiac function, and exercise endurance [25–29]. Most existing studies use a single measure to intervene in patients and test whether it can improve their quality of life and other outcome indicators, while some investigations have begun integrating new information technologies, such as the “Internet Plus” initiative and WeChat platforms, into intervention strategies. In the future, healthcare providers should be committed to the development of high-quality interventions, the combination of multiple interventions, and the promotion of positive and effective interventions in clinical practice to improve the quality of life in patients with CHF and promote their rehabilitation.

Analysis of the differences in research hotspots of quality of life in patients with CHF between Chinese and English literatures

Analysis of the keywords related to research on the quality of life in patients with CHF reveals that current studies primarily focused on elderly patients, negative emotions, comorbidities, disease treatment effects, and prognosis. In terms of intervention and management, research in China has focused on intervention measures led by medical staff. For instance, a randomized controlled study found that the implementation of nursing interventions helped reduce patients’ anxiety and depression [30], and improve their quality of life. In contrast, studies in English have focused more on patients’ self-management abilities. For example, Abbasi [31] used a variety of methods and self-management education programs based on media to improve the quality of life in patients with CHF. There are three reasons for this. The first is the cultural difference. Western countries emphasize personal responsibility and autonomy, and medical staff serve only as guides. Chinese culture emphasizes deference to authority, and patients tend to entrust health management to medical staff [32]. The second is the difference in the medical system. Western countries have universal basic medical care and a family doctor system that supports patients’ self-management. In China, medical resources are concentrated in large hospitals, and primary care services are weak, so it is more feasible to be led by hospital-based medical staff. The third is the difference in patients’ health literacy. Public health education is carried out early in Western countries, and patients’ health literacy is high. Chinese HF patients are mostly aged, with low education level, a lack of health knowledge, and limited self-management ability. Comfort nursing suggests that Chinese researchers pay attention to patients’ treatment experience and feelings, and emphasize the comfort of patients during the treatment. Chen reported that comfort nursing not only greatly promoted the relief of symptoms but also improved the emotional state and quality of life of patients [33]. The prominence of keywords such as “exercise tolerance” and “trial” highlights that researchers of the English literature pay more attention to patients’ exercise capacity and clinical trials. A systematic review has confirmed that exercise training can enhance exercise tolerance, shorten hospital stays, and improve the quality of life of patients [34]. In view of the above findings, the Chinese literature can learn from the experience of patient self-management in the English literature, but it needs to be localized and applied in practice according to the characteristics of culture and the medical system, and evidence-based research, such as exercise rehabilitation, should be strengthened.

Frontier analysis of quality of life in patients with CHF

Insight into and grasp of the research trends and frontiers of a discipline field are helpful in promoting the innovative development of research in this field. Based on the emergent results, keywords such as “social support,” “nursing intervention,” and “community” began to emerge around 2010 and remained prominent through 2016. During this period, researchers conducted surveys using these keywords and obtained a large number of research results. Patients with CHF require long-term treatment. Studies have shown that good social support has a positive effect on individual psychological health and is an important factor affecting the quality of life [35]. Social support can be categorized as either formal or informal. Formal social support includes the government at all levels, institutions, enterprises, communities, and other formal organizations, which mainly provide social security system support [36]. Communities are an important platform for social support, playing a key role in integrating social resources and coordinating social support. As understanding of patients’ conditions deepened, clinical experience accumulated, and medical research progressed, emergent keywords including “comprehensive Nursing,” “psychological Intervention,” and “compliance behavior” began to be taken seriously. Researchers have shifted their focus from paying attention to the intervention of one aspect of the disease in the early stage to the comprehensive management of multidimensional diseases, which means that the health management for patients has become comprehensive and systematic. The shift from a focus on physical interventions to psychological interventions also reflects a shift in the medical model from a biomedical model to a biopsychological model, with a greater emphasis on the mental health and overall well-being of patients. Since 2022, keywords such as “exercise endurance,” “Baduanjin,” “acupoint application,” “self-care ability,” and “sleep quality” have become increasingly prominent, indicating a growing recognition among healthcare workers of the importance of exercise and sleep in quality of life. This trend also highlights the integration of traditional Chinese medicine therapies and the adoption of more comprehensive, multidisciplinary intervention strategies. In the future, we should rely on community and multidimensional social support, integrate psychological interventions, exercise, sleep management, and characteristic treatments in traditional Chinese medicine, and conduct systematic and comprehensive research and practice in the health management of CHF.

Conclusions

This study systematically retrieved literature related to the quality of life in patients with CHF included in the CNKI and WOS databases and used Citespace6.4.R1 software to review the literature published from the establishment of the database to November 1, 2024. The authors, institutions, and keyword co-occurrence, clustering, and emergence were described and visualized. The study found a large difference in the number of articles published in Chinese and English, and that research had entered a period of decline after a period of rapid development. The research also found that cooperation among Chinese literature publishing institutions was weak. In terms of research hotspots, both English and Chinese literature focus on the elderly and patients with HF and chronic diseases, but the Chinese literature focuses on nursing interventions led by medical staff, whereas the English literature focuses on patient self-management. The research frontier showed an evolutionary trend from single-symptom management to multidimensional, comprehensive management. Future research should strengthen cooperation among institutions; personalize research for different patient types; explore multidisciplinary, multidimensional nursing interventions and comprehensive management research and practice, so as to improve patients’ quality of life.

Limitations

However, this study still has the following limitations: CiteSpace software was used to analyze the node type of literature, which was limited to author, author institution, and keyword analysis, and the co-citation and author contribution of the literature were not analyzed. This study included only English and Chinese literature, and the search terms did not cover all subtypes of “heart failure,” such as diastolic heart failure, or all quality-of-life-related expressions, such as “daily activities” and “self-efficacy”. Future research can use a more comprehensive search strategy for analysis.