Mediating effect of moral resilience between good-death perception and coping with death competence of ICU nurses: a cross-sectional study

Abstract

Background

Moral resilience can help Intensive Care Unit (ICU) nurses overcome moral dilemmas caused by the death of patients, while enhancing their competence to cope with death. Death-coping competence is also related to the cognitive level of nurses about good death. However, few studies have focused on the relationships among between moral resilience, perception of good death, and death-coping competence.

Research aim

To explore the focused on the relationships among ICU nurses’ moral resilience, good-death perception and death-coping competence, and to examine the mediating role of moral resilience between good-death perception and death-coping competence.

Research design

This was a quantitative study with a cross-sectional descriptive correlational design. The participants completed an online survey in which moral resilience, perception of good death and coping with death competence using the Rushton Moral Resilience Scale for nurses, Chinese version of Good Death Inventory and coping with death scale-short version, respectively.

Participants and research context

A total of 254 ICU nurses were recruited from five tertiary general hospitals in Shandong province of China to participate in the survey.

Ethical considerations

This study was approved by the ethics committee of Zibo Central Hospital. Informed consent was obtained from all the nurses.

Results

Good-death perception positively affects death-coping competence (r = 0.565, p < 0.01). The mediation analysis revealed that the direct effect of good-death perception on death-coping competence in the presence of the mediator was significant. Hence, moral resilience partially mediated the relationship between good-death perception and death-coping competence. And the mediating effect is 0.099.

Conclusions

ICU nurses’ good-death perceptions can directly predict their death-coping competence, and they can also indirectly affect their death-coping competence through the mediating effect of moral resilience.

Clinical trial number

not applicable.

Introduction

Nurses’ death-coping competence refers to the professional ability of nurses to deal with the dying or death of patients [1]. And the death-coping competence of clinical nurses mainly includes professional skills such as the ability to communicate with patients’ families about end-of-life affairs, the ability to provide end-of-life care to patients, and the ability to regulate one’s own emotions [2]. It is inevitable for nurses to encounter the death of terminal patients at times, and the death of patients may create moral dilemmas for nurses. After 3 decades of research documenting the existence of moral distress, there were few solutions [3]. It often brings different degrees of anxiety, fear, fatigue, helplessness, guilt and other negative moods to nurses [4, 5]. These negative emotions pose a significant challenge to nurses’ emotional regulation abilities, potentially impacting their mental health, reducing their confidence, and further affecting the professionalism of their work. Additionally, they may have a negative impact on nurses’ ability in delivering end-of-life care to patients and communicate with families regarding end-of-life matters [2, 6]. Lützén K called for the cultivation of moral resilience in response to the morally challenging situations nurses confront and as a means for addressing their moral distress [7]. Moral resilience refers to an individual’s ability to maintain, restore or boost his/her physical and mental health in the face of moral dilemmas, and it is also an ability to transform moral dilemmas from negative experiences into hopes and positive forward momentum [8]. On the other hand, good death is the core goal of hospice care services and reflects nurses’ respect for life [9, 10]. A profound understanding of the intricate interplay among moral resilience, perception of good death and coping with death competence among ICU nurses is essential for enhancing nurses’ confidence and assisting them in maintaining professionalism in their work of coping with patients’ deaths [2, 11].

Background

In end-of-life care, how to alleviate the suffering of terminally ill patients, improve the quality of their death, and enable them to depart with dignity during their final stages is an important issue worthy of profound consideration by contemporary healthcare professionals [12]. The conception of good death provides an excellent answer to this question. A good death is defined as a patient dying with dignity in an environment where their suffering is minimized. It involves decreasing a patient’s symptoms, avoiding invasive procedures, protecting their freedom, autonomy, and privacy, maintaining respect and meaningful relationships with their loved ones, and providing them with dignified and peaceful care in a safe environment [13]. This concept is a multi-dimensional, individualized, and dynamically changing process. It is necessary to evaluate the perception of good death based on the cultural background, individual wishes, and living environment of the dying person [14, 15]. Nurses are among the professionals that care for terminal patients in their final days. The Hierarchical Model of Death Competence (HMDC) proposed by Gamino et al. [16] emphasizes that cognitive ability is a critical component of death-coping competence. Nurses should possess foundational knowledge to develop a mature and comprehensive death-coping capacity. Based on this, ICU nurses can regulate their emotions when facing the death of terminally ill patients, provide end-of-life care services to patients, understand the needs of patients and their families, and communicate well with families regarding end-of-life affairs [2]. End-of-life care education is regarded as a common approach to cultivating nurses’ cognition of death and their competence in coping with death [17]. However, hospice education in China is suffering from a late start and lagging development [18, 19]. In the process of caring for terminally ill patients, nurses inevitably encounter numerous ethical challenges, such as addressing financial constraints, and overcoming end-of-life communication barriers. Research has indicated that in managing death-related events, nurses often offer limited emotional support to patients and their families, conducting less proactive assessments and understanding of patients’ care and treatment preferences, thereby making it challenging to cope with patient death [20].

The coping with death competence is a construct that represents a wide range of this phenomenon may have implications for the manner in which nurses respond to patients’ mortality skills for facing death, as well as our beliefs and attitudes about these capacities [21]. It can assist nurses to actively cope with occupational stress and reduce job burnout, so as to participate in patients’ death preparedness and planning in an optimal state [22]. Studies have shown that in China, the scores of ICU nurses’ competence in coping with death are at a relatively low level, lower than those reported in foreign studies [23]. Therefore, it is necessary to find the influencing factors and pathways of action regarding nurses’ death-coping competence.

Several studies have explored the relationship between good-death perception and coping with death competence among nurses [15, 24, 25]. He et al. [26] found that perception of death could improve natural acceptance and then enhance the sense of meaning in life to positively predict nurses’ competence to cope with death. There are qualitative research results demonstrating that if nurses’ good-death perception is increased, their death-coping competence will be improved accordingly [27]. Good death is a kind of death consciousness that represents the progress of social civilization [10]. Studies have demonstrated that terminally ill patients exhibit varying needs during the end-of-life stage [19]. A thorough understanding of terminally ill patients’ expectations and needs regarding a dignified death can assist nurses in providing care that aligns with these end-stage requirements. This also improves nurses’ capacity to respond appropriately to patients’ needs during this critical period [28].

As a way to alleviate moral suffering, moral resilience can help ICU nurses overcome moral dilemmas when facing patients’ death, while reducing the risk of emotional exhaustion, job burnout, and decreased moral sensitivity, so as to reduce nurses’ tendency to leave their jobs [29, 30]. The nurse’s perception of good death can also help meet ethical challenges in caring for patients at the end of life [31, 32].

Moral resilience may have an impact on the competence of coping with death [33]. However, most of the existing studies dealing with moral resilience and death-coping competence are qualitative. As for other research types, the relevant studies are only between any two variables in moral resilience, good-death perception and death-coping competence [34]. And no study to date has been conducted to explore the associations between good-death perception and moral resilience and their interaction effects on coping with death competence.

Aim

This study aims to understand the current status of scores regarding ICU nurses’ competence in coping with death, and explore the relationship between ICU nurses’ moral resilience, good-death perception and death-coping competence. We hypothesize that ICU nurses’ good-death perception would affect the death-coping competence through the mediating effect of moral resilience. And examining the mediating effect of moral resilience is our central aim in exploring the three concepts.

Methods

Study design

This study adopts a cross-sectional research design and uses convenience sampling. The survey was conducted in June 2024, and ICU nurses were recruited from five tertiary general hospitals to participate.

Participants and sampling

According to the requirements for constructing a structural equation model [35], 200 is taken as a basis, and at least 5 people are added for each additional variable, and the number of variables investigated in this study is 5. Given the 10% non-response rate, the required sample size is at least 248 cases. A total of 254 cases were included in this study. The inclusion criteria are as follows: (a) Nurse professional qualification certificate, (b) ≥ 1 year for nursing work in ICU, (c) Informed consent and voluntary participation. Participants were excluded when they met the following situations: nurses studying in external schools, taking vacations, or not staying on the job in other ways.

Measures

Four instruments were used in this study. The use of the questionnaire has been authorized by the author or the translator.

Demographic information

ICU nurses who participated in this study were asked to provide information about their gender, age, education, marital status, years of experience, and job title.

Rushton moral resilience scale for nurses

The Chinese version of Rushton Moral Resilience Scale (RMRS) is a self-report tool for assessing the moral resilience of nurses [36]. It is a 17-item scale, including 3 dimensions: coping with moral dilemma in nursing and relational integrity (10 items), personal integrity (2 items), and moral efficacy (5 items), used for assessing the moral resilience of nurses. A 4-point Likert scale (1 = strongly disagree, 4 = strongly agree) was used, with higher scores indicating greater moral resilience of nurses. The Cronbach’s alpha of the Chinese version of Rushton Moral Resilience Scale for nurses is 0.829.

Chinese version of good death inventory

Good death inventory (GDI) was developed by Miyashita [37]. Zeng et al. [38] developed the Chinese version of GDI, which was tested to have good reliability and validity, thus widely adopted in China. It is a one-dimensional scale with a total of 18 items. The Likert 5-point scale (1 = strongly disagree, 5 = strongly agree) was used, with a minimum score of 18 and a maximum score of 90. The higher the score, the better the perception of good death. The Cronbach’s alpha of Chinese Version of Good Death Inventory is 0.894.

Coping with death scale short version

The Coping with Death Scale short version(CDS-9)was developed by Galiana et al. [21] to assess competence in coping with death. Li et al. [39] developed the Chinese version of CDS-9, which has one dimension and 9 items and has been proven to have good Chinese characteristics. Coping with Death Scale Short Version is a self-report instrument that assesses death-coping competence using a 7-point Likert scale from 1 (“strongly disagree”) to 7 (“strongly agree”). The higher the score, the better the death-coping competence. The reliability of the Coping with Death Scale-Short Version has been reported with Cronbach’s alpha of 0.925.

Data collection

The researcher created an electronic version of the questionnaire, and its first page is marked with the filling instructions, filling requirements, and informed consent. The researcher explained the purpose of the survey to the nurse managers; after obtaining the nurse managers’ understanding, the nurse managers invited the researchers to join the work group of ICU nurses. Then the researchers distributed the electronic questionnaire to ICU nurses through their online work groups, and they explained the purpose of the survey to the ICU nurses. The participants were informed that: (a) participation in the study was voluntary and anonymous, (b) the information would only be used for research, (c) they could withdraw before data analysis. The data collected were collated and checked by two researchers. If a questionnaire response was found an inconsistency between ages and working years, or too short answering time, or same choices for all the items, it would be considered unqualified and be eliminated.

Data analysis

SPSS 26.0 software and AMOS 26.0 software were used for statistical analysis. Descriptive analysis was performed for general data. The normally distributed measurement data were expressed as ±s, and the enumeration data were expressed in frequency and percentage. The normality test was performed using skewness and kurtosis values. The variable could be regarded as normally distributed with a skewness value ≤ 2 or a kurtosis value ≤ 4. The variables in this study were found to be normally distributed [40]. Pearson correlation analysis was used to test the correlation between moral resilience, perception of good death and coping with death competence. AMOS software was used to build the mediation model.

Ethical considerations

This study was approved by the ethics committee of Zibo Central Hospital (No. 2024142). All procedures were in accordance with the ethical standards of the 1964 Helsinki Declaration and its later amendments.

Results

Work and demographic characteristics of the participants

A total of 254 ICU nurses participated in this study. The majority of them were women (90.9%). Most of them had a Bachelor’s degree (86.2%). A large percentage of them had job experience of 3 ~ 5 years (35.8%). Regarding the professional title, 66.9% of the ICU nurses held a junior title (Table 1).

Table 1.

The characteristics of the study participants (N = 254)

Variables	Categories	Frequency	Percent(%)
Gender	Male	23	9.1
	Female	231	90.9
Age	≤ 30	169	66.5
	31–40	72	28.4
	≥ 40	13	5.1
Marital status	Married	87	34.3
	Single or divorced	167	65.8
Educational background	Diploma/higher diploma	13	5.1
	Bachelor degree	236	92.9
	Master degree	5	2.0
Years of experience as ICU nurses	≤ 3	59	23.2
	3–5	71	28.0
	5–10	73	28.7
	≥ 10	51	20.1
Professional title	primary title	106	41.7
	Intermediate title and above	142	55.9
	Senior title	6	2.4

Descriptive analysis

As shown in Table 2, the total score of the moral resilience was 47.54 ± 12.41, of which the coping with moral dilemma in nursing and relational integrity dimension was 25.02 ± 7.16, the personal integrity dimension was 5.02 ± 1.78, the moral efficacy dimension was 12.49 ± 3.65. The total score of good-death perception was 61.98 ± 14.94 and the total score of coping with death competence was 45.28 ± 11.79.

Table 2.

Descriptive results for good-death perception, moral resilience and coping with death competence (N = 254)

Items	Scores (Mean ± SD)	Means (Mean ± SD)
Moral resilience	47.54 ± 12.41	2.80 ± 0.73
coping with moral dilemma in nursing and relational integrity	25.02 ± 7.16	2.50 ± 0.72
Personal integrity	5.02 ± 1.78	2.51 ± 0.89
Moral efficacy	12.49 ± 3.65	2.50 ± 0.73
Good-death perception	61.98 ± 14.94	3.44 ± 0.83
Coping with death competence	45.28 ± 11.79	5.03 ± 1.31

Correlation analysis

As shown in Table 3, there was significant and positive correlation between good-death perception and moral resilience (r = 0.484, p < 0.001) and coping with death competence (r = 0.565, p < 0.001). Good-death perception increasing, both moral resilience and the competence of coping with death also increased. Additionally, moral resilience and coping with death competence also showed positive correlation (r = 0.453, p < 0.001). As moral resilience of ICU nurses has been enhanced, their ability to cope with death has also improved.

Table 3.

Relationships among good-death perception, moral resilience and coping with death competence (N = 254)

Variables	1	2	3
1.GDP	1
2.MR	0.484**	1
3.CDC	0.565**	0.453**	1

Abbreviations: GDP, Good-Death Perception; MR, Moral Resilience; CDC, Coping with Death Competence

Note: **p < 0.001

Common method bias test

Harman’s one-factor test method was used to test the common method bias. Following the completion of principal component analysis, 4 eigenvalues exceeding 1 were obtained, with the first factor explaining 38.12% of the variance, which falls below the critical threshold of 40% [41], suggesting the lack of substantial concerns associated with common method biases in this study.

Mediating effect of moral resilience on good death and coping with death competence

The structural equation model was established by taking good-death perception as the predictive variable, taking moral resilience as the mediating variable, and taking the total score of coping with death competence as the responsive variable. And the maximum likelihood method was used to modify and fit the model for testing the hypotheses. The model’s fit indexes are χ²/df = 1.592, RMSEA = 0.048, TLI = 0.945, CFI = 0.950, GFI = 0.863, all standing within the acceptable range. The mediating effect model is shown in Fig. 1.

Fig. 1.

The mediation model of good-death perception, moral resilience, and death-coping competence in ICU nurses

By using the Bootstrap method, 5000 repeated samplings were performed for testing the mediation effect. The results showed that 95% CI values of both the direct effect of on coping with death competence and the indirect effect of moral resilience did not include 0, suggesting that moral resilience has a significant mediating effect between good-death perception and coping with death competence (Table 4).

Table 4.

Mediation test

Effects	Paths	β	SE	p	95%CI
Direct effects	GDP - CDC	0.503	0.072	<0.001	[0.348, 0.633]
Indirect effects	GDP - MR - CDC	0.099	0.046	<0.001	[0.023, 0.204]
Total effects	GDP - MR - CDC	0.601	0.052	<0.001	[0.491, 0.696]

Discussion

This study completed the verification of the hypothesis, suggesting that moral resilience does plays a partial mediating role between good death and coping with death competence. Good-death perception can directly affect ICU nurses’ coping with death competence, and can also indirectly affect ICU nurses’ coping with death competence through the mediating effect of moral resilience.

Current status of ICU nurses’ coping with death competence

The score of ICU nurses’ coping with death competence was (5.03 ± 1.31), which is similar to the survey score of Gong et al. [42] Death education and training should be based on nurses’ characteristics and personal experiences, and provide personalized and targeted death education and training, and ultimately improve their competency in coping with death [42]. Managers can organize discussions on death-related topics and encourage ICU nurses to actively participate, so as to provide psychological counseling and an outlet for emotional release for ICU nurses and improve their competence to cope with death [16].

Correlations among good-death perception, moral resilience and coping with death competence among ICU nurses

It has been found that the perception of good death has a positive predictive effect on coping with death competence. Good-death perception reflects a positive death cognition level of ICU nurses. With the perception of good death, nurses can be psychologically prepared to face the death of patients at any time, while showcasing positive concern about the possible death of patients. Thus, they can overcome such negative emotions as death anxiety and death avoidance and actively respond to death [43, 44]. A high level of death cognition is the premise for improving coping with death competence. ICU nurses should not only learn the correct concept of death, but shall also enhance the level of death cognition. Death café projects implemented by some researchers in China can improve students’ cognition of death and boost students’ death-coping competence [45]. It has been shown that nurses’ hospice care training can deliver a positive impact on good-death perception [46].

Moral resilience has a positive predictive effect on coping with death competence. Research by Hossain et al. [33] also supports this finding. Moral resilience enables professionals to analyze situations, reflect, consider alternative paths, change approaches, step back, and continually rethink the best course of action [47]. When faced with a dilemma or challenge, people usually first try to regain control by removing the source of the pain and exerting force; and then, they may make pacification or disengagement, or may become numb by “going through the motions” or avoiding the cause of the pain altogether [30]. However, nurses’ primary duty is to be accountable to patients, so response to patients’ death is a moral challenge that ICU nurses must face. Most nurses still work while adhering to morality, because they have developed moral resilience in their jobs [48]. They must treat patients with “compassion and respect for the inherent dignity, worth and unique attributes of their patents”, recognize their sense of moral responsibility, and effectively deal with situations of complex, ambiguous or conflicting morality [30]. In this regard, it is first necessary to change the relationship with the dilemma that causes suffering. Nurses must explore it; and when they are in an inquiring mindset, they would become more inclined to remain positive, while shifting the focus from helplessness to resilience [30, 49]. Evidence suggests that resilience can diminish burnout and fatigue; enhance the skills for coping with work stress, the self-efficacy feeling, and the social support; and promote the development of resilience [49]. Managers can create a supportive environment and gradually introduce death-related discussions in it, such as through interactive workshops. Encouraging ICU nurses to engage in simulated patient interactions and self-reflection exercises on their own can help them manage emotions and build confidence [24]. Additionally, case-based learning, screening of documentaries, and guided discussions can make conversations about death more normalized. These interventions can ultimately enhance ICU nurses’ moral resilience [50].

The direct and indirect effects of good-death perception on coping with death competence were significant. ICU nurses can strengthen death education and training through such channels and methods as special lectures, symposiums, academic forums, and scenario simulation experience teaching [24]. Virtual reality technology can be used to complete course implementation designs with immersive, conceptual, and interactive characteristics, as part of the death-coping education program [42]. Attention shall be paid to discussions about death education practice, death communication skills [6]. A training program focused on developing death-coping skills also can be implemented to guide nurses in appropriately addressing issues of life and death. This will enable them to provide good-death services to patients, assisting patients in managing the end-of-life process [6]. Notably, the mediating effect of moral resilience between perception of good death and coping with death competence of ICU nurses is significant. Interventions to improve ICU nurses’ moral resilience can be implemented through the following approaches. First, it is crucial for ICU nurses to feel organizational support, which is essential for boosting their confidence. Membership in professional organizations should be considered a strategy to promoting moral resilience [51]. And we encourage ICU nurses to actively participate in relevant activities. Zhao et al. [30] organized nurses to conduct morality reflection through reviewing moral cases, further provided clinical ethical support, and alleviated their ethical pains caused by various negative emotions and mental states, such as loss of self-worth, anxiety, and depression, finally delivering the effect of enhancing the moral resilience of nurses [52]. Research indicates that psychological support plays a supplementary role in enhancing nurses’ competence to cope with death [6]. Second, ICU nurses need to engage in self-regulation [53]. Maintaining balance and harmony in one’s life needs to be a priority, a healthy diet, proper rest, and time for relaxation can foster the needed energy to cope with distressing situations. Several interventions can be implemented to “rewire” the brain [54], such as commitment therapy, mindfulness meditation, and ethical dilemma counselling, so as to strengthen the moral resilience of ICU nurses [48]. Finally, it has been suggested that ethics education is vital to the enhancement of nurses’ confidence and competence in addressing ethical challenges, but consensus on the most effective methods, formats and curricula has not yet been reached [53].

Moral resilience plays a partial mediating role between good-death perception and coping with death competence

The most significant finding of this study is the confirmation that moral resilience partially mediates the relationship between good-death perception and coping with death competence. In clinical nursing work, some nurses often lack the skills to manage dying processes, increasing their anxiety and reluctance to work in end-of-life care. Negative attitudes themselves pose a challenge to nurses’ emotional regulation ability when providing end-of-life care to terminally ill patients, and this emotional regulation ability is a crucial part of competence in coping with death. Analyzing the causes of negative attitudes, this reluctance may be reinforced by traditional Chinese beliefs [23]. In China, on the one hand, the moral dilemma brought about by death is influenced by “filial piety is the uppermost”; as a result, offspring often devotes all their efforts to cure their elders who are seriously ill; however, blindly “curing and rescuing” pays too much attention to the superficial aspect of filial piety ethics, to some extent, it overlooks patients’ genuine desire for comfortable treatment and care. On the other hand, under the influence of the idea of “bad living is better than good death”, China is extremely taboo to talk about life and death, and this deeply rooted traditional belief may result in the underestimation and inadequate attention to end-of-life issues [32, 55]. The essence of these ethical dilemmas stems from the contradiction to the principle of good death. In other words, the principle of good death serves as a fundamental prerequisite for addressing ethical dilemmas. Enhancing moral resilience can facilitate the resolution of moral challenges [29]. To enhance moral resilience, ICU nurses must embrace the concept of a good death to effectively address ethical dilemmas. ICU nurses fully comprehend the needs of end-stage patients for high-quality care and they can effectively address the challenges associated with death-related events [56]. To provide compassionate and appropriate care for terminally ill patients and their families, it is crucial to enhance nurses’ understanding of death-related concepts and improve their competence to cope with death. ICU nurses can conduct death reflection on their own, through simulated death experiences, while simultaneously engaging in perspective-taking from an observer’s viewpoint. This approach aims to stimulate internal psychological needs and reflect on the meaning of life, thereby fostering the positive development of death cognition [57]. Therefore, ICU nurses could overcome ethical dilemmas by cultivating the concept of ‘good death’ and enhancing their own moral resilience, thereby improving their ability to control their emotions when dealing with patient deaths, maintaining their professionalism, deeply understanding the needs of patients and their families, and promoting care for terminally ill patients and effective communication with their families.

Limitation

There are some limitations with this study. It only carried out a survey on nurses in five tertiary general hospitals. In the future, a multi-region and multi-center survey could be implemented, so as to provide a theoretical basis for the subsequent development of intervention programs to enhance the moral resilience and advantage expectations of ICU nurses, while improving their coping with death competence. Because this model used data from a cross-sectional survey, the results may be affected. It is recommended that a longitudinal study be conducted to confirm these results. In addition, the scores of death coping ability in this study were derived from nurses’ subjective evaluations, which may have certain biases. Future studies could adopt objective indicators for ability assessment.

Conclusion

Good death and moral resilience have a positive predictive effect on coping with death competence; moral resilience plays a mediating role between good-death perception and coping with death competence. Enhancing ICU nurses’ moral resilience is a new way to boost their coping with death competence. This prompts managers to advocate for improving ICU nurses’ competence in coping with death by enhancing their perception of good death and moral resilience. In the future, apart from directly conducting education or training related to death, researchers can focus on exploring methods to enhance the moral resilience of ICU nurses, thereby indirectly improving their coping with death competence. Investigating effective methods and formats for moral education is considered promising.

Abbreviations

ICU

Intensive Care Unit

Author contributions

XZ: Conceptualization, data curation, formal analysis, methodology, writing & original draft; YM: Conceptualization, data curation, formal analysis, methodology, writing & original draft; YL: Methodology, project administration, software formal Analysis; QL: Conceptualization, methodology, supervision, writing & review & editing; YC: methodology, project administration, supervision; YX: methodology, supervision, writing & review & editing; HX: software formal Analysis, supervision; XJ: Conceptualization, methodology, supervision, Writing & review & editing. All authors read and approved the final manuscript.

Funding

No funding.

Data availability

The datasets generated and/or analysed during the current study are not publicly available due to the participant confidentiality, but are available from the corresponding author on reasonable request.

Declarations

Ethical approval and informed consent statements

This study was approved by the ethics committee of Zibo Central Hospital. And informed consent was obtained from all nurses.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.