Abstract
Noninvasive ventilation (NIV) has been shown to be beneficial for patients with respiratory failure; however, many patients fail to tolerate it and require other interventions. The objective of this thematic synthesis was to describe the nature of NIV experiences in adults with hypercapnic respiratory failure. A systematic, computerised literature search of English-language databases was undertaken with no restriction on date of publication. A total of 99 papers was identified and screened for eligibility from databases including CINAHL, Medline and PsycINFO, and some were hand searched. 45 papers were critically appraised and 32 met our inclusion criteria. Thematic analysis identified six key themes: benefits of NIV; fear (of various categories, namely, fear of technology/mask, fear of death and dying, and fear of pain and suffering); adaptation to NIV machine; decision making; need for information; and relationship with healthcare professionals. For people using NIV treatment, the experience of being on the NIV machine is unexpected and can be stressful. Findings from this review offer healthcare professionals insights and understanding into the patient experience of NIV. Healthcare professionals may use these findings to implement new strategies in NIV provision and the exploration of the applicability of age-specific supportive care NIV guidelines.
Abstract
There is a need for research into the experiences of NIV in older people with hypercapnic respiratory failure http://ow.ly/10onBl
Introduction
Noninvasive ventilation (NIV) is an established treatment for hypercapnic respiratory failure (HRF) in older people in whom endotracheal intubation may not provide long-term benefits. A widely used definition of NIV is the delivery of mechanical ventilation without the use of an invasive artificial airway (endotracheal or tracheostomy tube) [1, 2]. HRF is defined as alveolar hypoventilation characterised by low levels of oxygen (hypoxia) and high levels of carbon dioxide (hypercapnia) [3, 4].
The use of NIV in HRF has been extensively researched since its inception, with a gradual improvement in the quality of the research produced. A meta-analysis of in-hospital trials for chronic obstructive pulmonary disease has shown that NIV is associated with reduced mortality and reduces the need for intubation [5]. Keenan et al. [6] found that the use of NIV decreased the need for endotracheal intubation by 38% and reduced mortality by 16%, leading to lower costs per patient admission. Despite the effectiveness of NIV in treating patients with HRF, older people present a large population of patients who have difficulties with NIV treatment acceptance as compared to young adults [7, 8]. The reasons for this difficulty, individual experiences and treatment preferences of older people receiving NIV remain unclear.
In the past, there has been emphasis on quantitative research designs in NIV treatment resulting in a significant lack of literature on the experiences of older people with HRF. There is need for a review of existing work about the experience of people with HRF receiving NIV [9].
Purpose
The aim of this review was to conduct a thematic synthesis of available qualitative studies that examined NIV experiences of adult people, regardless of the setting, age or the mode of NIV treatment. To our knowledge, there is no literature available that examines the NIV experiences of older people with HRF in acute settings. Therefore, broadening study selection was considered appropriate to provide a general perspective into the experiences of NIV in adult people with respiratory failure.
Methods
Search strategy
A systematic, computerised literature search of the electronic databases CINAHL, Medline (the primary database of the US National Library of Medicine) and PsycINFO (American Psychological Association), with PubMed as the search engine, was carried out in 2013 and updated in 2015 [10]. No restrictions of sex or date of publication were imposed; however, only studies written in English were considered. Publications that may have been missed were identified by searching the tables of contents of key respiratory journals (Chest, Thorax and the BMJ, the European Respiratory Journal, Respiratory Research, and the American Journal of Respiratory and Critical Care Medicine) and reference lists of articles identified as relevant were also reviewed by hand searching. Databases were searched using Boolean operators, field codes, truncation (*), wildcards (?) and quotation marks. The search was limited to articles that include an abstract and research articles only (supplementary table S1).
Study selection
Qualitative studies that provided insights into experiences of people aged 18 years and older who were receiving NIV treatment or had just been weaned off were included. Pre-defined inclusion and exclusion criteria were used (table 1). Studies were included if they: described the experience of being on continuous positive airway pressure (CPAP) or bilevel positive airway pressure (BiPAP) therapy; used qualitative methods or mixed methods with a qualitative section that met the inclusion criteria; were written in English; and were peer reviewed. The application of CPAP is usually on a long-term basis and BiPAP, which is used in people with acute HRF, is similar; hence, the inclusion of people using CPAP on long-term basis. Studies were not included if they: reported views of healthcare professionals and carers only, or children; or focussed on people receiving invasive ventilation. Study selection was broadened to include relevant studies linked to experiences of older people undergoing NIV.
Inclusion and exclusion criteria
Appraisal process/quality assessment
There are no universally accepted criteria by which to evaluate the quality of qualitative research studies and debate continues about the appropriateness of applying evaluation techniques primarily developed for reviewing quantitative research to reviews of qualitative research [11–13]. Nevertheless, qualitative studies for this review were assessed for methodological quality prior to inclusion in the review in accordance with the Critical Appraisal Skills Programme tool. Qualitative papers for this study were categorised using hierarchy of evidence for qualitative research [14]. This approach uses four levels of hierarchy: generalisable studies (level I), conceptual studies (level II), descriptive studies (level III) and single case studies (level IV) (table S1). The hierarchy of evidence used to evaluate literature retrieved for the purpose of this study does not include quantitative studies because it is specific to qualitative research methods.
Thematic analysis
The review was undertaken using the thematic synthesis approach as outlined by Thomas and Harden [15]. Thematic synthesis involves the identification of important or recurrent themes. The synthesis was in three stages, which overlapped to some degree: the free line-by-line coding of the findings of primary studies; the organisation of these “free codes” into related areas to construct “descriptive” themes; and the development of “analytical” themes [15]. Findings are summarised under thematic headings.
Studies identified
A total of 1423 results were identified, and after screening and removing duplicates, 99 full-text articles were retrieved. 45 studies were considered eligible and after reading the full articles, another 13 articles were excluded because they did not meet the criteria or they were not presented as research articles (e.g. discussion papers). One study [16] was excluded since it only had an abstract in English with the full document written in German. Another study [17] provided an abstract but the full article was unavailable. The remaining 32 studies were considered relevant for thematic analysis and were included in the review. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines was used in preparing this paper [18]. PRISMA has a 27-item checklist, and a four-phase study flow diagram reporting identification, screening, eligibility and inclusion. PRISMA checklist items that could not be used in this review, for example, questions 13 about summary measures and 21 about meta-analysis, are not presented. Figure 1 shows the flow of studies through the search process, including the rationale for exclusion [19].
Preferred Reporting Items for Systematic Reviews and Meta-Analyses study flow chart.
Studies included
The description and underpinning philosophy of all the studies varied significantly. Some studies clearly followed a particular theory through the analysis process, giving clarity and implying rigour in the data. Most studies used phenomenology, of which one utilised interpretive phenomenological analysis, five descriptive phenomenology with bracketing and two hermeneutics. Two studies by the same authors and another two from different authors used grounded theory [20–22]. One used a sequential explanatory mixed method, which employed quantitative methods and then a qualitative study [23], whilst another study used a concurrent nested mixed-method design in their longitudinal study [24]. Three studies that were very similar used almost the same sample [25–27]; as such, findings from these publications were used but categorised as one study.
All qualitative studies used interviews, with variations in the details given. Some interviews were semistructured, with examples of questions given; others were open-ended. Findings were clearly presented and discussed in most of the studies; however, some lacked clarity in acknowledging their limitations. There was variation in the way in which abstracts were presented, with some authors giving very little information. This could have been influenced by the policy of the journal and the space allocated.
The majority of the studies were Scandinavian, and conducted in a community setting on people using CPAP, as compared to eight studies that were conducted in a hospital setting. Of these eight studies, only two were undertaken on people using acute BiPAP. This may suggest that the majority of NIV treatment studies are performed with most of the participants recruited from community centres across the world. There was also variance in the representation of men and women: four studies included men only [28–31] and one study included females only [32]. The age ranges varied from 27–85 years. Cultural and ethnic differences were not described well except for one study, which refers to white/European, Southeast Asian and Middle Eastern backgrounds. A summary of the characteristics of the articles reviewed is provided in table 2.
Studies included
Results
The literature provided 285 free codes that were organised in hierarchical order under a range of 10 descriptive themes: initial reactions to NIV treatment; fear of death and dying, technology, and pain and suffering; relationships; emotional impact; hope; coping; support; information; finding meaning; and personal values. In the final synthesis stage, six analytical themes emerged: benefits of NIV; fear; adaption to the NIV machine; decision making; need for information; and relationship with health professionals. Within the key themes, there were subthemes pertaining to fear.
Benefits of NIV
The benefits of NIV are well established. In several studies, participants reported experiencing benefits from NIV treatment, mainly due to improved breathing, a sense of immediate relief, good sleeping patterns, increased alertness and less snoring [32, 34, 35, 40, 46, 47, 49]. In one study, patients described NIV treatment as a life-saving treatment, adding that it improves quality of life and that it is an absolute necessity for survival [43]. Nevertheless, findings from other studies suggest that benefits of NIV depend on the underlying condition and its influence. For example, patients who received NIV for the treatment of HRF regarded the NIV machine as a lifesaver [46] and in case of obstructive sleep apnoea [43], the ventilator was a necessity for survival. There is evidence that NIV is effective and it improves the quality of life for people with respiratory failure [53].
Despite these benefits, it is clear that most of the studies' findings identified and focused on problematic experiences of adult people undergoing NIV treatment [22, 24, 27, 30, 33, 34, 50, 51]. These problematic experiences were common themes across the studies and they are explored in the next sections.
Fear
Fear in people undergoing NIV treatment was a recurrent theme. In all but four studies [38, 40, 42, 51], participants expressed a fear of being on NIV. From the studies, it appears there are a number of factors, which triggers fear in people with HRF undergoing NIV, which may lead to NIV rejection. Evidence indicates that patient fear is the most common disorder among NIV patients, and that it may interfere with the whole treatment process. In almost half of the studies, fear is also described as anxiety. None of the studies reviewed gave a clear definition of fear but it appears that fear is an unpleasant and uncomfortable feeling related to a certain source [46]. However, across the studies, fear was described by participants in various ways. Specific categories of fears described included fear of technology/mask, fear of death and dying, and fear of pain and suffering. The following section will explore these subthemes in more detail.
Fear of technology
A number of studies highlighted that most of patients with acute respiratory failure usually have a fear of technology, how it works and possible adverse effects [22, 23, 29, 39, 40, 43, 47]. Fear of technology was highlighted in the study by Ingadóttir and Jonsdottir [43], in which most of the participants experienced fear of the ventilation machine. There is a clear indication across the studies that discussed fear that patients view the NIV machine as a meaningless exercise and a burden due to the negative side of the experience, which then manifests as fear. Studies also revealed that participants questioned survival as well as relying on technology and others for survival, they felt out of control and reliant on technology, creating feelings of doom, fear of death and concerns over survival. The fact that they needed to be on an NIV machine meant that they felt at times that they would not survive.
Fear of death and dying
Another form of fear identified in many studies was the fear of death and dying. The majority of the studies found that patients' fears may be linked to iatrogenic outcomes and death, and that the fear of death and dying in patients using NIV is not uncommon [38, 44–46, 49, 50]. Studies also revealed that respiratory failure, choking, running out of air, discomfort, disease progression, and how and when they will die were the main fears that affected all the patients' decisions about NIV [44, 45, 50]. One study suggested that the fear of death in some patients not only manifests as a result of having the NIV mask strapped around their face but also as a result of not having the NIV treatment as a means of supporting their life [30].
Fear of pain and suffering
Some studies suggest that fear extends to fear of pain and suffering [20, 33]. None of the studies described the fear of pain and suffering explicitly; however, a few of them highlighted fear of pain and suffering as a barrier to NIV treatment. A grounded theory study by SØrensen et al. [20] reported that as a result of the fear of pain and suffering, participants usually considered NIV discomfort intolerable as soon as they found that the treatment worsened their breathlessness, making them unable to cope with the sensation of being ventilated and restrained by the mask. Studies indicate that NIV side-effects such as claustrophobia, stomach distension, nose sores, throat dryness and nasal problems can be very frightening and unbearable for most NIV patients, and this was related to suffering [24–27, 33].
Adaptation to the NIV machine
Adaptation to the NIV machine was discussed in some of the studies and there is clear evidence that acceptance of NIV is a lengthy process that involves gradual orientation with the machine [22, 24, 27, 30, 34, 39, 41, 45]. Findings from these studies suggest that adaptation to the NIV machine can be very challenging and that it is intervention specific. Most the studies did not attempt to define adaptation in the context of NIV treatment. The few studies that attempted to define adaptation to the NIV machine described it with reference to adherence or tolerance, as these two concepts form an integral part of the intervention process [21, 31, 52, 54]. Thus, from this given description and related discussions in other studies, adaptation in the context of NIV treatment may be defined as a gradual familiarisation process by which patients make alterations to conform to a newly introduced medical device.
Evident throughout all the studies was the fact that when patients are started on NIV treatment, they usually have difficulties and troubles in becoming accustomed to the machine. These difficulties are related to significant air leaks, feelings of claustrophobia, aerophagia, presence or risk of facial injuries and respiratory distress [35, 36, 38, 51].
In one study, all the patients who took part in the research chose not to continue using their CPAP machine as prescribed because of various problems that they encountered, such as mask tightness, skin lesions, noise, leaks and airway lesions as result of the treatment [31].
However, regardless of the difficulties and troubles that most of the participants across the studies encountered at the inception of the treatment, they persisted through these difficulties [31, 34, 39, 46, 48]. Studies in this review suggest that persistence and tolerance were two mechanisms adopted by participants in the process of developing an adjustment to being attached to the NIV machine. Furthermore, others studies indicate that adaptation or acceptance of the NIV machine can be promoted by developing specific education about adverse effects and benefits of the treatment, which patients need to be aware of prior to being started on NIV treatment [41, 45, 52].
Decision making
The present literature review specifically addressed experiences of patients with acute respiratory failure on NIV treatment, and did not extend to studies which focused primarily on decision making. Nonetheless, the issue of decision making was apparent in some of the papers reviewed, particularly with regard to treatment preference [45]. Across the studies, there is no clear definition of decision making or description of the process of decision making. However, it appears that decision making is important and that it is about recognising patients' autonomy, respect and giving the patients the right to have a final say in determining how they wish to be treated. Decision making has been described widely as an important component of the relationship between healthcare professionals and patients, and was an aspect of some of these studies [20, 45, 46].
Interestingly, the majority of studies found that patients are not always involved in decisions relating to their NIV [30, 43, 46, 55]. It appears that the choice about ventilation is always made for patients by healthcare professionals, families or carers, without them being fully involved. In some studies, there is a consensus among the participants that the decision-making process is not simple, especially when participants felt that they were being led onto the treatment without making a final decision. In one particular study [46], most participants who took part in the interviews revealed that they were relieved to be able to make a decision to discontinue the NIV treatment if they found ventilation intolerable or did not see sufficient benefit, thus implying that it is important to allow the patients to retain the right to make their own decision by weighing up evidence and reaching their own conclusions.
Similarly, as in other studies, the finding of Piggin [46] in relation to decision making confirm that patients are under-consulted and they lack understanding of the whole process of decision making [34, 37, 45]. Arguably, this evidence may justify why another study suggested the need for patients receiving NIV therapy to have better understanding and involvement with their treatment in order to promote them as active participants in their own care [47]. The Declaration of Helsinki has informed recent human rights in relation to patient rights and, currently, healthcare policy and guidelines direct clinicians to engage in shared decision making [55]. However, findings from some, but not all, of the studies raise concerns about the process of gaining informed consent from NIV patients. From the studies that explored decision making, the process has been widely described as an important component of the relationship between healthcare professionals and patients. However, there is consistency among these studies, which appear to suggest that patients are under-consulted when it comes to making decision about their treatment. From all these studies, none appears to give a clear definition of decision making or the process of decision making; nevertheless, it appears that decision making in NIV treatment refers to patient ownership over treatment preferences and the decision to ventilate.
The need for information
The need for information among NIV patients with acute respiratory failure is well documented in most of the studies. Evidence from these studies suggest that when people with acute respiratory failure become ill, as seen in other illnesses, they develop much interest in knowing more about their condition, how it will be managed and the impact that will have on their wellbeing [25, 35, 36, 39, 41, 46]. It is also evident from the studies that information is regarded by patients as a means to feel empowered and in control [22, 37–39, 41, 52]. Based on the literature, information need in NIV treatment may be defined as a state or process that begins when a patient becomes aware that there is disparity between the information and knowledge available to manage their illness and the actual solution of the illness. However, in information-related themes, participants who took part in most of the studies raised concerns with the manner in which diagnosis was delivered. The concerns were directed at the communication style of the healthcare professional and systemic issues in experiencing convoluted pathways to arrive at a diagnosis. Findings suggest that healthcare professionals do not always give their patients enough information about their illness, prognosis and the nature of treatment intervention.
Based on participants' narratives, it appears that patients are more interested in knowing how the NIV machine works, the likely duration of the intervention or any other alternative intervention. However, it appears that healthcare professionals do not routinely provide such information to NIV patients. Patients feel that healthcare professionals provide them with “too little” information, which then leave them with no option but to resort to seeking the information themselves from varied sources, such as the internet [22, 32]. Findings suggested that having access to information assists the patient in making decisions and overcoming barriers to adapt to the device [40].
Nevertheless, it is also interesting to note that NIV patients are very particular with the amount of information that they wish to receive; for instance, some participants in the study by Shoukry et al. [32] felt that they were being bombarded with “too much” information, which they thought was not necessary, especially at a time where they were still assimilating their diagnosis. The findings of Bakker et al. [36] and Lemoignan and Ells [45] suggest that patients do not want information about their treatment before it is needed to make decisions, because information tends to discourage them. Furthermore, the studies revealed that the positivity aspect of information was an issue rated more highly by patients than by healthcare professionals. This may then suggest that healthcare professionals overlook the patients' need for information, which could then affect treatment adherence in NIV patients.
Relationship with healthcare professionals
A number of studies that examined patients' experiences of NIV in hospital settings make reference to healthcare professionals, as they are an integral part of the support structure. Nurses and doctors are essential for the care and management of patients on NIV because they provide 24-h, direct care and support, and their collaboration with patients may influence the success of NIV [35, 49]. Sørensen et al. [20] refer to patient–healthcare professional relationships in terms of patient involvement, patient participation in decisions, trust, respect and professional intimacy, and the appropriate use of authority or power.
Across the studies, it appears that the relationship between healthcare professionals and patients is characterised by various forms of asymmetry. These studies indicate that healthcare professionals may have both theoretical and practical knowledge; patients usually have neither. Studies also reveal that at times, healthcare professionals may need to act in the best interest of a patient when considering NIV; however, some patients viewed this as an act of being controlled, characterised by components of coercion and pressure [20, 51]. Nevertheless, there is a general consensus across the studies that patients often relinquished control of their treatment to healthcare professionals, but they were still able to interact positively with healthcare professionals and to take part actively in their treatment [44, 45].
Results from seven studies suggest that when patients are diagnosed with acute respiratory failure, they usually put their lives, and often decision making, in the hands of healthcare professionals [24, 25, 30, 35, 45–47]. It is evident from findings across these studies that breathlessness and being on an NIV machine make patients perceive that they have to be completely dependent on clinicians during the intervention. These findings may imply that patients find themselves in a situation characterised by a low degree of patient power and patient participation. Nevertheless, it was also found that although the patients trusted clinicians in helping them with NIV, there were care deficiencies: situations in which they felt neglected and not involved in their treatment. In the conclusions of their study, Sørensen et al. [20] suggest that in future, research projects should investigate the effect of patient–nurse interactions to promote our understanding of how to influence tolerance and succeed with NIV.
Discussion
This review illustrates the nature of experience that people undergoing NIV treatment face. The six themes give a holistic description of the experience of patients undergoing NIV, which includes benefits of NIV, the psychological reactions to NIV treatment, coping strategies, decision making, provision of information, and the relationship with healthcare providers.
This review reveals that the benefits of NIV are well documented. NIV is effective and improves the quality of life for people with respiratory failure [53]. However, the review showed that participants were more interested in sharing the negative aspects of being on the NIV machine rather than the positive aspects of the machine. Thus, the review shows that fear, sometimes described as anxiety, is the most common disorder among NIV patients and, as such, interferes with the whole treatment process. Patients usually experience fear due to not only being on the NIV machine but also fear of death and dying, and pain and suffering. It is clear that fear of technology has an impact on how patients perceive NIV treatment. There is a general consensus across the studies that the fear, be it of technology, death and dying, or pain and suffering, is often overlooked by healthcare providers, and that when a patient has intense fear, it can be very damaging to their health because the patient may not be willing to use the machine [30]. Perhaps healthcare professionals need to devise alternative measures that can assist patients to alleviate the fear in NIV treatment. Literature indicates that to date, there have been no qualitative studies conducted to investigate the fear of NIV machines in older people.
Challenges associated with adaptation to the NIV machine were a significant finding in this review. Due to various problems that patients encounter as a result of the treatment, such as mask tightness, skin lesions, noise, leaks and airway lesions, most patients find it difficult to adapt to the NIV machine. Thus, patients often have a negative experience of the machine. However, some patients are not deterred by these problems; instead, they find a need to persist through difficulties in order to survive. Positive coping styles, such as being actively engaged with problems of adaptation and acceptance, could be key factors for psychological wellbeing and better adherence to NIV [40]. Furthermore, there is a realisation that adaptation or acceptance of the NIV machine can be promoted by developing specific education about adverse effects and benefits of the treatment, which patients need to be aware of prior to being started on NIV treatment [39]. Thus, healthcare professionals need to provide more information and education to NIV patients in order to promote NIV tolerance.
Another significant finding in this review was that the choice about ventilation is always made for patients by healthcare professionals, their families or carers, without them being fully involved. Patients feel that healthcare professionals do not include them in decision making and that they are often put onto the treatment without making a final decision [49]. Patients can feel relieved to be able to make a decision to discontinue the NIV treatment if they find ventilation intolerable or do not see sufficient benefit. Results pertaining to decision making also revealed that patients need to be consulted regarding treatment preference, so that they can decide a preferred mode of NIV treatment for themselves [45]. Lack of treatment involvement in decision making leaves patients with no other option but to comply with the available treatment method.
Furthermore, this review also revealed that the need for information was an issue rated more highly by patients than by healthcare professionals. Patients need information in order to inform their decision and they regard information as fundamental. Healthcare providers should therefore play a pivotal role in providing such information. Findings clearly suggest that patients are likely to accommodate NIV treatment if they are educated and given enough information on NIV prior to treatment inception [37].
The importance of the relationship between NIV patients and healthcare professionals is acknowledged. Trust is essential in the relationship between the healthcare professional and NIV patient. Trust between patient and nurse is necessary for reducing patients' anxiety, and enabling them to regain a sense of control.
Strengths and limitations
All the studies in this review are of high quality and authored by experts in the field. They were conducted in various countries, populations and situations, and different sociocultural structures, albeit that they were limited to studies written in English. Even though studies written in other languages were not considered, it seems sensible to affirm that the findings of this study can be applied to other countries in a similar healthcare context. Nevertheless, the transferability of the review is restricted by the nature of included studies: most include participants receiving NIV as CPAP for diseases such as sleep apnoea and motor neurone disease, except for two that considered patients receiving NIV as BiPAP.
Conclusion
For people with HRF using NIV treatment, the experience of being on an NIV machine is unexpected and can be stressful. Underlying much of the reviewed studies was participants' desire for more information or other interventions that could make their experience much better.
This review identified the need for research into the experiences of NIV in older people with HRF, as the majority of the studies in this review focused on mixed populations aged 27–85 years. This suggests that our knowledge of NIV treatment in patients with HRF is predominantly based on the experiences of patients of varied age groups recruited from communities rather than older people admitted in hospital. It is therefore useful to conduct a study that explicitly focuses on HRF in people aged 65 years or more who are admitted for the purpose of NIV treatment, in order to better understand acceptance issues in this population. This may give healthcare professionals the basis to implement new strategies in NIV provision and the exploration of the applicability of age specific supportive care NIV guidelines.
Supplementary material
Supplementary Material
Please note: supplementary material is not edited by the Editorial Office, and is uploaded as it has been supplied by the author.
Tables S1 and S2 ERR-0002-2016_Online_supplement
Disclosures
Acknowledgements
All authors were responsible for the overall study conception and study design, analysed the data and drafted the manuscript. All authors approved the final manuscript.
Footnotes
This article has supplementary material available from err.ersjournals.com
Support statement: This study was undertaken as part of lead author's research project for his postgraduate research degree at Cardiff University. H. Ngandu received a studentship from the Research Capacity Building Collaboration Wales in an open, peer-reviewed process. Funding information for this article has been deposited with the Open Funder Registry.
Conflict of interest: Disclosures can be found alongside this article at err.ersjournals.com
Provenance: Submitted article, peer reviewed.
- Received January 13, 2016.
- Accepted March 21, 2016.
- Copyright ©ERS 2016.
ERR articles are open access and distributed under the terms of the Creative Commons Attribution Non-Commercial Licence 4.0.
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