Chapter 17
Future directions
Christian B. Laursen1,2,3, Najib M. Rahman4,5,6 and Giovanni Volpicelli7
1Dept of Respiratory Medicine, Odense University Hospital, Odense, Denmark. 2Centre for Thoracic Oncology, Odense University Hospital, Odense, Denmark. 3Institute for Clinical Research, SDU, Odense, Denmark. 4Oxford Centre for Respiratory Medicine, Oxford University Hospitals NHS Foundation Trust, Oxford, UK. 5Oxford Respiratory Trials Unit, Nuffield Dept of Medicine, University of Oxford, Oxford, UK. 6Oxford NIHR Biomedical Research Centre, Oxford, UK. 7Dept of Emergency Medicine, San Luigi Gonzaga University Hospital, Torino, Italy.
Correspondence: Christian B. Laursen, Dept of Respiratory Medicine, Odense University Hospital, Sdr. Boulevard 29, 5000 Odense C, Denmark. E-mail: Christian.b.laursen@rsyd.dk
Although the number of published studies describing the clinical use of TUS is steadily increasing, several aspects are yet to be studied and assessed in clinical trials. Most importantly, the use of TUS is yet to find its place in major guidelines describing some of the conditions that can be assessed using US. It is hoped that future additional research will help to provide an evidence-based approach that indicates how TUS should be incorporated into the relevant guidelines.
Cite as: Laursen CB, Rahman NM, Volpicelli G. Future directions. In: Laursen CB, Rahman NM, Volpicelli G, eds. Thoracic Ultrasound (ERS Monograph). Sheffield, European Respiratory Society, 2018; pp. 244–252 [https://doi.org/10.1183/2312508X.10007617].
This chapter focuses on some of the areas in which further knowledge is needed to establish an evidence-based TUS approach and which may help incorporate TUS into the relevant clinical guidelines.
Nomenclature
The publication of the international evidence-based recommendations for point-of-care LUS was a milestone in the field of TUS [1]. Not only did it summarise existing knowledge and provide evidence-based recommendations based on that knowledge, it also for the first time tried to establish a consensus on nomenclature and definitions of some of the pivotal artefacts. This was a major improvement as it provided a framework on which future studies and research could be based. Despite the recommendations, there is still no international consensus on many terms and principles. This is most clearly demonstrated by the fact that there is no consensus on what US of the lungs and chest should be called (e.g. TUS, LUS, chest sonography). This may seem a trivial discussion but it is of importance if respiratory physicians and other specialities are to claim professional ownership of an US competency in the same magnitude as echocardiography or abdominal US. Some new definitions of artefacts are also emerging, such as different origins of B-lines and signs seen in complex pneumothorax (PTX) [2]. Hopefully, in time, the European Respiratory Society (ERS) will be able to provide the international network needed to help facilitate further consensus on nomenclature and principles.
Training and competency assessment
The extent to which the use of TUS has been implemented among respiratory physicians varies significantly from country to country. This may be due to differences in the extent to which the various specialities have taken ownership of the modality and whether US machines are generally available. Based on current knowledge, use of TUS should be considered a core skill for the vast majority of respiratory physicians and physicians from other specialities handling emergencies and patients with respiratory symptoms. One of the future challenges will therefore be to provide an evidence-based framework that may help achieve this.
The ERS is yet to describe recommendations for training and competency assessment in TUS. Other national and international societies have previously provided such recommendations [3–5]. The guidelines generally describe different competency levels and a minimum number of supervised procedures to achieve competency. The major limitation is that there are not enough studies that can provide the number of procedures required in order to obtain competancy in TUS. The existing guidelines do not describe or use any validated tools for competency assessment. As expertise increases within the respiratory community, it is likely that respiratory societies themselves will define competence and training within TUS (as opposed to radiologists), given that the majority of LUS/TUS is essentially delivered by non-radiologists.
Some published studies describe training in TUS but the majority provide low levels of evidence and take the form of one-group pre- and post-tests studies [6–17]. This type of study design does not provide any evidence showing how to build a training curriculum and is considered obsolete [18–21]. The learning curves of novices in TUS have been described as being steep but with good overall agreement with supervisor findings [16]. However, learning curves of focused LUS- and US-guided transthoracic procedures indicate that even though most physicians can obtain the skills quickly, there are some outliers who improve their skills at a slower rate, or do not improve at all [22, 23]. Using a validated tool for competency assessment could help to identify such outliers. Such a tool should encompass a range of aspects, including understanding TUS, image acquisition and image interpretation. Two studies have gathered validity evidence for such instruments [24, 25]. The developed tools have not yet been validated elsewhere, but studies in abdominal US using similar principles found such an assessment tool to be both valid and reliable [26]. Further studies are required in order to establish a test that assesses theoretical competency and, ideally, randomised clinical trials that assess both the optimal training programme and the effect on clinical outcome of patients when implemented.
In 2016, the ERS launched a structured training programme in EBUS [27]. Hopefully, additional future studies in TUS education, training and competency assessment will be able to provide the knowledge needed to establish a similar and evidence-based ERS training programme in TUS.
Pleural diseases
The use of TUS in the diagnosis of pleural fluid, and for guidance in pleural procedures, is well established. Early studies suggested using TUS in the diagnosis of pleural disease aetiology (including malignant pleural effusion) and pleural infection [28]. Exciting potential future areas include the use of TUS in predicting outcomes in pleural disease; for example, response to therapy in those with pleural infection according to septation presence or fluid characteristics, and in malignant effusion where advanced M-mode analysis may predict the presence of unexpandable lung [29]. There is also evidence that US may reduce complications further through detection of the intercostal artery, potentially preventing laceration during procedures [30]. All of these indications require further, focussed prospective work.
Pneumothorax
Several studies have assessed the use of TUS for diagnosis of PTX and found the diagnostic accuracy to be superior to conventional CXR [31–37]. Despite these studies, TUS is still not integrated into clinical PTX guidelines [38]. This is mainly due to the fact that whether TUS can accurately be used to estimate the size of a PTX is still an issue of debate [1]. Some studies have assessed this issue, but further studies are required that assess the clinical impact when TUS is implemented either as an adjunct to or a complete replacement of conventional CXR when diagnosing, managing and monitoring patients with PTX in a variety of settings (e.g. spontaneous, traumatic, iatrogenic) [39–41]. However, the widespread use of conventional radiology that is recommended by the existing societal guidelines should at least be questioned by the growing evidence of the efficacy of LUS [33–37, 40, 41], which is in contrast with the absence of original trials demonstrating the efficacy of CXR for confirmation and monitoring of PTX.
Lung parenchymal pathology
Several studies have discussed the use of TUS in the assessment of diseases affecting the lung parenchyma (e.g. pneumonia, pulmonary embolism, contusion, malignancy, acute respiratory distress syndrome, pulmonary oedema, interstitial lung diseases (ILD)) [1, 42–44]. However, the vast majority are feasibility studies or studies assessing diagnostic accuracy, rather than clinical impact and safety when TUS is fully integrated into clinical practice. A single randomised trial has assessed the safety of replacing CXR with TUS in children with suspected community-acquired pneumonia; the study found it feasible and safe to replace the standard CXR with TUS [45]. Similar studies are warranted in other settings (e.g. adult, emergency department, intensive care unit, outpatient clinic) and for the other diseases affecting the lung parenchyma.
Obstructive pulmonary diseases
It has been proposed that US is the visual stethoscope of the 21st century, and the diagnostic accuracy of TUS is far better than lung auscultation for a wide range of conditions [46, 47]. Even though some indirect signs may be present, TUS cannot be used to diagnose obstructive pulmonary diseases such as COPD and asthma [48]. It has been suggested that a focused TUS examination with no abnormal findings is consistent with a COPD exacerbation in a patient with acute respiratory failure in an intensive care setting [49]. This assumption does not seem to be applicable in the emergency department [50]. Stethoscope and lung auscultation may therefore still be of use in clinical practice and cannot be entirely replaced by TUS. Future studies will hopefully be able to clarify whether TUS has a potential role in patients with obstructive pulmonary diseases.
Interstitial lung diseases
Several studies have described the use of TUS in the assessment of various ILD [51–57]. It does not seem to be possible to visualise rare cystic lung diseases with TUS, indicating that it cannot be used as an efficient tool to rule out all types of ILD [58]. However, it does still have a number of possible clinical applications: it can be used as a means of screening for ILD subtypes in various populations and for monitoring purposes [57].
Acute respiratory failure
Acute respiratory failure may represent a complex diagnostic challenge, as this condition could be the result of many different diseases. The physician has to consider a number of possible differential diagnoses. Despite several milestone advances in diagnostic modalities since Laennec invented the stethoscope, a significant proportion of patients with respiratory failure are not diagnosed correctly and receives inappropriate treatment [59, 60]. Several diagnostic accuracy studies have been published supporting the use of TUS when assessing patients with respiratory failure [49, 50, 61–64]. The clinical impact of routine US assessment has, however, only been assessed in one randomised single-centre clinical trial. The study demonstrated that, when compared to routine diagnostics without the use of focused US, the US group had a significantly higher proportion of patients who were correctly diagnosed and treated within 4 h of admission to the emergency department. There were no differences in outcome, such as mortality and length of hospital stay, but the study was not powered to show such differences [65]. Larger studies focusing on outcome and patient-reported outcomes are required.
Advanced TUS
A variety of different advanced US modalities are available and have been used by other specialities for the assessment of other organs and anatomical structures. Some of these modalities could also have a clinical use in the field of TUS.
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