Methods of Evaluation of Respiratory Muscle Endurance - A Scoping Review

Introduction

Lung ventilation is a mechanical process in which the respiratory muscles are working together to circulate air in and out of the lungs. Any alteration in the performance of the respiratory muscles may decrease the effectiveness of ventilation.¹ Ventilation consists of inspiration, which is an active process and expiration, which is a passive process. The primary muscles of inspiration are the diaphragm and the external intercostal muscle. The passive expiration occurs through elastic recoil and active expiration is performed by internal intercostal and abdominal muscles. The respiratory muscles require strength and endurance to work skilfully and fulfil the basic physiological need and also to perform high-intensity functional tasks.

Respiratory Muscle Endurance (RME), can be defined as the ability of a muscle to sustain a particular task over time. Impaired RME could reduce the ability to tolerate exercise and contribute to respiratory failure.² In clinical practice, RME assessment is used to determine muscle fatigue, quantify the severity of certain diseases, and for its prognostic value.³ Respiratory muscle endurance is important to identify early clinical changes in respiratory muscle function in pathological conditions.³ Evaluating respiratory muscle endurance is necessary for characterizing respiratory muscle abnormality. Although plenty of tests are used to evaluate respiratory muscle endurance, not all of them are applicable in clinical practice.⁴ Hence the objective of the study was to identify a reliable method of assessment of respiratory muscle endurance.

Materials and Methods

Permission from the institutional research committee (JSSCPT/IRC/179/2020-2021) was obtained before commencement of the study. We performed the scoping review to identify a reliable method of assessment of respiratory muscle endurance. Articles were selected based on the Joanna Briggs Institute (JBI) scores. For the identification of articles for review, initial search of PubMed was done to find keywords. The search strategy consisted of keywords according to medical subject headings MeSh. This search strategy was conducted across PubMed and Google Scholar for articles published from 2016-2021. The final search strategy was conducted using the search terms and Boolean logic and only full-text articles were included in the study.

Study selection

The articles were selected based on the following inclusion criteria:

• Articles published in the past five years.
• Assessment methods used to evaluate respiratory muscle endurance in a healthy individual and conditions like obstructive and restrictive respiratory disorders. 
• Randomized controlled trials, observational studies, and systematic reviews.

All search results were imported into the reference management software Mendeley and duplicates found were removed. The title and abstracts of articles were screened. The quality of the articles included was checked using the Joanna Briggs Institute (JBI) Critical Appraisal Score.

Data charting

A data extraction chart was developed to find the key characteristics of each study as well as relevant information about the methods of respiratory muscle endurance. The variables included title, authors, publication year, method, result/conclusion, and Joanna Briggs Institute (JBI) Critical Appraisal Score.

Results

The literature review was mainly focused on the different methods to assess respiratory muscle endurance. The databases used were PubMed, Google Scholar. Keywords used were “Respiratory muscles, inspiratory muscles, physical endurance, respiratory function test, assessment, testing”. The search of the databases yielded a total of 8,117 articles. Based on the year of publication 4,112 articles were excluded. Based on inclusion, exclusion criteria and considering only full text articles, around 3,981 articles were excluded. Based on the title and abstract, 13 articles were excluded and a total of 11 articles were finally included in this review.

The articles reviewed were divided based on the device used, age group, and health status. A total of five devices were used across all the studies, PowerBreathe, Spirotiger, PrO2, Inspiratory muscle trainer, and RT2 device. The most commonly used one was the PowerBreathe.^4,5–7 It is a versatile device as it is used clinically and also on healthy individuals. Based on the health status, the articles were categorized as people suffering from a respiratory disorder and healthy individuals. The most commonly found articles on respiratory disorder were on COPD;^4,6–8 among the four studies, three had used the PowerBreathe Device^4,6,7 while one used the PrO2 device,⁸ with the test of incremental respiratory endurance (TIRE) as their protocol for reliability and validity, in conclusion, found it to be reliable. One study was conducted on children and adolescents with cystic fibrosis⁹ and the inspiratory muscle trainer was used to measure the respiratory muscle endurance. Articles based on healthy individuals were five which included participants from all age groups like children, adolescents (3), adults who were obese (10) and smokers (15).^3,5,10,11,12 Spiro tiger device was used in two studies^10,12 to measure respiratory muscle endurance and the study, in conclusion, recommended more research to be conducted on inspiratory resistive breathing and isocapnic hyperpnea endurance test. The other three studies used PowerBreathe,⁵ Inspiratory muscle trainer³ and RT2 device,¹¹ respectively. Based on the age group, two articles^3,9 included children and adolescents as their participants and both the studies used the Threshold inspiratory muscle trainer with the ‘incremental threshold loading protocol’ as their device

for testing and have shown promising results. However, they recommended that further studies be conducted with this device including a larger sample size.

Eight articles were found with adults as study population and these studies used devices like PowerBreathe, Spirotiger, PrO2, as well as the RT2 device.^5-8,10,11,12 All involved non-invasive methods of testing of respiratory muscle endurance. Out of all the devices used in the adult population, PowerBreathe was the most commonly used and was reported to be the reliable device.^4,5–7

Discussion

Generally, there was limited research available for the objective of the review. Using the keywords, 11 articles were included, while a large number of articles had to be excluded due to poor quality, unavailability of full-text and also based on the inclusion and exclusion criteria. The articles included in the study had moderate to high quality as assessed by the Joanna Briggs Institute (JBI) critical appraisal tool. The JBI critical appraisal tool was preferred for analytical cross-sectional studies compared to the National Institutes of Health (NIH) and the Appraisal tool for cross-sectional studies (AXIS) tool.¹³ The cut-off used for the inclusion of the articles was 60% of the JBI score. The study designs obtained in the review seemed to be appropriate to use based on the objective of the review. Based on the articles reviewed, the PowerBreathe, Spirotiger and the Threshold inspiratory muscle trainer are the devices used to train respiratory muscles and work on the principle of resistance training. Out of the three devices, the PowerBreathe and Spirotiger are costly compared to the Threshold inspiratory muscle trainer.^4,5,5–7,12 From the review of articles, the Inspiratory muscle trainer though found to be affordable was only used among children and adolescents by modifying the device available in the market and increasing the resistance using a higher resistance spring for measuring endurance.^3,9 Combining all the above data, the PowerBreathe, though expensive was used clinically as well as on healthy adults in multiple articles and can be considered as the device of choice for measuring respiratory muscle endurance.^4,5–7

Strengths

The stringent criteria used for the inclusion of articles, which was based on the type of the article, the quality of the article, and the year of publication were the strengths of this review.

Limitations

Non-inclusion of articles that were published in a language other than English and the non-availability of translated articles were the limitations. This review only included the findings which were published by the authors and did not conduct a meta-analysis.

Future directions

Though there was evidence of using the PowerBreathe device for respiratory muscle endurance in patients, there was no mention of its reliability in healthy individuals and people of different age groups. Further studies testing this device among different age groups and a larger sample size are recommended. More isolated studies on assessment of respiratory muscle endurance comparing different devices are advised along with a meta-analysis.

Conclusion

This scoping review showed moderate-quality evidence on different methods of respiratory muscle endurance. The few devices which were found were not sufficiently tested on various populations and were not specific for assessment as they are endurance training devices. The studies testing the reliability of the devices are lacking, and the studies were found to be inconclusive. Future studies are recommended on the measurement of respiratory muscle endurance.

Conflict of Interest

Nil