Introduction

The diaphragm is the primary respiratory muscle,¹ however, a broad range of pathologies and clinical situations may cause diaphragmatic dysfunction, which can negatively affect patient outcomes.² Diaphragm dysfunction is present in 63-80% of patients mechanically ventilated for over 48 hours in intensive care (ICU),^3–5 and is associated with prolonged ventilation, increased mortality, and readmission to ICU.^6–11 In Chronic Obstructive Pulmonary Disease (COPD), diaphragm dysfunction is exaggerated by hyperinflation, as the disease progresses, the diaphragm switches to type 1 fibres, becoming shorter and more horizontal, leading to ineffective mechanical function, causing breathlessness and functional limitations.^12–14 Pre-operatively, patients with respiratory muscle weakness have a higher risk of post-operative pulmonary complications.¹⁵ Spinal cord injury can dramatically impair diaphragm function, resulting in the need for mechanical ventilation, causing further atrophy.^16–18 In stroke, respiratory function may be compromised due a direct result of central nervous system impairments, or restriction in chest wall movement.¹⁹ Respiratory muscle weakness is present in 30-50% of patients with heart failure, impairing exercise capacity and peripheral muscle strength and causing dyspnoea.^20,21

IMT aims to improve the strength and endurance of the diaphragm and inspiratory muscles through the application of resistance during inhalation using a mechanical or electronic device.²² IMT aims to improve clinical outcomes such as function, exercise tolerance, breathlessness, quality of life, hospital length of stay, and ventilator weaning time.^23–26 For example, in ICU or spinal cord injury, IMT may ameliorate the effects of prolonged mechanical ventilation by increasing respiratory muscle strength and reducing dyspnoea.^17,23,27,28 In COPD, IMT may improve exercise capacity and reduce dyspnoea,^29,30 thought to occur by influencing the respiratory metaboreflex.^31–33

IMT has been widely studied in a large range of clinical populations, yet the strict inclusion criteria inherent to systematic reviews limit the comprehensive exploration of the literature; in contrast, a scoping review can summarise findings from a large body of literature that is heterogeneous in methods or discipline.³⁴ This scoping review aimed to comprehensively investigate the extent, range and nature of literature investigating IMT in a range of clinical populations. The objectives of this scoping review were to: (1) explore the methodological characteristics of IMT in clinical populations, (2) report main study results by patient population, (3) identify research gaps or limitations in the literature to aid in the planning of future research or guidance.

Methods

This scoping review was conducted according to the methods by Arksey and O’Malley³⁵ and Levac et al.,³⁶ and is reported in accordance with the PRISMA reporting guidelines for scoping reviews.³⁴

Protocol and registration

The protocol was registered on the Open Research Framework (ORF) website on 20/01/2023. Registration number: osf.io/j27hk.

Article selection

Articles were included if they investigated IMT in adult patients with any disease process, including, but not limited to: respiratory conditions (COPD, interstitial lung disease, bronchiectasis, cystic fibrosis); critically ill patients; neurological conditions (neuromuscular conditions, spinal cord injury, stroke, Parkinson’s disease); prehabilitation prior to, and following surgery, and heart failure. Studies using objective outcome measures such as respiratory strength or function, Quality of life (QoL), physical function, exercise tolerance, or time to ventilator liberation were included. We included articles published between 01/01/2013 to 01/01/2025 in English language only. Articles were excluded if they were conducted on healthy participants, athletes, paediatric patients, or animals. We excluded opinion and commentary papers, papers using qualitative methodology only, and where no full text was available.

Information sources and search

An experienced university librarian (LB) conducted searches of AMED, CINAHL, EMBASE, and MEDLINE in January 2023 and January 2025. Systematic reviews (SRs), meta-analyses (MAs) and narrative reviews referenced in included articles were also included. The search strategy was adapted for each database (Appendix 1).

Selection of sources of evidence

References identified from the search were uploaded to Rayyan.³⁷ Following removal of duplicates, titles and abstracts were screened by two blinded reviewers against the eligibility criteria. Where the title and abstract were relevant, eligibility of the article was assessed by reading the full text. Disagreements were resolved by a third senior reviewer (AL or TJ).

Data charting process

Study design, participants, intervention, comparator (if applicable), outcome measures and main findings were independently extracted onto a charting form, using a combination of hand extraction and AI assisted extraction (Elicit, Covina, United States), with human validation. Inconsistencies in charting was addressed by a lead reviewer in each subject area (AE, SKM, AT, FB, TJ). Quality of articles was not assessed as the objective was to conduct a scoping review, not a systematic review.

Synthesis of results

For result synthesis we grouped the included articles according to disease process including: neurological conditions, chronic and other respiratory conditions, surgery, heart failure, ICU and other. We then explored: study methodology (study design and patient group); intensity, type, frequency and duration of IMT; effect of IMT on outcome measures.

Results

Selection of sources of evidence

The initial search yielded 3,068 articles; the updated search yielded 1,100 articles, totalling 4168 articles. Following removal of 424 duplicate records, titles and abstracts were screened. 3,330 articles did not fulfil the inclusion criteria. Full texts were retrieved for 404 articles; 10 articles were unavailable for full text review; reference searches yielded an additional 30 articles. 79 were excluded. Subsequently, 355 articles were included in this review (figure 1).

Figure 1.PRISMA literature search process

Characteristics of sources of evidence

Figure 2 details the extent of evidence for each subject area.

Figure 2.Characteristics of sources of evidence for each subject area.

Percentage of articles for each evidence source is shown where sufficient space exists in the pie chart.; RCT: Randomised Controlled Trial; NMD: Neuro Muscular Disease; ICU: Intensive Care Unit.

Results of individual sources of evidence by each subject area

Neurological conditions

Neuro Muscular Disease (NMD)

Nine articles investigated IMT in NMD: 3 observational studies, 2 SRs, 2 MAs, 1 narrative review, and 1 scoping review. Disease processes included motor neuron disease (MND), Amyotrophic Lateral Sclerosis, Becker muscular dystrophy, myotonic dystrophy, Duchenne muscular dystrophy, multiple sclerosis (MS) and myasthenia gravis.

IMT protocols used threshold devices set at an intensity of 15-60% Maximal Inspiratory Pressure (MIP). Sessions were daily and lasted 10-15 minutes. Training duration was 3-4 months. Some studies combined IMT with other interventions, including peripheral muscle strength and endurance training.

Outcome measures included pulmonary function (Forced Vital Capacity (FVC)), Forced Expiratory Volume in 1 second (FEV₁), Peak Cough Flow (PCF)), respiratory muscle strength (MIP, Maximal Expiratory Strength (MEP)), maximal voluntary ventilation (MVV), Six Minute Walk Test (6MWT), dyspnoea, QoL, and survival.

IMT significantly improved MIP and PCF in neuromuscular patients. IMT enhanced respiratory muscle strength and ventilatory function in MS and MND patients. Evidence was inconclusive regarding the impact of IMT on survival time in MND patients. Some studies observed improvements in quality of life and reductions in dyspnoea.

Parkinson’s disease

Eight articles investigated IMT in Parkinson’s disease: 2 Randomised Controlled Trials (RCT’s), 3 observational cohort studies, 2 SRs and 1 MA. IMT protocols largely used threshold devices. Intensity ranged from 15-75% MIP, daily to several times per week, training duration was between 2 months to 12 weeks. Some studies included Expiratory Muscle Training (EMT).

Outcomes included: respiratory function (MIP, MEP, FVC, FEV₁); cardiovascular autonomic function (heart rate variability (HRV), baroreflex sensitivity); functional outcomes (Unified Parkinson’s Disease Rating Scale, Hoehn and Yahr stage); speech and swallow; and cough ability.

Significant improvements in MIP, pulmonary function and physical function were observed following IMT. Cardiovascular autonomic function improvements were noted, though some benefits diminished after cessation of training. Speech and swallowing function improved, reducing the risk of aspiration pneumonia.

Spinal cord injury (SCI)

Twenty-nine articles investigated IMT in SCI. Eight were RCT’s, (1 pilot), 8 observational cohort studies, 4 SRs, 2 MAs, 1 narrative review and 6 case reports. IMT protocols varied, including IMT, EMT, game-based IMT, and IMT as part of rehabilitation programs. Most programs started at 20-50% of MIP, increased to 40-80% based on tolerance. Programs typically involved 3-7 sets of 5-30 breaths or 2-10 minutes per session, conducted 2-7 times per week. Duration of IMT ranged from four weeks to six months, mostly lasting 6-8 weeks.

Common outcomes included FVC, FEV₁, MIP, and MEP. Most studies reported significant improvements in respiratory muscle strength and pulmonary function. Impact on QoL was less consistent. No studies measured pulmonary infections or mortality.

Stroke

Forty-one articles were included: 24 RCT’s (two pilot RCT’s), 11 MAs, 2 SRs, 3 retrospective case control and 1 narrative review. Protocols included IMT, EMT, or both. IMT typically lasted 20-40 minutes, 3 to 7 times per week. Duration ranged from four to eight weeks. Some studies incorporated additional rehabilitation methods such as neurodevelopmental treatment or trunk stabilization exercises.

Outcomes included FVC, FEV₁, Peak Expiratory Flow (PEF), MIP, MEP, and 6MWT. A majority of studies reported significant improvements in all outcomes. There were conflicting results regarding reductions in respiratory complications, such as pneumonia, enhancements in speech and swallowing, QoL, and activities of daily living.

Chronic and other respiratory conditions

Eighty-nine articles investigated IMT in COPD (n=51), COVID-19 (n=14), obstructive sleep apnoea (n=7), cystic fibrosis (n=2), asthma (n=3), bronchiectasis (n=4), interstitial lung disease (n=5) and other (n=4). 9 systematic review and meta-analyses (including 2 Cochrane reviews), 43 RCT’s, 2 systematic reviews, 7 narrative reviews, 2 pilot studies, 2 non-randomised trials, 2 mixed-methods study, 4 case series, 1 case report, 2 cohort studies, 1 case control, 1 scoping review, 2 preliminary studies, 4 observational studies, 1 retrospective study and 6 other studies were included.

IMT load was set between 30-75% MIP, lasting between 5-30 minutes per day, 2-7 days per week. Some studies combined IMT with other interventions such as cycle ergometry or pulmonary rehabilitation. Outcome measures included breathlessness (the Borg scale, the Medical Research Council (MRC), and the modified MRC dyspnoea scale); function (6MWT, 12 Minute Walk Test and Incremental Shuttle Walk Test); QoL; oxygen saturation, blood pressure and HRV; sleep questionnaires (Epworth Sleepiness Scale and the Pittsburgh sleep quality index); and balance. IMT significantly improved MIP in a majority of studies, with varied improvements in breathlessness, function, QoL and physiological parameters.

Surgery

Eighty-six articles investigated IMT in the surgery pathway, including pre- or post-operative and following discharge in cardiac, thoracic, abdominal, bariatric surgery, cancer, and lung transplantation. This included 31 RCT’s, 33 SRs and MAs, 5 pilot RCT’s, 4 secondary data analysis, 2 quasi experimental trials, 2 case reports, 3 observational cohort studies, 1 case series, 1 Cochrane review, 2 scoping reviews, and 2 narrative reviews.

IMT was commonly prescribed at 30-40% MIP (range 15-80%), with some not reporting intensity. Load was sometimes increased as clinically able by healthcare professionals. IMT was compared to usual care, chest physiotherapy or sham IMT. IMT was often combined with usual physiotherapeutic post-operative respiratory care or with a prehabilitation intervention including peripheral muscle strength and endurance training.

Outcome measures frequently included MIP, post-operative pulmonary complications (PPCs), pulmonary function, length of stay (LOS), and 6MWT. Training groups showed significant improvement in MIP. Some studies showed reduction in PPCs, improvement in pulmonary function, LOS and quality of life.

Heart failure

Forty-three articles were included: 25 RCT’s, 10 SRs and MAs, 2 SRs, 3 narrative reviews, 1 randomised crossover trial, 1 cohort study, and 1 case report. IMT intensity ranged from 15-60% MIP. In RCT’s, IMT was compared to usual care or sham IMT. Some RCT’s combined IMT with aerobic, high intensity interval training or peripheral muscle training.

Outcome measures included physical function, walk distance, MIP, oxygen consumption (VO₂), QoL, haemodynamic response, and parasympathetic control. Training groups showed significant improvements in MIP. Some studies found improvements in peak or maximum VO₂, QoL, function, walk distance and parasympathetic control, but this was not consistent across all studies. The addition of aerobic and/or resistance training resulted in greater magnitude of improvement in outcome measures.

ICU

Thirty-four articles investigated IMT in critical care: 18 RCT’s, 1 pilot RCT, 2 SRs, 3 MAs, 4 narrative reviews, 2 observational studies, 2 case reports, 1 mixed methods study and 1 case series.

Protocols used threshold or electronic resistance devices set at 15-50% MIP or the load where the patient could just complete the sixth breath. Many protocols increased intensity daily. One RCT performed IMT by increasing trigger sensitivity on the ventilator. Both meta-analyses included papers that performed unclassified inspiratory muscle training (defined as “any physical intervention employed with the goal of improving inspiratory muscle strength”). IMT was compared to usual care or sham IMT.

Frequently included outcome measures included MIP, weaning and mechanical ventilation duration, and rapid shallow breathing index (RSBI). Some studies measured QoL, dyspnoea and lung function. A majority of studies found significant improvements in MIP. Improvements in weaning duration or duration of mechanical ventilation were less consistent. Most studies that measured RSBI, lung function, QoL and dyspnoea found improvements in these outcomes.

Other

Sixteen articles investigated IMT in myocardial infarction, lung cancer (non-surgical), continuous veno-venous hemofiltration, urinary stress incontinence, diabetes, chronic kidney disease, back and neck pain and obesity. These comprised of narrative reviews, SRs and MAs, RCT’s, pilot studies, cohort studies, and case studies. IMT protocols and clinical outcomes varied considerably.

A detailed summary of all included articles is presented in the study review tables (Appendix 2).

Synthesis of results

Table 1 demonstrates the significant variability in IMT regimes.

Discussion

We present the current literature investigating IMT in a range of clinical populations. We found a large diversity in study design, and in training intensity, frequency, duration and type, within and between different patient populations. The effect of IMT on clinical outcomes was varied. Understandably, outcome measures used between different populations varied considerably.

This review highlights the substantial heterogeneity of training intensity, frequency, and duration of IMT (table 1), making it challenging for clinicians to prescribe evidence based IMT in practice. IMT regimes included high intensity training using mechanical threshold or tapered flow resistance devices, or, endurance focussed training. Some studies increased intensity without re-assessing MIP. In some regimes, IMT was delivered by adjusting ventilator trigger sensitivity. Device choice may influence training load and compliance, and consequently the training effect of IMT; for example, tapered flow resistance devices maintain resistance throughout inhalation and reduce unpleasantness compared to mechanical threshold devices.^38–40 To the authors knowledge, no studies have measured adherence to IMT comparing direct clinician supervision versus a home-based programme, but adherence to home-based IMT can be poor due to lack of direct supervision,^38,41 potentially effecting effectiveness of the intervention. However, digital based interventions can improve both adherence and outcomes during home-based programmes.⁴¹

A majority of studies suggest that IMT improves MIP, however, since the MIP manoeuvre has many similarities with the IMT manoeuvre, these improvements could represent a learning effect of the intervention.^42,43 furthermore, the minimally clinically important difference for MIP has only been defined in patients with COPD undertaking pulmonary rehabilitation.⁴⁴ This review highlights the variable effects of IMT on clinical and patient related outcomes within and between patient populations. This may be explained by the large diversity in study design and training regimes in the literature, also making it difficult for systematic reviews and meta-analyses to come to conclusions regarding the efficacy of IMT. There are calls for the optimisation of the IMT stimulus^2,31 using the principles of muscle physiology and exercise prescription, however, to the authors knowledge, only a few studies have investigated this.^45–47 Some guidance on IMT prescription exists for the ICU population,⁴⁸ however, there is no consensus on the optimal training regimes for specific patient populations.

Limitations

The scope of this review, and the heterogeneity of IMT interventions mean the authors are unable to give recommendations for clinical practice, only advise clinicians of the extent and type of current literature investigating IMT in respiratory clinical populations. Clinicians are advised to appraise specific evidence relating to their patient population, with consideration of the primary goals of IMT when implementing in practice. We included all articles with IMT as an intervention; in some, IMT was combined with other interventions, making it difficult to determine the isolated effect of IMT. Additionally, some systematic reviews and meta-analyses included studies employing methods such as: adjusting ventilator trigger sensitivity, diaphragmatic breathing exercises, biofeedback on breathing pattern, or incentive spirometry^49–51; this limitation in methodological rigour makes conclusions difficult.

Recommendations

Despite a high volume of studies investigating IMT, there is significant diversity in study design, and in training intensity, frequency, duration, and outcomes measured. This review highlights the need for (1) optimisation of the IMT stimulus (2) well conducted trials with robust reporting and clinically relevant outcome measures (3) further guidance for clinicians on prescription and monitoring of IMT in practice.

Conclusions

This scoping review has identified a vast number of articles investigating IMT in clinical populations. There is significant variability in IMT regimes within and between patient populations. IMT improves measures of respiratory strength and pulmonary function in a majority of studies, however, there is inconsistent evidence suggesting it can improve clinically relevant outcome measures. Clinicians are advised to appraise specific evidence relating to their patient population, with consideration of the primary goals of IMT when implementing in practice.

Declaration of interest

TOJ received grants from the Royal Brompton and Harefield Hospital Charity and the National institute for Health Research outside of the submitted work. LB, FB, EC, AE, JH, MH, SKM, CM, LM, CS, LB, AT and AL report no funding or conflict of interest related to this work.

Funding

All authors gave their time voluntarily.

Acknowledgements

The authors gratefully acknowledge the ACPRC Evidence and Guidance Group for their help during this scoping review.

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