The aim of the present study was to investigate the effects of an interval-based high-intensity inspiratory muscle training (H-IMT) programme on inspiratory muscle function, exercise capacity, dyspnoea and health-related quality of life (QoL) in subjects with chronic obstructive pulmonary disease.
A double-blind randomised controlled trial was performed. Sixteen subjects (11 males, mean forced expiratory volume in one second (FEV1) 37.4±12.5%) underwent H-IMT performed at the highest tolerable inspiratory threshold load (increasing to 101% of baseline maximum inspiratory pressure). Seventeen subjects (11 males, mean FEV1 36.5±11.5%) underwent sham inspiratory muscle training (S-IMT) at 10% of maximum inspiratory pressure. Training took place three times a week for 8 weeks and was fully supervised. Pre- and post-training measurements of lung function, maximum inspiratory pressure, maximum threshold pressure, exercise capacity, dyspnoea and QoL (Chronic Respiratory Disease Questionnaire; CRDQ) were obtained.
H-IMT increased maximum inspiratory pressure by 29%, maximum threshold pressure by 56%, 6-min walk distance by 27 m, and improved dyspnoea and fatigue (CRDQ) by 1.4 and 0.9 points per item, respectively. These changes were significantly greater than any seen following S-IMT.
In conclusion, high-intensity inspiratory muscle training improves inspiratory muscle function in subjects with moderate-to-severe chronic obstructive pulmonary disease, yielding meaningful reductions in dyspnoea and fatigue.
Several studies have investigated the effects of inspiratory muscle training (IMT) on pulmonary function, inspiratory muscle function, exercise capacity, dyspnoea and health-related quality of life (QoL) in subjects with chronic obstructive pulmonary disease (COPD) 1–9. However, results have been mixed due to differences in study designs, subject selection, assessment tools and training protocols. A recent meta-analysis of randomised controlled trials of IMT in COPD concluded that improvements in inspiratory muscle function and dyspnoea occur following such training. However, the effects on exercise capacity and QoL remain unclear 7 and, subsequently, debate still exists on the role, if any, IMT may play in the rehabilitation of COPD subjects. Currently, IMT is not recommended as a standard component of a pulmonary rehabilitation programme 10–12.
An optimal IMT protocol for COPD subjects has yet to be defined. To date, most studies have applied training programmes characterised by repeated inspiratory efforts against sub-maximal workloads, prolonged training periods and slow progression of training loads 13, 14. Most IMT studies achieve loads equivalent to <75% of the baseline maximum inspiratory pressure 1, 3, 4, 8, 15–20. Attaining greater loads may be beneficial as it appears improvements in inspiratory muscle function are dependent on the magnitude of the inspiratory load, with more substantial gains achieved in subjects training at higher loads 15, 21. Furthermore, greater improvements in dyspnoea and exercise capacity may be possible following IMT programmes that achieve larger gains in inspiratory muscle function 14, 15. In COPD subjects, achieving very high training loads is difficult with continuous-based training programmes, probably due to the onset of intolerable dyspnoea 22. In contrast, interval-based training programmes offer the opportunity for COPD subjects to potentially achieve very high training loads with less dyspnoea 23. The current authors recently demonstrated the feasibility of interval-based high-intensity IMT (H-IMT) in subjects with moderate-to-severe COPD 24.
The primary objective of the current study was to determine the effect of H-IMT on inspiratory muscle function, exercise capacity, dyspnoea and QoL in a COPD population. It was hypothesised that H-IMT would allow a very high training load to be tolerated, yielding rapid, pronounced gains in inspiratory muscle strength and endurance, reductions in dyspnoea and improvements in exercise capacity and QoL in subjects with moderate-to-severe COPD.
Subjects who had a diagnosis of COPD, a smoking history >10 pack-yrs and a forced expiratory volume in one second (FEV1) ranging 15–70% of predicted normal were recruited. Exclusion criteria included: 1) comorbid conditions likely to reduce exercise capacity (e.g. symptomatic ischaemic heart disease, body mass index (BMI) >35 kg·m-2); 2) previous lung surgery; 3) use of long-term oxygen therapy; and 4) weaning doses of oral corticosteroids. The study was approved by the appropriate ethics committees and informed consent was obtained. Subjects were recruited from referrals to pulmonary rehabilitation programmes and in response to local advertising.
In total, 55 subjects (34 males) were screened for suitability to participate in the study. Of these, 17 were excluded during the “screening and familiarisation” phase: nine could not complete the assessments due to comorbid conditions or intercurrent illness; six did not meet inclusion criteria based on formal lung function testing; and two had a BMI >35 kg·m-2. Three subjects withdrew of their own accord during the screening and familiarisation phase. The majority of those who were excluded or withdrew during the screening and familiarisation phase were recruited from the general community in response to local advertisements and were not under the care of a respiratory physician. A total of 35 subjects were randomised to receive either H-IMT or sham IMT (S-IMT).
A prospective, double-blind, randomised, controlled design was used with a treatment group undergoing 8 weeks of H-IMT and a control group undergoing 8 weeks of S-IMT (fig. 1⇓). Prior to randomisation, all subjects entered a 2-week screening and familiarisation phase, during which time baseline measurements were recorded: 1) resting lung function; 2) inspiratory muscle strength and endurance; 3) exercise capacity; 4) dyspnoea; and 5) QoL. To account for any improvements due to familiarisation 26, assessments of inspiratory muscle strength and endurance were performed on four separate occasions (>24 h apart) with the best test result recorded as the baseline measure. Subjects were allocated to H-IMT or S-IMT groups according to a computer-generated, random number sequence with randomisation stratified for sex and severity of airflow obstruction. On completion of the 8-week training period, measurements were repeated in both groups.
Neither the investigator responsible for the collecting the pre- and post-training data nor the subjects were aware of whether an individual had been allocated to the H-IMT or S-IMT group. Blinding of subjects was maintained throughout the study by ensuring H-IMT and S-IMT groups trained at different times. Subject training and assessments were performed by different investigators.
Training device and protocols
Subjects attended supervised training sessions three times per week for 8 weeks. Each session lasted 21 min and comprised seven cycles of 2 min of breathing on an inspiratory threshold loading device (Threshold IMT; Respironics, Cedar Grove, NJ, USA) followed by 1 min of rest. The threshold training device was modified to allow imposition of loads as large as -103 cmH2O (for H-IMT) and as small as -3 cmH2O (for S-IMT; fig. 2⇓). H-IMT was performed at the maximum load tolerable for each 2-min work interval and was progressively increased over the training period 24. S-IMT was prescribed at 10% of baseline maximum peak inspiratory mouth pressure (PI,max), and remained at this level during all training sessions. Throughout training, subjects were permitted to choose their own breathing pattern. Inspiratory mouth pressure was continuously recorded (PowerLab/8sp; ADInstruments Pty Ltd, Castle Hill, NSW, Australia) and the pressure–time integral (area subtended by the pressure–time curve; ∫Pdt) 27 was calculated for every training session in each subject.
Resting lung function
Measurements of the following were obtained: 1) lung volumes (Medgraphics Elite Series DX plethysmograph; Medical Graphics Corporation, St Paul, MN, USA); 2) FEV1, forced vital capacity (digital pneumotachograph, Vertek Series; Hewlett-Packard, Palo Alto, CA, USA); and 3) single-breath diffusing capacity for carbon monoxide (P.K. Morgan Ltd, Chatham, UK).
Inspiratory muscle strength
The PI,max generated against an occluded airway (differential pressure transducer; Honeywell, Morristown, NJ, USA) was measured from functional residual capacity (FRC) 27. Upon each testing occasion, the highest PI,max within 5% of two others was recorded.
Care was taken to ensure that measurements of PI,max were taken at similar end-expiratory lung volumes. In addition to the measurement of FRC before and after the training period (see Resting lung function), end-expiratory lung volumes were monitored via measurements of inspiratory capacity 28 and via respiratory inductance pneumography (Respitrace Corporation, Ardsley, NY, USA) 25, 29 during both the fourth pre-training and the post-training assessment sessions. Only PI,max measures collected during these assessment sessions were included in the analyses.
Inspiratory muscle endurance
Inspiratory muscle endurance was assessed via a progressive loading protocol 30, which required subjects to breathe against threshold loads 31, 32 that were increased each minute by 10% of baseline PI,max until voluntary task failure. Standard verbal encouragement was provided throughout. Inspiratory muscle endurance was defined as the maximum threshold pressure (Pth,max) sustained for a minimum of 30 s 25. Subjects were seated, wore a nose-clip, and were permitted to choose their own breathing pattern. Mouth pressure (differential pressure transducer; Honeywell), inspiratory flow and volume (Fleisch 2 pneumotachograph and differential pressure transducer; Validyne Engineering, Northridge CA, USA) were continually recorded (Powerlab/16s; ADInstruments Pty Ltd). Dyspnoea and respiratory effort scores (modified Borg category ratio scale) were recorded on completion of the test 33.
Whole-body exercise capacity
Exercise capacity was assessed via a cycle ergometry test and 6-min walk test (6MWT). Scores for dyspnoea were recorded every minute throughout both tests. On completion of each test, scores for dyspnoea and leg fatigue were obtained 33.
The symptom-limited cycle ergometry test was performed on an electronically braked bicycle ergometer (ER 900; Jaeger, Hoechberg, Germany). Work rate was increased by 10 W·min-1 until voluntary cessation. Throughout the test, breath-by-breath measurements of ventilation, breathing pattern and gas exchange were collected using an automated exercise metabolic system (Medgraphics CardiO2; Medical Graphics Corporation).
The 6MWT was performed in accordance with current recommendations 34, but modified to include the following throughout: 1) standardised encouragement every 15 s during rest periods to continue walking when able; and 2) monitoring of arterial oxygen saturation by pulse oximetry (Sp,O2; Ohmeda 3700; GE Healthcare, Louisville, CO, USA) and heart rate (Polar a1 heart rate monitor; Polar Electro Oy, Kempele, Finland). On each occasion, two tests were performed, separated by 30 min, with the greatest 6-min walk distance (6MWD) recorded. In the event that Sp,O2decreased to <80% during a test, the investigator instructed the subject to rest until Sp,O2had increased to >80%. The test was then continued. Data from tests obtained in subjects demonstrating arterial oxygen desaturation <80% were excluded from subsequent analyses.
This was assessed via the Chronic Respiratory Disease Questionnaire (CRDQ), which was administered via interview 35. The CRDQ assessed four domains: 1) dyspnoea during activities of daily living (ADL); 2) fatigue; 3) mastery; and 4) emotional function. Subjects were required to answer between four and seven questions (items) within each domain. Overall QoL score was defined as the average score from all items. Domain-specific scores were defined as the average score from all items within that domain. During post-training interviews, subjects were informed of their pre-training responses 36.
Baseline measurements were compared between groups using unpaired t-tests or Chi-squared tests. Pre- and post-training differences between H-IMT and S-IMT groups were analysed using two-way repeated measures of ANOVA. Pre- and post-training differences within H-IMT and S-IMT groups were analysed using paired t-tests. A p-value ≤0.05 was considered significant. Initial analyses for sample size and statistical power were based on the results from the pulmonary rehabilitation programme conducted at Sir Charles Gairdner Hospital (Nedlands, WA, Australia), pilot studies and data available in the literature.
In total, 35 subjects were randomised to H-IMT or S-IMT. Two subjects (allocated to the H-IMT group) withdrew during the study (one due to prolonged intercurrent illness and one to transportation difficulties). Results are presented for the 33 subjects (22 males) who completed the study.
There were no significant differences between the H-IMT and S-IMT groups in baseline measures of resting lung function, inspiratory muscle function, exercise capacity, dyspnoea and QoL (tables 1⇓–⇓3⇓).
With H-IMT and S-IMT groups considered together, during the screening and familiarisation phase, PI,max and Pth,max increased by 22% (p<0.001) and 56% (p<0.001), respectively. PI,max expressed as a percentage of the predicted normal value 41 increased from 65±20 to 81±26% (p<0.001). Pth,max expressed as a proportion of PI,max (Pth,max/PI,max) increased from 49±22 to 61±17% (p = 0.001).
Over the 8-week period, the training load for the H-IMT group increased from 45±13% (session 1) to 101±26% (session 24) of baseline PI,max (p<0.001; fig. 3⇓). When expressed in terms of the ∫Pdt during each training session, training load increased from 8,265±3,218 to 11,432±5,291 cmH2O×s (p = 0.037).
In the S-IMT group, the inspiratory load remained unchanged over the 8-week training period, being 14±4% (session 1) and 11±3% (session 24) of baseline PI,max. The ∫Pdt during the S-IMT decreased over the training period from 2,420±786 to 2,017±624 cmH2O·s (p = 0.035).
Resting lung function
With the exception of a 90-mL decrease in FEV1 in the S-IMT group (p = 0.023), no changes were observed in any measure of pulmonary function. Inspiratory capacity was unchanged over the 8-week training period in both groups.
Inspiratory muscle strength and endurance
In the H-IMT group, PI,max and Pth,max increased by 29% (p<0.001) and 56% (p<0.001), respectively. Pth,max/PI,max increased from 63 to 75% (p = 0.01). Following S-IMT, PI,max increased by 8% (p = 0.023), while Pth,max and Pth,max/PI,max remained unchanged.
Notably, the magnitude of the observed increases in PI,max (p = 0.002), Pth,max (p<0.001) and Pth,max/PI,max (p = 0.012) were greater in the H-IMT group when compared with changes seen following S-IMT.
The maximum work rate achieved during cycle ergometry testing remained unchanged in both groups. No between-group differences were observed in any cardiopulmonary measure collected at maximum work rate.
Sub-maximal cardiopulmonary responses were compared between pre- and post-training cycle ergometry tests at 50% of the maximum work rate achieved during the baseline assessment. No significant differences between the groups were observed in any cardiopulmonary measure.
Data from three subjects randomised to the H-IMT group and one to the S-IMT group were excluded from the 6MWD analyses due to arterial oxygen desaturation. In the remaining subjects, 6MWD increased by 27 m in the H-IMT group (p<0.001) and was unchanged in the S-IMT group. Compared with S-IMT, the magnitude of the change in 6MWD was greater following H-IMT (p = 0.018).
Overall QoL scores improved by 0.8 and 0.4 points per item in the H-IMT group and the S-IMT group, respectively (table 2⇑). The magnitude of change was not different between the groups. Following H-IMT, improvements were seen in all four domains of the CRDQ (p<0.02). Following S-IMT, improvements were seen in dyspnoea during ADL (p<0.001) and mastery (p = 0.043). Compared with S-IMT, the magnitude of the improvement in dyspnoea during ADL (p = 0.026) and fatigue (p = 0.01) was greater following H-IMT.
The present study has demonstrated that in subjects with moderate-to-severe COPD, 8 weeks of interval-based H-IMT yields significant improvements in inspiratory muscle strength, inspiratory muscle endurance, dyspnoea during ADL and fatigue, with modest gains in functional exercise capacity. When compared with previous studies of IMT in COPD, this study has employed a novel, particularly rigorous methodology. First, the study was double-blind, adequately powered to detect changes in inspiratory muscle function, dyspnoea and 6MWD, with well-matched treatment and control groups. Secondly, great care was taken to ensure familiarisation with assessments of inspiratory muscle function and the 6MWT prior to initiating training. Thirdly, the training programme was fully supervised, thereby ensuring compliance and employed regular increases in training load. Fourthly, the length of each training session (21 min), and frequency and duration of the training programme (three times per week for 8 weeks) falls within the current recommendations for outpatient whole-body exercise training programmes 11. Therefore, it is more likely to be clinically feasible than other IMT protocols that have required prolonged training periods. Finally, outcome measures were selected that were both task specific (inspiratory muscle function) and non-task specific (exercise capacity and QoL), laboratory based (cycle ergometry testing) and field based (6MWT).
An interval-based high-intensity protocol was used to maximise any potential training-related gains in inspiratory muscle strength and endurance. Consistent with the current authors’ previous experience 24, H-IMT was well tolerated, with subjects achieving the highest training loads reported in any COPD population to date (101% of baseline PI,max). The current authors attribute this to the characteristics of an interval-based training programme, with the frequent rest periods permitting relief of dyspnoea and local muscle fatigue, thereby allowing achievement of greater loads than would be possible when breathing against constant inspiratory loads with a continuous-based training protocol 24.
Training intensity, expressed as a percentage of baseline PI,max, improved by 56% over the 8-week training period. The increase in ∫Pdt, a measure of inspiratory muscle work independent of breathing pattern, was more modest (38%; fig. 3⇑), suggesting that subjects altered their breathing pattern in a manner which lessened inspiratory muscle work. However, such a strategy could only partially compensate for the training load, which systematically increased over the 8-week period. Subjects undergoing S-IMT also appeared to adopt a strategy of manipulating breathing pattern to minimise inspiratory muscle work, as evidenced by a decrease in ∫Pdt despite no change in training intensity.
Improvements in inspiratory muscle function
PI,max increased by 29% and Pth,max by 56% following 8 weeks of H-IMT. The maximum load achieved during the progressive loading test, when expressed as a percentage of the maximum inspiratory pressure (Pth,max/PI,max), also increased, suggesting that improvements in Pth,max following H-IMT were not solely the result of gains in inspiratory muscle strength. These improvements in inspiratory muscle strength and endurance demonstrated following H-IMT are notable for three reasons. First, they are of similar or greater magnitude than the increases described in most studies published over the last 15 yrs, which have reported significant increases in inspiratory muscle strength (measured as PI,max 1–6, 8, 9) and inspiratory muscle endurance (measured using progressive loading protocols 4, 8, 9, 17, 42, 43). Secondly, the improvements in the current study were observed following a rigorous pre-training familiarisation period, during which time inspiratory muscle function improved substantially. It is possible that these improvements during the screening and familiarisation phase were the result of training-induced increases in inherent muscle contractility. However, the current authors believe this to be unlikely because changes that characterise neuromuscular adaptation with training, such as an increased number of motor units firing, appear to require a more intense training regimen over a longer period of time, being weeks rather than days 25, 44. Muscle hypertrophy requires even longer training periods 44. A more likely mechanism is neurosensory conditioning resulting from repeated exposures to the same task 25, 45. Regardless of the precise mechanism, the current study demonstrated improvements in inspiratory muscle strength and endurance following H-IMT over and above any changes resulting from familiarisation. Thirdly, the improvements were achieved over a relatively brief total training period (see Training efficiency; fig. 4⇓). Although the present study did not examine the length of time these gains were maintained after completing the H-IMT programme, a recent study has shown that training-related improvements in inspiratory muscle strength and endurance gradually decline over a 12-month period without adherence to a maintenance programme 46.
Only three previous studies have reported substantially greater increases in PI,max than observed in the current study (>40% improvement from baseline measures). However, each provided less rigorous familiarisation procedures for their assessments than the present study 17, 18, 42; thus, some of the improvement ascribed to the training programmes may be attributable to familiarisation with the assessment procedures. Only one study has reported a substantially greater improvement in Pth,max (143% improvement from baseline measures) 2, a finding which may be explained by differences in assessment protocol (e.g. increases in threshold pressure every 15 s versus 1 min in the present study) and possibly incomplete familiarisation to the task prior to initiating training.
The interval-based nature of the H-IMT programme optimised the time spent breathing against an inspiratory load for any given increase in PI,max (i.e. training efficiency). Figure 4⇑ compares training efficiency of the current H-IMT programme to that calculated for previous randomised controlled trials of IMT in COPD, which have reported significant improvements in inspiratory muscle strength above baseline measures 1–6, 8, 9, 16–18, 21, 42, 47. The protocol prescribed in the present study resulted in a mean gain of 3.2 cmH2O in PI,max for every hour spent training (i.e. time spent loaded breathing). The two studies with the closest training efficiencies also employed interval-based training protocols 2, 8.
Improvements in QoL, dyspnoea, fatigue and exercise capacity
Overall QoL increased in both groups, largely due to improvements in dyspnoea during ADL. The increases of 1.4 points per item (95% confidence interval (CI) 1.9–1.0) in dyspnoea during ADL and 0.9 points per item (95% CI 1.2–0.6) in fatigue following H-IMT exceed the threshold for a clinically meaningful improvement (0.5 points per item) 48. The improvement in dyspnoea during ADL is comparable with gains reported by others after 4 6 and 6 months of IMT 17. While of smaller magnitude than the H-IMT group, significant improvements in overall QoL and dyspnoea during ADL were also seen following 8 weeks of S-IMT. Although it is possible that these gains are related to the modest increase in inspiratory muscle strength following S-IMT, the absence of change in any other physiological measure of inspiratory muscle function or exercise capacity suggests that the improvement in questionnaire-based assessments is probably due to acquiescence bias 49, “Hawthorne” or “Rosenthal” effects 50. This finding highlights the importance of a control group when employing questionnaire-based outcome measures.
Relative to the changes seen in the S-IMT group, both dyspnoea during ADL and fatigue improved a further 0.6 points per item following H-IMT, a difference that exceeds the threshold for a clinically meaningful improvement 48. It is likely that the improvement in dyspnoea during ADL following H-IMT is related to greater inspiratory muscle strength, with less dyspnoea resulting from a reduction in respiratory motor output and perception of inspiratory effort 51, 52. Such a mechanism may also play a part in the improvements demonstrated in the CRDQ domain of fatigue.
H-IMT had little effect on whole-body exercise capacity. When compared with S-IMT, no change in any measure collected at maximum or sub-maximum work rates during cycle ergometry testing was demonstrated following H-IMT. Furthermore, the increase in 6MWD following H-IMT was modest (27 m), and although statistically significant may not be clinically meaningful 53. These findings are consistent with the results of previous randomised controlled trials of IMT in COPD, which, despite reporting gains in inspiratory muscle function 1–3, 15, 17, 18, 43, 54 and changes in external intercostal muscle morphology 8, have generally failed to demonstrate substantial improvements in exercise capacity measured via incremental cycle ergometry tests 3, 8, 17, 54, time to exhaustion exercising at a constant sub-maximal workload 43, 54, or field walking tests 1, 2, 8, 15, 18, 54. Furthermore, the observation that subjects undergoing H-IMT achieved maximum ventilation during the cycle ergometry test equal to 97.5±14.8% of their estimated maximum voluntary ventilation does not support the contention that COPD subjects with a ventilatory limitation to exercise are more likely to demonstrate large improvements in exercise capacity following IMT 7.
Conclusions, clinical implications and future directions
The present authors have demonstrated that interval-based, progressively increased, fully supervised H-IMT allows subjects with moderate-to-severe COPD to achieve very high training loads, yielding large improvements in inspiratory muscle strength and endurance over a relatively brief total training period. Despite the significant improvement in inspiratory muscle function, it appears unlikely that IMT alone, even when optimised and applied to subjects with a ventilatory limitation during exercise, is capable of yielding meaningful improvements in exercise capacity in COPD.
However, high-intensity inspiratory muscle training does result in significant and clinically meaningful improvements in dyspnoea during activities of daily living and fatigue. Therefore, it may be of particular benefit to chronic obstructive pulmonary disease patients who report dyspnoea during activities of daily living and/or fatigue, but are unable to effectively participate in whole-body exercise training because of comorbid conditions, such as musculo-skeletal impairments. Further study is required to investigate the effects of combining high-intensity inspiratory muscle training and whole-body exercise training on dyspnoea during activities of daily living and fatigue.
The authors would like to thank all subjects for their participation. They would also like to thank M. Phillips, R. Gupta, T. van der Touw, G. Sturdy, E. Gabbay, P. Winship, the Dept of Respiratory Medicine and the Dept of Physiotherapy, Royal Perth Hospital (Perth, Western Australia) for their assistance.
- Received September 8, 2005.
- Accepted February 5, 2006.
- © ERS Journals Ltd