Changes in Diaphragm Thickness and 6-min Walking Distance Improvement After Inspiratory Muscle Training in Patients With Chronic Obstructive Pulmonary Disease

Background: The Global Initiative for Chronic Obstructive Lung Disease guidelines (http://www.goldcopd.org, accessed January 16, 2020) reported the use of inspiratory muscle training (IMT) as a part of comprehensive respiratory rehabilitation programs for patients with chronic obstructive pulmonary disease (COPD). However, some studies have indicated that the effect of such training is uncertain. Moreover, it is unclear whether IMT effects are caused by improvement in central or peripheral factors. Few studies examining IMT effects have used new evaluation items. We aimed to clarify the effects of IMT by additionally measuring the airway-occlusion pressure at 0.1 s after the start of inspiratory �ow (P 0.1 ), as an index of respiratory central output, and by evaluating diaphragm movement based on the thickness of the diaphragm muscle (Tdi) using ultrasound. Methods: Thirteen patients with COPD participated in the study. IMT was performed using the POWER breathe® Medic Plus breathing trainer in combination with each participant’s outpatient rehabilitation regimen. Starting at 20% of the maximal inspiratory pressure (PImax) and increasing to 50%, the participants performed 30 IMT repetitions twice a day for 2 months. Respiratory muscle strength, P 0.1 , 6-min walking distance (6MWD), and Tdi were measured before and after IMT. Dyspnea, lower limb fatigue (assessed using the Borg Scale), and respiratory rate (RR) were measured before and after the 6-min walk test (6MWT). Results: PImax and 6MWD signi�cantly increased after training. Tdi at resting inspiration and expiration and maximal inspiration also signi�cantly increased after training. In addition, the Borg Scale scores for dyspnea and leg fatigue and the RR of the 1-min recovery period after the 6MWT signi�cantly decreased. There was no signi�cant difference in P 0.1 . Conclusions: We examined the combined effects of IMT, incorporating the evaluation of P 0.1 and Tdi. We found that the PImax, 6MWD, and Tdi signi�cantly increased, but no signi�cant difference was observed in P 0.1 after training. These results suggest that


Background
Pulmonary rehabilitation is centered on exercise therapy and is a combination of conditioning and daily living training activities.Respiratory muscle training is one of the fundamental disciplines in exercise therapy [1] and includes inspiratory (IMT) and expiratory muscle training; however, IMT is mainly performed [2,3].
IMT is expected to improve respiratory muscle strength and endurance, exercise tolerance, dyspnea, respiratory function, daily living activities, and health-related quality of life (HRQOL) [4].Currently, there are two IMT methods: one using an abdominal pad, and one using speci c equipment.The abdominal pad method, which is well established, involves the placement of a heavy object on the abdomen while the patient is in the supine position to add inspiratory resistance [5,6].There are two equipment methods: the inspiratory resistance load method and the hyperventilation method.The rst method aims to increase the inspiratory muscle strength by contraction of the muscle at high strength and low speed.Devices, such as the P-ex, Threshold-IMT, and POWER breathe, are used in such cases [7].In contrast, the second method aims to improve the endurance of the inspiratory muscle by contraction of the muscle at low intensity and high speed; devices, such as the incentive spirometer and Tri o , are used in such cases.
Recently, a new tapered IMT device has been developed, in which the load pressure gradually decreases with the start of inspiration, while the air ow increases [8].Because the pressure decreases gradually, the sense of effort also decreases.Therefore, the tapered IMT device gradually increases the resistance during the training period, thus, enhancing the strength and endurance of the inspiratory muscle more e ciently compared to a threshold-type device [9].Currently, chronic obstructive pulmonary disease (COPD) is the most common target for respiratory muscle training [10,11]; however, thoracic restrictive disease and neuromuscular diseases, such as muscular dystrophy, have also been reported as targets [12][13][14].The Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines (http://www.goldcopd.org,accessed January 16, 2020) recommended the incorporation of respiratory muscle training as part of a comprehensive respiratory rehabilitation program for COPD.In their metaanalysis of 32 studies including 830 cases of COPD, Gosselink et al. [15] revealed signi cant improvements in the maximal inspiratory pressure (PImax), respiratory muscle endurance, progressive load pressure, exercise tolerance, Borg Scale scores, dyspnea, and HRQOL after IMT.However, the effectiveness of IMT was limited and not as high as that of exercise therapy; nevertheless, IMT was effective in cases where the PImax was ≤ 60 cmH 2 O and the load intensity was ≥ 30% of the PImax [15].
In another meta-analysis of 37 studies including 1,427 cases of COPD, which was conducted by Beaumont et al. [16], dyspnea, inspiratory muscle strength, and 6-min walking distance (6MWD) improved following IMT.However, it was reported that improvements in dyspnea were not related to respiratory muscle strength [16].Langer et al. [17] reported that IMT improved the PImax and endurance in patients with COPD who have low PImax, and that it also reduced diaphragm electromyographic activity and respiratory discomfort.
Although the effect of IMT alone is recognized, some studies have examined the effect of respiratory muscle training in combination with exercise training [1].In 2018, Charususin et al. [18] reported that PImax and respiratory muscle endurance improved after a combination of exercise and IMT.In contrast, other studies have reported no improvement in 6MWD, maximal exercise capacity, shortness of breath, HRQOL, grip strength, or number of steps.Beaumont et al. [19] discovered that, in patients diagnosed with GOLD "III" to "IV" COPD, only PImax differed between those who underwent respiratory rehabilitation and those who underwent IMT plus respiratory rehabilitation.Schultz et al. [20] compared respiratory rehabilitation with IMT in a cohort of 300 patients with COPD and 302 controls.They reported that only PImax and forced expiratory volume in 1 s (FEV 1.0 ) were signi cantly better in the combination group.
However, no signi cant differences were found in the 6MWD, COPD assessment test (CAT) scores, St.
George's Respiratory Questionnaire scores, and Clinical COPD Questionnaire scores between the two methods [20].
Besides the contradictory results of previous studies regarding the effects of respiratory muscle training in combination with exercise training, it remains unclear whether the IMT effects are attributed to central or peripheral improvement.Previous studies have only used typical parameters, such as respiratory function, respiratory muscle strength, 6MWD, and HRQOL to evaluate the effects of IMT [15,16,[18][19][20].
In this study, we aimed to clarify the effects of IMT and evaluate these effects from multiple perspectives, by using two novel parameters: the airway-occlusion pressure at 0.1 s after the start of inspiratory ow (P 0.1 ), as an index of respiratory central output, and the thickness of the diaphragm muscle (Tdi), as a proxy for diaphragm movement, assessed by ultrasound.
Patients who could walk without using a cane and did not need to inhale oxygen while walking were included.Patients with severe orthopedic disease, cardiovascular disease, central nervous system disorders, and cognitive impairment were excluded.After obtaining approval from our Institutional Ethics Committee (approval no.29 − 10 in 2017), the purpose and content of this study, and the method of personal information management, were explained to all participants.All the participants provided written informed consent.

Procedures
We used a threshold-type IMT device, POWER breathe® Medic Plus (POWER breathe International Ltd., Southam, United Kingdom) (Fig. 2), for IMT in combination with the outpatient rehabilitation regimens.
PImax measurements were performed twice, starting with a load of 20% of the highest PImax obtained and subsequently increasing the load to 50%.IMT was performed for 30 repetitions twice a day for 2 months, and was recorded every day for 2 months using a checklist.The respiratory function, respiratory muscle strength, P 0.1 , 6MWD, Tdi, and CAT scores were measured before and after 2 months of IMT (Fig. 3).
The peripheral artery oxygen saturation (SpO 2 ), heart rate (HR), respiratory rate (RR), as well as dyspnea and lower limb fatigue (using the Borg Scale) were measured before and after the 6-min walk test (6MWT).These measurements were also recorded during a 2-min recovery period [21].

Respiratory function
We used an electronic spirometer (AS-507, Minato Medical Science, Osaka, Japan,) to evaluate respiratory function, vital capacity, forced vital capacity (FVC), FEV 1.0 , and FEV 1.0 /FVC ratio (FEV 1.0 %).For respiratory muscle strength, the PImax and maximum expiratory pressure (PEmax) were used to measure the intraoral pressure [22,23].The PImax at residual capacity in the sitting position and the PEmax at whole lung capacity were measured according to the method described by Black and Hyatt [24].The participants were maintained pressure for at least 1.5 s and the maneuver was repeated three times; the respiratory muscle strength was determined as the highest of the three recorded values [25].P 0.1 measurement P 0.1 has been used as an indicator of respiratory center output related to dyspnea [26,27].In our study, an airway occlusion system (model 9326, Hans Rudolph, Shawnee, KS, USA; dead space 48.9 mL) was used to measure P 0.1 (Fig. 4).
Airway pressure was measured using a differential pressure meter (DP-10, Validyne, Northridge, CA, USA), while ventilation parameters were measured using a metabolic gas analyzer (AE-300S, Minato Medical Science).A hot wire ow transducer was connected to the outlet of the airway occlusion system, and a mask (MAS0215, Minato Medical Science) was connected to the mouth port.A differential pressure transducer was connected to the airway occlusion system via a tube (4-mm diameter) to measure the airway pressure.Entrance to the airway obstruction system was manually occluded using a balloon at the end of each expiration and held until the start of inspiration.The P 0.1 measurement was obtained by closing the inhalation port with a balloon shutter at the end of expiration and by measuring the intraoral pressure after 100 ms, when the intraoral pressure became negative at the start of resting inhalation [28].P 0.1 was randomly measured ve times, and the average calculated from four stable measurements was recorded, as previously reported [29].Analog signals from raw ow pressure were downloaded to a personal computer via an analog-to-digital converter (PowerLab 16/30, ADInstruments, Sydney, Australia) at a sampling frequency of 1,000 Hz.These signals were analyzed using commercially available software (Chart 5.3, ADInstruments) to calculate P 0.1 .COPD was characterized by decreased inspiratory muscle strength, and P 0.1 was corrected to PImax (P 0.1 /PImax).

Tdi measurement
The Tdi was measured via a linear probe in B mode using an ultrasonic diagnostic imaging apparatus (ARIETTA Prologue, Hitachi, Ltd., Tokyo, Japan).The measurement position was between the eighth and tenth ribs on the right axillary line.The muscle thickness at the zone of apposition of the diaphragm (the part of the rib diaphragm angle in contact with the chest wall at the origin) was measured by placing a probe coated with an ultrasonic gel on the body surface.The parameters measured were resting inspiration, resting expiration, maximal inspiration, and maximal expiration.

6MWT
The 6MWT is a eld walking test for assessing exercise capacity and is an essential evaluation item for respiratory rehabilitation.The guidelines for the 6MWT were published by the American Thoracic Society in 2002 [30].According to these guidelines, the walkway was used by folding back a at straight course of 30 m, and the participants were instructed to "walk a long distance as fast as possible in 6 min," and a call was made every min.The primary evaluation item of the 6MWT was the 6MWD [30].

Other measurements
Dyspnea and lower limb fatigue were assessed using the Borg Scale; the RR, SpO 2 , and HR were measured before and after the 6MWT and after the 1-and 2-min recovery periods.

Statistical analysis
SPSS version 21.0 (IBM Corporation, Armonk, NY, USA) was used for all statistical analyses.A paired ttest was used to compare the values of each item before and after training and to determine whether there was a correlation between the speci c measurement items.All values are expressed as means ± standard deviations, and statistical signi cance was set at P < .05.

Discussion
In this study, we used the improved version of the POWER breathe model, POWER breathe® MEDIC PLUS, to evaluate the combination of IMT with individual training conducted during outpatient rehabilitation.
PImax and 6MWD signi cantly increased.Regarding the Tdi, resting inspiration, resting expiration, and maximal inspiration also increased signi cantly after training.The Borg Scale scores and RR were signi cantly lower during the 1-min recovery period following the 6MWT.The loading pressure increased from 20-50% of the PImax, and IMT was performed 30 times, twice a day, for 2 months.IMT load pressure was classi ed as follows: low-strength load (low load), < 30% of the PImax; moderate-pressure load (medium load), 30% to < 60% of the PImax; and high-strength pressure load (high load), ≥ 60% of the PImax [35][36][37].For IMT, the optimum load pressure, at which an effect is reliably obtained, is a medium load of ≥ 30% of the PImax [35,38].At present, performing IMT for 15 min twice a day at a load pressure of ≥ 30% of the PImax is a standard practice for patients with COPD [35].However, it was reported that IMT focused on the number of sessions, and not on the duration, and that the maximum inspiratory pressure increased during one session, in which training was performed 30 times [9].Considering previous reports [35,38], in this study we increased the load pressure from 20-50% and asked the participants to perform 30 repetitions twice a day.Interestingly, improvements were also observed in the PImax and 6MWD, similar to the results of the meta-analysis conducted by Beaumont et al. [16].
An ultrasound imaging system has recently been established as a method for evaluating diaphragm movement, in which Tdi can be measured in the B mode [39][40][41].Baria et al. [42] reported that there was no signi cant difference in Tdi between patients with COPD and healthy individuals, and also reported a lower limit of normal muscle thickness at rest (i.e., 1.5 mm) in both healthy individuals and patients with COPD.
Furthermore, it has been reported that Tdi increases with an increase in pulmonary volume in healthy individuals [43,44] and that Tdi decreases signi cantly in diaphragmatic paralysis [45].In our study, the resting muscle thickness was 1.39 mm, which was comparable to that reported in a previous study [42].
Additionally, there was a signi cant increase in resting inspiration, resting expiration, and maximal inspiration after training.This resulted from an increase in FVC and FEV1.0; therefore, in our view, training improved respiratory function and increased Tdi.
In this study, the Tdi was measured using ultrasound before and after IMT, and it was con rmed that IMT affected the respiratory muscle strength, respiratory function, and Tdi.DiNino et al. [46] reported that Tdi assessment could be a criterion for ventilator weaning.Thus, we consider that Tdi evaluation may be used as an index for respiratory rehabilitation in the future.
In an ATS/ERS systematic review [47], there was a moderate to strong correlation (coe cient .38-.85) between the 6MWD and physical activity and a strong correlation between low 6MWD (300-450 m) and high mortality; however, prognosis was poor when the 6MWD was < 300 m.The results of our study demonstrated that 6MWD increased signi cantly after training; although the 6MWD was classi ed as low, an increase from 354 to 384 m was observed.A further increase in 6MWD is desirable.
The CAT is an eight-item questionnaire that can be used to comprehensively evaluate patients' HRQOL, and has a clear and strong correlation with St George's Respiratory Questionnaire, which is widely used in the evaluation of quality of life in clinical trials [34].It has been translated and used in several countries worldwide, including Japan; the same correlation was shown for the Japanese version, which was previously validated [31].There were no signi cant differences in the CAT scores before and after training in our study.These results suggested that although the performance of IMT for 2 months improved the respiratory muscle strength and 6MWD, it did not lead to improvement in HRQOL.
The normal range of P 0.1 is 1-2 cmH 2 O [28, 48], and it increases with increased respiratory central activity.In patients with COPD, the value is much higher than that in healthy individuals [49,50], and increases with COPD severity [51].The P 0.1 /PImax index is obtained by normalizing the P 0.1 based on differences in inspiratory muscle strength among individuals.P 0.1 and P 0.1 /PImax are used as prediction indicators for weaning [52] and are regarded as a more accurate predictor compared to PImax [52].A recent study using P 0.1 for COPD showed that P 0.1 and P 0.1 /PImax signi cantly increased when the severity increased in various relaxation positions [53]; interestingly, we obtained similar results.In our study, there was no signi cant difference in P 0.1 before and after training, but a signi cant decrease in P 0.1 /PImax was observed, re ecting the increase in the PImax.In addition, as no signi cant difference was observed in P 0.1 , we considered that the improvement in respiratory function by IMT was probably attributed to improvements in peripheral rather than in central functions.This was also supported by a previous study using P 0.1 before and after manual chest wall compression in patients with COPD, which showed that VO 2 , VCO 2 , and dyspnea severity signi cantly decreased after chest compression, whereas P 0.1 and P 0.1 /PImax did not change [54].Improvement in respiratory function and dyspnea because of manual chest wall compression may be attributed to peripheral rather than central improvement [54].
Dyspnea improvement is also important in respiratory rehabilitation.The dyspnea sensing mechanism includes not only chemoreceptors and mechanoreceptors, but also motor command [55] and neuromechanical dissociation [56, 57], involving various factors, such as motor outputs and sensory projections in the central nervous system [58].Polkey et al. [59] reported that classic IMT may control muscle and central circuits and is particularly useful for patients with neurological disorders and for those with reduced lung volume because of stroke [59].However, it remains unclear whether dyspnea improvement and IMT effects are central or peripheral.In this study, during the 1-min recovery period following the 6MWT, the Borg Scale scores for dyspnea and lower limb fatigue and the RR signi cantly decreased.Furthermore, a negative correlation was found between the 6MWD and the Borg Scale dyspnea score (after the 6MWT), suggesting that dyspnea is one of the factors that limit performance in the 6MWT.Therefore, dyspnea improvement is necessary for continuous movement.Moreover, the fact that the PImax, 6MWD, and Tdi signi cantly increased while P 0.1 did not change after IMT further suggested that the IMT effects are attributed to improvements in peripheral factors rather than to improvements of central factors.
One of the strengths of this study was the nding that the improvement in respiratory function by IMT was likely to be attributed to peripheral factors.Elucidating whether improvements in dyspnea and respiratory function are attributed to central or peripheral components would contribute to the design of more effective respiratory rehabilitation intervention methods, help determine more accurately the effects of respiratory rehabilitation, and help identify more appropriate candidates for rehabilitation.
This study had several limitations, including the small sample size, outpatient-based IMT, lack of a control group, patient compliance validation, and technical outcome measurements.In a future research, we will further increase the number of participants and use a control group to examine the differences among different IMT devices.

Conclusion
We examined the effects of IMT in combination with individual training and evaluated the P 0.1 and Tdi.
We observed a signi cant increase in PImax and 6MWD, and an improvement in Tdi.However, there was no signi cant difference in P 0.1 , suggesting that the effect of IMT was attributed to the improvement of peripheral factors rather to the improvement of central factors.

Table 1
Comparison of respiratory function before and after inspiratory muscle training

Table 2
Comparison of the respiratory muscle strength before and after inspiratory muscle training

Table 5
Comparison of Borg Scale scores, RR, SpO 2 , and HR during 6MWD before and after IMT ES: effect size; IMT: inspiratory muscle training; SpO 2 : oxygen saturation of peripheral artery; HR: heart rate; RR: respiratory rate; 6MWD: 6-min walking distance