Reproducibility and minimal detectable change of sustained maximal inspiratory pressure in healthy adults

Abstract

[Purpose] This study aimed to verify the reproducibility and minimal detectable change of sustained maximal inspiratory pressure measured using the test of incremental respiratory endurance, establishing it as an index of respiratory muscle function. [Participants and Methods] Thirty healthy young adults (mean age: 24.8 ± 2.7 years) participated. Sustained maximal inspiratory pressure was measured two or three times using the test of incremental respiratory endurance. Its relative and absolute reliabilities were assessed using the intraclass correlation coefficient and Bland–Altman analysis. The minimal detectable change and percentage of minimal detectable change were also calculated. [Results] The intraclass correlation coefficients for sustained maximal inspiratory pressure were 0.937 and 0.969, respectively, indicating high reliability across both re-measurement criteria. Bland–Altman analysis showed no systematic bias. Minimal detectable change values were 124.7 and 88.4 pressure time units, with corresponding percentage of minimal detectable change values of 23.8% and 16.9%, respectively. [Conclusion] Sustained maximal inspiratory pressure demonstrated high reliability and no systematic bias, suggesting its potential as a valid measure of respiratory muscle endurance. Using this measure as a reference for repeated measurements may enhance accuracy. Future research should assess its applicability in older adults and clinical populations.

INTRODUCTION

In recent years, population aging has become a growing concern as sarcopenia, characterized by physical dysfunction and the loss of limb skeletal muscle mass and strength associated with aging, contributed to the decline in activities of daily living and affects long-term prognosis. Similar changes have also been reported in respiratory muscles¹⁾. However, muscle-specific changes in morphology in early aging may differ between locomotor and respiratory muscles²⁾, highlighting the need to examine respiratory muscles separately from limb skeletal muscles.

Conventionally, respiratory muscle function is assessed using maximum inspiratory pressure (MIP), maximum expiratory pressure, and peak expiratory flow, which serve as indices of respiratory muscle strength^3,4,5,6,7). However, as many life activities require sustained effort, it is essential to evaluate both endurance and instantaneous strength. Given that MIP, maximum expiratory pressure, and peak expiratory flow primarily assess instantaneous muscle strength, they are unsuitable for assessing muscle endurance. Several methods exist for evaluating respiratory muscle endurance, including the relative value of peak inspiratory pressure obtained from the incremental threshold loading divided by MIP, maximal voluntary ventilation, and sustained maximal inspiratory pressure (SMIP) obtained from the test of incremental respiratory endurance (TIRE)^8,9,10,11). The intermittent inspiratory load titration method requires specialized equipment and can be physically demanding due to the prolonged loading phase. Moreover, as maximal voluntary ventilation is influenced by airway resistance, some studies have reported that it may not be effective for measuring inspiratory muscle endurance in patients with chronic obstructive pulmonary disease¹²⁾. In contrast, TIRE offers a cost-effective, less physically demanding alternative to incremental threshold loading. Additionally, TIRE is a measurement method that requires a maximal, sustained inspiratory effort beginning at maximal expiration, thereby allowing assessment of muscular endurance as well as instantaneous muscle strength^{13, 14)}. Respiratory muscle endurance in chronic obstructive pulmonary disease has been shown to predict prognosis, and studies conducted in the United States have demonstrated the utility of SMIP in assessing respiratory muscle function in chronic obstructive pulmonary disease patients¹⁴⁾. SMIP, which incorporates a temporal element and reflects endurance, has been reported to be more effective at identifying respiratory muscle dysfunction than traditional MIP, which only reflects instantaneous inspiratory muscle strength. Additionally, SMIP has shown a stronger correlation with peak oxygen uptake^{15, 16)}. These findings suggest that endurance should be considered alongside instantaneous muscle strength, reinforcing the growing recognition of respiratory muscle endurance assessment. However, SMIP has not yet been studied in Japan. Therefore, the purpose of this study was to verify the reproducibility and minimal detectable change of SMIP to establish its utility as a measure of respiratory muscle function.

PARTICIPANTS AND METHODS

The study included 30 healthy Japanese young adults aged 21–30 years; all participants were non-smokers with no history of respiratory or neuromuscular disease. Their mean age, height, and weight were 24.8 ± 2.7 years, 165.0 ± 7.4 cm, and 59.6 ± 9.1 kg, respectively. Participant characteristics are shown in Table 1. Written informed consent was obtained from all the participants. The ethics committees of Kawasaki Medical School and Kawasaki Medical School Hospital approved all procedures (approval number: 5232-03).

Table 1. Participant characteristics.

	Volunteers (n=30)
Age [years]	24.8 ± 2.7
Sex (males/females) [n]	15/15
Height [cm]	165.0 ± 7.4
Weight [kg]	59.6 ± 9.1
BMI [kg/m2]	21.9 ± 3.0
%VC [%]	93.7 ± 10.9
FEV1.0% [%]	88.8 ± 6.0
MIP [cmH2O]	89.7 ± 21.4

Mean ± SD.

BMI: body mass index; VC: vital capacity; %VC: percent vital capacity; FEV1.0%: forced expiratory volume in one second divided by forced vital capacity; MIP: maximal inspiratory pressure.

SMIP was measured using TIRE with a PrO2 Fit device (PrO2 Health Incorporated, Smithfield, RI, USA). The PrO2 Fit device is equipped with a real-time pressure–time graph that enables instant monitoring of inspiratory pressure¹⁷⁾. Furthermore, similar to general respiratory muscle pressure-measurement devices, it incorporates a fixed 2-mm leak to prevent artifacts and has an extensive track record as a reliable tool for measuring respiratory muscle strength^{14, 15, 17,18,19)}. The TIRE measurement method was explained using photographs before participants were instructed to perform maximal and sustained inspiratory efforts following full expiration in a sitting position. The TIRE automatically terminated when inspiratory pressure returned to baseline (0 cmH₂O) following inspiration. Measurements were conducted twice at a one-minute interval, with visual feedback provided using the actual measurement waveform. If MIP differed by more than 20% between the two measurements, a third measurement was conducted³⁾. SMIP was recorded in pressure time units (PTU), representing the area under the curve from the start to the end of inspiration (Fig. 1). The relative and absolute reliabilities of SMIP were examined under two condition. The first condition, SMIP_MIP, was defined as cases where the difference in MIP was within 20%, a threshold chosen in accordance with the ATS/ERS statement on respiratory muscle testing³⁾. The second condition, SMIP₂₀, was defined as cases where the difference in SMIP was within 20%.

Fig. 1.

The Test of Incremental Respiratory Endurance (TIRE) method evaluates respiratory muscle endurance by measuring pressure over time. The x-axis represents time (seconds), while the y-axis represents pressure (cmH₂O). The shaded area under the curve represents SMIP (sustained maximal inspiratory pressure), reflecting the total work performed during the test.

Data normality was confirmed using the Shapiro–Wilk test. Relative reliability was assessed using the intraclass correlation coefficient (ICC) (1, 1) for a one-way random-effects model with a 95% confidence interval (CI). Absolute reliability was evaluated using Bland–Altman analysis to identify the presence or absence of systematic bias, including fixed bias, proportional bias, and minimal detectable change (MDC). Fixed bias was determined based on the 95% CI of the difference between measurements, and proportional bias was assessed using the Pearson product-moment correlation coefficient. If no systematic bias was found, MDC₉₅, the 95% CI of MDC, and %MDC were calculated, where %MDC was derived by dividing MDC₉₅ by the mean value and multiplying by 100. Statistical analysis was performed using SPSS statistics version 24 (IBM Corp., Armonk, NY, USA) for the ICC and the Shapiro–Wilk test, and R version 2.8.1 (The R Foundation of Statistical Computing, Vienna, Austria) for B-A analysis. The significance level was set at <5%.

RESULTS

The relative and absolute reliabilities of the SMIP are presented in Table 2 and Fig. 2. The measured values of SMIP_MIP and SMIP₂₀ were 521.4 ± 176.9 PTU and 519.7 ± 182.3 PTU, respectively, for the first trial, and 527.3 ± 178.0 PTU and 528.0 ± 182.3 PTU, respectively, for the second trial. ICC (1,1) values were 0.937 for SMIP_MIP with a 95% CI of 0.874 to 0.970, and 0.969 for SMIP₂₀ with a 95% CI of 0.936 to 0.986. The lower limits exceeded 0.8 and 0.9, respectively. Bland–Altman analysis confirmed the absence of fixed or proportional bias for both SMIP_MIP and SMIP₂₀. MDC₉₅ values were 124.7 PTU and 88.4 PTU for SMIP_MIP and SMIP₂₀, respectively, with corresponding %MDC values of 23.8% and 16.9%.

Table 2. Relative and absolute reliability of SMIP.

	SMIP [PTU] 1st, 2nd	ICC (95% CI)	Fixed bias		Proportional bias		MDC95 (%MDC)
			95% CI	existence	p-value	existence
SMIPMIP	521.4 ± 176.9	0.937	−29.62 to 17.88	No	0.93	No	124.7
(n=30)	527.3 ± 178.0	(0.874 to 0.970)					(23.8%)
SMIP20	519.7 ± 182.3	0.969	−25.71 to 9.26	No	0.99	No	88.4
(n=28)	528.0 ± 182.3	(0.936 to 0.986)					(16.9%)

Mean ± SD.

SMIP: sustained maximal inspiratory pressure; PTU: pressure time units; ICC: intraclass correlation coefficient; 95% CI: 95% confidence interval; MDC₉₅: 95% minimal detectable change; %MDC: percentage minimal detectable change; MIP: maximal inspiratory pressure; SMIP_MIP: Cases in which the difference in maximal inspiratory pressure between trials is within 20%; SMIP₂₀: Cases in which the difference in sustained maximal inspiratory pressure between trials is within 20%.

Fig. 2.

Bland–Altman plots of sustained maximal inspiratory pressure (SMIP) measured by the Test of Incremental Respiratory Endurance (TIRE): (A) Measurement based on MIP (SMIP_MIP), (B) Measurement based on SMIP (SMIP₂₀). The central line represents the mean difference between the two trials. PTU: pressure time units. SMIP_MIP: Cases in which the difference in MIP between trials is within 20%. SMIP₂₀: Cases in which the difference in SMIP between trials is within 20%.

DISCUSSION

This study verified the measurement accuracy of SMIP using TIRE and examined variability in MDC₉₅ based on different measurement conditions. According to Formiga et al.¹⁵⁾, test-retest reliability of SMIP was evaluated using three to five consecutive trials with a 30- to 60 min interval, adopting the maximum value. The results yielded an ICC greater than 0.9, indicating high reliability. In the present study, measurement accuracy was evaluated in consecutive trials. The ICC, which is considered desirable at 0.8 or higher²⁰⁾, was found to be ≥0.9, with the lower limit also exceeding 0.8, both indicating high reliability. Furthermore, Bland–Altman analysis showed that the 95% CI included zero, and no significant correlation was observed. These findings suggest the absence of systematic bias that could affect evaluation validity. High reliability in SMIP measurement was demonstrated, similar to that observed in MIP measurements using the method proposed by the ATS/ERS³⁾, where two measurements were performed, and remeasurement was conducted when a difference of more than 20% was observed. It is important to note that this method was originally designed for MIP, considering MIP as the standard for remeasurement. However, in the case of TIRE, using SMIP the primary index as the standard for remeasurement may enable more accurate measurement. The ICC was as high as 0.969 when SMIP was used as the criterion for repeated measurements. In light of these findings, using SMIP as the standard for repeated measurements may allow for more accurate measurement when evaluating SMIP using TIRE.

A feature of the TIRE is that its sustained inspiration through a fixed 2-mm leak challenges the inspiratory muscles across their full volume range, thereby yielding a comprehensive pressure-time integral (i.e., SMIP). Although this design necessarily allows a gradual increase in lung volume, the volume drift does not compromise the validity of the SMIP as a surrogate measure of inspiratory-muscle endurance. As lung volume rises, diaphragm shortening and altered thoracic mechanics lower inspiratory pressure. Participants with greater inspiratory-muscle strength and inspiratory-muscle endurance resist this decline, sustaining pressure for longer and producing a higher SMIP; those with lower endurance show a rapid pressure drop and a smaller SMIP. Thus, SMIP reflects inspiratory-muscle performance—rather than lung volume—by quantifying the ability to generate and maintain pressure throughout the breath. Furthermore, a randomized controlled trial has shown that increases in SMIP following inspiratory-muscle training occur without concomitant changes in vital capacity or total lung capacity, lending further support to this concept²¹⁾.

When applying this evaluation index as an outcome measure in research or clinical applications, it is essential to assess its effectiveness while considering random errors in pre- and post-intervention comparisons and effect assessments. In this study, MDC₉₅ was used to quantify random error. MDC₉₅ represents the threshold beyond which changes between two measurements can be attributed to factors other than measurement error. If a change remains within the MDC₉₅ range, it is considered indistinguishable from measurement error. Conversely, a change exceeding this threshold suggests a “true change”. However, MDC₉₅ is population-specific, and measurement values may vary significantly among individuals, limiting its generalizability. In contrast, %MDC accounts for individual differences, making it more widely applicable. In this study, %MDC of SMIP was 16.9% when used as a reference for retesting. Based on these findings, when using SMIP as a dependent variable for comparisons, any observed difference greater than 88.4 PTU or a relative change exceeding 16.9% between two measurements whether pre- or post-intervention or between groups can be considered beyond measurement error and attributed to other factors. This threshold may be useful in future studies for determining clinically meaningful changes in SMIP.

This study has certain limitations. The participants were young and healthy adults, making it unclear whether these findings can be generalized to older adults or patients with disease. Additionally, many unknown factors may influence this index, warranting further investigation of related factors for future clinical applications. Future research should explore related physiological and methodological factors that may impact SMIP measurements, particularly in diverse populations.

Funding

This study was supported by JSPS KAKENHI (grant number 21K17540).

Conflict of interest

The authors have no relevant financial or non-financial interests to disclose.