Material and methods
Study design and sample size
A cross-sectional study was conducted between March 2020 and July 2021.
For this study, the methodology and sample size were defined following Consensus-based Standards for the selection of health status Measurement Instruments (COSMIN) guidelines.20 Moreover, validity and reliability were also defined according to COSMIN recommendations. Validity is commonly defined as the extent to which the test can measure the concept it was designed to measure, that is, if it relates to the gold-standard measure (criterion validity) or other measures that assess the same construct (hypotheses testing for construct validity).19 21 Based on hypotheses testing for construct validity, a comparison with another outcome measurement instrument was assessed (convergent validity) in this study, by analysing the correlation between the number of steps taken in the IST and the 6 min walk distance (6MWD), and the number of repetitions in the 1 min sit-to-stand (1MSTS) test. Due to restrictions imposed by the COVID-19 pandemic, we were unable to conduct our original study, namely the comparison of the IST with the CPET, the gold standard for exercise capacity,10 to determine its criterion validity, and with the ISWT to determine its convergent validity. As an alternative, we chose to use the 6MWT, which is a valid and reliable test of exercise capacity for people with chronic lung disease, due to its strong correlations with measures of peak work capacity on a CPET.8 It is also considered the most widely used field walking test for the assessment of outpatients with COPD.9 The 1MSTS was also chosen because it is reliable, valid and responsive, and showed a comparable end-exercise cardiorespiratory response to the 6MWT22 and can induce a similar cardiorespiratory stress to that of CPET.23
Reliability refers to the consistency of a measure and its ability to replicate the score from one assessment or rater to another.19 24 Measurement error was also considered for reliability, and it is defined as the systematic and random error of a participant’s performance that is not attributed to true changes in the construct to be measured.19
A minimum of 50 participants were aimed to be included in this study since this is the sample size suggested by the COSMIN guidelines to determine the construct validity and reliability of measurement instruments with adequate methodological quality.19 20
Participants
Patients with COPD were recruited by pulmonologists from two hospitals and two clinics in Portugal. Pulmonologists identified potential participants and ensured the fulfilment of the eligibility criteria. Patients were considered eligible if they had an established diagnosis of COPD based on the Global Initiative for Chronic Obstructive Lung Disease (GOLD) criteria—postbronchodilator forced expiratory volume in 1 s (FEV1)/forced vital capacity ratio <70%,4 were clinically stable over the past month (ie, no hospital admissions or exacerbations), and the presence of an ECG record at rest with no significant changes. Patients were excluded if they had other lung diseases, presence of a significant cardiovascular (eg, symptomatic ischaemic cardiac disease), neurological (eg, neuromuscular dystrophy disease), musculoskeletal disease, signs of cognitive impairment or significant risk of fall.
Participants who agreed to participate were contacted by researchers to schedule the appointments for assessment sessions at their homes to provide more information about the study and collect data.
Data collection
Participants recruited by pulmonologists were asked to accept two visits for the assessment sessions, performed by one physiotherapist, at their homes, within 5–7 days apart. Lung function tests (spirometry)25 were collected from all participants and, according to the GOLD guidelines, the airflow limitation of COPD (GOLD I, II, III, IV) of each participant was classified according to the FEV1(%) values.4 Of these participants, those who performed the 6MWT, according to the American Thoracic Society/European Respiratory Society guidelines,9 over the last month were also identified and the respective report (with the main outcome: total distance—6MWD) was also collected.
During the first home visit, sociodemographic (age, sex) and clinical data (medication, comorbidities, smoking status, long-term oxygen, non-invasive ventilation, number of exacerbations, unscheduled consultations, emergency department admission and hospitalisations in the previous year, vital signs, peripheral oxygen saturation—%SpO2, fatigue and dyspnoea at rest with the modified Borg scale—mBorg) were collected. Anthropometric data (height, weight and body mass index) were collected using a measuring tape and bioelectrical impedance measure—Tanita BC-545 N (Tanita, Amsterdam, The Netherlands). Then, participants performed the 1MSTS once, after a training attempt, with a 5 min period of rest. The first IST (IST-1) was performed after another resting period of at least 20 min to allow for recovery of participant’s vital signs, fatigue, and dyspnoea to their baseline values. During this resting period, patient-reported outcomes measures were collected, namely the Modified Medical Research Council (mMRC)26 and the COPD Assessment Test (CAT).27 28
During the second home visit, 5–7 days afterwards, the vital signs, %SpO2, fatigue and dyspnoea (mBorg) at rest were collected, and the second IST (IST-2) was performed. The test conditions were similar to the first visit for IST measurements (eg, environment, instructions, same platform to perform the test) and COPD disease stability in participants was also guaranteed.
Incremental step test
IST was designed to provide an incremental profile by using a digital recording with timed metronome step cadence, and with a 20 cm tall platform (Max Aerobic step, Mambo, Tisselt, Belgium). The number of levels, and duration of each level (increment) were based on the characteristics of the ISWT.13 The original protocol of the ISWT consists of 12 levels; however, as suggested by the literature, we can add more levels to the protocol (total of 15 levels) to allow its future application for other clinical populations, in order to prevent the ceiling effect.29 Therefore, IST consists in 15 levels, each of 1 min duration. The timed metronome set the step cadence which starts at 10 steps/min and increases 2 steps/min every 1 min, with a step cadence maximum of 38 steps/min (level 15). The maximum test duration is 15 min. Heart rate (HR) and SpO2 (%) was monitored and registered during the test with a pulse oximeter (PalmSAT 2500 Series, Nonin Medical, Minnesota, USA). The perceived dyspnoea and leg fatigue during the test were also registered with the mBorg scale. The blood pressure was not assessed due to the difficulty of measuring during the stepping.
The criteria to stop the test were: not able to maintain the required step cadence for 10 s, SpO2 falls to ≤85%, when requested by the participant, or when symptoms were reported (chest pain, intolerable dyspnoea, leg cramps, diaphoresis and a pale or ashen appearance). The main outcome measure of the IST was the total number of steps performed. Maximal step cadence reached, and duration of the test were also collected.
The instructions to perform the IST and a reporting form are available as online supplemental material.
6 min walk test
6MWT was performed according to the American Thoracic Society/European Respiratory Society guidelines.9 The 6MWT is a valid test in people with COPD (moderate to strong correlation with maximum oxygen uptake and peak work on CPET, r=0.40 to 0.93).8 The 6MWD was the main outcome. HR, SpO2(%), perceived dyspnoea and leg fatigue (mBorg scale) were monitored during the test.
1 min sit-to-stand
1MSTS was performed on a normal chair available at the participant’s home. Standardised instructions and encouragement were used according to Vaidya et al.30 This test is a valid in people with COPD (moderate to strong correlations with peak cycling work capacity and one-repetition maximum, r=0.36–0.63, p<0.05; and positive and strong correlation with the 6MWT, r=0.57 to 0.72, p<0.05).22 23 30–32 The main outcome measure of the 1MSTS was the total number of repetitions performed. HR, SpO2(%), perceived dyspnoea and leg fatigue (mBorg scale) were monitored during the test.
Patient-reported outcomes measures
The mMRC26 and the CAT27 28 were used to assess dyspnoea and the impact of the COPD disease, respectively. The mMRC is a 5-point scale with scores ranging between 0 and 4, where higher scores indicate greater dyspnoea severity. Whereas, the CAT is an 8-item scale developed to assess the impact of COPD through symptoms in patients’ life (cough, sputum, chest tightness, dyspnoea during stair climbing, limitations on home daily activities, confidence to live home, sleep, and energy). Scores range from 0 to 40 and higher scores indicate greater impact of the disease on the patients’ life. The Portuguese versions of the tests are available through the Directorate-General of Health of Portugal website.33
The application of these two instruments, along with the collected information of the number of exacerbations, non-programmed consultations, emergency admission and hospitalisations in the previous year, allowed the application of the GOLD ABCD assessment tool and classification of participants for the assessment of symptoms and risk of exacerbation, according to the GOLD guidelines.4
Data analysis
Data analysis was performed using IBM SPSS Statistics V.27.0 (IBM). The level of significance was set at 0.05. Continuous variables were tested for normality with the Kolmogorov-Smirnov and Shapiro-Wilk tests. Descriptive statistics were used, and data are presented as mean±SD, median (percentile 25–75) or frequencies (percentage).
For the assessment of validity, the construct validity34 35 was analysed through the correlation between the number of steps in the best IST and the 6 min walk distance (6MWD), and the number of repetitions in the 1MSTS, using the Spearman correlation coefficient. According to COSMIN recommendations, a ‘positive’ rating to qualify construct validity is determined if the correlation coefficient is equal to or above 0.5.19 In addition, the strength of correlations was classified according to British Medical Journal guidelines: significant correlation coefficients of 0–0.19 as very weak, 0.2–0.39 as weak, 0.4–0.59 as moderate, 0.6 0.79 as strong and 0.8–1.0 as very strong.36
Reliability was determined by intraclass correlation coefficient (ICC) model 2 (two-way random effects), absolute agreement, with a single rater (ICC2,1), and with 95% CI.37 According to COSMIN recommendations, a ‘positive’ rating to qualify reliability is determined if the ICC value is above 0.70.19 Measurement error was determined calculating the SE of measurement (SEM) and the minimal detectable change at 95% CI (MDC95).24 The SEM was measured according to the following equation:
where SD is the SD of the performances obtained from all participants (IST-1 and IST-2). The %SEM was calculated as:
where ‘mean’ is the mean of the performances obtained in IST-1 and IST-2. The MDC95 was calculated as follows:
The %MDC95 was calculated as:
where ‘mean’ is the mean of the performances obtained in in IST-1 and IST-2. A %MDC95 of less than 30% was considered acceptable.38
The learning effect was explored using Wilcoxon signed-rank test or paired t-test to compare the performance (number of steps) between the two attempts of the IST. The same tests were used to compare other variable performances (duration, step cadence reached) and physiological response (HR, %SpO2, dyspnoea and leg fatigue) between the IST-1 and IST-2. The same test was used to compare the HR, %SpO2, dyspnoea and leg fatigue before and after the completion of the tests.