The evaluation of exercise capacity is fundamental in the clinical routine of patients with chronic obstructive pulmonary disease (COPD),1 and field tests are viable, simple, and inexpensive alternatives. The 6-min walk test (6MWT) is one of the most widely used tests2; however, it requires a track of 30 m. In this sense, tests with reduced space requirements, such as the 6-min step test (6MST), may be more applicable in clinical practice and home-based environments.3
The 6MST is a valid4 and reliable test5 that is performed at a self-paced speed.3 To increase safety of the 6MST, criteria based on heart rate (HR) and pulse oxygen saturation (SpO2) for interruption and resumption can be adopted.5,6 A 6MST protocol that did not adhere to these criteria showed a peak oxygen uptake (
O2peak) close to the one in the maximal cardiopulmonary exercise test.7 It is expected that, when using the safety criteria, 6MST physiological responses are submaximal, similar to the 6MWT. However, these responses during the 6MST have been poorly investigated.7
The analysis of physiological responses allows us to identify how and when the disease may interfere with test performance and thus to understand the mechanisms of functional limitations. It has already been demonstrated that oxygen uptake (
O2) in the 6MWT reaches a steady state from the third minute of testing onward,8–10 similar to what occurs in activities of daily living. Since the 6MWT has substantially lower ventilatory requirements than the maximal cardiopulmonary exercise test, its tolerability in adults with chronic respiratory disease and its sensitivity for identifying exercise-induced desaturation are larger.2 Therefore, if the 6MST induces submaximal physiological responses, we may postulate that the 6MST would show similar responses as the 6MWT, with the advantage of being easily applied in environments with limited spaces.
O2 reflects the aerobic capacity of patients, muscular oxygenation variables explain the capacity for oxygen utilization in the peripheral muscles. In cycle ergometer tests, a correlation between these variables has already been demonstrated between
O2peak and the slope of tissue saturation index (TSI),11
O2 and TSI, and deoxyhemoglobin (HHb) and total hemoglobin (THb) during incremental exercise12 and between
O2 and oxyhemoglobin (O2Hb) on recovery.13 In the 6MST, these associations have not been fully investigated. Furthermore, it is not known whether interruptions during the test and COPD severity induce different physiological responses to the 6MST. This would indicate whether the test could be intolerable for some patients and whether the interruptions could interfere in the test physiological measurements. In addition, it is not yet known whether performing a second test after a short period of time could impose a higher physiological overload.
Therefore, the objectives of the present study were (1) to describe the physiological responses to the 6MST in patients with COPD, (2) to investigate whether COPD severity and test interruptions could determine different physiological responses, and (3) to test the reproducibility of 6MST performance.
Patients with COPD enrolled in the Center for Assistance, Education and Research in Pulmonary Rehabilitation (NuReab) of the State University of Santa Catarina (UDESC) participated in the study. The inclusion and exclusion criteria are presented in Table 1. The study was approved by the Human Research Ethics Committee of the UDESC (CAAE: 51369015.5.0000.0118). All participants gave informed consent.
Data collection (from July 2016 to May 2017) was done in 2 days in the morning, and it had to be completed, at most, in 4 days. Pulmonary function, dyspnea, health status, and functional status (day 1) and physiological variables during the 6MST (day 2) were measured.
PULMONARY FUNCTION, DYSPNEA, HEALTH STATUS, AND FUNCTIONAL STATUS
The total body plethysmograph MasterScreen Body (Erich Jaeger) was used to evaluate lung function according to American Thoracic Society/European Respiratory Society guidelines.14,15 The forced expiratory volume in the first second of expiration (FEV1), forced vital capacity (FVC), FEV1/FVC ratio, residual volume (RV), total lung capacity (TLC), and the RV/TLC ratio in percentage (%) and percentage of predicted (%pred) were analyzed. The Brazilian reference values were considered,16,17 and patients were categorized according to the Global Initiative for Chronic Obstructive Lung Disease (GOLD).18
The modified Medical Research Council (mMRC) scale19 was used to assess dyspnea and to compose the COPD multidimensional classification.18 The COPD Assessment Test (CAT) questionnaire20 was used to evaluate the impact of COPD on health status. The functional limitation was assessed by the London Chest Activity of Daily Living (LCADL) scale.21
6-MIN STEP TEST
Two 6MSTs (6MST1 and 6MST2) were performed, with an interval of 30 min, using a 20-cm high step. All patients received standardized instructions before starting the test5 and 6MWT verbal incentive during the 6MST.2 They were instructed to perform the maximum number of steps in 6 min. The 6MST was interrupted when HR exceeded 85% of the predicted maximum HR22 (HRsubmax) or when SpO2 dropped to less than 85% during the test and was resumed when HR reached 10 beats/min below HRsubmax or when SpO2 reached 88% or more.5,6 The patients could also stop the test at any time and resume as soon as they felt able to do so. There was not a limit for the interruptions. The best test result was used for data analysis.
Physiological responses during the 6MSTs were evaluated using a K4b2 gas analyzer (Cosmed) by the breath-by-breath method. A turbine was positioned on a face mask to measure the expired gas, and oxygen and carbon dioxide concentrations were determined by rapid response analyzers.23 The predicted maximal voluntary ventilation (MVVpred) was calculated16 and the variables measured were respiratory rate (RR), tidal volume (VT), minute ventilation (
E), inspiratory volume (Vins),
O2, production of carbon dioxide (
CO2), and ventilatory demand as the ratio between
E and MVV (
The PortaMon near-infrared spectroscopy device (Artinis Medical Systems) was used for peripheral muscle oxygenation measurements during 6MSTs. It was positioned on the vastus lateralis muscle of the dominant lower limb, as recommended by the Surface ElectroMyoGraphy for the Non-Invasive Assessment of Muscles (SENIAM) Project, approximately 10 cm from the knee.13 O2Hb, HHb, THb, and TSI were measured, and the baseline values, except TSI, were converted to zero and adjusted to other minutes.
The mean of the final 15 sec of each minute and the variation (Δ) between minute 6 and baseline values were used for analysis.
Data were analyzed using the SPSS Statistics 20.0 (IBM). Data distribution was checked by the Shapiro-Wilk test. Repeated-measures analysis of variance (ANOVA) (Bonferroni post hoc test) was used to analyze the physiological variables pre-6MST, post-6MST, and at each minute of the 6MST. The mixed-model ANOVA was used to analyze the time versus group interaction of each physiological variable to determine whether the profile was significantly different over the 6MST between patients who interrupted and did not interrupt the test. The intraclass correlation coefficient (ICC) was used to check the reproducibility of the 6MST and its physiological responses, which was and classified as follows: low (ICC< 0.40), moderate (ICC 0.41-0.74), and high (ICC > 0.75).24 The standard error of measurement [SEM = SD√(1 − ICC)] and the minimal detectable change (MDC = 1.96√2SEM) were calculated, considering SD as the standard deviation of the 6MST1.25 The paired t test or Wilcoxon test was used to compare the physiological variables between the 6MST1 and the 6MST2, and the independent t test or the Mann-Whitney U test was performed to compare all variables between groups. Spearman's or Pearson's coefficient was used to test the correlation between variables. The level of significance adopted was P < .05.
O2 response to the 6MWT found by Karloh et al9 was used for the sample calculation. The lowest mean difference (88.3 mL/min) and its SD (99.5 mL/min) until the stabilization (minute 3), assuming 80% power and α = .05, resulted in an estimated sample size of 36 patients.
Forty-two patients were potentially eligible for this study. Of these, 6 were excluded: intense discomfort during the use of the K4b2 (1 patient), exacerbation of COPD during the protocol (3 patients), and hip pain during the 6MST (2 patients). Therefore, 36 patients (29 men) completed the study and Table 2 describes their characteristics.
PERFORMANCE AND PHYSIOLOGICAL RESPONSES
The total sample performed 80 ± 29 steps and showed, on average, better 6MST2 performance when compared with the 6MST1 (79 ± 29 vs 74 ± 30 steps, P < .001), presenting a learning effect of 6.28%. Only 6 (16.6%) patients did not perform the 6MST2 better than the 6MST1. The number of steps presented high reproducibility (ICC = 0.98, P < .001) with an SEM of 4.27 and an MDC of 11.8. Twenty-one patients interrupted the 6MST (1.50 ± 0.88 min): 10 patients interrupted the test once; 9 patients twice; 1 patient 3 times; and 1 patient 4 times. The reasons for the interruptions were severe dyspnea (13 patients), HR > HRsubmax (7 patients), and SpO2 < 85% (1 patient). Dyspnea (4 [1-10] vs 0 [0-2], P < .001) and all physiological variables, except THb (Table 3), were different at the end of the 6MST when compared with the beginning. The number of interruptions and the durations showed high reproducibility (ICC = 0.97, P < .001 for both).
After the first interruption, it was observed that all analyzed variables showed statistically significant reduction. However, on average,
O2, RR, and
E/MVV remained higher than baseline values while TSI returned to baseline values (Table 4). 6MST performance was similar between patients who interrupted and did not interrupt the 6MST (74 ± 35 vs 88 ± 17, P = .11). Figure 1 shows the comparisons between patients who interrupted and did not interrupt the test, as well as time versus group interaction of each physiological variable. There were no statistically significant differences in the physiological variables and dyspnea at the end of the 6MST between those who interrupted and did not interrupt the test (P > .05), except for
E/MVV and HR. There was a statistically significant difference in the
E/MVV in all test minutes, as well as in HR, in the first and second minutes between patients who interrupted and did not interrupt the 6MST (P < .05). It was also observed that patients who interrupted the 6MST presented lower FEV1 (1.37 ± 0.37 L vs 1.82 ± 0.41 L, P = .002, and 47.2 ± 13.2%pred vs 56.6 ± 12.40%pred, P = .04) than those who did not interrupt the test, and the RV/TLC showed a borderline P value (49.9 ± 8.10% vs 44.3 ± 8.08%, P = .05).
There were no statistically significant differences in the physiological variables at the end of the test between the 6MST1 and the 6MST2, except for HR (mean difference = −2.83 ± 6.42; 95% CI, −5.00 to −0.66; P = .01). However, reproducibility of all the physiological variables ranged between moderate and high (ICC = 0.74-0.98).
ΔTSI presented modest correlations with Δ
O2 (r = −0.36, P = .03; Figure 2), smoking load (r = 0.48, P = .003), FEV1 %pred (r = −0.39, P = .02), RV/TLC (r = 0.35, P = .04), and ΔHR (r = −0.39, P = .02). ΔO2Hb also correlated modestly with Δ
O2 (r = −0.34, P = .04), Δ
E (r = 0.36, P = .03), and Δ
O2/HR (r = −0.35, P = .04), whereas ΔHHb showed a correlation only with ΔHR (r = 0.40, P = .02).
6MST performance presented correlations with FEV1 (L) (r = 0.40, P = .01), LCADL score (r = −0.38, P = .02), mMRC score (r = −0.57, P < .001), and CAT score (r = −0.51, P = .001).
PERFORMANCE, PHYSIOLOGICAL VARIABLES, AND STAGING OF COPD
No statistically significant difference in 6MST performance was found between GOLD 2 and GOLD 3-4 patients (87 ± 26 vs 74 ± 31, P = .21), whereas the duration and frequency of interruption were greater for GOLD 3-4 patients than for GOLD 2 patients (1.15 ± 1.06 min vs 0.53 ± 0.84 min, P = .03, and 15 vs 6, Cramer's V = 0.38, P = .02). In addition, 8 of the 11 patients who interrupted the 6MST more than once were GOLD 3-4 patients. GOLD 3-4 patients presented lower ΔVT (0.44 ± 0.30 L vs 0.73 ± 0.36 L, P = .01), Δ
E (20.9 ± 6.93 L vs 27.4 ± 9.45 L, P = .02), and ΔVins (0.39 ± 0.26 L vs 0.65 ± 0.27 L, P = .01) than GOLD 2 patients. On the contrary, GOLD 2 patients tended to present greater reduction in TSI (−9.10 ± 4.15% vs −6.03 ± 5.77%, P = .01) and O2Hb (−7.53 ± 5.50% vs 3.80 ± 6.18%, P = .07) than GOLD 3-4 patients.
The main findings of this study were that the 6MST shows stabilization of most physiological variables between the second and fourth minutes whereas the RR and HR stabilized after the fifth minute. The physiological overload was similar between patients who interrupted and did not interrupt the test, except for
E/mvv (all test minutes) and HR (first and second minutes), which were higher in patients who interrupted the 6MST. These patients also had worse lung function compared with patients who did not interrupt the 6MST. In addition, 6MST performance is reliable and a second test after a 30-min interval improves performance without imposing a greater physiological overload.
Although self-paced tests show a submaximal profile, a study analyzed briefly 6MST ergoespirometric responses and showed a
O2peak close to the one in the maximal cardiopulmonary test in patients with COPD.7 Our study showed a submaximal profile of the 6MST in patients with COPD as the 6MWT has shown previously and different from stair or step tests with incremental protocols.8,26,27 The fact that some patients did not interrupt the test might have contributed to the late stabilization of the RR and HR. However, for most variables, the interruptions seem to have not interfered with the physiological responses, demonstrating that they not only provide greater test safety and tolerance for some patients but also have no interference in the test ability to measure the physiological responses. In addition, 6MST performance was reproducible and most of the physiological variables did not differ at end of the test. The only variables that differed between the patients who discontinued the test were
E/MVV and HR. Probably, the high ventilatory demand during the test was a contributing factor for some test interruptions due to dyspnea, while in other patients, high HR exceeded the safety limits in the first minutes of the test, forcing the evaluator to stop. Given that the patients who interrupted the 6MST showed worse pulmonary function and greater static hyperinflation, the need for interruption is possibly related to an important ventilatory limitation at rest and during exercise. Patients with COPD with lower ventilatory reserve quickly reach their ventilatory ceiling because they have difficulty increasing the VT before exercise.28 Thus, the increase in ventilation predominantly occurs at the expense of increased RR, which makes the expiratory time insufficient, contributing to dynamic hyperinflation.29 One finding that may endorse this hypothesis in the present study is that patients with worse pulmonary function had lower VT,
E, and Vins variations. They also interrupted the 6MST more often and spent more time on these interruptions.
6MST performance did not differ between patients who interrupted and those who did not interrupt the test, as well as between GOLD 2 and GOLD 3-4 patients. However, GOLD 2 patients tended to show a greater reduction in TSI than GOLD 3-4 patients, which was unexpected, since it has already been demonstrated that patients with COPD with severe static hyperinflation and greater airflow obstruction present lower blood flow and increased vascular resistance in the peripheral musculature even at rest.30 However, we speculate that GOLD 2 patients probably performed the 6MST with higher intensity, requiring more oxygen extraction. In addition, GOLD 3-4 patients interrupted the 6MST more often and the present study demonstrated that the first interruption was sufficient for a significant recovery of TSI, which remained higher at post-6MST when compared with immediately after interruption. Perhaps, the responses in patients who interrupted the 6MST have been similar to the interval aerobic training, in which patients reduce ventilatory overload during the low load or rest phase,31 reducing air trapping during the rest. Thus, it is possible to infer that the tendency for a greater reduction in TSI in GOLD 2 patients is related to a lower rate of 6MST interruption, which would lead to higher dynamic hyperinflation, resulting in a similar performance for GOLD 3-4 patients. Furthermore, a low correlation between ΔTSI and Δ
O2 was found, diverging from what had been previously demonstrated for patients with COPD. Studies with incremental maximal tests and no return after interruption protocols demonstrated a relationship between a reduction in TSI and peak
O2(r = 0.54, P < .05),11 between TSI and
O2 (0.47 ≤ r ≥ −0.98, P < .05),12 and between the recovery of O2Hb and
O2 (r = 0.68, P < .05).13 The 6MST protocol used in the present study differs from those used in these studies, and the interruptions may be responsible for the low correlation found. One possible explanation for this finding is that, faced with an increased ventilatory demand and by interrupting the test, the patients could rest their peripheral muscles, thereby reducing their energy requirement. Therefore, patients probably decided to interrupt the test due to intolerant dyspnea instead of due to peripheral physiological factors, such as TSI reduction. On the contrary, the high but stable
O2 would still maintain the demand on ventilatory muscles to continue ventilation because, even at rest, patients with COPD have a higher
O2 than healthy individuals as a result of increased respiratory work caused by airflow obstruction.32 The results of the first interruption analysis reinforce this hypothesis, since they demonstrated that TSI returned to baseline values with the interruption while ventilation variables and
O2 did not.
To our knowledge, this is the first study to detail the physiological responses induced by the 6MST in patients with COPD and to establish relationships with their performance. Our results demonstrated that the 6MST can be used as a tool to evaluate functional capacity in patients with COPD, since it correlates with clinical and functional outcomes related to the disease, confirming validity of the test.4 Moreover, the 6MST presented similar physiological responses to the 6MWT and can be used to investigate not only functional capacity but also exercise-induced desaturation, which is associated with impaired daily physical activity, faster FEV1 decline, and worse prognosis.33,34 Interruptions can be allowed and performed to make the test safer, without significantly compromising the assessment of physiological responses. In addition, the physiological responses demonstrated by the 6MST probably reflect what occurs in step-down training with constant load, which may be an interesting alternative intervention in pulmonary rehabilitation programs.
The small number of GOLD 4 patients in the sample may be considered a limitation of the study and therefore these findings should not be extrapolated to patients with greater impairment of lung function. In addition, our results may not reflect the physiological responses in patients with functional impairment because the 6MST performance was above the cutoff point of impaired functional capacity.4 The spectrometer was positioned in the dominant lower limb, but patients were instructed to step first with the lower limb that they considered more comfortable, with the possibility of changing the side if they decided so. Therefore, activation of the vastus lateralis during the 6MST, at times concentrically and eccentrically at other times, may have produced a confounding effect in the results. However, we believe that this aspect does not compromise future comparisons, since this instruction is part of the standardized protocol and is justified by reflecting activities of daily living in a more accurate manner.6 Furthermore, a type II error may have compromised comparisons of 6MST performance between GOLD 2 and GOLD 3-4 patients, as well as between those who interrupted and those who did not interrupt the test. However, a previous sample calculation was applied focusing on the main objective of this study.
The 6MST shows that stabilization of the main physiological variables and the interruptions do not significantly interfere in the analysis of the physiological responses of the test. The patients who interrupted the 6MST had worse pulmonary function and presented with increased ventilatory demand during the test. Regardless of these findings, 6MST performance was similar between these groups, as well as among patients with different degrees of airflow obstruction. Moreover, the 6MST is a reliable test to evaluate the functional capacity reflecting important outcomes in COPD.
This study was funded, in part, by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, Brazil (CAPES), Finance Code 001, and by the Fundação de Amparo à Pesquisa e Inovação do Estado de Santa Catarina, FAPESC/Brazil (PAP UDESC, Chamada Pública N° 01/2016, Termo de Outorga 2017TR645).
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