Six-minute walk test may be a reliable predictor of peak oxygen uptake in patients undergoing hemodialysis

Cardiorespiratory fitness seems to play an important role in the general health of patients undergoing hemodialysis (HD). However, the prediction of peak oxygen uptake (V̇O2peak) in a clinical setting is not widely adopted for these patients. Evaluate the agreement and reliability between directly and indirectly V̇O2peak measurements in patients undergoing HD. This is a cross-sectional study with patients undergoing HD that performed a cardiopulmonary exercise test (CPET) with 5/10 watts incremental load in each minute using a cycle ergometry to directly evaluate the V̇O2peak, and the 6-min walk test (6MWT) in a 30-m corridor to indirect measures it. Both tests were performed on a midweek non-dialysis day. Bland–Altman analysis of agreement limits was used with direct and indirect V̇O2peak values. Intraclass correlation coefficient (ICC) and Cronbach’s Alpha was used to evaluate the reproducibility and reliability between direct and indirect V̇O2peak values. Twenty-six patients (54.4 ± 14.5 years, 53.8% of male) were evaluated. The V̇O2peak direct mean obtained through CPET was 15.91 ± 5.26 (ml/kg/min), while the indirect mean obtained through 6MWT was V̇O2peak of 14.89 ± 4.21 (ml/kg/min). There was a strong positive correlation between both V̇O2peak values (r = 0.734; p < 0.001). The Bland–Altman analysis demonstrated that the methods agreed with each other (p = 0.103). Also, the ICC (0.829) and Cronbach's Alpha (0.846) showed excellent reproducibility and reliability. 6MWT is a reliable tool for estimating V̇O2peak in patients undergoing HD.


Introduction
Patients with chronic kidney disease (CKD) may experience physical deconditioning due to a high sedentary behaviour and impairments of musculoskeletal, cardiopulmonary, and cardiovascular systems caused by the hydroelectrolytic imbalance, and the subclinical proinflammatory state [1][2][3]. The fatigue symptoms commonly seen in a patient with CKD are exacerbated by the hemodialysis (HD) treatment and impact the physical deconditioning [4]. In the long term, it may deteriorate cardiorespiratory fitness and increase the risk of mortality due to the high incidence of cardiovascular events [5][6][7].
Cardiorespiratory fitness is accurately evaluated by peak oxygen uptake (VȮ 2peak ) [8]. The VȮ 2peak reflects the greatest oxygen-carrying capacity of the blood, oxygen extraction at the tissues, and oxygen uptake by the musculoskeletal system [5,7,8]. Patients undergoing HD commonly show lower cardiorespiratory fitness and VȮ 2peak than healthy individuals [9]. Peak oxygen uptake values ≤ 17.5 ml/min/kg have been previously associated with lower survival in patients undergoing HD, indicating the clinical importance of evaluating cardiorespiratory fitness in this population [6].
Generally, the most popular clinical tests to evaluate cardiorespiratory fitness are the cardiopulmonary exercise test (CPET) and the 6-min walk test (6MWT) [10,11]. The CPET is the gold standard method to assess functional impairment related to cardiorespiratory health and provides a direct VȮ 2peak measurement. However, advanced and high-cost equipment is required, as well as a special room for sudden medical emergencies. Moreover, a specialized team must be present [8]. Thus, simple, widely available, reliable, and economically feasible tests to routinely evaluate cardiorespiratory fitness in the clinical setting are necessary, such as the 6MWT.
The 6MWT is a field widely tool applied in different populations, such as patients with CKD, for functional capacity measurement. Its performance demands a 30-m corridor and portable equipment to evaluate blood pressure, oxygen saturation (SpO 2 ), and heart rate (HR) [12]. Therefore, 6MWT appears to be a low-cost and easyapplication test to indirectly estimate cardiorespiratory fitness and may be done more frequently than the CPET test in the clinical routine of patients undergoing HD [11].
The main difference between both tests is that CPET is a lab test that provides maximum exercise exertion, directly measuring the VȮ 2peak , whereas the 6MWT is a field submaximal exercise test that indirectly estimates VȮ 2peak by the distance walked [13]. Despite that, indirect evaluations of VȮ 2peak in patients undergoing HD in clinical setting is not widely adopted. Therefore, this study aimed to evaluate the agreement and reliability between directly and indirectly VȮ 2peak measurements in patients undergoing HD.

Methods
This is a cross-sectional study that enrolled patients with CKD from two HD centres in Porto Alegre, Brazil (Division of Nephrology of Hospital de Clínicas de Porto Alegre-HCPA and Instituto de Doenças Renais-IDR), who were invited to participate from September 2016 to August 2019. This study was approved by the Ethics and Research Committee of the Hospital de Clínicas de Porto Alegre (40,167,014.3.0000.5327), and it was conducted in accordance with the Declaration of Helsinki. All participants provided their written informed consent.
All eligible patients had been diagnosed with CKD ≥ 6 months, undergoing HD > 3 months, clinically stable (absence of any hospitalization in the last 30 days), ≥ 18 years old, and did not suffer from any musculoskeletal impairment such as sequelae by stroke or walking damage, and had a serum haemoglobin level ≥ 10.0 g/dl (100 g/l). Patients who had acute myocardial infarction within 3 months, an inflammatory process in treatment with anti-inflammatory or antibiotics drugs in the last 30 days, decompensated coronary artery disease, symptomatic peripheral arterial disease, and lower-limb arteriovenous fistula, arteriovenous grafts, or tunnelled catheter due to the unknown risk were not eligible. All patients performed CPET and 6MWT to be included in the analysis.

Cardiopulmonary exercise test
The CPET was performed by a trained physiotherapist and physician in a cycle ergometer to directly evaluate the VȮ 2peak (ml/kg/min). It has been applied an incremental load protocol with 5 or 10 Watts (W) per minute [8]. The incremental load protocol was defined by the authors respecting the American Thoracic Society (ATS) and the American College of Chest Physicians (ACCP) [8] guidelines and according to CKD cause. Those with suspicion of hypertension as CKD cause were submitted to a 5 W incremental due to possible hemodynamic and cardiovascular acute adverse events. Patients with other CKD causes had a 10 W increase protocol.
CPET was performed at Vmax ® Encore metabolic cart system (CareFusion, San Diego, California, USA) using a gas analyzer. The patients used a 10-lead electrocardiogram cardiosoft (V1, V2, V3, V4, V5, V6, RL, RA, LL, LA) to evaluate the heart electrical function and to measure the HR and maximum HR percentage (%HR max ) and were also monitored during the whole CPET through pulse oximetry to obtain SpO 2 , and a manual sphygmomanometer in the non-fistulated arm to obtain blood pressure.
Patients were verbally encouraged before and during the whole CPET to obtain a maximum physiological exertion (respiratory exchange ratio [RER] > 1.0) [8,15]. CPET consisted of 4 phases: (a) three-minute rest in which was verified the absence of hyperventilation; (b) a warm-up unloaded cycling (0 W for 2-min); (c) an incremental phase of exercise every minute (5/10 W-cycling rate at 60-65 revolutions per minute) until the patient reaches volitional exhaustion (RER > 1.0) or the test is terminated by the physician. If the subject did not reach an RER > 1.0, they were encouraged to continue the test; d) an active recovery unloaded (0 W for 1 min).
If necessary, CPET was interrupted as suggested by the ATS/ACCP under the supervision of a physician [8]. In the end, HR, %HR max , SpO 2 , SBP, and DBP delta values were registered.

Six-minute walk test
6MWT was performed using a standard protocol according to ATS guideline [12]. It was performed in a 30-m corridor with signs every three-meter and delimitated by cones. Immediately before the test's beginning and after its end, HR and SpO 2 were evaluated through pulse oximetry, SBP, and DBP through a manual sphygmomanometer in the non-fistulated arm, and dyspnea and lower limbs fatigue perception were evaluated by Borg CR10 Scale [12,14].
The patient observed the researcher performing the test and then was instructed to walk as far as possible for 6-min and maintain a normal walking pace. Patients were able to interrupt the test if necessary. At each minute of walking the researcher communicated clearly and calmly how many minutes remained until the end of the test. In the end, the walked distance was measured. Besides, HR, %HR max , SpO 2 , SBP, and DBP delta values were registered.
The reference equation to predict the 6MWT distance was proposed by Enright and Sherrill [16]. The formula proposed by Cahalin et al. [13] was used to indirectly estimate VȮ 2peak by the 6MWT: Both tests were performed on the same non-dialysis day, by the same physiotherapists' evaluator. The 6MWT was performed after a 20-min CPET considering when the patient's vital signs had returned to baseline levels, as previously proposed by Costa et al. [17]. It used the Karvonen method to predict the maximal HR and then calculated the %HR max achieved after both tests [18].

Statistical analysis
All data were tested for normality using the Shapiro-Wilk test. Normally distributed data are presented as mean ± standard deviation (SD), and non-normally are presented as median (minimum; maximum values). Paired t-test was used to compare directly VȮ 2peak obtained by CPET and indirectly VȮ 2peak obtained by 6MWT. Pearson's correlation test was used to correlate directly and indirectly VȮ 2peak . Spearman correlation test was used to correlate the difference between the vital sign and Borg CR10 Scale before and after both tests. The correlation size was classified as very low (r < 0.300, low (r = 0.300 to 0.500), moderate (r = 0.500 to 0.700), high (r = 0.700 to 0.900) and very high (r = 0.900 to 1.000) [19]. Chi-square was used to compare the dyspnea perception between both tests.
Bland-Altman analysis was used to evaluate the agreement limits between directly and indirectly VȮ 2peak values. The intraclass correlation coefficient (ICC) and Cronbach's Alpha were calculated to evaluate the reproducibility and reliability between both VȮ 2peak values.

Results
Thirty-one patients were invited to participate. One patient was not eligible due to the lower-limb arteriovenous fistula and four patients were not included in the final analysis for not performing the 6MWT. Thus, the final sample consisted of 26 patients. The study flowchart may be seen in Fig. 1. Table 1 shows the general characteristics of the patients. The majority of the patients performed the CPET until the volitional exhaustion and only one patient had the test interrupted by the physician due to ischemic electrocardiographic abnormalities.

Association between CPET and 6MWT
There was no significant difference between directly and indirectly VȮ 2peak (p = 0.160) confirming that the measurements have similarities. Table 2 shows the correlation between directly and indirectly VȮ 2peak , HR, %HR max , SpO 2 , SBP, and DBP variation, and Borg CR10 Scale. As seen, there was a significant strong positive correlation between directly and indirectly VȮ 2peak (r = 0.734; p < 0.001). Also, there was a low positive correlation in the dyspnea perception between both tests (r = 0.443; p = 0.023). This indicates that self-perception of shortness of breath is present in both the maximal test and the submaximal exercise test with the same behaviour.
Also, there was no significant correlation in vital signs' behaviour (HR, SpO 2 , SBP, and DBP), %HR max , and lower limbs fatigue self-perception between both tests. Similar behaviour on dyspnea perception after both tests according to the Chi-square test was seen (CPET 30.8% vs 6MWT 11.5%, p = 0.090).
The Bland-Altman analysis to evaluate the agreement between directly and indirectly VȮ 2peak measurements is shown in Fig. 2. The difference between VȮ 2peak mean in both tests was not significantly different from zero (p = 0.103). The majority of the values remained within the 95% CI. Besides, there was no significant proportion bias which means that the distribution of the values is homogeneous both above and below average (p = 0.119). Thus, both VȮ 2peak measurements (directly and indirectly) agreed with each other.

Discussion
The present study aimed to evaluate the agreement and reliability between directly and indirectly VȮ 2peak measurements in patients undergoing HD. Our results indicated a strong positive correlation and a high agreement between directly and indirectly VȮ 2peak , measured by CPET and 6MWT, respectively. Thus, 6MWT seems to be a reliable predictor of VȮ 2peak and could be routinely used to evaluate cardiorespiratory fitness in patients undergoing HD.
The VȮ 2peak is the most reliable and accurate index of the cardiorespiratory fitness in many clinical populations. CPET provides reliable cardiorespiratory fitness values as it is a maximum physiological exertion test, leading the patient to a high limit of physical, cardiac, respiratory, and cardiovascular tolerance [8]. However, CPET is an unaffordable test in many health settings, as it requires a spacious room and a specialized medical team, turning it into a high-cost exam [8]. On the other hand, the 6MWT is a submaximal test with a predetermined time and standardized procedures which barely expose the patient to physical exhaustion, although it evaluates the global and integrated responses of all the systems involved during exercise (pulmonary, cardiovascular, neuromuscular and musculoskeletal) [12].
In this sense, several submaximal functional tests reflecting daily life activities and incorporating elements of lower-limb functionality and gait speed have been proposed to be alternatives to evaluate improvements in rehabilitation programs or functional impairments due to chronic diseases, such as CKD [11][12][13][21][22][23]. Furthermore, as CPET, the 6MWT also presents a strong correlate of mortality in CKD patients [6,24].
People with chronic diseases show higher physical impairment than healthy individuals and many studies have been carried out to demonstrate the importance of cardiorespiratory fitness evaluation [3,25]. Although CPET is often used in several studies because it accurately measures cardiorespiratory fitness, this is not widely seen in clinical routine, where the 6MWT is more commonly adopted [11]. Besides, the 6MWT seems to show an advantage to evaluate the functional capacity in clinical populations because usually the maximal exercise tests are more difficult to be performed and understood by the patients [26,27].
Carvalho et al. (2011) [28] also evaluated the agreement from directly and indirectly VȮ 2peak in heart failure patients evaluated by CPET and 6MWT, respectively, and found similar results to ours. They concluded that the 6MWT is a reliable tool to systematically identify cardiorespiratory fitness adaptations following clinical exercise programs through indirect VȮ 2peak [29].
Ross et al. [11] also examined the relationship between directly and indirectly VȮ 2peak in different clinical populations and all prediction equations were able to estimate VȮ 2peak from 6MWT with minimal loss of accuracy. Their study did not include studies estimating VȮ 2peak from 6MWT in patients with CKD and we believe that it may occur due to the absence of studies evaluating this outcome in patients with CKD. Additionally, in terms of clinical routine application, a recent scoping review showed that several studies have been using the 6MWT to evaluate rehabilitation programs' effectiveness, as well as cardiorespiratory fitness in patients with CKD [10]. To the authors, is necessary further investigation into the validity, reliability, and practicality of outcome measures. Besides, defining consistent, relevant, and patient-important outcome measures allow for more meaningful conclusions to guide clinical decisions [10].
The correlation and Chi-square analysis have found similar behaviour on dyspnea self-perception after both tests, even with 30.8% of patients reporting very severe respiratory effort after CPET while 11.5% after 6MWT. We hypothesized that vital signs and self-perception of lower limbs fatigue and dyspnea would not agree due to the physiological differences between both tests (maximum versus submaximal exertion). However, we were surprised with our results that demonstrated similar behaviour in dyspnea perception.
According to the ATS, the Borg CR10 Scale is the main tool to quantify symptoms during both exercise tests (CPET and 6MWT) and it incorporates a nonlinear spacing of severity verbal descriptors which corresponds to specific numbers. Its reproducibility for dyspnea measurement during exercise has shown good results [8,12]. However, as a symptom, dyspnea is a multidimensional perceptual experience of breathing discomfort that consists of qualitatively distinct sensations [30]. It is often associated with fluid overload and coexisting morbidities and HD seems to cause a definite regression of these symptoms turning its quantification difficult with ambiguous results [30,31].
This study has some strengths and limitations. The strengths are: (a) our findings, showing that the 6MWT may predict VȮ 2peak , strengthen the clinical importance of our results and indicate the 6MWT as a reliable tool to evaluate the cardiorespiratory fitness in patients with CKD; (b) to the best of our knowledge, this is the first study showing the agreement between a direct and indirect VȮ 2peak, evaluated by CPET and 6MWT in patients undergoing HD.
The main limitations are: (a) the small sample size; (b) despite VȮ 2peak being 5-10% higher in cycle ergometer test than in treadmill test, and safer to clinical individuals, a treadmill test could have provided a better relevant comparison to 6MWT [32]; (c) the performance of both tests on the same day may have underestimated 6MWT results, which has been corrected by a 20-min resting whereas the patient's vital signs had returned to baseline levels; and d) we recognized that the formula proposed by Cahalin et al. (1996) [13] was described to be used in heart failure patients, however, due to heart failure being one of the main comorbidities associated with the fluid overload caused by CKD and the excellent reproducibility and reliability when compared to the CPET, we decided to present it to the scientific community as an alternative to evaluating the cardiorespiratory fitness through a field test in patients undergoing HD.

Conclusion
Thus, we conclude that the 6MWT may be used to estimate VȮ 2peak in patients undergoing HD, as it shows excellent reproducibility and reliability when compared to the CPET (i.e., gold standard method). Also, the 6MWT seems to be an important, practical, and low-cost test for evaluating and monitoring cardiorespiratory fitness (i.e., VȮ 2peak) in dialysis centres. We believe that more studies may be performed with the major sample size, aiming to propose new formulas to indirectly evaluate VȮ 2peak through functional capacity submaximal tests in patients undergoing HD.