Rev Bras Fisiol Exerc 2022;21(5):312-21

doi: 10.33233/rbfex.v21i5.5405

ORIGINAL ARTICLE

Correlation between the 6-minute walking test and the cardiopulmonary exercise test in individuals with heart failure

Correlação entre o teste de caminhada de 6 minutos e o teste de exercício cardiopulmonar em indivíduos com insuficiência cardíaca

Valéria Cristina de Faria¹, Ana Carolina Campos Ferreira², Dayana Magalhães Drummond Oliveira¹, Danielle Aparecida Gomes Pereira¹

¹Universidade Federal de Minas Gerais (UFMG), Belo Horizonte, MG, Brazil

²Hospital das Clínicas da Universidade Federal Minas Gerais (UFMG), Belo Horizonte, MG, Brazil

Received: September 5, 2022; Accepted: October 13, 2022.

Correspondence: Danielle Aparecida Gomes Pereira, E-mail: danielleufmg@gmail.com

How to cite

Faria VC, Ferreira ACC, Oliveira DMD, Pereira DAG. Correlation between the 6-minute walking test and the cardiopulmonary exercise test in individuals with heart failure. Rev Bras Fisiol Exerc. 2022;21(5):312-21. doi: 10.33233/rbfex.v21i5.5405

Abstract

Objective: This study aimed to verify the correlation between the 6-minute walking test (6MWT) and the cardiopulmonary exercise test (CPET) in individuals with heart failure (HF), and to analyze the sensitivity of 6MWT in differentiating individuals from classes I, II, and III of the New York Heart Association (NYHA) functional classification system. Methods: Seventeen individuals (3 class I, 8 class II, and 6 class III) aged 48.09 ± 12.91 years with a left ventricular ejection fraction of 44 ± 11% and VO_2peak of 25.18 ± 6.05 mL·kg^-1·min^-1 were assessed. Volunteers performed CPET on a treadmill; after an interval of 48 h to 6 months, they performed 6MWT. Pearson and Spearman correlations were used, and ANOVA was used to compare 6MWT variables between NYHA functional classes. An alpha of 5% was adopted as significant. Results: Significant correlations were found between distance walked in 6MWT and VO_2peak (r = 0.70). Significant differences were found between NYHA functional classes regarding the distance walked in 6MWT. For walking performance, significant differences were found between NYHA classes I and II (p = 0.0001) and between classes I and III (p = 0.0001). Conclusion: The 6MWT is a simple and reliable method to assess the functional capacity of individuals with HF, proving to be a valid instrument for clinical practice.

Keywords: stress test; heart failure; 6-minute walking test.

Resumo

Objetivo: Este estudo teve como objetivo verificar a correlação entre o teste de caminhada de 6 minutos (TC6) e o teste de exercício cardiopulmonar (TECP) em indivíduos com insuficiência cardíaca (IC), e analisar a sensibilidade do TC6 em diferenciar indivíduos das classes I, II e III do sistema de classificação funcional da New York Heart Association (NYHA). Métodos: Dezessete indivíduos (3 classe I, 8 classe II e 6 classe III) com idade de 48,09 ± 12,91 anos, fração de ejeção do ventrículo esquerdo de 44 ± 11% e VO_2pico de 25,18 ± 6,05 mL·kg^-1·min^-1 foram avaliados. Os voluntários realizaram TECP em esteira; após um intervalo de 48h a 6 meses, realizaram o TC6. Correlações de Pearson e Spearman foram usadas, e ANOVA foi usada para comparar as variáveis do TC6 entre as classes funcionais da NYHA. Um alfa de 5% foi adotado como significativo. Resultados: Foram encontradas correlações significativas entre a distância percorrida no TC6 e o VO_2pico (r = 0,70). Diferenças significativas foram encontradas entre as classes funcionais da NYHA em relação à distância percorrida no TC6. Para o desempenho da caminhada, foram encontradas diferenças significativas entre as classes NYHA I e II (p = 0,0001) e entre as classes I e III (p = 0,0001). Conclusão: O TC6 é um método simples e confiável para avaliar a capacidade funcional de indivíduos com IC, mostrando-se um instrumento válido para a prática clínica.

Palavras-chave: teste de esforço; insuficiência cardíaca; teste de caminhada de 6 minutos.

Introduction

Heart failure (HF) is a clinical systemic syndrome characterized by cardiac dysfunction, which leads to a blood supply insufficient for metabolic needs of tissue [1]. It is the later stage of most diseases that attack the heart. These diseases are often associated with disabling symptoms, such as dyspnea and fatigue, which lead to exercise intolerance [2]. Exercise intolerance leads to a reduction of functional capacity and is directly associated with the prognosis of the disease [3]. Therefore, valid and reliable methods are vital for the prescription of appropriate exercise during the rehabilitation process of these patients.

In HF, 2 methods are most commonly used to define the degree of exercise limitation: the cardiopulmonary exercise test (CPET) and the 6-minute walking test (6MWT) [3]. The CPET is considered the gold standard for the assessment of functional capacity and prognosis of individuals with HF [4]. It is a non-invasive assessment procedure and provides important information on the individual’s aerobic capacity [4]. The CPET is highly reliable in patients with HF and is a well-accepted method of assessment in this population [5].

The CPET aims to assess the clinical, hemodynamic, electromyographic, metabolic, and ventilatory responses to effort [6]. It is a test that has the primary variables oxygen consumption (VO₂), carbon dioxide (CO₂) production, respiratory rate (RR), tidal volume (VT), and heart rate (HR) [5]. With these parameters, it is possible to calculate secondary variables such as pulmonary ventilation (VE), respiratory exchange rate (RER = VCO₂/VO₂), expired oxygen fraction (%FEO₂), carbon dioxide expired fraction (%FECO₂), oxygen pulse (VO₂/FC), oxygen ventilatory equivalent (VE/VO₂), and CO₂ ventilatory equivalent (VE/VCO₂). Some of these variables have been described in the literature as predictors of the prognosis of individuals with HF. Values of VE/VCO₂ less than 35 mL·kg^-1·min^-1 indicate the worst prognosis in these individuals [1,6].

Although the CPET is the gold standard for HF, many individuals have difficulties in being evaluated by a maximum test, especially those who are more impaired, unfit, or elderly. The equipment required for this test is expensive and requires specialized staff. Furthermore, the maximum exercise performed may not reflect the difficulties of individuals with HF in performing daily activities [4]. Thus, the CPET may not be the most appropriate test to assess the functional capacity of these individuals in situations of lower effort.

The 6MWT has been proposed as another tool for the assessment of the physical capacity of individuals with HF. It is a safe, valid (r = 0.687, p < 0.001) [7], and reliable test. It is easily performed, has low costs, and predicts the survival of individuals with the disease [6]. The effort during the 6MWT is described as similar to activities of daily living, enabling the individual to determine the walking rhythm he/she tolerates; this is an advantage for those with a greater degree of impairment who would not tolerate the CPET [2,3,6,8,9,10,11,12]. The main variable analyzed in the 6MWT is the distance walked [6]. In addition, the speed is assessed and the hemodynamic response to effort is analyzed (HR, BP, and SpO₂).

Based on previous evidence, both the 6MWT and the CPET are indicated to assess functional capacity and establish the prognosis of individuals with HF [3,6]. However, in the Brazilian population, few studies have correlated the variables obtained in both tests [13]. In a recent study, Carvalho et al. [13] verified the reproducibility of the 6MWT in a sample of Brazilian individuals with HF and correlated the VO_2peak obtained by the CPET with the variables of the 6MWT. No correlations with other cardiorespiratory variables of important prognostic value for HF, such as VE/VCO₂, were established. Therefore, more studies that investigate the correlation between the variables used in the CPET and the 6MWT are necessary in order to confirm the similarity of the results of both tests when performed on individuals with HF. Furthermore, it is important to analyze if the existing submaximal tests, such as the 6MWT, are sensitive enough to stratify individuals with different functional capacity levels.

The present study aimed to verify the correlation between the variables used in the 6MWT and the CPET in a Brazilian sample of individuals with HF, attended at the public health service, as well as analyze if the 6MWT is sensitive enough to differentiate between individuals of the New York Heart Association (NYHA) functional classes I, II, and III.

Methods

Sample

A transversal study, approved by the Ethics in Research Committee of the Universidade Federal de Minas Gerais (Protocol number 050/09), was conducted. Individuals with HF having symptoms that indicated effort limitation (NYHA classes I, II and III), independent of gender and ethnic group, were included. Individuals with HF were selected by convenience in an ambulatory cardiology service associated with the institution.

The study included individuals aged between 30 and 59 years with the following characteristics: presence of HF for at least 6 months, as confirmed by the ambulatory cardiology service; clinically stable condition for at least 2 months prior to the study; left ventricular ejection fraction (LVEF) at rest < 45%, recently assessed by the bidimensional echocardiogram (up to 6 months); and classification as NYHA class I, II, or III and HF stage C, according to the American College of Cardiology/American Heart Association guidelines [14]. The individuals included in the study were prescribed optimized doses of beta-blockers [15,16], ECA inhibitors [15], or angiotensin receptor blockers and had a body mass index (BMI) < 30 kg/m².

Individuals who did not meet the criteria that define the effort test as maximum [6], who presented a peripheral oxygen saturation (SpO₂) lower than 85% during exercise, or who presented any type of complication such as the flu or a cold, hospital admission, or alterations in medication prescription between the CPET and the 6MWT were excluded from the study.

Procedures and data collection

The volunteers attended LabCare. After signing the written consent form, they were assessed by the CPET (Medical Graphics® CPX Ultima, Miami, FL, USA) on a treadmill (Millenium Classic CI®, Inbramed/Inbrasport, Brazil) using a ramp protocol [17], according to the Brazilian Cardiology Society on Ergometric Test Guidelines, in order to determine the cardiorespiratory parameters. During the test, HR was monitored by an electrocardiogram (Welch Allyn® Cardioperfect, USA) connected to an ergospirometer. Blood pressure was measured using a sphygmomanometer (Diasyst®, São José dos Campos, SP, Brazil) and a stethoscope (Litmann Classic II S.E 3M®, USA). SpO₂ was continuously measured by an oximeter (Model 300 Series MEDIAD INC., California, USA) connected to the ergospirometer. The subjective effort perception for dyspnea and lower limb fatigue was assessed by the modified Borg scale.

After a minimum interval of 48 h and a maximum of 6 months, the volunteers returned to LabCare to perform the 6MWT. The test was performed in a 30-m-long hallway, where the volunteer was instructed to walk the longest distance possible in 6 min, without running or trotting. The route was delimited by plastic cones and standard encouraging phrases, such as “very good, go on” and “let’s go, continue walking as fast as possible,” were given every minute [11,18]. Before and after the test, blood pressure was measured using a sphygmomanometer (Diasyst®, São José dos Campos, SP, Brazil) and a stethoscope (Litmann Classic II S.E 3M®, USA). The subjective effort perception for dyspnea and lower limb fatigue was assessed at the end of the test by the modified Borg scale. The SpO₂ was continuously measured during the test by an oximeter (Ohmeda Tuffsat®), and the HR by a cardiofrequencimeter (Polar S810®).

The test was interrupted if any of the following criteria were observed: precordial pain, unbearable dyspnea, oxygen saturation lower than 85%, muscle cramps, limping gait, discomfort, or dizziness and paleness [19]. Two tests, with a 15-min interval between them, were performed. If the difference between the distance walked in the tests was higher than 10%, a third test was performed [11,18]. Data obtained from the 6MWT were registered in a patient identification form, which contained personal data and information on life habits.

Variables

Although the CPET provides information on the cardiorespiratory system’s response to effort, the variables with higher interest for this study were VO_2peak (primary variable), VE/VCO₂, and PO₂ (secondary variables). The distance walked was the primary variable obtained by the 6MWT with most interest. The following variables were considered secondary variables: speed, HR, and walking performance (distance/HR). All these variables were studied in order to establish a correlation between the tests.

Statistical analysis

Data were presented as measures of central tendency and dispersion. Data distribution was analyzed by the Shapiro–Wilk test. The correlations were performed using the Pearson correlation coefficient or the Spearman correlation coefficient, for variables with normal and non-normal distribution, respectively. One-way ANOVA was used to compare the 6MWT variables between NYHA functional classes I, II, and III. In all statistical tests, alpha values of 5% were considered significant. The software Statistical Package for the Social Sciences® (SPSS, Chicago, IL, USA) version 17.0 was used for data analysis.

Results

Seventeen individuals (5 women and 12 men) with HF (3 NYHA class I, 8 class II, and 6 class III) participated in the study, with an average age of 48.09 ± 12.91 years (95% CI = 39.42–56.76), LVEF of 44% ± 11% (95% CI = 0.36 - 0.51), and BMI of 25.86 ± 2.86 kg/m² (95% CI = 23.93–27.98). Data from the results obtained for the CPET and 6MWT are shown in Table I.

Table I - Results of the variables assessed in the cardiopulmonary exercise test and the 6-minute walking test (N = 17)

SD = standard deviation; VO_2peak = oxygen consumption at peak effort; VE/VCO₂ = ventilatory equivalent for carbon dioxide; PO₂ = oxygen pulse; RER = respiratory exchange rate; D_6MWT = distance walked in the 6-minute walking test; V_6MWT = velocity obtained in the 6-minute walking test; D/HR = walking performance

Significant and strong correlations were found between speed and distance walked in the 6MWT and VO_2peak and between walking performance (D_6MWT/PR) and PO₂ (Table II).

Table II - Correlation between the variables of the 6-minute walking test and the cardiopulmonary exercise test

VO_2peak = oxygen consumption at peak effort; VE/VCO₂ = ventilatory equivalent for carbon dioxide; RER = respiratory exchange rate; PO₂ = oxygen pulse; D_6MWT = distance walked in the six-minute walking test; V_6MWT = velocity obtained in the 6-minute walking test; D/HR = walking performance. a = Pearson correlation coefficient; b = Spearman correlation coefficient; * = p < 0.005

Significant differences were found between NYHA functional classes with respect to the distance walked in the 6MWT, speed obtained in the 6MWT, and walking performance (Table III).

Table III - Comparison of the variables of the 6-minute walking test between the different NYHA functional classes

Mean ± SD; SD = standard deviation; D_6MWT = distance walked in the 6-minute walking test; V_6MWT = velocity obtained in the 6-minute walking test; D/HR= walking performance. Class I = no symptomatology; Class II = symptoms during daily activities; Class III = symptoms during small efforts; F = ANOVA; * = p < 0.05 between classes I and II; # = p < 0.05 between classes I and III; † = p < 0.05 between classes II and III

There were no significant differences between the individuals from different NYHA functional classes in relation to age, BMI, and LVEF.

Discussion

The main finding of the present study was the strong correlation between the distance walked and the speed reached in 6MWT and the VO_2peak obtained in CPET; this demonstrated the ability of 6MWT to objectively assess the functional capacity of individuals with HF, especially considering that the sample represents individuals assisted by the public health service.

Standard tests are useful to compare and document changes in the functional capacity of an individual through time [2]. In individuals with HF, 6MWT has been a submaximal test widely used to assess the degree of limitation to exercise. The advantages of this test, such as simplicity, viability, and insignificant cost, have been repeatedly reported [2,3,6,8,9,10,11,12]. Moreover, since it is a submaximal test in which the individual determines the walking rhythm he or she tolerates, it has been suggested that the 6MWT is highly applicable to activities of daily living.

Several authors have assessed the use and clinical applicability of the 6MWT in different populations with diverse levels of severity [6,12,20,21,22,23,24,25,26,27]. The present study assessed individuals with HF of mild severity (VO_2max > 20 mL·kg^-1·min^-1), according to the classification proposed by Weber [28].

The results of the present study agree with the findings of Guazzi et al. [3], who assessed 253 individuals with HF NYHA classes II and III. In their study, the distance walked in the 6MWT was strongly correlated with the VO_2peak (r = 0.788; p = 0.001), and there was a weak, but significant, correlation between the 6MWT and VE/VCO₂ (r = 0.46; p = 0.001). Moreover, the distance walked in the 6MWT decreased significantly with the progression of the functional classes proposed by Weber et al. [28].

Other authors have also verified these correlations by finding similar results. Cahalin et al. [20] assessed 45 individuals with HF and obtained correlations of moderate magnitude between the distance walked in the 6MWT and VO_2peak (r = 0.64; p = 0.001). According to these authors, the 6MWT was a strong predictor for the occurrence of undesirable events, such as death and hospitalization. Compared to individuals who walked longer distances, individuals who walked less than 300 meters in the 6MWT showed an increased death risk in 6 months. A study by Pulz et al. [27] assessed the ability of submaximal tests, such as the 6MWT and the shuttle walking test (SWT), in predicting a decrease in O₂ consumption and establishing a prognosis in individuals with HF. Significant correlations were found between VO_2peak and the distance walked in the 6MWT (r = 0.76; p = 0.001) and the SWT (r = 0.79; p = 0.001). The sensitivity of both tests in predicting reduced levels of O₂ consumption (VO_2max < 14 mL·kg^-1·min^-1) was 490 m, predominantly in individuals with mild functional impairment (Weber classes A and B), and submaximal tests were not considered good predictors of mortality in this population.

Although other authors have also reported significant correlations between the distance in the 6MWT and other cardiorespiratory variables with important prognostic value, such as VE/VCO₂ [3,20], the results of the present study did not exhibit these correlations. A larger number of individuals with HF may be necessary to reach a sufficient size to obtain statistical significance. The sample calculation of the present study considered the number of individuals necessary to establish a correlation between the 6MWT and VO_2peak of the CPET, the primary variables of the study.

Another CPET variable with clinical prognostic and mortality value for HF is PO2 [29,30,31]. PO₂ is considered one of the most important variables assessed by ergospirometry. The variable is related to the systolic volume and the arteriovenous O₂ difference, representing an indirect index of the O₂ transport at each cardiac systole [31,32,33]. In our study, the walking performance variable showed a strong correlation with the PO₂ determined by ergospirometry and may be related to the tissue efficiency in using the offered O₂. In other populations, such as individuals with peripheral obstructive arterial disease, this variable has been used to infer the cardiovascular conditioning [34] and is a variable of interest in the assessment of therapeutic interventions. Thus, if an individual is able to walk a longer distance while reaching lower HR values after physical training, it can be inferred that the physical capacity has been enhanced.

Functional capacity has shown a significant correlation with HF severity [35]. A simple and widely used method to quantify the impact of HF on the individual’s daily life is the classification proposed by NYHA. According to this classification, individuals with HF can be categorized into 1 of 4 classes according to the intensity of the symptoms and the degree of limitation in performing daily activities [1]. The NYHA functional classification is a subjective estimation of the individual’s real functional capacity and is significantly correlated with the objective measure of VO₂ determined by the CPET [36].

The CPET is sensitive enough to differentiate between individuals of NYHA functional classes I, II, and III [36]; however, limited data are available on the sensitivity of the 6MWT in stratifying individuals from different functional classes. In the present study, it was possible to observe significant differences between NYHA functional classes I and III, and between classes II and III in the comparisons that involved distance and speed in the 6MWT. No significant differences were found between classes I and II. This finding may be related to the fact that individuals from functional classes I and II are in the initial stages of the disease, when there are fewer repercussions for functional capacity. Another aspect that may have contributed to the limitation of the 6MWT in stratifying these individuals was the overall percentage of NYHA I individuals, which was considered relatively small (17%) when compared to the other functional classes. A significant difference was observed between classes I and II for the variable of walking performance. This finding may be explained by the fact that this variable has additional distance values compared to the HR data. This may have increased the variable’s sensitivity and ability to stratify these individuals, even in initial stages of the disease when there are fewer negative repercussions for function.

One limitation of our study was that the sample was composed predominantly of individuals with HF and mild functional impairment (Weber classes A and B). Therefore, other studies are necessary to demonstrate such results in individuals with HF and moderate-to-severe functional impairment.

Conclusion

In the present study, the 6MWT correlated with the direct measure of VO_2peak (gold standard for functional capacity assessment) and was able to stratify individuals with different levels of functional capacity according to the classification proposed by NYHA. Thus, the 6MWT proved to be a simple and valid clinical tool for the assessment of individuals with HF, and with special external validity for individuals assisted by the public health service.

Conflict of interest

There are no conflicts of interest.

Financing

There was no funding.

Authors' contribution:

Conception and design of the research: Ferreira ACC and Pereira DAG; Data collection: Ferreira ACC and Oliveira DMD; Analysis and interpretation of data: Ferreira ACC, Oliveira DMD and Pereira DAG; Statistical analysis: Ferreira ACC and Pereira DAG; Manuscript writing: Ferreira ACC and Oliveira DMD; Critical review of the manuscript for important intellectual content: Faria VC and Pereira DAG

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