IJCS | Volume 32, Nº2, March/April 2019

140 Morais and Rassi Six-minute walk test in heart failure Int J Cardiovasc Sci. 2019;32(2)134-142 Original Article sex, older age, lower FVC and higher mMRC, which simultaneously explained 37% of the variance for the 6MWD. Additional five variables showed bivariate correlation with the 6MWD – MIP, MEP, HGS, Borg scale, and Charlson index. Previous studies have reported several predictive variables of the 6MWD, including age, sex, weight, BMI, LVEF, anxiety, depression, quality of life, etiology of HF, functional class (NYHA), N-terminal (NT)-pro hormone BNP (NT-proBNP), glomerular filtration rate, resting HR and maximal power in the maximal effort test. 18-24 Female sexwas a predictor of shorter 6MWD in a study conductedwith 571 HF patients and 668 patients without HF. 19 Similar findings were reported in other studies on patients with HF. 18,22 On the other hand, Bajraktari et al., 23 did not report any difference in the 6MWD betweenmen andwomen. This result may be explained by the younger age and better cardiac function (indicated by higher LVEF) in female subjects than in men in their study. 23 With respect to age, previous studies have reported findings similar to ours. Age was found to be an independent predictive factor of the 6MWD in a study conducted in Poland, 20 including 243menwithHF, and in two studies from the United Kingdom 21,22 on HF patients aged from 71 to 80.5 years. Frankenstein et al., 19 evaluated 1,035 patients with HF, mean age of 54.9 years, and found a bivariate correlation between age and 6MWD (r = -0.32, p < 0.01), although no significant correlation was found in the multivariate analysis. In the study by Adel et al., 18 involving 40 HF patients (mean age of 55.6 years; 72.5% male), age was not associatedwith 6MWD. According to the authors, the small sample size contributed to these results. Increasing age is a contributing factor to a shorter 6MWD. 25 In young adults, peak VO 2 decreases by 8-10% per decade, which is exacerbated with increasing age, leading to cardiovascular and pulmonary dysfunction. Besides, other factors that may negatively affect functional capacity decreases with age, such as psychological components and neuromuscular function (e.g. sarcopenia, and decrease in muscle strength, flexibility, balance and cognition). 3 The effect of sex on 6MWD may be attributed to differences in biological and structural features between men and women, including higher muscle strength, muscle mass, and height in men than in women. 26 In studies conducted in the United Kingdom 21 and in the Netherlands, 24 pulmonary function was not a predictive variable of the 6MWD. The Dutch study showed only an association of FEV1, the FEV1/FVC ratio and total lung capacity with the distance covered by HF patients in the 6MWT. Agrawal et al., 27 correlated the 6MWD with spirometric parameters in 130 patients with chronic obstructive pulmonary disease (COPD) with characteristics similar to our study regarding age (mean of 55.6 years) and sex (58.4% male) and found a correlation of the distance covered with FEV1 and FVC. Pulmonary functionmay be altered inHF, particularly when combined with cardiomegaly, a condition characterized by enlargement of the heart. In this case, enlarged heart compete for intrathoracic space, causing compression of the lungs and limiting their expansion. 28 In a systematic review, Silva et al., 28 reported an association between cardiomegaly and reduced MIP, FVC and FEV1 in HF patients. In the present study, FVC was an independent predictor of six-minute walk distance. We did not measure cardiothoracic index in the study group for the presence of cardiomegaly. However, HF was caused by Chagas disease in most patients, a condition frequently associated with cardiomegaly. Besides, other factors may be associated with changes in pulmonary function, such as respiratorymuscle weakness, chronic pulmonary congestion and pleural effusion, which decrease pulmonary compliance and increase respiratory work. 29 Our study group showed reducedMIP, which correlated with 6MWD. This, together with cardiomegaly, may have contributed to altered pulmonary function. Further explanation on the influence of pulmonary function on 6MWD in patients with HF is needed. In the present study, mMRCwas a predictor of 6MWD. Valadares et al., 29 reported a correlation of mMRC with the London Chest Activity of Daily Living (LCAD) (r = -0.68, p < 0.05), another instrument for assessment of ADLs, that showed a strong correlation with the 6MWT (r = -0.83, p < 0.05). Camargo et al., 30 found a moderate correlation between mMRC and 6MWD in 50 patients with COPD. Similar to our findings, 80% of the patients were classified as mMRC I and II, and the mean 6MWD was 435 m; nevertheless, in their study, mMRC was not a predictive factor in the multivariate analysis. Although mMRC has been rarely used in patients with HF, the instrument has been shown to be both easy to apply and easy to understand in patients with COPD. Thus, mMRC may be a fast, simple alternative tool to evaluate functional status in HF patients.