IJCS | Volume 31, Nº2, March / April 2018

147 160 140 120 100 80 60 40 20 T1 T2 T3 T4 T5 T6 T7 108 – HR at theAT HFNEF 126 – maxHR in the CPET 108.9 HR (bpm) Figure 1 – Longitudinal progression of the heart rate (HR) during the second 6-minute walk test (6MWT2) in the study participants. HFNEF: heart failure with normal ejection fraction; CPET: cardiopulmonary exercise test; maxHR: maximum heart rate; AT: CPET anaerobic threshold; T1: time zero of the 6MWT (baseline conditions); T2, T3, T4, T5, T6, T7: first, second, third, fourth, fifth, and sixth minutes of the 6MWT, respectively. Teixeira et al. Gas Analysis in the 6-MinuteWalkTest in HFNEF Int J Cardiovasc Sci. 2018;31(2)143-151 Original Article during the 6MWT2 (108.9 bpm) was similar to the HR at the CPET AT (108 bpm), shown by the dashed line (Figure 1), and 85.7% comparedwith the CPETmaximum HR (126 bpm), highlighted in the Figure. The longitudinal progression of the relative VO 2 (mL.kg -1 .min -1 ) during the 6MWT2 in the study participants is shown in Figure 2. Note in the dashed line the VO 2 value of the CPET AT (11.76 mL.kg -1 . min -1 ) and in the highlighted area, the value of 15.9 mL.kg -1 . min -1 of the CPET peak VO 2 (Figure 2). The longitudinal progression of the production of carbon dioxide (VCO 2 ) during the 6MWT2 occurred in a similar way to that of the VO 2 , reachingmaximumvalues in the 6MWT2 of 11.5 mL.kg -1 .min -1 (not displayed). Pearson's correlation (r) levels found between the maximum walked distance in the 6MWT2 with the peak VO 2 obtained in the same test and between the walked distance with the CPET peak VO 2 were r = 0.528 (p = 0.014) and r = 0.532 (p = 0.013), respectively. Discussion A review of the literature on the subject of this study – 6MWT and HFNEF – has shown that most studies in this area are focused on HFREF. Therefore, it became difficult to compare the results found in the present study, which included patients with HFNEF, with data from the literature. Due to that, the discussion belowwill be based on similar results found in studies with HFREF. Riley et al., 6 Foray et al., 7 Faggiano et al., 8 and Kervio et al. 10 analyzed the kinetics of variables using a portable gas analyzer and demonstrated that the kinetics and other cardiovascular parameters are decreased in HFREF in response to the effort imposed by the 6MWT. However, no studies have described how the kinetics of patients with HFREF respond during this test. Among the study patients, there was a predominance of patients of the female gender, with hypertension, and with an increased body mass index (BMI), 3,11 characteristics that confirm the typical clinical profile of patients with HFNEF. The average distance walked is within the values that indicate a good prognosis, i.e. , above 300 meters. 5,18 A significant percentage (81.81%) equaled or increased in the 6MWT2 the distance walked during the 6MWT1 but did not obtain values of clinical significance, i.e. , above 50 meters. 5 This reinforces the importance of conducting at least one learning and familiarization test due to a potential influence on the results of the 6MWT. 6,19 The distances obtained are aligned with the average values found in studies with patients with HFREF, such as those by Kervio et al. 10 (452.6 ± 18.7 meters), Faggiano et al. 8 (419 ± 120 meters), and Guimarães et al. 9 (470 ± 48 meters). The correlation levels found between the 6MWT2 and the peak VO 2 in the 6MWT1 and with the CPET peak VO 2 are aligned with findings in the literature, despite some discrepancies. Some authors report a good correlation between the distance of the 6MWT and the peak VO 2 , with an average of 0.73 in patients with HF 6,19 (all with HFREF). Riley et al. 6 found a high correlation (r = 0.63), whereas Lucas et al. 20 and Roul et al. 21 found a