IJCS | Volume 31, Nº2, March / April 2018

149 Teixeira et al. Gas Analysis in the 6-MinuteWalkTest in HFNEF Int J Cardiovasc Sci. 2018;31(2)143-151 Original Article RQ > 1.0. Other authors agree and claim that the greater the functional deficiency of the studied cohort, the more the 6MWT would be executed close to the maximum. 6-8 In the analysis of the VE/VO 2 , the curve first descended and then ascended, which became significant after the fifth minute of the test (T6 and T7). Such progression is one of the criteria used to identify the CPET AT. 13,24 When this concept is transferred to the 6MWT, it confirms that it represents an effort similar to or slightly above the AT. In regards to the VE/VCO 2 , the values stabilized after the secondminute of effort (T3), with peak values slightly above those of the CPET AT (24.5 ± 3.1 in the 6MWT versus 23.6 of the CPET AT).The VE/VCO 2 stabilized in values that do not reflect ventilatory inefficiency. The relative VO 2 in the 6MWT showed stable values and its progression in patients with HFNEF did not differ from that found in similar studies in patients with HFREF, both in terms of progression during the 6MWT as well as in regards to the percentages obtained in relation to the CPET: 6-8,10 it reached values above the VO 2 corresponding to the CPET AT and of significant percentages of the CPET peak VO 2 (86.45%). The variable VCO 2 tended to stabilize after T5 (the fourth minute of the 6MWT), which contrasts with the data presented by Kervio et al., 10 in which this variable did not reach a stable state until the end of the 6MWT in patients with HFREF. The patients withHFREF in the study by Kervio et al. 10 performed their 6MWT above the relative VO 2 of the CPET AT, which is alignedwith findings in patients with HFNEF in this study. The VO 2 at the CPET AT obtained by Kervio et al. 10 (11.7 ± 0.6 mL.kg -1 .min -1 ) is also very similar to that found in this study (11.76 mL.kg -1 .min -1 ). It is interesting to note that 7 out of 22 patients in the present study had peak VO 2 in the 6MWT equal to or greater than the CPET peak VO 2 , representing 31.8% of the total sample. This percentage value is aligned and supersedes those by Faggiano et al. 8 (27.0%) in a HFREF population. The high intensity of the 6MWT for patients with HF is reinforced by findings from the study by Kervio et al., 10 Faggiano et al., 8 and Foray et al., 7 which demonstrated that the 6MWT leads to a demand above 85%of the values of the CPET relative peak VO 2 . Faggiano et al. 8 found a peak VO 2 at 86% of the CPET, which represented 73.0% of the VO 2 of the CPET AT. The present study showed similar percentages in relation to the CPET, but the VO 2 at the CPET AT presented higher percentages, corresponding to 85.03% of the peak VO 2 in the 6MWT in patients with HFNEF. Guimarães et al. 9 analyzed the results of a single CPET and 6MWT on a treadmill, with the participants connected to a gas analyzer and, using an incentive, found a peak VO 2 at 90% of the CPET. The values of the CPET relative peak VO 2 are aligned with the findings by Guazzi et al. 27 These authors found CPET peak VO 2 values of 15 mL.kg -1 .min -1 in HFNEF patients. The oxygen uptake efficiency slope (OUES) showed lower values in the 6MWT, but no difference in values was observed between the 6MWT and the CPET. Although the literature indicates that reduced values in bothHFREF andHFNEF, 13 theHFNEF patients evaluated obtained values above 1.2, which are considered to be of poor prognosis. 28 The metabolic variable with the most difference was the kinetics of oxygen consumption during recovery (T1/2), which showed a significantly greater recovery time in the CPET. The T1/2 values in the 6MWT did not fulfill the criteria of poor prognosis. 13,24,29 Considering that the value of 90 seconds 29 in the CPET, the study participants exceeded this value. Conclusion There is an actual possibility of patients with HFNEF to be are able to perform a 6MWT at maximum or almost maximum intensity. This estimate is based on the following observations: high percentages obtained in peak values in the 6MWT in relation to the maximum value of the variables HR (85.7%) and relative VO 2 (86.4%) in the CPET, similar RQ values, similar peak VO 2 values in the 6MWT and at the CPET AT, and the progression of the VE/VO 2 , which after reaching a nadir, showed a trend to curve upward. It should be noted that in relation to the reviewed studies, all conducted in patients with HFREF, the assessed variables in HFNEF showed, on average, an equal profile during the 6MWT. Study limitations Some limitations in this study are identified: • Reduced size of the study sample. Further studies with an increased number of patients are suggested to validate some of the conclusions.