Moreover, absolute DS changes during exercise, so that also the AZD6244 VEYint value is likely close but different from the rest value. Indeed, we showed that VD tended to increase in HF patients and to reduce in healthy subjects during exercise without added DS. However, we suggest using VEYint as a tool to evaluate the presence of an increased DS, regardless of its physiological meaning with respect to rest and exercise. The adding of DS significantly reduced the external work produced in HF patients, while a not significant reduction was observed in normal subjects. Peak VO2 remained unchanged in both groups after adding DS; this finding suggests that added DS was associated to an increased work of breathing which, as a percentage of total work, seems to be greater in HF patients than in normal subjects. However, the ratio varies during exercise, so that which exercise VE/VCO2 ratio value should be considered is still a matter of debate. Moreover, while the behaviour of VE/VCO2 ratio during exercise is well described in normal and HF individuals, its behaviour in COPD or in patients with HF and COPD is less characteristic and not used as a diagnostics/prognostic tool. To avoid the above-mentioned uncertainties, many authors prefer to study the VE vs. VCO2 relationship throughout the exercise or up to the respiratory compensation point. To do so, the slope of the VE vs. VCO2 relationship is calculated, but no attention is dedicated to the intercept of this relationship on the VE axis. However, the increase of the slope of VE vs. VCO2 relationship may be blunted when COPD is associated to HF. Notably, the presence of COPD in HF may be difficult to be defined because some lung impairment is typical of HF and particularly in more advanced cases regardless of COPD. In the present study, we showed that a DS increase is parallel to the VEYint increase, so that its value should be taken into account when analyzing the VE vs. VCO2 relationship. Indeed, VEYint differences were observed even by adding a relatively small DS, which corresponded to 1/10 of peak VT in healthy subjects. It is recognized, however, that whilst the means of estimated and measured VD are similar, the individual values differ up to 60% in case of no added DS and up to,20% when 500 mL DS were added. This suggests caution when analyzing specific individual data, particularly in the presence of no or modest lung disease. In the present study, we added 250 mL and 500 mL of DS during exercise. To confirm that VEYint increase was related to DS increase, we calculated VDYint. To do so, we need to divide VE by RR, but the value of RR to be chosen is an open question. We used the intercept of the RR vs. VCO2 relationship on the RR axis because this is the RR value corresponding to VEYint. Interestingly, the changes of VDYint values with added DS were very similar to the amount of added DS. In conclusion, we provide the rational basis for the assessment of VEYint during exercise as a tool to evaluate DS. Further studies are needed to confirm and to analyze the clinical meaning of the present observation.