Alveolar gas exchange kinetics in humans during moderate and supra-maximal intensity exercise.

In this thesis the characteristics and the physiological determinants of gas exchange kinetics in humans (Chapters I.2) during exercise performed at different intensities are illustrated and discussed on the basis of the recent data presented in the literature (Chapters I.3), including the findings regarding the dynamic responses of cardiac output and muscle oxygen extraction at the onset of exercise (Chapter I.4). After this introduction, the results obtained by investigating the possible role of systemic oxygen delivery in limiting gas exchange kinetics are summarized. To this aim, the model of the head down tilt bed rest and its consequences on cardiovascular deconditioning were exploited (Chapter II). Cardiovascular deconditioning slowed oxygen delivery response, without affecting gas exchange kinetics at the onset of exercise. The invariance of oxygen uptake kinetics after HDTBR suggests and confirms the conclusion that muscular O2 uptake during moderate intensity exercise is mainly controlled by peripheral mechanisms, rather than by the bulk O2 delivery to the muscles. Subsequently, the results obtained by evaluating in parallel pulmonary O2 uptake, cardiovascular O2 delivery, as measured with cardiac output (Q’), and muscular deoxygenation kinetics, as assessed by means of NIRS, during supra-maximal intensity and moderate intensity cycling exercises are illustrated (Chapter III). Supra-maximal exercise has been selected as a valuable tool, in these extreme exercise conditions, for examining the dynamic coupling between oxygen delivery and oxygen uptake and for inferring on the relative role of central and peripheral limitations of oxidative metabolism as a function of the exercise intensity. This study showed that the time constants of the Primary Phase of V’O2 kinetics was significantly shorter during supra-maximal than during moderate-intensity exercise when the data were fitted with a non-linear curve-fitting procedure. In contrast, the time constants of the Primary Phase of V’O2 kinetics resulted significantly larger during supra-maximal than at moderate-intensity exercise when a semilogarithmic transformation was applied to the data. The findings seem to disprove the hypothesis that Q’ kinetics at the onset of supra-maximal exercise accelerates as the metabolic power request increases. This would also imply that the system controlling O2 uptake does not behave linearly in the supra-maximal intensity domain. Concerning systemic O2 delivery, Q’ response at the beginning of supra-maximal exercise was faster than at the onset of moderate exercise and it was also faster than V’O2 kinetics at supra-maximal intensity. At the microvascular level, this scenario changed, as the delay occurring at the onset of the exercise before the increase in HHb was significantly shorter during supra-maximal than during moderate exercise. After this delay, the kinetics of muscle deoxygenation was rapid. These findings suggest that the muscular O2 consumption-to-perfusion ratio was characterised by a greater rate of change during the 120%Wmax tasks than during 80%LT exercise trials.