1. Breath-by-breath (BB) oxygen and carbon dioxide transfer (VO2 and VCO2) at the mouth, together with respiratory pattern, were determined over sequences of 100 consecutive breaths in six subjects at rest and during cycloergometric exercise (75 W). 2. BBVO2 and VCO2 at alveolar level were calculated taking the changes of pulmonary gas stores throughout each single ith breath into account. This requires knowledge of the alveolar volume at the beginning of the breath (VA.i-1), which cannot be directly measured on a BB basis at present. Therefore a constant nominal value, ranging from 0 to 5 l, was attributed to VA,i-1. 3. It was found that: (i) all nominal VA.i-1 values employed for the estimate of steady-state alveolar gas exchange yield the same average value but greatly reduce BB variability with respect to gas exchange as determined at the mouth; (ii) no unequivocal criterion was identified to select the appropriate VA.i-1 value; and (iii) the apparent kinetics of alveolar VO2 after work onset may be influenced by the VA.i-1 value selected for the calculation, and by the change in lung volume at the onset of exercise. 4. It was concluded that: (i) as long as VA.i-1 cannot be measured for each single breath, 'true' alveolar gas exchange remains elusive; hence, (ii) it is not possible to establish the extent to which the alveolar gas exchange variability depends on physiological phenomena using available methods.
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