New Approaches in Measuring to the Calculations and Analysis Modelling of Breath-by-Breath Alveolar Gas Exchanges in Humans

Abstract

The pros and cons of different algorithms developed for estimating breath-by-breath (B-by-B) alveolar O₂ transfer (${\dot{\text{V}}}$O_2A) will be detailed and discussed. ${\dot{\text{V}}}$O_2,A is the difference between O₂ uptake at the mouth and changes in alveolar O₂ stores (∆VO_2,si), which, for any given breath, are equal to the alveolar volume change at constant ${\text{F}}_{{{\rm AO}_{2} }} \left[ {\left( {{\text{F}}_{{{\rm A}i{\text{O}}_{2} }} \times \, \Delta {\text{V}}_{{{\rm A}i}} } \right)} \right]$ plus the O₂ alveolar fraction change at constant alveola volume $[{\text{V}}_{{{\rm A}i - 1}} \times \left( {{\rm F}_{{{\text{A}}i}} {-}{\rm F}_{{{\text{A}}i - 1}} } \right)_{{{\text{O}}_{2} }} ]$, where V_Ai−1 is the alveolar volume at the beginning of a breath. Therefore, ${\dot{\text{V}}}$O_2,A can be determined B-by-B if V_Ai−1 is: (i) set equal to the subject’s FRC (algorithm of Auchincloss, A) or to zero; (ii) measured (optoelectronic plethysmography, OEP); (iii) selected according to a procedure that minimises B-by-B variability (algorithm of Busso and Robbins, BR). Alternatively, the respiratory cycle can be redefined as the time between equal FO₂ in two subsequent breaths (algorithm of Grønlund, G), making any assumption of V_Ai−1 unnecessary. All the above methods allow an unbiased estimate of ${\dot{\text{V}}}$O_2,A at steady state, albeit with different precision. However, the algorithms “per se” affect the parameters describing the B-by-B kinetics during exercise transitions. Among these approaches, BR and G, by increasing the signal to noise ratio of the measurements, reduce the number of exercise repetitions necessary to study ${\dot{\text{V}}}$O_2,A kinetics, compared to A approach. OEP and G (though technically challenging and conceptually still debated), thanks to their ability to track ∆VO_2s changes during the early phase of exercise transitions, appear rather promising for investigating B-by-B gas exchange.