Assessing breath-by-breath alveolar gas exchange: is the contiguity in time of breaths mandatory?

Abstract

A new algorithm is illustrated for the determination of breath-by-breath alveolar gas exchange that neglects the contiguity in time of breaths, i.e. it allows the breaths to be partially superimposed or disjoined in time. Traces of oxygen, carbon dioxide fractions, and ventilatory flow were recorded continuously over 20min in 15 healthy subjects in resting conditions; at 5-min intervals, subjects voluntarily hyperventilated for ~ 30s to induce abrupt changes in lung gas stores. Gas exchange data were calculated applying the new algorithm and were compared to those yielded by a reference algorithm, also providing values at the alveolar level. Average O2 uptakes (V'O2) obtained with the two algorithms were similar during quiet breathing (0.28 ± 0.06 vs. 0.29 ± 0.06L/min; two-sided paired t test, n = 45, p = NS); during hyperventilation, average V'O2 was significantly lower applying the new algorithm compared to the reference algorithm (0.57 ± 0.15 vs. 0.65 ± 0.17L/min; difference - 0.077 ± 0.048L/min; two-sided paired t test, n = 45, p < 0.001). The first breath of each hyperventilation manoeuvre showed the greatest difference in V'O2 (- 0.25 ± 0.23L/min, z test against zero, n = 45, p < 0.001). The volumes of O2 considered twice (or neglected) because of the lack of contiguity of breaths were overall small (maximum of 3mL) and, if accounted for, had only a slight softening effect on the fluctuations of the O2 uptake. The new algorithm, which assumes each breath as the leading subject, was able to effectively account for changes in lung gas stores without requiring any predetermined value or off-line optimisation procedure.