Breath‐by‐breath measurement of alveolar gas exchange must preserve mass balance and conform to a physiological definition of a breath

Figures

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  FIGURE 1. (a) Gas flow, where positive deflections occur during the expiration phase and negative deflections during the inspiration phase. The boundaries of the grey and white areas represent a single breathing cycle according to the classical definition (i.e., the time interval between the start of two consecutive inspiration phases). (b) Intra‐breath trace of oxygen fractional concentration (FO2). (c) Intra‐breath trace of nitrogen fractional concentration (FN2). Of note, FN2 is not typically measured directly by commercial breath‐by‐breath systems; rather, it is estimated as FN2 = 1 − (FO2 + FCO2). Accurate estimation of FN2 requires properly aligned intra‐breath traces of FO2 and FCO2, necessitating the use of fast‐response O2 and CO2 sensors. Misalignment introduces distortion in the estimated FN2 trace, especially during inspiration–expiration transitions (and vice versa). (d) Intra‐breath trace of the FO2 to FN2 ratio. This trace is used to identify the breathing cycle when applying the IND algorithm. The panel shows the breathing cycles identified by the IND algorithm (thick horizontal black line) and those identified by the classical definition (grey and white areas). Notably,…

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  FIGURE 2. (a) For the IND algorithm, the reference value t 1 is chosen according to specific criteria, whereas t 2 is chosen to yield FO2/FN2 equal to that at t 1. The breathing cycle is defined as the period elapsing between t 1 and t 2. ‘Exp’ and ‘Insp’ mark the expiration and inspiration phases, respectively, of a conventional breath. (b) Example of overlaps and gaps between breathing cycles identified using the IND algorithm. The red and blue vertical areas represent the overlap and gap between two breaths, respectively. (c) Series of breathing cycles with overlaps and gaps identified using the IND algorithm. The red and blue squares depict when an overlap or gap occurs, respectively. Traces were collected and edited from a healthy, physically active male adult during steady‐state moderate‐intensity work‐rate cycle ergometry.

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  FIGURE 3. (a) O2 volume computed during each overlap (red) and gap (blue) observed during high‐intensity step‐wise exercise transition. The protocol consisted of 3 min of rest, followed by 6 min at 20 W, 6 min at 250 W (which was above critical power for this individual), and 6 min recovery at 20 W. Grey lines represent the cumulative O2 volume for overlaps (bottom) and gaps (top). The IND algorithm identified a total of 412 breathing cycles, of which 376 presented with gaps and overlaps. Note the uneven distribution of gaps and overlaps immediately at the onset and offset of the step transitions. Despite the same exercise duration (i.e., 6 min), the total number of overlaps and gaps differed between the pre‐ and post‐250 W bouts (pre‐bout: gaps n = 30, overlaps n = 21; 250 W bout: gaps n = 55, overlaps n = 91; post‐bout: gaps n = 68, overlaps n = 79). Nineteen gaps and 13 overlaps were identified during the initial 3 min of rest. (b) Zoomed‐in view of the first 2 min of the onset of the step‐wise transition phase (overlaps in red, gaps in blue, breaths without gaps/overlaps in black). Note the prevalence of overlaps during this phase.