Introduction Plethysmographic Specific Airways Resistance (sRaw) is measured during tidal breathing from the relationship between simultaneous measurements of airflow and change of plethysmographic pressure without need for airway occlusion.1 sRaw has been shown to be useful for discriminating lung disease in young children. It has previously been shown that breathing frequency (BF) can have a marked impact on measured values on sRaw,2 and recent recommendations have suggested BF should be maintained at 30–45 bpm.3 However, we hypothesise that the true impact of breathing pattern may relate more to pattern of breathing and flows attained than breathing frequency per se.
Aim To determine the influence of flow and breathing frequency on specific effective airways resistance (sReff) measurements.
Methods 12 healthy adults (age 17–56 year) performed repeated measures of sReff in the plethysmographic body box (V5.01, Cardinal Health) at either 30 bpm or 60 bpm at low flows (ie, quiet, natural breathing) or high flows (ie, forced breathing) in a random order. Paired t-tests were used to determine the impact of flow and BF on sReff.
Results There was a significant increase in sReff (mean difference: 0.3 kPa·s (95% CI of the difference: 0.2 to 0.4) p<0.0001) when flows were doubled from approximately ±1 l/s to ∼±2 l/s, while maintaining a constant BF. By contrast, when BF was doubled while maintaining a constant flow there was no significant change in sReff (mean difference: 0.06 kPa·s (95%CI of difference: 0 to 0.1), p=0.21) (Abstract P126 Figure 1).
Conclusion The true impact of breathing pattern on sRaw relates more to flows attained than to breathing frequency. To facilitate improved repeatability and minimise inter-subject and inter-centre variability, it is essential that subjects are encouraged to breathe quietly and naturally during plethysmographic recordings of sRaw. Software adaptations that allow accurate display of flows during data collection and analysis are required before guidelines pertaining to flow can be generated.
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