Reversibility in plethysmographic airway and specific airway resistance in children

Plethysmographic airway resistance (R _aw) and specific airway resistance (sR _aw) are widely used to assess baseline lung function and bronchodilator reversibility in children unable to perform spirometry. However, no consensus exists on cut-offs for significant reversibility.

We conducted a retrospective study on a large cohort of asthmatic children tested during panting without electronic thermal compensation, to determine optimal R _aw and sR _aw cut-offs for detecting significant forced expiratory volume in 1 s (FEV₁) reversibility. A smaller cohort undergoing tidal breathing plethysmography with electronic thermal compensation was also analysed.

From 2436 tests without thermal compensation, receiver operating characteristic curve analysis identified cut-offs of -34.4% (R _awtot) and -36.0% (sR _awtot) from baseline to detect significant FEV₁ reversibility, with sensitivities of 70% (95% CI 65-74%) and 72% (95% CI 67-76%) and specificities of 70% (95% CI 68-72%) and 73% (95% CI 71-75%), respectively. Expressing reversibility as a percentage of predicted did not improve accuracy, but required larger cut-offs (-42.6% for R _awtot, -56.1% for sR _awtot) and increased the influence of baseline value. In the 106 tests performed using electronic thermal compensation, R _awtot/eff and sR _awtot/eff cut-offs were lower than that without electronic thermal compensation (-25% of baseline) with sensitivities and specificities ranging from 63% to 70%.

R _awtot and sR _awtot reversibility effectively identify FEV₁ reversibility with cut-offs around -35% of the baseline value, showing no added benefit when using larger cut-offs expressed as a percentage of the predicted value. When electronic thermal compensation is applied, it may necessitate lower cut-offs of -25% of the baseline, compared to those determined without it.