In a previous study we assessed the feasibility of measuring bronchial-reactivity (BHR) in young asthmatic children by the determination of PC₂₀-FEV₁ along with clinical end-of-test criteria during a methacholine challenge test (MCT). The end-point was associated with a significant reduction in both flow and vital capacity values. The findings could be due to the children's loss of motivation, which may preclude use of this test. Alternatively, if it reflects air trapping during airway obstruction, it might reinforce its applicability in preschool age children.

To elucidate the mechanism of low vital capacity at PC₂₀-FEV₁ in preschool age children.

Twenty-eight children (3.3–6.9 years) with recurrent respiratory symptoms.

An MCT was carried out using tripling doses (0.06–13.9mg/ml) delivered by a dosimeter. Spirometry was measured at baseline and after each inhalation in duplicate sets. Whole body plethysmography was measured at baseline and at end-of-test (defined by clinical criteria) according to the recommendations for older populations.

Plethysmography was reliably performed by 20 children before and after MCT. At baseline, lung function was within the healthy range. At end-of-test (PC₂₀-FEV₁=4.02±3.47mg/ml), the spirometry parameters and specific conductance values were markedly reduced in correlation with a significant increase in residual volume and resistance.

The study shows that diminished vital capacity is due to the increase in FRC at end-of-test. Our findings support the use of PC₂₀-FEV₁ during BHR in young children and suggest that lung volume measurement by a plethysmograph may be feasible in early childhood. Larger studies should be performed to establish the clinical applicability of PC20-FEV1 determination in the preschool age.