Respir Med. 2013 Jul;107(7):975-80.

Forced expiratory decay in asthmatic preschool children–is it adult type?

Vilozni D, Hakim F, Livnat G, Bentur L.

The Edmond and Lili Safra Children’s Hospital, Sheba Medical Center, Ramat-Gan, Affiliated with Sackler Medical School, Tel-Aviv, Israel.



BACKGROUND: The forced expiratory decay in healthy preschool children portrays a convex shape that differs from the linear decay in the older healthy population. The “adult-type” expiratory decay during airway obstruction is concave. The study objective was to determine if the expiratory decay in young asthmatic children is “adult-type”.

METHODS: Among 245 children (age 3-7 yrs), 178 had asthma (asthmatics) and 67 were non-asthmatic (controls). The expiratory flow decay was inspected by FEF25-75/FVC ratio (=1.0 when linear). Values were compared to those of our formerly studied (n = 108) healthy children. A meaningful obstruction in FEF25-75/FVC ratio was defined as 2-zScores from healthy. A meaningful response to bronchodilators was related to asthmatics and non-asthmatics.

RESULTS: In healthy subjects FEF25-75/FVC ratio declined with age from 1.73 ± 0.17 to 1.28 ± 0.11. Non-asthmatics portrayed ratio values similar to those of healthy subjects. In asthmatics, 118/178 displayed a convex to linear expiratory decay (FEF25-75/FVC = 1.33 ± 0.22). Sixty/178 asthmatics portrayed concavity (FEF25-75/FVC-0.79 ± 0.16) that appeared when FEF50 was 43.4 ± 12%healthy. Concavity appearance was also age-dependent (30.4% of 3-4 y old and 59.1% of 6-7 y). Vital-Capacity decreased in either decays, forming a visually petit curve. Most asthmatic children respond to bronchodilators by a meaningful elevation in FEF25-75/FVC values and by a visual change in the shape of the curve. Other common spirometry indices also increased meaningfully.

CONCLUSION: Most asthmatic preschool children portray convex to linear expiratory decay with diminished vital-capacity, resulting in a visually smaller than healthy curve, with seemingly normal expiratory decay. These curves may be misinterpreted as “normal” or as “no-cooperation” and may lead to misinterpretation. In response to bronchodilators, FEF25-75/FVC value increases in asthmatics and the curve changes from concave to linear or from linear to convex contour. Copyright © 2013 Elsevier Ltd.

PMID: 23664671



Advances in spirometry measurement techniques have made it possible to obtain flow/volume curves in children as young as 3 years of age. However, in practice, the interpretation of the flow volume curve in these children is often based on recommendations for adults which include:

  1. The forced expiratory volume in first second of expiration (FEV1).
  2. The ratio between FEV1 and the forced vital capacity
  3. The concavity of the expiratory decay.

These parameters may not be suitable for preschool children for the following reasons:

  1. Preschool children will exhale all their vital capacity within less than a second, therefore FEV1 and by extension FEV1/FVC, are invalid [1]
  2. The airways of a preschool child are smaller in caliber than that of adults. Indeed we previously found that preschool children with asthma who had no current respiratory symptoms and normal flow/volume curve showed elevated FRC and RV in relation to the healthy [2].  Therefore it appears that these children cannot take a deep breath due to air trapping which in turn will limit expiratory volume and produce a small vital capacity.
  3. The airways of preschool children are fully developed at birth while the lung parenchyma is not. This anatomical structure renders the expiratory flow/volume ratio higher than that of adults (convex flow/volume curve decay rather than linear in shape) and therefore during obstruction we should not expect concavity of the curve. [3]

Indeed we found that most of our asthmatic preschool children displayed a linear expiratory decay (measured by FEF25-75/FVC ratio). Values were significantly lower than in healthy controls of similar age. Our interpretation is that a small flow/volume curve with a linear expiratory decay indicates airway obstruction which is reversible by bronchodilators.



  1. Vilozni D, Barak A, Efrati O, Augarten A, Springer C, Yahav Y, Bentur L. The role of computer games in measuring spirometry in healthy and “asthmatic” preschool children. Chest. 2005 Sep;128(3):1146-55.
  2. Vilozni D, Efrati O, Hakim F, Adler A, Livnat G, Bentur L. FRC measurements using body plethysmography in young children. Pediatr Pulmonol. 2009 Sep;44(9):885-91.
  3. Vilozni D, Hakim F, Livnat G, Bentur L. Forced expiratory decay in asthmatic preschool children–is it adult type? Respir Med. 2013 Jul;107(7):975-80
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