Objective      To develop and evaluate the reference equation of tidal breathing parameters for Chinese infants and young children up to 3 years old in the Southwest.  Methods      Pulmonary function tests were measured in 166 infants using the Jaeger BabyBody system with standard protocols. Twelve tidal breathing parameters were analyzed. SPSS software (version 18.0) was used for statistical analyses. Association between each parameter and related variables on lung function were quantified using Pearson correlation. The impact of Dichotomous variables on lung function was analyzed by independent sample t test. Reference equations adjusted for independent variables

i.e.

body weight (W) and length (L)

age (A)

and sex (S) were built using stepwise regression model. A comparison between our estimates and those published reference equations was done by the mean of relative prediction error (RPE%)，which was defined as RPE%=Σ∣predicted value-observed value∣/ predicted value*100%/n.  Results      The following equations described the correlation of tidal breathing parameters to the variables W

L

A and S: VT (tidal volume)=20.80+0.34×A+4.38×W+4.91×S; RR (respiratory rate)=30.30-0.06×A; PTEF/VT (peak tidal expiratory flow to tidal volume)=130.72-0.27×A-8.98×S; PEF (peak expiratory flow)=25.62+6.88×W；TEF75 (tidal expiratory flow when 75% of tidal volume remained in the lungs)=20.33+6.99×W；TEF50=23.07+6.39×W；TEF25=0.07+2.57×W+0.55×L. Among them

RR and PTEF/VT were negative linear correlated to age. The correlation coefficient r were -0.5 and -0.4

respectively. In contrast

a positive linear relationship was found between VT and age (r=0.8) as well as expiratory flow (including both PEF and TEF) and body weight (r=0.5

0.5

0.6

and 0.6 for PEF

TEF75

TEF50

and TEF25). The RPE% analysis indicated that our developed reference equations and equations reported by Nguyen TT were more applicable to 0~3 years’ old infants and children of our population.  Conclusion      Our newly defined reference equations for tidal breathing parameters are applicable for infants and young children up to 3 years old in Southwest China by using commercially available equipment. The tidal breathing parameters are linear related to age and/or weight.