Background and aims Current definitions for bronchopulmonary dysplasia (BPD) lack objectivity. A physiological definition for BPD where the level of shunt and the reduction in ventilation-perfusion ratio serve as an objective grading of severity has been suggested. Shunt and reduced VA:Q can be measured non-invasively by determining the relationship of arterial oxygen saturations (SpO2) to the fraction of inspired oxygen (FiO2). Our aims were to: 1. quantify shunt and reduced VA:Q in infants with BPD and in preterm infants without BPD. 2.correlate shunt and VA:Q to clinical grading of severity where possible

Methods The group study population consisted of 10 infants (two with ‘No BPD’, two with ‘Mild BPD’ and six with ‘Severe BPD’) based on the NIH grades of BPD severity. Stepwise alterations in FiO2 were made, whilst ensuring infants stayed within the Monash Newborn SpO2 alarm limits. A two compartmental model of gas exchange was used to derive the SpO2 vs. FiO2 curves and values for shunt and VA:Q.

Results Five out of six infants with ‘Severe BPD’ and one infant with ‘Mild BPD’ had VA:Q well below normal, range 0.34 to 0.56. Two infants with ‘No BPD’ and two infants with BPD, had SpO2 vs. FiO2 curves suggesting no impairment in gas exchange. The level of shunt and reduction in VA:Q did not consistently reflect the clinical grading of BPD.

Conclusions Our results reinforce the need for a more objective definition of BPD as the possibility of misclassification using the clinical definition occurred on three occasions.