Abstract

Background: An objective definition of bronchopulmonary dysplasia (BPD) is required to interpret trial outcomes and provide a baseline for prognostic studies. Current definitions do not quantify disease severity. The cardinal measures of impaired gas exchange are a reduced ventilation:perfusion ratio (V_A:Q) and increased right to left shunt. These can be determined non-invasively by plotting arterial oxygen saturation (Spo₂) against inspired oxygen pressure (PIo₂).

Aims: To describe the reduced V_A:Q and shunt in infants with BPD and evaluate these as graded measures of pulmonary dysfunction.

Methods: 21 preterm infants with BPD were studied. PIo₂ was changed stepwise to vary Spo₂ between 86% and 94%. Pairs of PIo₂ and Spo₂ data points for each infant were plotted and analysed to derive reduced V_A:Q ratio and shunt.

Results: In every infant, the Spo₂ versus PIo₂ curve was shifted to the right of the normal because of a reduced V_A:Q. The mean (SD) shift was 16.5 (4.7) kPa (normal 6 kPa). Varying degrees of shunt were also present, but these were less important in determining Spo₂ within the studied range. The degree of shift was strongly predictive of the PIo₂ required to achieve any Spo₂ within the range 86–94% (R²>0.9), permitting shift and V_A:Q to be determined from a single pair of PIo₂ and SpO₂ values in this range.

Conclusions: The predominant gas exchange impairment in BPD is a reduced V_A:Q, described by the right shift of the Spo₂ versus PIo₂ relationship. This provides a simpler method for defining BPD, which can grade disease severity.