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 (VA: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 VA: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 VA: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 VA: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 VA: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 VA: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.
- BPD, bronchopulmonary dysplasia
- Pco2, arterial carbon dioxide tension
- Po2, arterial oxygen tension
- PIo2, partial pressure of inspired oxygen
- Spo2, arterial oxygen saturation
- VA:Q, ventilation:perfusion ratio
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