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Non-invasive measurement of reduced ventilation:perfusion ratio and shunt in infants with bronchopulmonary dysplasia: a physiological definition of the disease

¹ Neonatal Unit, Royal Infirmary of Edinburgh, Edinburgh, UK
² Department of Medicine, University of Cambridge Clinical School, Cambridge, UK

Correspondence to:
Correspondence to:
B J Stenson
Neonatal Unit, Simpson Centre for Reproductive Health, Royal Infirmary of Edinburgh, Edinburgh EH16 4SU, UK; ben.stenson{at}luht.scot.nhs.uk

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.

Abbreviations: BPD, bronchopulmonary dysplasia; PCO₂, arterial carbon dioxide tension; PO₂, arterial oxygen tension; PIO₂, partial pressure of inspired oxygen; SpO₂, arterial oxygen saturation; V_A:Q, ventilation:perfusion ratio

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