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Increased lung water and the location of lung damage in bronchopulmonary dysplasia

Presented at the Neonatal Society 2003 Summer Meeting (programme).

Adams E1, Harrison M1, Counsell S2, Allsop J2, Kennea N1, Hajnal J2, Thornton A3, Duggan P1, Edwards D1,2

1 Department of Paediatrics, Imperial College, Hammersmith Hospital, London, UK
2 Robert Steiner MRI Unit, MRC Clinical Sciences Centre, Hammersmith Hospital, London, UK
3 Department of Radiology, Queen Mary's University Hospital, London, UK

Background: The aetiology of Bronchopulmonary dysplasia (BPD) is multifactorial and incompletely understood but may be in part due to inflammation following either under or over-distension during mechanical ventilation. We have previously demonstrated that in acute preterm lung disease, lung water content is increased, particularly in dependent regions (1). We hypothesised that this would persist in infants with BPD and be associated with non-uniform lung tissue damage, as collapse of gravity-dependent lung could lead to non-uniform ventilation.

Objective: To test this hypothesis we used magnetic resonance (MR) imaging to:
(a) measure lung water content and distribution in preterm infants with and without BPD
(b) and to obtain three-dimensional information on the type, location and extent of lung tissue injury.

Design / Methods: MR images were obtained from 35 preterm infants (gestational age 23 - 33 weeks), 15 with severe, 13 with mild and 7 with no BPD. Multi-slice axial T1 and Proton Density weighted scans were obtained with the infants lying supine or prone according to their position prior to scanning. Relative proton density was quantified to give an index of lung water content and distribution. Images were scored to reflect the location and extent of focal abnormalities indicative of lung damage:
(a) increased focal densities, probably representing atelectasis, inflammation or fibrosis
(b) cyst-like appearances, representing tissue destruction.

Results: (a) Proton density was unevenly distributed and significantly higher in dependent than non-dependent lung in all groups (p<0.02). Average proton density was significantly greater in severe BPD compared to no BPD (p<0.003).
(b) There were significantly higher scores for both focal densities and cyst-like lesions in the severe group compared with groups with mild or no disease (p<0.005) In infants with severe BPD there were significantly higher scores in the dorsal compared with ventral regions of lung (p<0.05). Cephalo-caudal and left-right distribution of lesions was not significantly different. Multivariate analysis revealed a positive correlation between the number of days of mechanical ventilation and the severity of focal abnormality.

Conclusions: Severe BPD is associated with increased water burden, dependent atelectasis and/or alveolar flooding and more extensive focal lung damage. Focal damage is inhomogeneous in nature and predominately seen in dorsal regions. This might be due to several factors, including the effects of gravity on the lung and different modes of ventilation.

1. Adams E, Counsell S, Magnetic resonance imaging of lung water content and distribution in term and preterm infants. Am.J.Respir.Crit Care Med 2002; 166:397-402.

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