Prematurity and BPD
Prematurely born infants, particularly those who had BPD, are at increased risk of chronic respiratory morbidity. BPD is diagnosed in infants who are dependent on oxygen for at least 28 days after birth. Prematurely born infants are classified at 36 weeks’ post-menstrual age as having mild, moderate or severe BPD according to their respiratory support requirement at that date. In the past, infants who developed BPD had frequently suffered severe respiratory failure, necessitating both high inflating pressures and supplementary oxygen. Nowadays, BPD can occur in very prematurely born infants who initially had minimal or even no signs of lung disease: the so-called ‘new’ BPD. In new, compared with ‘old’, BPD, there is less interstitial fibrosis, but there is an arrest in acinar development resulting in fewer and larger alveoli; there is also a reduction in the number of arteries. It has been suggested that abnormal vascular development may lead to abnormalities in lung growth and that new BPD is a maldevelopment sequence resulting from interference with or interruption of normal developmental signalling for terminal maturation and alveolarisation of the lungs of very pre-term infants.
BPD has a multi-factorial aetiology (figure 1). It can occur in infants born at term who had severe respiratory failure, but it is commonest in very prematurely born infants. It was originally thought that BPD was caused by oxygen toxicity: prematurely born infants are deficient in antioxidant enzyme systems at birth and have low levels of antioxidants, making them more vulnerable to oxygen toxicity. High airway pressures have been associated with the development of BPD, with an inverse relationship between carbon dioxide levels and BPD development. Volutrauma in the first minutes after birth may be injurious to the lungs. BPD is commoner in infants who develop a patent ductus arteriosus, particularly if this is temporally related to infection. Chorioamnionitis can increase the risk of BPD, but only if associated with ‘other hits’ such as post-natal infection and a requirement for mechanical ventilation for more than 7 days. Some infants have a family history of BPD and certain genetic polymorphisms have been associated with the development of BPD.