Respiratory distress syndrome and its treatment with surfactant.

Abstract

Respiratory distress syndrome (RDS) is the single most common serious condition affecting newborn babies in the U.K. (incidence about 1.5%). Most of them are premature. The incidence of RDS is related to the immaturity of the baby. It is almost invariable in babies born before 31 weeks. With modern management very few of the babies who are born after 29 weeks die. Below this age the mortality is inversely related to the gestational age. The major problem is the difficulty these small babies have in expanding their lungs, which fill with exuded protein and become atelectatic. On X-ray the lungs have a granular appearance. In the most severe disease they are so dense that it is not possible to distinguish the lungs from the heart. RDS has been thought to be due to a deficiency of surfactant on the inside of the lungs. However, these babies have a large number of compounding problems all due to the immaturity of the baby and lungs: (1) They have difficulty clearing the fluid from the lungs after birth. (2) The lung structure is very immature with virtually no alveoli. (3) The ductus arteriosus which connects the aorta with the pulmonary artery does not close, the lungs become oedematous and pulmonary hypertension develops. (4) The babies have poor muscle tone and often cannot generate sufficient pressure to expand their stiff lungs. When they do breath hard their ribs are so soft that the chest wall is drawn in, reducing the amount of air entering the chest. (5) They have immature surfactant but it is important to realize that they are not completely deficient in surfactant. The total lung phospholipid is about half that of a full-term baby. It has an abnormal composition with a reduction in the phosphatidylcholine content from about 80% of the total phospholipid to approx. 60%. This has less saturated fatty acids than usual. It is important to note that although the incidence of RDS increases as the amount of surfactant decreases, babies with low levels of surfactant do not invariably develop RDS. (6) There is a substantial leak of protein through the immature capillaries on to the lung surface. Histology of the lungs of babies who die with RDS shows the airways to be lined with hyaline membranes, which are the exuded proteins and degradation products from epithelial cell destruction. Babies with RDS have many complications which may lead to death or handicap. Approx. 30% develop pneumothoraces, which can lead to profound hypoxia unless treated rapidly. Haemorrhage into the cerebral ventricles occurs in about half of the sickest babies and is the commonest cause of death or handicap in RDS. In the long term approx. 20% of the smallest babies die, 20% of them require prolonged oxygen therapy and a few develop severe chronic lung disease. Their care is very expensive and time consuming and emotionally exhausting for their parents. In the face of such a severe problem, neonatologists have been keen to try to improve the condition of these babies by treating them with exogenous surfactant. They have been encouraged in this view by work on