HYPERINFLATION,
SURFACTANT AND NEONATAL LUNG INJURY
Bengt
Robertson
Laboratory
for Surfactant Research, Department of Surgical Sciences, Karolinska
Hospital, Stockholm, Sweden
Neonatal lung adaptation
implies that the liquid expanding in the fetal lung at a volume roughly
equivalent to functional residual capacity is replaced with approximately
the same volume of gas. This requires rapid transepithelial absorption of
fetal lung liquid from the airspaces to the interstitial space and a
simultaneous uniform distribution of gas in terminal airspaces, stabilized
by a film of surfactant. Hyperexpansion of a liquid filled immature lung at
birth, for example by ventilation with excessive tidal volumes during a
vigorous resuscitation maneuver, may lead to epithelial disruption with
recruitment of inflammatory cells and permeation of plasma proteins to the
airspaces, compromising the effect of subsequent surfactant therapy.
Experiments on immature lambs with a gestational age of 127-128 days have
confirmed that manual ventilation (‘bagging’) with just a few large breaths
(35-40 ml/kg) soon after birth interferes with the response to surfactant
therapy 30 min later. Control animals not undergoing the bagging maneuver
showed a significant improvement of lung compliance and alveolar expansion
after receiving surfactant. Animals subjected to the bagging procedure at
birth showed no improvement in lung compliance and alveolar expansion;
instead these lambs had wide-spread alveolar collapse associated with
influx of leukocytes to the airspaces, epithelial necrosis in terminal
conducting airways, and formation of hyaline membranes (Bjorklund et al,
Pediatr Res 42:348-355, 1997). In other experiments, in which different
volumes were inflated during a similar bagging maneuver, we found that lung
injury could be induced by volumes as low as 16 ml/kg (Bjorklund et al,
Acta Anaesthesiol Scand, in press). Administration of surfactant at birth
without preceding hyperinflation of the lungs had the anticipated
beneficial effect. In babies with respiratory distress syndrome, surfactant
should probably be administered as soon as possible after the diagnosis has
been established, preferably before the baby becomes dependent on
mechanical ventilation. Randomized clinical trials have revealed that
babies with respiratory distress syndrome breathing spontaneously can be
treated with surfactant during a brief period of intubation and that this
intervention significantly reduces the need for mechanical ventilation
during the subsequent course of the disease.