From RICHARD HARDING
I would like to add some comments to the article by Jonathan Beard (‘How
lungfulls of liquid can save a tiny life’, 19 March) about ‘ventilating’
premature babies with an oxygenated liquid perfluorochemical. This is potentially
a very valuable technique as it eliminates the need for damaging high gas
pressures and high oxygen tensions (which can cause oxygen toxicity). The
new technique does not, however, completely replicate the situation in the
fetus as stated in the article.
Based on data from sheep, our understanding is that the lungs of the
fetus contain a fluid which, like fetal blood, has a rather low oxygen tension.
The fetal lungs contain 35 to 40 millilitres of this liquid for each kilogram
of body weight.
A common misconception, repeated in the article, is that the lungs contain
amniotic fluid. The lungs actively secrete their own fluid (essentially
sodium chloride solution) which is either swallowed by the fetus or flows
into the amniotic sac. Due to a valve-like action of the upper respiratory
tract of the fetus, amniotic fluid rarely enters the lungs, unless the fetus
is deprived of oxygen and begins gasping. If amniotic fluid enters the lungs,
it may impair the production of surfactant, which is necessary after birth
for lung expansion.
In fetal life it is highly unlikely that oxygen is taken up by the lungs
because the oxygen tension and oxygen content of ‘lung liquid’ is low and
there is very little blood flow through the lungs. Once the baby is born
and is exposed to atmospheric oxygen, blood flow through the lungs greatly
increases, allowing gas exchange to take place. The new technique will only
be effective if this increased blood flow has occurred.
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Richard Harding Monash University Australia
