MadSci Network: Medicine

Re: symptoms that may appear if a newborn's ductus arteriosus fails to close?

Date: Wed Mar 16 15:37:32 2005
Posted By: Jens Peter Bork, M.D., Internal Medicine, Erlangen University Hospital
Area of science: Medicine
ID: 1110134142.Me

Dear Kelli, 

many thanks for your question regarding problems resulting from non-closure
of the Ductus arteriosus (or Ductus Botalli, after the Italian anatomist
Leonardo Botallo, b.1519 in Asti, Italy, d. 1588 Paris, France). It brings
back some memories of revising for my final exams in Medicine – congenital
heart defects were something of a dread for all of us (so many, so
complicated...), and everyone hoped and prayed they would be spared this
particularly thorny area of Pediatrics...

Fortunately, though, non-closure of the ductus arteriosus (often called
patent Ductus arteriosus, PDA), is one of the less complicated anomalies,
so don’t worry too much. 
Slightly less than 1 % of all newborns suffer from a congenital („inborn“)
heart defect, and of these, PDA is about the fourth most frequent, at 10 %
of all defects (ventricular septum defect being the most frequent at 25).
The Ductus Botalli is an absolute necessity for the fetus before birth.
Consider: After birth the two chambers of the heart transport exactly the
same amount of blood through the pulmonary (lung) circulation as through
the rest of the body, respectively. During passage through the pulmonary
capillaries, the blood is saturated with oxygen. Now in the fetus, there is
of course no breathing – the lungs are still inactive, and the body is
supplied with oxygen (and everything else) through the placenta. The lungs
are collapsed (i. e. not filled with  air), the vessels in the lung contain
only small amounts of blood and have a high resistance. The problem now is:
Where to put all the blood? The right ventricle (chamber) of the heart must
transport blood at the same rate as the left ventricle, because all the
blood the left ventricle pumps into the body will eventually end up at the
right ventricle, so the two must pump at an equal rate. On the other hand
it would not do to pump all this blood into the pulmonary circulation – too
much resistance, and (for a fetus) far too little usefulness. The solution
to the problem is the Ductus Botalli. It is a short cut (a so-called shunt)
between the pulmonary circulation and that of the rest of the body. It
links the main stem of the pulmonary artery with the Aorta – thus, blood
from the right ventricle is shunted through the Ductus arteriosus into the
„left“ circulation (there is a second such „escape valve“, namely a hole in
the wall separating the two atria of the heart, the so-called Foramen
ovale, or oval hole. But that does not matter here).

After birth, things change rapidly: The lungs are filled with air, vessel
resistance in the pulmonary circulation drops, oxygen content in the blood
returning from the pulmonary capillaries rises sharply – and from one
minute to the next, the pulmonary circulation receives – and must receive –
the same amount of blood as the circulation of the rest of the body.
Accordingly, the Ductus arteriosus serves no function any more, and it
obliterates within the first few hours of extrauterine (outside-the-womb)
life. Within weeks, it shrinks down to form a little ligament (the
ligamentum arteriosum) linking the pulmonary artery and the Aorta. 

But what if it doesn’t? You might think, since the left and the right
ventricle transport the same amount of blood, there would just be no flow
between the two systems, and that would be that – the Ductus would just sit
there like an unused road. Unfortunately, that is not the case: While both
ventricles transport the same amount of blood per minute, the left
ventricle does so at a much higher pressure than the right – compare it to
the narrows of a  river and a broad lake: While both might transport the
same amount of water per second seawards, the water will flow much more
rapidly through the narrow bed of the river than through the broad one of
the lake owing to higher pressure – and if you’d create a link between the
two, water would spill over from the narrows to the lake simply because the
pressure in the narrow river is higher.

And that is exactly what happens – the flow in the Ductus is not stopped
but reversed creating a left-right shunt. As a consequence of this, the
pulmonary circulation has to sustain a higher blood volume, and the right
ventricle has to pump against a higher resistance (every time it beats, it
has to pump the blood not only against the resistance of the lungs but also
against the force of the left ventricle pressing blood into the pulmonary
circulation). This leads to a condition known as pulmonary hypertension. In
many ways, it resembles arterial hypertension in the general circulation:
The heart muscle (of the right ventricle) has to grow stronger to meet the
added resistance, and the blood vessels in the pulmonary circulation become
thicker and more rigid in response to it. This can lead to a vicious circle
– in the final stages of the disease, the shunt flow can even re-reverse
because the pressure in the pulmonary circulation has risen so much that it
rises above the pressure in the general circulation. When this happens,
things deteriorate rapidly, as the blood coming from the right ventricle is
low in oxygen ( after all, it is on its way to the lungs to be
re-oxygenated). If this blood is mixed into the oxygen-rich blood in the
general circulation, the organism is weakened due to lack of oxygen. The
de-oxygenated blood lends a bluish tinge to the skin (called cyanosis, from
Greek „kyanos“, blue). 

Generally, a small PDA is not itself dangerous – the heart can cope. It
only carries a relatively high risk of endocarditis, a bacterial infection
of the heart valves. Of the newborns with a large PDA, however, 30 % die
in infancy; mean life expectancy is under 40 years. 

If the condition is not treated! Fortunately, of all the heart anomalies it
is the easiest to treat (and historically, was the first to be treated) –
physicians can try to force natural closure with fluid restriction,
diuretics (drugs that enhance urine flow) and the aspirin-like drug
indomethacine. If you have to proceed to surgery, it is still a relatively
light procedure: Open the thoracic cavity, visualize the duct, place a
thread around it and pull it shut (Of course, it is not that easy – the
duct is usually resected these days to prevent reopening. But since you do
not have to operate on the heart itself and you do not have to open any
large vessels, it can be repaired far more easily than other heart defects). 

So, as for your project – PDA is not really prominent among the problems
affecting children after birth – 10 % of 1 % of all newborns is not that
much. But if you want to talk about heart anomalies, PDA is a very good
example because it shows how massive the reorganization of the circulation
after birth really is and how marvellously our physiology deals with this
enormous task. And it shows how a small vessel (less than half an inch in
length, actually) can have very grave consequences.

So, lots of fun with your project!
Yours truly 
Jens Peter Bork

PDA is standard fare for any Pediatrics textbook. Also, googling for
„Ductus arteriosus“ or „Botalli“ will give you lots of material (both in
images and texts).
(located at
the University of Kansas)
has a nice image of the patent Ductus arteriosus (and all the other
congenital heart defects) plus an article on how the problem is cured.

So does
of the
American Heart Association.

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