|MadSci Network: Medicine|
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 References: 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). http://www.kumc.edu/kumcpeds/cardiology/pedcardiodiagrams.html (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 http://www.americanheart.org/presenter.jhtml?identifier=1672 of the American Heart Association.
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