PHYSIOLOGIC CONSEQUENCES OF CONGENITAL HEART DISEASE 



439 



aperture of the foramen ovale requires many weeks 

 or years. In fact, about 25 per cent of all adults have 

 at least probe patency of this orifice (184), a phe- 

 nomenon without functional significance unless pres- 

 sure in the right atrium subsequently becomes higher 

 than that in the left. Under these conditions the in- 

 teratrial communication will be restored and may 

 even enlarge until significant cjuantities of venous 

 blood are shunted into the left atrium. 



In the fetus the tunica media of the ductus arteriosus 

 is loose in structure and composed of elastic fibers 

 and smooth muscle (142). This histologic pattern is 

 cjuite different from that of the compact tunica media 

 of the other arterial trunks. Kennedy & Clark (149) 

 reported that functional closure of the ductus 

 arteriosus is probably due to contraction of smooth 

 muscle within its walls. Barclay and co-workers (17) 

 reported angiocardiographic studies indicating that 

 the ductus arteriosus is functionally closed 5 or 7 

 min after respiration begins, although contrast 

 medium was observed to flow intermittently through 

 the channel for a considerable period. However, 

 E\Trett & Johnson (98), employing sensitive radio- 

 isotope techniques, found some reduction in ductus 

 arteriosus flow i to 2 hours after birth, but a greater 

 reduction after 9 hours. Eldridge and collaborators 

 (93) found e\'idence of right-to-left shunting through 

 the ductus arteriosus during the first several hours 

 and days of life, since the simultaneously determined 

 arterial oxygen saturation in the foot was frequently 

 lower than that in the hand. Prec & Cassels (193), 

 using dye-dilution curves as a method for determining 

 cardiac output, also found evidence for shunting 

 which they reasoned was probably at the level of the 

 ductus arteriosus. These studies were done in the 

 quiet state and were indicative of a left-to-right shunt 

 rather than a right-to-left shunt. 



Direct observations by others (4, 202), using the 

 right-heart catheterization technique, have confirmed 

 the patency of the ductus arteriosus and the presence 

 of pulmonary hypertension lasting up to se\eral days 

 of age. Adams & Lind (4) found that the left-to-right 

 shunt through the ductus was frequently quite large. 

 The pulmonary hypertension observed by the above- 

 mentioned authors was often of such a degree that 

 pulmonary and systemic systolic pressures were 

 equal. James & Rowe (143) later found that short 

 periods of hypoxia with 10 per cent oxygen would 

 increase pressure in the pulmonary artery if the 

 arterial oxygen saturation fell to fetal levels, and 

 Eldridge & Hultgren (92) showed that in room air, in 

 those normal infants with a venoarterial shunt. 



hypoxia increased the flow through the venoarterial 

 shimt. 



Recently, interest has arisen in evaluating the 

 effect of drugs and gas mixtures on the neonatal 

 cardio\ascular system of both animals and man (3, 

 6). Relatively small amounts of acetylciioline were 

 effectixe in eliciting a response in newborn animals; 

 but in humans, on the other hand, even large amounts 

 of acetylcholine produced no drop in pulmonary- 

 artery pressure or in heart rate. These studies suggest 

 that the human neonatal pulmonary vascular system 

 is capable of vasoconstriction, but as yet there is no 

 evidence that it is capable of vasorelaxation as has 

 been demonstrated in older patients with left-to- 

 right shunts. 



It is thus possible that the pulmonary hypertension 

 in the newborn infant is due to two principal factors: 

 /) increased resistance due to thick fetal pulmonary 

 arteries, and _') increased pulmonars' blood flow due 

 to the large left-to-right shunt through the ductus 

 arteriosus. As soon as the ductus arteriosus closes, the 

 pulmonary hypertension apparently disappears, so 

 that probably the anatomic nature of the fetal pul- 

 monary arteries is not the major factor in the produc- 

 tion of the pulmonary hypertension (5). Histologic 

 studies of the structure of the pulmonary \'essels of 

 fetuses and infants has shown that the relative medial 

 thickness of the muscular arteries is high during fetal 

 life. After birth there is a pronounced drop in the 

 relative medial thickness, which continues for about 

 2 weeks. A further but gradual decrease in medial 

 thickness occurs until about 1^2 years of age, when 

 a constant le\'el is reached which is similar to that 

 found in older children and in adults (58). 



Xormal Adult Circulation 



PRESSURES. Although essentially equal amounts of 

 blood flow through the systemic and pulmonary 

 circulations in normal man, the forces required to 

 drive blood through these circulations are markedly 

 different. The systemic circulation is a relatively high- 

 resistance system requiring systolic pressures in the 

 left ventricle of more than 100 mm of mercury to 

 maintain a normal blood flow. Right ventricular 

 pressure, however, is approximately one-sixth that in 

 the left ventricle, indicating that the resistance to 

 flow through the normal pulmonary vascular bed is 

 much lower than that in the systemic bed. 



Values for cardiac output and pressure in the right 

 heart chambers were determined by Barratt-Boyes & 

 Wood (20) in 26 normal subjects ranging from 13 to 



