One thing that is usually not made clear when talking about fetal circulation is why it is specifically set up the way it is. We know that the placenta, interfacing with mom’s circulation, is acting as a substitute for the lungs (gas exchange), and the kidneys and intestines (nutrient and waste exchange). The lungs are full of fluid and consequently have high vascular resistance, so they receive very little of the total amount of blood pumped out by the heart, while the placenta has low vascular resistance to encourage more blood to flow through it. Fetal hemoglobin also plays a part, as it has a greater affinity for oxygen, allowing it to load oxygen from the placenta at the same low O2 saturation that in mom’s adult hemoglobin causes the unloading of oxygen.
Theoretically, the placenta could perform molecule exchange much like a dialysis machine does, by simply taking blood from a blood vessel, performing the exchange, and then delivering the blood back to the systemic circulation via the same or a different vessel. The rest of the circulation would not need to be any different from an adult’s. But it is. Because what’s left out of most discussions is that in addition to gas and nutrient exchange, the fetal circulation is responsible for the preferential delivery of oxygenated blood to the most important organs; the brain, heart and liver. Fetal circulation first drops off a significant portion of oxygenated blood straight to the liver, and then shunts the remainder of oxygenated blood directly to the brain and heart, while it shunts deoxygenated blood past these organs. And this requires three modifications:
1. Ductus venosus (connects umbilical vein directly to the IVC)
2. Foramen ovale (an opening between the shared wall of the left and right atria)
3. Ductus arteriosus (connecting the pulmonary artery to the descending aorta)
Fresh, oxygenated, nutrient-rich blood coming from the placenta via the umbilical vein is divided up between the developing liver and the ductus venosus, which connects to the IVC. This blood from the IVC streams across the right atrium, and is shunted straight through the foramen ovale to the left atrium, where it ends being pumped by the left ventricle to the aortic arch, directly perfusing the brain and heart.
At the same time, the deoxygenated blood from the rest of the body sluggishly enters the right atrium, via the SVC and the IVC distal to the ductus venosus. This blood ends up getting pumped by the right ventricle into the pulmonary artery. Most of it bypasses the lungs and the aortic arch via the ductus arteriosus, and then mixes with the highly oxygenated blood from the aortic arch at the descending aorta, to perfuse the rest of the body.
To recap, despite the fact that highly oxygenated blood from the placenta enters the the right atrium via the IVC, the same place where the rest of the systemic circulation ALSO enters via the SVC and IVC, the anatomy actually encourages ‘preferential streaming’ of the highly oxygenated blood through the foramen ovale into the left atrium and consequently to the brain and myocardium. The deoxygenated blood from the systemic circulation bypasses the aortic arch, therefore never reaching the brain. They only come together at the descending aorta after the brain and heart have received the most oxygen-rich blood, perfusing the rest of the body and flowing via the umbilical arteries back to the placenta, where CO2 and waste products are removed, and O2 and nutrients are picked up.
DUCTAL DEPENDENT LESIONS
After birth, fluid in the lungs is cleared and placental circulation is clamped off. Pulmonary vascular resistance decreases, and blood starts flowing into the pulmonary artery, causing a decrease in RA pressure. Blood from the lungs returns to the LA via the pulmonary vein, increasing pressure there. As RA pressures decrease and LA pressures increase, the right to left flow across the foramen ovale and ductus arteriosus decreases, and they both close off soon after birth. The right side of the heart pumps blood to the lungs, the left side to the rest of the body, and congenital heart lesions (structural/anatomic defects of the heart and major blood vessels) involve problems with how these two parts of the circulatory system, driven by different sides of the heart, connect. These lesions can be broken down into three functional categories:
1. circulation bypasses the lungs
(pulmonary stenosis, pulmonary atresia, tricuspid atresia, tetralogy of fallot)
2. circulation bypasses the body
(aortic stenosis, aortic coarct, hypoplastic left heart syndrome)
3. circulation between the lungs and the body is completely disconnected
(transposition of the great arteries)
Lesions in all three of these categories can cause parts of the body to not receive any oxygenated blood, evident as cyanosis, and some of these lesions are termed ‘ductal dependent’. This means that the effects of the lesion (poor or no mixing of the two parts of the circulatory system) are mitigated while the ductus arteriosus remains open, (maintaining some mixing of the pulmonary and systemic circulation).*
If a neonate develops cyanosis or dyspnea that is not responsive to supplemental oxygen, then the differential includes problems with oxygen delivery (i.e. lung issue) or problems with circulation itself. This is when it becomes important to assess whether the infant has a ductal dependent heart lesion. Aside from getting an echocardiogram to evaluate the lesion, this is done by monitoring preductal and postductal oxygen saturations, i.e. pulse oximetry of a preductal extremity (one that is supplied by the aorta proximal to where the ductus arteriosus inserts, classically the right arm but also the left) and a postductal extremity (one of the lower legs). A difference of >10% O2 saturation between the two extremities indicates that they are likely getting blood from different parts of the circulation, and the ductus arteriosus is still patent. If there is cyanosia AND the test is positive (i.e. there is an actual difference in oxygen saturations between the extremities), there is a high chance that they have a ductal-dependent cardiac lesion. This means that any worsening cyanosis signifies relative hypoxia of those parts of the body that will soon be getting only deoxygenated blood if the ductus arteriosus closes off completely.
Identifying a ductal dependent lesion is important because we can do something about it: we can medically prevent the ductus from closing using a continuous IV infusion of Prostaglandin E, which relaxes the smooth muscle in the walls of the vasculature. Consequently one of the major side effects of PGE is hypotension, and the infant has to be observed closely while on the drip, with resuscitative fluids and inotropes available. Usually they will require intubation and ventilation as the other major side effect is apnea. (NSAIDS inhibit the COX enzymes that make prostaglandins, so they are absolutely contraindicated.)
*Note that just as lesions are ductal dependent, many lesions are only compatible with life due to additional shunts between the right/pulmonary and left/systemic circulation, e.g. atrial and ventricular septal defects.