PDA, PFO and Fetal Circulation

Question:

If a neonate has a patent ductus arteriosus (PDA), blood flows from the aorta to the pulmonary artery. Yet, a PDA is said to cause a right-to-left shunt. I don't understand this. Since I think blood flows from the heart's left side to the right, I would think a left-to-right shunt would exist. Please help me understand.

Answer:

This can be confusing, as the PDA can facilitate either a right-to-left or a left-to-right shunt. The shunt direction is usually opposite pre-delivery and post-delivery. Here is a little more detail.

From the placenta comes the umbilical cord, which has one vein (carrying oxygenated blood) and two arteries (carrying deoxygenated blood). These vessels are responsible for oxygen and nutrient delivery and removing carbon dioxide and waste products. As can be seen, the fetus is dependent on the placenta.

Fetal circulation in utero is markedly different from post-delivery. In utero, only 10% of blood flows through the fetal lungs due to the high pulmonary resistance owing to alveolar collapse (fluid-filled) and compression of blood vessels, low PaO₂, and pH of blood flowing through. This leads the blood from the right ventricle (RV) on the path of least resistance, bypassing the fetal lungs and flowing through the patent ductus arteriosus (PDA) into the descending aorta. Around 90-95% of the blood from the RV uses this pathway, causing a right-to-left shunt. This also explains fetal circulation's low systemic vascular resistance (SVR). This low SVR benefits the unborn baby's heart, decreasing the work required and, hence, low left atrial (LA) pressure.

The ductus arteriosus is a small vessel between the pulmonary artery and the descending aorta. This is an extracardiac shunt. If the ductus arteriosus does not close after birth, it becomes a patent ductus arteriosus (PDA), which is kept open by the presence of prostaglandins.

The ductus arteriosus usually closes in two phases. The first phase happens within the first 24-48 hrs. It is due to smooth muscle contraction due to increased PaO₂>50%, along with decreases in prostaglandin E2 (PGE2) and prostacyclin (PGI2) due to placental removal and pulmonary metabolism. Two to three weeks later, permanent closure happens due to hypoxia in the media of the vessel, leading to endothelial proliferation, subintimal disruption, and fibrosis. [Davis 10^th p.754].

Suppose the PDA remains open post-delivery, seen especially in preterm newborns. In that case, the baby will have a left-to-right shunt, which behaves much like a ventricular septal defect, leading to left ventricular volume overload and dilation. [Baysinger 2^nd p.521] The severity depends on the pressure gradient from the aorta to the pulmonary artery (the pulmonary/systemic vascular resistance ratio) and the diameter and length of the ductus arteriosus. [Hines 7^th p.133]. As the left-to-right shunt brings more blood to the pulmonary system, which has more blood to pump forward, it can lead to fluid overload in the left atrium and ventricle. If this overload is not addressed, it further develops into congestive heart failure. In longstanding disease, Eisenmenger syndrome may ensue.

If the PDA does not close, it can be either attempted to be closed by medical or surgical intervention. Medical intervention focuses on inhibiting prostaglandin synthesis using either indomethacin, ibuprofen, or acetaminophen, of which indomethacin is considered first-line treatment.

Most infants born before 28 weeks gestation will require treatment to close the PDA, and the majority will respond well to medical treatment. [Hines 7^th p.134]

If medical treatment fails or is contraindicated, surgical intervention may be necessary to ligate the ductus arteriosus.

The Foramen Ovale (FO) is a hole in the atrial septum between the right and left atrium. This hole has a small tissue flap that can fully cover the passage of blood. When the pressure is higher in the right atrium, the flow will go from right to left and into the left atrium. This is an intracardiac shunt. After birth, the left arterial pressure increases, and the tissue flap is pressed over the foramen ovale. This usually happens within the first hour of life. Anatomic closure usually occurs within the first year of life but may remain probe-patent into adulthood in 10-20% of patients.  If the flap is missing, the baby will have a Patent Foramen Ovale (PFO), which could need surgical repair.

This flap can reopen with increased RA pressures due to increased pulmonary resistance or fluid overload. In 10-20% of patients, it may remain open into adulthood and, if symptomatic, require surgical closure.

References

Davis, C: Smith’s Anesthesia for Infants and Children 10^th ed. p.754

Hines, R: Stoelting’s Anesthesia and CO-existing Disease 7^th ed.p.133-134

Baysinger, Bucklin, Gambling: "A Practical Approach to Obstetric Anesthesia" 2^nd ed.p.521

Barash: "Clinical Anesthesia" 8^th ed. p.1179-1180

Elisha: "Nurse Anesthesia" 7^th ed.p.1207-1209