PDA, PFO and fetal circulation
Question: If a neonate has a PDA that means 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'm thinking since the blood is flowing from the left side of the heart to the right side of the heart. I would think a left-to-right shunt would exist. I'm sure I am not getting something. Please help me to understand.
Answer: This can be a confusing subject as the PDA can facilitate both a right to left or a left to right shunt. The shunt direction will usually be opposite pre-delivery and post-delivery. Let me explain in a little more detail.
While the baby is in the uterus, the baby is relying on the mothers blood supply. It is not possible for the unborn baby to oxygenate its own blood. Therefore the unborn baby's pulmonary vascular resistance (PVR) is high, causing minimal blood flow through the lungs, and the systemic vascular resistance (SVR) is low, making it easy for the blood to flow through the systemic vasculature. This low SVR is beneficial for the unborn baby's heart, decreasing the work required, hence low left atrial (LA) pressure.
The ductus arteriosus is a small vessel between the aorta and the pulmonary artery. It is needed in the unborn baby to shunt blood that leaves the right side of the heart back to the left side, since there is no benefit for the blood to pass through the lungs for oxygenation. Therefore most of the blood leaving the right side of the heart is shunted back to the left side in order to bypass the pulmonary circulation. This is the first cardiac shunt. If the ductus arteriosus does not close after birth it is a patent ductus arteriosus (PDA).
The Foramen Ovale (FO) is a hole in the atrial septum between the right and left side. This hole has a small 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, into the left atrium. This is the second cardiac shunt. When the pressure becomes higher in the left atrium (after birth) the flap will be forced to close the hole. It the flap is missing the baby will have a patent Foramen Ovale (PFO), which could need surgical repair.
Fetal circulation is characterized by
- Increased PVR
- Decreased pulmonary blood flow
- Decreased SVR
- Decreased LA pressure
- R to L blood flow via PDA and Foramen Ovale
Post birth circulation is characterized by
- Decreased PVR
- Increased pulmonary blood flow
- Increased SVR
- Increased LA pressure
FO usually closes with the higher LA pressure as there is a flap of tissue that covers the FO. This flap can reopen with increased RA pressures. In 10-20% of patients it may remain open into adulthood and may require surgical closure if symptomatic.
In the term newborn the PDA usually closes 24-48 hours post-delivery, with PaO2 >50% and increased mean arterial pressure (MAP).[Barash p.1172-1173]
If the PDA remains open post-delivery, seen especially in preterm newborns, the baby will have a left to right shunt. This is all depending 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. [Stoelting 6th p.52]. Stoelting writes, that it’s estimated that 70% of preterm infants delivered before 28 weeks require medical or surgical closure of the PDA. Prostaglandins will keep the PDA open, whereas inhibition of prostaglandin synthesis appears to be an effective medical treatment for closure of the PDA. NSAID's are the most used choice for prostaglandin inhibition. Indomethacin is the usual drug of choice, but can cause decreased mesenteric, renal, and cerebral blood flow. Sometimes Ibuprofen can be used with less side-effects. [Stoelting 6th p.53]
References
Barash "Clinical Anesthesia" 6th edition p.1172-1173
Stoelting "Anesthesia and Co-existing Disease" 6th edition p.52-53
Stoelting "Anesthesia and Co-existing Disease" 5th edition p.47-48
Bucklin, Gambling and Wlody "A Practical Approach to Obstetric Anesthesia" p.423-424