2.1 Fetal Circulation, PDA, PFO, and Transitional Physiology
Key Takeaways
- Fetal circulation uses three shunts—ductus venosus, foramen ovale, and ductus arteriosus—because pulmonary vascular resistance is high and the placenta keeps systemic vascular resistance low; the most oxygenated umbilical venous blood is streamed through the foramen ovale toward the brain and coronaries.
- Transition requires a PVR drop with lung expansion and oxygen plus an SVR rise after cord clamp; the foramen ovale functionally closes when left-atrial pressure exceeds right-atrial pressure, and the ductus arteriosus functionally closes as PaO2 rises and prostaglandin E2 falls, often by 24–72 hours in a healthy term infant.
- PGE1 (alprostadil) keeps a needed duct open (typical start 0.05–0.1 mcg/kg/min, wean toward 0.01–0.05 mcg/kg/min; apnea is the signature adverse effect). Indomethacin, ibuprofen, acetaminophen, or later device/ligation close an unwanted duct.
- After PVR falls, a large preterm PDA is left-to-right: wide pulse pressure, bounding pulses, pulmonary edema, and feeding intolerance from diastolic steal. A murmur in the first day of life is a finding, not a diagnosis.
- PPHN is failure of PVR to fall, with right-to-left shunting at the foramen ovale and ductus. Right-hand (pre-ductal) versus foot (post-ductal) saturations and a UAC (post-ductal) sample show the mechanism; full iNO, HFOV, and ECMO management is taught with respiratory PPHN.
Fetal Circulation, PDA, PFO, and Transitional Physiology
Quick Answer: The fetus uses three shunts—ductus venosus, foramen ovale, and ductus arteriosus—because the placenta, not the lung, is the organ of gas exchange and because pulmonary vascular resistance (PVR) is high while systemic vascular resistance (SVR) is low. At birth, lung expansion and oxygen drop PVR, cord clamp raises SVR, the foramen ovale functionally closes when left-atrial pressure exceeds right-atrial pressure, and the ductus arteriosus functionally closes as PaO2 rises and prostaglandin E2 falls. PGE1 (alprostadil) keeps a needed duct open; NSAIDs, acetaminophen, or a device/ligation close an unwanted duct. Persistent pulmonary hypertension of the newborn (PPHN) is the transitional failure in which PVR does not fall. Full inhaled nitric oxide, high-frequency ventilation, and ECMO management belong with respiratory PPHN teaching; this section owns the shunt mechanism.
Cardiovascular problems account for 10% of the current Neonatal CCRN Test Plan (exams on or after November 12, 2025). This OpenExamPrep chapter is independent teaching on alteration in the transition to extrauterine life, congenital heart defects, and neonatal cardiac surgery—the three leaves grouped here. It does not claim endorsement by AACN Certification Corporation.
Why the fetus cannot use adult circulation
In adult series circulation the right ventricle (RV) sends the entire cardiac output through a low-resistance pulmonary bed, and the left ventricle (LV) sends an equal output through a higher-resistance systemic bed. The fetus cannot do that. The lungs are fluid-filled, relatively hypoxic, and vasoconstricted, so PVR exceeds SVR. The placenta is a large, low-resistance vascular bed that both oxygenates fetal blood and keeps SVR low. Combined ventricular output is therefore arranged around three anatomic shunts that bypass the liver and the lungs and deliver the most oxygenated blood to the developing brain and coronary arteries.
A useful bedside image: fetal circulation is a parallel circuit, not a series circuit. The two ventricles pump into a shared downstream world. Most RV stroke volume never reaches an alveolus. Most of the best-oxygenated umbilical venous blood never fully mixes with the most desaturated superior vena cava (SVC) stream. When a neonate “keeps fetal pathways,” you are watching those same holes run in the fetal direction or reverse after PVR falls.
The three fetal shunts and the two resistance beds
Ductus venosus. Oxygenated blood returns from the placenta in the umbilical vein. A portion enters hepatic sinusoids, but a streaming fraction is shunted through the ductus venosus into the inferior vena cava (IVC). Umbilical venous saturation is the highest in the fetal body—often near 80%, with a partial pressure of oxygen commonly cited around 30–35 mm Hg. That is still hypoxemic by postnatal standards, but it is the best blood the fetus has.
Foramen ovale (FO). The IVC stream is directed across the foramen ovale into the left atrium (LA) and LV. That preferential streaming is why the brain and myocardium receive relatively oxygen-rich blood. SVC blood, which is more desaturated, is directed toward the tricuspid valve, the RV, and the pulmonary artery (PA). Mixing occurs, but it is incomplete; the fetus uses streaming, not a blender.
Ductus arteriosus (DA). Because PVR is high, only a small fraction of RV output enters the pulmonary microcirculation. The majority of RV output crosses the ductus arteriosus into the descending aorta and then returns to the placenta through two umbilical arteries. In fetal life the DA is a right-to-left (pulmonary-to-systemic) pathway.
Typical fetal combined-output numbers help you reason rather than decorate a flashcard: the RV often contributes about 60–65% of combined ventricular output, and placental flow may be on the order of 40% of that combined output. Those ratios explain why an infant who still depends on a closing duct can look either blue (right-to-left at FO/DA) or gray and shocky (systemic flow still depending on the DA).
| Structure | Fetal job | What should happen after birth |
|---|---|---|
| Ductus venosus | Bypasses hepatic sinusoids; delivers umbilical venous blood toward the heart | Flow ceases when the umbilical vein is interrupted; the ligamentum venosum forms |
| Foramen ovale | Right-to-left atrial shunt that favors brain and coronary perfusion | Left-atrial pressure rises after pulmonary venous return increases; the septum primum flap functionally closes |
| Ductus arteriosus | Right-to-left conduit from PA to descending aorta while PVR is high | Rising PaO2 and falling prostaglandin E2 produce constriction; functional close, then anatomic close |
| High PVR / low SVR | Keeps lung flow low and placental flow high | Lung expansion plus oxygen drop PVR; cord clamp removes the placenta and raises SVR |
What must change in the first minutes and the first days
Transition is a coordinated rise in SVR and fall in PVR.
Cord clamping removes the low-resistance placenta. SVR rises abruptly. The LV now faces a postnatal afterload, and systemic blood pressure becomes the infant’s problem rather than the placenta’s.
The first effective breaths expand the lung, create a gas–liquid interface, raise alveolar oxygen, and release hypoxic pulmonary vasoconstriction. PVR falls within minutes. Pulmonary blood flow can increase several-fold. That extra pulmonary venous return fills the LA. When LA pressure exceeds RA pressure, the foramen ovale flap is pressed against the septum secundum. That is functional FO closure. Anatomic fusion, if it occurs, happens later. A probe-patent foramen remains common in neonates and is not automatically a congenital heart defect.
Ductal constriction is slower than the PVR drop. Rising arterial oxygen tension constricts mature ductal smooth muscle. The placenta, a major prostaglandin source, is gone, and the newly ventilated lung metabolizes circulating prostaglandin E2 (PGE2) more effectively. In a healthy term infant, functional ductal closure is often underway by 12–24 hours and commonly complete by 48–72 hours. Anatomic closure by intimal cushioning and fibrosis follows over 2–3 weeks, leaving the ligamentum arteriosum. Premature ductal tissue is thinner, less muscular, and more prostaglandin-sensitive, so the same oxygen signal often fails.
Ductus venosus flow stops when the umbilical venous circuit is interrupted. Functional close is usually measured in hours to a few days.
If you remember only one sequence for the bedside: lungs open → PVR falls → pulmonary venous return rises → FO functionally closes; oxygen up and prostaglandin down → DA functionally closes; placenta gone → SVR rises. Anything that blocks a step—asphyxia, meconium, sepsis, hypothermia, extreme prematurity, or congenital heart disease—produces a delayed or disordered transition.
Delayed transition is a physiology, not a single diagnosis
Delayed transition means PVR stays higher than it should, SVR is not yet a stable postnatal afterload, or a fetal shunt remains the dominant pathway. The infant may have labile hypoxemia, a murmur that comes and goes, a pre- and post-ductal saturation gap, or a combination of pulmonary hypertension and a still-open duct. Do not treat every delayed transition as “just PPHN” or “just PDA.” Ask which resistance bed and which hole are still behaving like a fetus.
A 39-week infant with perinatal acidemia who remains 88% on 40% oxygen at 90 minutes of life, with a 6-point pre/post-ductal gap and a loud single or narrowly split S2, is telling you PVR has not fallen. A 25-week infant who is pink on CPAP on day 1 and then develops bounding pulses and pulmonary edema on day 4 is telling you PVR did fall and a large PDA is now left-to-right. Same anatomic holes, opposite physiology.
Patent ductus arteriosus: preterm versus term
After PVR falls, the pressure gradient across a PDA reverses. Blood that once went from PA to aorta now goes from the descending aorta into the PA—a left-to-right shunt. The lungs see extra volume. The systemic bed, especially the gut, kidneys, and brain in diastole, can be stolen.
Preterm PDA is common. In infants under 28 weeks or under 1000 g, a hemodynamically significant duct is frequent enough that you should look for it rather than be surprised by it. Immature ductal tissue, ongoing hypoxia or hypercarbia, infection, and liberal fluid loads all favor patency. The classic picture of a hemodynamically significant PDA (hsPDA) after the first few postnatal days includes:
- Wide pulse pressure (for example systolic 58 mm Hg with diastolic 22 mm Hg) and bounding palmar or pedal pulses
- Active precordium, sometimes a harsh systolic or continuous murmur at the left upper sternal border
- Rising oxygen or CPAP/ventilator need, hazy lung fields, and pulmonary edema
- Feeding intolerance, residuals, or a rising concern for necrotizing enterocolitis because diastolic steal reduces mesenteric flow
- Oliguria or a creeping creatinine as renal diastolic flow falls
- Metabolic acidosis that is not explained by a lung problem alone
Term PDA is a different conversation. An isolated PDA in a term infant can still steal and flood, but a duct that remains widely open in a cyanotic or shocked term neonate is more often a clue to congenital heart disease or PPHN than a primary “preterm duct” problem. Right-to-left ductal flow in a term infant is PPHN or left-heart obstruction until proven otherwise—not an invitation to give indomethacin.
Not every murmur is pathologic. Flow across a closing duct, a PFO, or branch pulmonary arteries in the first 24–48 hours is common. Pair the sound with perfusion, pre/post saturations, liver size, urine output, and the chest radiograph before you treat a murmur.
Keep the duct open versus close the duct
This is the highest-yield medication trap on this topic.
PGE1 (alprostadil) is an exogenous prostaglandin used to keep the ductus arteriosus open when systemic or pulmonary blood flow depends on it. Typical starting infusions are 0.05–0.1 mcg/kg/min, with many units weaning toward 0.01–0.05 mcg/kg/min once the duct is open and perfusion or saturations improve. Expect apnea (have a bag and an intubation plan, especially under 2 kg), fever, flushing, and vasodilation. PGE1 is an emergency drug for a closing-duct lesion. It is the wrong drug for a flooding preterm PDA.
Indomethacin and ibuprofen are cyclooxygenase inhibitors that lower endogenous prostaglandin and favor ductal constriction. They are used when the clinical goal is to close a PDA. Acetaminophen is used on many units as a medical-closure option with a different hepatic safety profile (often 15 mg/kg every 6 hours for several days—follow unit protocol and liver enzymes). Transcatheter device closure or surgical ligation/clip is reserved when medical therapy fails or is contraindicated.
Do not give a prostaglandin-lowering drug to an infant whose systemic output is PDA-dependent. Medical closure is also deferred or avoided with active necrotizing enterocolitis, severe thrombocytopenia or bleeding, and significant renal failure.
| Goal | Typical tools | Clinical picture |
|---|---|---|
| Keep DA open | PGE1 infusion; avoid NSAID or acetaminophen closure | Ductal-dependent congenital heart disease; closing-duct shock or cyanosis |
| Close DA | Indomethacin, ibuprofen, acetaminophen; later device or ligation | Preterm hsPDA with steal and overcirculation after PVR has fallen |
| Support PVR drop | Lung recruitment, treat acidosis and hypothermia, oxygen as indicated; iNO pathway if PPHN | Labile hypoxemia with right-to-left atrial or ductal shunt |
Patent foramen ovale
A patent foramen ovale (PFO) is a residual flap communication after birth. In a quiet term neonate it is often left-to-right and silent. It becomes important when right-atrial pressure is high—PPHN, severe lung disease, or right-heart obstruction—because the shunt can flip right-to-left and deepen cyanosis. Isolated PFO is not balloon atrial septostomy territory. Septostomy is a mixing procedure for selected cyanotic lesions, especially transposition with a restrictive atrial communication, and is taught in the next section.
PPHN as a failed fall in PVR
Persistent pulmonary hypertension of the newborn is the name we give when postnatal PVR stays near fetal levels. The RV continues to dump blood right-to-left across the FO and DA. The infant is hypoxemic, often labile, and may show a pre-ductal (right hand) saturation higher than a post-ductal (foot) saturation because deoxygenated PA blood enters the descending aorta through the DA. That gradient is a mechanism clue, not a complete treatment plan.
Triggers you will meet include meconium aspiration, congenital diaphragmatic hernia, sepsis or pneumonia, asphyxia, and idiopathic maladaptation. Incidence is on the order of 2 per 1000 live births in many series. Titration of inhaled nitric oxide, high-frequency oscillatory ventilation, and ECMO belongs with the respiratory PPHN section. What you must own here is the transitional diagnosis: the pulmonary vascular bed did not switch from high-resistance fetal behavior to low-resistance postnatal behavior, so fetal shunts keep running in the fetal direction.
UAC samples and pre- versus post-ductal saturations
The right hand is the usual pre-ductal pulse-oximetry site. It samples blood that has left the aorta before the ductal insertion. A foot is post-ductal. A left-hand reading is ambiguous because the left subclavian origin versus ductal insertion varies. Dual-site monitoring is how you catch right-to-left ductal shunting and how critical congenital heart disease screening is built.
An umbilical arterial catheter (UAC) tip in the descending aorta samples post-ductal arterial blood. That matters when you compare a UAC PaO2 with a right-radial (pre-ductal) PaO2 in suspected PPHN or coarctation physiology. High (thoracic, often T6–T9) versus low (below the renal arteries, often L3–L4) UAC position is a safety and mesenteric/renal discussion; it does not change the pre/post conceptual rule.
Worked bedside example: a 40-week infant with meconium-stained fluid has a right-hand SpO2 of 94% and a foot SpO2 of 86% on 60% oxygen, with a loud second heart sound. That pattern supports right-to-left ductal flow from high PVR. A 26-week infant on day 6 with identical pre- and post-ductal saturations of 96%, a pulse pressure of 40 mm Hg, and a liver 3 cm below the costal margin is not PPHN—look at the PDA and the chest radiograph.
Exam traps to lock in
- PGE1 versus indomethacin, ibuprofen, or acetaminophen are opposite intents. Name the goal (open versus close) before you name the drug.
- A murmur in the first day of life is a finding, not a diagnosis. Perfusion, saturations, and work of breathing decide urgency.
- A large left-to-right PDA can steal from the gut and flood the lung. Wide pulse pressure and feeding intolerance are not “just prematurity.”
- PPHN is failed PVR drop with right-to-left shunting. Do not close the duct while you are still using it as a pop-off, and do not start a full iNO/ECMO algorithm from this section alone.
When you want mixed-item practice after you study this physiology, use the independent OpenExamPrep bank at /practice/ccrn-neonatal. Pediatric CCRN cardiac pages at /study-guides/ccrn-pediatric cover older infants; the transitional numbers and ductal decisions in this section are neonatal.
A 26-week, 780 g infant is on nasal CPAP on day 5 of life. Heart rate is 172, blood pressure is 56/20 mm Hg, pulses are bounding, the precordium is active, and the chest radiograph shows pulmonary edema. The nurse notes increasing gastric residuals and a rising lactate. Which interpretation should guide the next conversation with the team?
A 3.4 kg term infant becomes mottled at 36 hours of life with weak femoral pulses, a rising lactate, and a metabolic acidosis. Cardiology suspects critical coarctation as the duct constricts. Which medication intent is correct while the infant is prepared for transfer?
Which description best matches fetal streaming of the most oxygenated blood?
A 40-week infant with meconium-stained fluid remains hypoxemic on 80% oxygen. The right-hand SpO2 is 93% and the foot SpO2 is 84%. The second heart sound is loud. Which transitional mechanism explains this pattern?