This chapter explains the newborn transition from fetal circulation to postnatal circulation and provides a practical bedside framework for oxygen delivery, cardiac output, pulmonary and systemic blood flow, and evaluation of suspected cardiac disease.
At birth, the infant must rapidly transition from placenta-based gas exchange to lung-based oxygenation.
| Structure | Role |
|---|---|
| Placenta | Main organ for fetal gas exchange; receives ~50–55% of fetal cardiac output |
| Umbilical vein | Carries oxygenated blood from placenta to fetus |
| Ductus venosus | Allows oxygen-rich blood to bypass most of the liver |
| Foramen ovale | Allows O₂-rich blood to cross RA → LA, preferentially supplying brain and myocardium |
| Ductus arteriosus | Allows RV output to bypass high-resistance fetal lungs → descending aorta |
| Fetal lungs | High PVR: fluid-filled alveoli + relative hypoxia keep pulmonary resistance elevated |
| Event | Physiologic Effect |
|---|---|
| First breaths inflate lungs | Alveolar O₂ rises → PVR falls |
| Pulmonary blood flow increases | More blood returns to LA and LV |
| Cord clamping removes placenta | SVR rises |
| Left-sided pressure rises | Functional closure of foramen ovale begins |
| Rising O₂ + falling ductal flow | Functional ductal closure begins |
| Normal transition | Mostly complete within 24 hours |
| Abnormal transition | May persist 3–10 days; may be associated with PPHN, PDA, CHD, or lung disease |
| Shunt Direction | Common Physiology | Bedside Concern |
|---|---|---|
| Right-to-left | PVR is high or pulmonary blood flow is limited | Cyanosis and hypoxemia |
| Left-to-right | SVR is high or PVR is low with open shunt | Pulmonary overcirculation, tachypnea, pulmonary edema, CHF |
| Bidirectional | Transitional or unstable PVR/SVR relationship | Labile saturations and variable pre/postductal difference |
Oxygen delivery (DO₂) is the amount of oxygen delivered to the body per minute.
| Variable | Meaning |
|---|---|
| DO₂ | Oxygen delivery (mL O₂/min) |
| CO | Cardiac output (L/min) |
| CaO₂ | Arterial oxygen content (mL O₂/dL blood) |
| Hb | Hemoglobin (g/dL) |
| SaO₂ | Arterial oxygen saturation |
| PaO₂ | Arterial oxygen tension (mmHg) |
The most important contributors to oxygen content are hemoglobin and oxygen saturation. Dissolved oxygen (0.003 × PaO₂) contributes minimally in most clinical situations.
Delivery only matters relative to what the tissues are using. The Fick relationship ties cardiac output, oxygen consumption and the arterial–venous content difference together.
| Variable | Meaning | Units |
|---|---|---|
| VO₂ | Oxygen consumption — what the tissues actually use per minute | mL O₂/min |
| CO | Cardiac output | L/min (or mL/kg/min in neonates) |
| CaO₂ | Arterial oxygen content | mL O₂/dL |
| CvO₂ | Mixed venous oxygen content | mL O₂/dL |
| CaO₂ − CvO₂ | What the tissues extracted in one pass | mL O₂/dL |
Baylor Ed. 33 prints "CO = VO₂ × (CaO₂ − CvO₂)" and "VO₂ = CO / (CaO₂ − CvO₂)". Both are the wrong way round: multiplying by the content difference would make cardiac output rise as extraction worsens, which is the opposite of what happens. Use the corrected forms above — CO = VO₂ ÷ (CaO₂ − CvO₂) and VO₂ = CO × (CaO₂ − CvO₂) — which are the standard Fick equations. Do not reproduce the printed version on rounds or in teaching.
A healthy neonate extracts roughly 25% of delivered oxygen, leaving a large reserve. A rising extraction ratio is the earliest haemodynamic sign that delivery is failing to keep up with consumption — it moves before lactate, which is a late marker (see section 9 for the OER bands and section 10 for lactate and NIRS).
| Problem | Practical Intervention |
|---|---|
| Low hemoglobin | Consider RBC transfusion if clinically indicated |
| Atelectasis | Optimize lung volume and respiratory support |
| Overinflation | Reduce excessive MAP or volume if impairing venous return / cardiac output |
| Poor cardiac output | Correct acidosis, optimize ventilation, give volume only if indicated, consider inotropes |
| Ductal-dependent systemic flow | Start PGE₁ when outflow obstruction is suspected |
| Cause of High VO₂ | Bedside Action |
|---|---|
| Hypothermia or hyperthermia | Maintain normothermia |
| Pain or agitation | Treat pain and reduce stress |
| Increased work of breathing | Provide adequate respiratory support |
| Arrhythmia | Diagnose and treat rhythm disturbance |
| Sepsis or comorbidity | Treat underlying disease |
| Seizures | Control seizures promptly |
OER assesses the balance between oxygen delivery and oxygen consumption.
| OER Value | Interpretation |
|---|---|
| 25–30% | Normal |
| 30–40% | Elevated extraction |
| 40–50% | Impending shock |
| >50% | Shock, tissue hypoxia, lactate accumulation |
Cardiac output is the volume of blood ejected by the left ventricle per minute.
| Variable | Meaning |
|---|---|
| CO | Cardiac output |
| SV | Stroke volume |
| HR | Heart rate |
| BP | Blood pressure |
| SVR | Systemic vascular resistance |
Neonates depend mainly on heart rate and preload to increase cardiac output — contractility reserve is more limited than in older children.
| Factor | Meaning | Clinical Implication |
|---|---|---|
| Preload | Ventricular filling before contraction | Too little → reduced CO; too much → may worsen pulmonary edema |
| Contractility | Strength and speed of contraction | Poor contractility may require inotrope support — confirm with echo first |
| Afterload | Resistance against ventricular ejection | High afterload reduces stroke volume — consider afterload reduction |
| AV synchrony | Coordinated atrial and ventricular contraction | Arrhythmias can reduce CO even when BP appears acceptable |
Normally pulmonary and systemic blood flow are balanced (Qp:Qs ≈ 1).
| Qp:Qs Pattern | Meaning | Example |
|---|---|---|
| <1 | Too little pulmonary blood flow | Pulmonary atresia or severe right-sided obstruction with cyanosis |
| ≈1 | Balanced flow | Normal transition or balanced physiology |
| >1 | Excess pulmonary blood flow | Large VSD or large left-to-right shunt with CHF physiology |
| Variable | Meaning | Where it comes from |
|---|---|---|
| Aortic O₂ sat | Systemic arterial saturation | Catheterization sample or arterial line |
| SVC O₂ sat | Systemic venous (mixed venous surrogate) saturation | Superior vena cava sample |
| Pulmonary venous O₂ sat | Saturation of blood returning from the lungs | Measured, or assumed ~95–100% when lungs are normal |
| Pulmonary artery O₂ sat | Saturation entering the lungs | Pulmonary artery sample |
| LAP / RAP | Mean left and right atrial pressure | Catheterization |
The saturation-based ratio assumes a steady state and accurate samples from each site — a sample drawn during agitation, from a mispositioned catheter, or with the pulmonary venous saturation simply assumed to be 100% in a baby with lung disease will give a misleading number. The resistance equations require true blood-flow values, not estimates, so they belong to the catheterization laboratory rather than the bedside.
Ductal shunt direction depends entirely on the SVR:PVR relationship. Interventions that change SVR or PVR change shunt direction — and can convert a left-to-right ductal shunt into right-to-left, worsening hypoxemia.
| Finding | Possible Meaning |
|---|---|
| Weak pulses, delayed capillary refill, hypotension | Systemic hypoperfusion or ductal-dependent systemic blood flow |
| Delayed femoral pulses | Coarctation of the aorta |
| Central cyanosis | Arterial desaturation |
| Pink despite apparent hypoxemia | May occur with anemia |
| Peripheral cyanosis only | May reflect cold stress rather than arterial hypoxemia |
| Tachypnea or respiratory distress | Acidosis, systemic hypoperfusion, or pulmonary overcirculation |
| Right-sided cardiac impulse | Dextrocardia |
| Single S2 | May be associated with PPHN, TGA, or pulmonary atresia |
| No murmur | Does NOT exclude critical congenital heart disease |
| Hepatomegaly | Elevated systemic venous pressure or heart failure |
The hyperoxia test helps distinguish pulmonary from cardiac causes of hypoxemia, but it does not rule out CHD.
Place the infant on 100% oxygen for at least 10 minutes → obtain preductal ABG from the right radial artery → compare PaO₂ and SpO₂ with pre-test values.
| Result | Interpretation |
|---|---|
| PaO₂ rises >20–30 mmHg (often >150 mmHg), or SpO₂ rises ≥10% | Pulmonary cause more likely |
| Minimal PaO₂ rise, often <100 mmHg | Fixed right-to-left cardiac shunt or mixing lesion more likely |
| Equivocal | Cardiac disease is still possible — obtain echocardiography |
Some lesions may partially "pass" the hyperoxia test. If clinical suspicion remains, echocardiography is mandatory regardless of the test result.
| Test | Use |
|---|---|
| CBC | Anemia, polycythemia, infection clues |
| ABG + Lactate | Oxygenation, ventilation, acidosis, tissue hypoperfusion |
| Chemistry panel | Electrolytes, renal status, metabolic contributors |
| Chest X-ray | Heart size and pulmonary vascularity. Compare the width of the cardiothymic silhouette with the width of the chest: cardiomegaly if the ratio exceeds 0.65. The thymus makes this less reliable in the newborn than in older children, so read it alongside the vascular pattern rather than alone |
| ECG | Helpful mainly for arrhythmia; rarely diagnostic for structural CHD in newborns |
| Echocardiogram | Gold standard for defining cardiac anatomy — do not delay if CHD suspected |
| CT angiogram | Coronary anatomy, pulmonary arteries, ductal anatomy, vascular relationships |
| Cardiac MRI | Ventricular function, blood flow quantification, cardiomyopathy evaluation |
If the clinical picture strongly suggests ductal-dependent disease and the infant is deteriorating, start PGE₁ before waiting for echo confirmation. An echo should still follow promptly to define anatomy.
| Mistake | Why It Is Risky | Better Action |
|---|---|---|
| Relying on a murmur to exclude CHD | Severe CHD may have no murmur at all | Use saturation pattern, perfusion, pulses, lactate, and echo |
| Treating blood pressure alone as the target | BP may not reflect organ blood flow or oxygen delivery | Assess perfusion, urine output, lactate, OER, and echo |
| Missing reverse differential cyanosis | Occurs in D-TGA with coarctation or pulmonary hypertension — can be missed if only one site is checked | Always compare right hand and foot saturation simultaneously |
| Overusing volume boluses for poor cardiac output | Poor CO may be from poor contractility, high afterload, ductal-dependent flow, or PPHN — not preload | Use clinical exam and echocardiography to guide therapy |
| Assuming hyperoxia test excludes heart disease | Some lesions may partially respond to O₂ without excluding CHD | Get echocardiography if clinical suspicion remains despite a positive hyperoxia test |
"The baby's heart and lungs normally change their circulation pattern right after birth, but sometimes that transition is incomplete or abnormal, and we need to support the heart and lungs while the change happens."
"We compare oxygen levels in the right hand and foot because this shows us how blood is moving through a special vessel called the ductus arteriosus, which normally closes after birth. A difference between the two sites can give us important information about what is happening."
"If we are worried about a heart problem, the most important test is an echocardiogram — an ultrasound of the heart that shows us the structure and how well the heart is pumping, without any radiation."