3.1 Acyanotic & Obstructive Heart Lesions (VSD, ASD, PDA, Coarctation, AS)

Acyanotic & Obstructive Heart Lesions: Pathophysiology, Hemodynamics, and Acute Management

Quick Answer: Large left-to-right shunts may cause pulmonary overcirculation as pulmonary vascular resistance falls after birth. A newborn with weak femoral pulses, acidosis and poor perfusion may have critical arch obstruction. Assess the anatomy urgently and start alprostadil promptly when systemic blood flow may depend on the ductus.

Left-to-Right Shunts and Pulmonary-to-Systemic Flow

Pulmonary flow ($Q_p$) and systemic flow ($Q_s$) are approximately equal in a normal circulation. A net left-to-right shunt recirculates some oxygenated blood through the lungs. Flow across a defect depends on its size and location, the pressure difference over the cardiac cycle and the downstream circulation. In an atrial septal defect, relative ventricular filling properties also matter. A small restrictive ventricular septal defect (VSD) can generate a high-velocity jet despite a small shunt volume.

Using the Fick principle

Under stable sampling conditions, the systemic and pulmonary oxygen-content differences are used to estimate the flows. The same whole-body oxygen consumption appears in each flow calculation and cancels in the ratio:

$\frac{Q_p}{Q_s}=\frac{C_{aO_2}-C_{\bar vO_2}}{C_{pvO_2}-C_{paO_2}}$

$C_{aO_2}$ is systemic arterial content, $C_{\bar vO_2}$ is representative mixed systemic venous content before shunted blood enters, $C_{pvO_2}$ is pulmonary venous content and $C_{paO_2}$ is representative pulmonary arterial content after mixing with any relevant shunted blood. Use representative samples for the actual anatomy. A single venous sample or an assumed pulmonary venous saturation can mislead with streaming, uneven flow or impaired lung oxygenation. With similar hemoglobin across samples and negligible dissolved oxygen, saturation differences approximate the content differences. Use full oxygen content when dissolved oxygen is important, including during high inspired oxygen.

Worked example: With a representative systemic arterial saturation of 98%, mixed systemic venous saturation of 68%, pulmonary venous saturation of 98% and pulmonary arterial saturation of 83%, the saturation approximation yields $(98-68)/(98-83)=30/15=2.0$. Under the stated assumptions, pulmonary flow is twice systemic flow. The ratio alone cannot tell whether systemic flow is adequate or pulmonary vascular disease is reversible. Pediatric ACC guidance uses $Q_p:Q_s \ge 1.5:1$ as one finding that can prompt referral for closure evaluation. Symptoms, chamber dilation and complications also matter. PVRI pediatric catheterization consensus · ACC pediatric shunt guidance

Recognizing the common shunts

ACC pediatric shunt guidance · ASE pediatric echocardiography · STS complete AVSD cohort

Postnatal changes and acute heart failure

As pulmonary vascular resistance (PVR) falls after birth, a large post-tricuspid shunt can carry more blood to the lungs. The infant may initially feed well and then develop tachypnea, feeding fatigue or diaphoresis, poor weight gain and hepatomegaly over the ensuing weeks. Significant AV valve regurgitation can add congestion. Assess the trajectory, work of breathing, hydration, perfusion and concurrent infection. Echo assesses shunt direction, chamber dilation, valve regurgitation and ventricular function.

Oxygen and breathing: A large left-to-right shunt often preserves systemic arterial saturation. Desaturation should prompt evaluation for pulmonary edema, lung disease, increased PVR with bidirectional flow, or another cause. Supplemental oxygen can lower PVR and may increase pulmonary flow, depending on anatomy and baseline hemodynamics. Titrate oxygen and respiratory support to the measured need and the child's physiology. Avoid routinely driving saturations to 100%, and treat clinically important hypoxemia without accepting an arbitrary low target. This biventricular physiology differs from the parallel circulation after Norwood palliation. PVRI pediatric shunt consensus · AAP neonatal preoperative cardiac care

Bridging to intervention: When congestion is present and perfusion permits, a loop diuretic can reduce symptoms. Monitor weight, intake, output, renal function and electrolytes. An ACE inhibitor can be considered for selected hemodynamics with cardiology. Its effects depend on systemic and pulmonary resistance and ventricular function. Monitor blood pressure, renal function and potassium. Have the feeding team assess endurance and aspiration risk, and individualize caloric fortification or tube support. Persistent symptoms or inadequate growth despite treatment warrant timely review for closure or repair. ACC pediatric shunt guidance · Infant captopril hemodynamic study · Neonatal cardiac nutrition guidance

PDA in preterm infants

Use echocardiography and the infant's clinical course to assess significance. AAP guidance recommends against prophylactic or routine early closure in the first two weeks. For a persistent hemodynamically significant PDA, discuss expectant management versus selected treatment with neonatology. If pharmacologic treatment is chosen, ibuprofen is commonly preferred, with acetaminophen or indomethacin as alternatives depending on the patient. Persistent important shunting despite therapy, or a contraindication to therapy, can prompt consideration of transcatheter or surgical closure. A later multicenter trial in infants born at 22–28 weeks found no improvement in the composite of death or bronchopulmonary dysplasia with active treatment compared with expectant management. AAP preterm PDA clinical report · PDA randomized trial

Pulmonary vascular disease and Eisenmenger physiology

Long-standing large shunts can promote pulmonary vascular remodeling. As resistance rises, net left-to-right flow can decline and become bidirectional or right-to-left. A quieter murmur can therefore coexist with rising pulmonary pressure or worsening cyanosis. Secondary erythrocytosis can accompany chronic hypoxemia. Established Eisenmenger physiology requires congenital cardiology and pulmonary hypertension care. Avoid shunt closure in established Eisenmenger physiology because it can precipitate right ventricular failure. In borderline cases, assessment of operability integrates catheter hemodynamics, anatomy and the clinical response in an experienced center. AHA pulmonary hypertension in congenital heart disease · PVRI pediatric shunt consensus

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Critical Outflow and Arch Obstruction

AHA pediatric catheter intervention statement · ASE pediatric echocardiography

Interpreting the gradient

A substantial right arm-to-leg systolic difference supports arch obstruction when pulses and clinical findings agree. A patent duct, low cardiac output, collateral flow, branch-vessel anatomy or cuff variability can alter the measured difference. A small or absent difference does not rule out critical neonatal disease. Look on echo for anatomic narrowing, increased flow velocity, continued antegrade flow in diastole and a damped abdominal aortic waveform in context. In follow-up of repaired coarctation, the pediatric ACC pathway uses a cuff or arch Doppler mean systolic gradient around 20 mmHg as a trigger to evaluate further. A newborn in shock needs action based on perfusion and suspected anatomy.

Worked example: A well-aligned Doppler jet of 3 m/s across a stenotic valve estimates a peak instantaneous pressure difference of $4(3^2)=36\text{ mmHg}$ if proximal velocity is negligible. This is neither a limb cuff difference nor the difference between nonsimultaneous catheter systolic pressure peaks. In a neonate with depressed left ventricular function and suspected critical valvar stenosis, evaluate the valve, ventricular function and ductal flow even if the gradient is modest. Neonatal four-limb pressure study · ASE Doppler recommendations · ACC repaired coarctation algorithm