Why does left to right shunting occur in a pediatric patient with Patent Ductus Arteriosus (PDA)?

Why Left-to-Right Shunting Occurs in Patent Ductus Arteriosus

Left-to-right shunting occurs in PDA because systemic arterial pressure in the aorta is higher than pulmonary arterial pressure, creating a pressure gradient that drives oxygenated blood from the aorta through the patent ductus into the pulmonary artery. 1

Fundamental Hemodynamic Mechanism

The direction and magnitude of shunting through a PDA depends entirely on the relative resistances between the systemic and pulmonary vascular beds 1, 2:

• In the normal postnatal state, systemic vascular resistance (SVR) is significantly higher than pulmonary vascular resistance (PVR), which drops dramatically after birth when the lungs expand and pulmonary vessels dilate 3
• This pressure differential (aortic pressure > pulmonary artery pressure) creates continuous flow from the high-pressure aorta into the lower-pressure pulmonary circulation throughout both systole and diastole 2, 4
• The classic "machinery murmur" heard at the left infraclavicular area reflects this continuous bidirectional flow pattern during the entire cardiac cycle 1, 5

Consequences of Left-to-Right Shunting

The hemodynamic impact depends on the size of the PDA and the volume of blood shunted 5:

Small PDA

• No significant hemodynamic consequences with normal left ventricular size and function 5
• Normal pulmonary artery pressures are maintained 5
• Patients remain asymptomatic but retain lifetime endocarditis risk 2

Moderate to Large PDA

• Pulmonary overcirculation occurs as excess blood volume recirculates through the lungs 1, 6
• Left ventricular volume overload develops from the increased pulmonary venous return, causing left atrial and left ventricular dilatation 1, 5
• Wide pulse pressure and bounding pulses result from rapid diastolic runoff from the aorta into the pulmonary artery 1, 5, 2
• Clinical manifestations include congestive heart failure, failure to thrive, dyspnea, and fatigue 1, 2, 4

Progressive Pathophysiology Leading to Shunt Reversal

A critical pitfall is missing the transition from left-to-right to right-to-left shunting 1:

• Chronic pulmonary overcirculation from a large unrepaired PDA causes progressive pulmonary vascular remodeling and increased PVR 2, 3
• When PVR exceeds SVR, the pressure gradient reverses, causing right-to-left (bidirectional or reversed) shunting—this is Eisenmenger physiology 1
• Differential cyanosis becomes the hallmark finding: deoxygenated blood from the pulmonary artery enters the aorta distal to the left subclavian artery, causing cyanosis and clubbing predominantly in the lower extremities and sometimes the left arm, while the right hand remains pink 1, 5
• Measurement of oxygen saturation in both hands and feet is mandatory to detect this pattern 1

Clinical Assessment of Shunt Direction

The physical examination and diagnostic findings directly reflect the shunt physiology 1, 5:

• Continuous machinery murmur indicates preserved left-to-right shunting with lower pulmonary pressures 1, 5
• Loss of the diastolic component (systolic murmur only) suggests developing pulmonary hypertension with equalization of diastolic pressures 1, 5
• Hyperactive precordium with laterally displaced apex beat reflects left ventricular volume overload from significant left-to-right shunting 5
• Echocardiography with color Doppler in the parasternal short-axis view directly visualizes shunt direction and estimates pulmonary artery pressure via the transpulmonary gradient 1, 5

When Cardiac Catheterization Is Essential

Invasive hemodynamic assessment becomes necessary when pulmonary hypertension is suspected 1, 7:

• Cardiac catheterization should be performed to measure PVR and assess pulmonary vasoreactivity when PA systolic pressure appears elevated on echocardiography 1, 7, 5
• This determines operability: PDA closure is recommended when PA systolic pressure is <50% systemic and PVR is <1/3 systemic with net left-to-right shunt 1
• PDA closure is contraindicated (Class III: Harm) when there is net right-to-left shunt with PA systolic pressure >2/3 systemic or PVR >2/3 systemic, as the ductus provides essential decompression for the failing right ventricle 1, 7