A 6-year-old boy is found to have a continuous 'machinery' murmur on cardiac auscultation during a routine school health check-up in Delhi. Echocardiography confirms patent ductus arteriosus (PDA). His mother asks how this differs from ventricular septal defect (VSD), which was diagnosed in her cousin's child. Which clinical feature best distinguishes PDA from VSD?

Explanation

## Distinguishing PDA from VSD ### Hemodynamic Signature of PDA **Key Point:** Patent ductus arteriosus creates a continuous left-to-right shunt throughout the cardiac cycle, resulting in a characteristic wide pulse pressure with bounding (hyperkinetic) pulses and a collapsing diastolic pressure. ### Comparison Table: PDA vs VSD | Feature | PDA | VSD | |---------|-----|-----| | **Murmur character** | Continuous 'machinery' (systole + diastole) | Holosystolic (pansystolic) | | **Pulse pressure** | Wide (bounding pulses) | Normal | | **Diastolic BP** | Low (collapsing pulse) | Normal | | **Location of shunt** | Ductus arteriosus (extracardiac) | Ventricular septum (intracardiac) | | **Systolic + diastolic flow** | Yes (continuous) | Systolic only | | **Bounding pulses** | Prominent | Absent | ### Mechanism of Wide Pulse Pressure in PDA The continuous shunt from the aorta to the pulmonary artery (especially during diastole when aortic pressure exceeds PA pressure) causes: 1. Increased diastolic runoff into the PA 2. Rapid fall in aortic diastolic pressure 3. Elevated systolic pressure (increased stroke volume) 4. Result: **Wide pulse pressure** with **bounding pulses** and **low diastolic BP** **Clinical Pearl:** The combination of a continuous murmur + wide pulse pressure + bounding pulses is virtually pathognomonic for PDA. This hemodynamic pattern is NOT seen in VSD, which has a normal or even narrow pulse pressure. **High-Yield:** On physical examination, palpate the radial or femoral pulse while auscultating—if you feel a prominent, bounding pulse coinciding with a continuous murmur, think PDA immediately. ### Why VSD Does Not Produce Wide Pulse Pressure VSD shunting occurs only during systole (when RV pressure exceeds LV pressure transiently). The shunt closes during diastole, so there is no diastolic runoff. Pulse pressure remains normal because diastolic pressure is not significantly reduced. [cite:Park 26e Ch 12]