Got this question wrong and googled an explanation: (All credit goes to http://www.usmleforum.com/files/forum/2013/2/746009.php)
This pt most likely got " VSD " :
-In order for a murmur to be present Blood has to flow across the defect from Higher Pressure Chamber to Lower Pressure Chamber.
-If the Pressure b/w two chambers is equal or nearly equal then there will be NO Flow across the defect ------ Hence NO Murmur.
-At Birth "Pulmonary Vascular Resistance is High" due to the
"Hypertrophy of Pulmonary Arterioles" during Fetal life keeping the pulmonary arterial pressure higher than normal that produce RV Hypertrophy.
*and cz of this High Pulmonary Vascular Resistance which transmit high pressure back to RV, result in "NO Shunting Across VSD"------ and this is the reason NO Murmur is detected on initial examination.
-Following birth "Involution Of Pulmonary Vasculature" start and Pulmonary Vascular Resistance start to decline as the arteriolar muscular hypertrophy regress----- and Resistance Falls to it's lowest point by the 6th wk of life
-and it is then (in a few wks after birth) when there is more shunting of blood across VSD producing a Holosystolic Murmur.
-This is the reason VSD detected in a few wks after the birth frequently on the first pediatrician's visit.
An infant with a large VSD will fail to thrive and become sweaty and tachypnoeic (breathe faster) with feeds.
First, I guess everyone was correct to diagnose it was a TOF case with VSD.
All murmur follows V shape relation to flow and intensity. TOo much flow (for eg, Such a big VSD that equalize pressure) - Weak murmur.
Too little flow (like severe stenosis, right after birth hish pulmonary resistance) - Weak murmur.
If you decide ur murmur is VSD ans option comes down to A & C. C is not realistic.
submitted by โkeyseph(99)
Gonna write another answer to this since the other answer is so long.
Basically, holosystolic murmur over lower left sternal border in a newborn is most likely a VSD. The murmur from a VSD is due to left-to-right shunt from LV to RV.
In utero and early after birth, pulmonary arterial pressure is high, so the pressure gradient between LV and RV is low. The left-to-right shunt is therefore not that substantial.
As pulmonary growth progresses, the pulmonary arterial pressure decreases, which increases left-to-right shunting and increases the murmur.