A short systolic murmur over pulmonary area (ie Left second inter coastal space ) is listed among 6 other auscultatory feature of pulmonary arterial hypertension.Though it is an accepted sign many would question the existence of such a murmur or its relevance in PHT.
Why does it occur ?
Acoustics principle tells us whenever velocity of blood flow exceeds a critical point(Raynolds number*) in a specific anatomical territory , a turbulent zone is created and a murmur could be generated .This is why many physiological situations like pregnancy, anemia, and some benign outflow murmurs occur.
In pulmonary hypertension , three things are thought to contribute for the murmur generation
- Dilated pulmonary artery promotes Raynauld turbulence
- Increased flow velocity (This is correlated with pulmonary artery acceleration time in Doppler)
- RV contractility (A normally functioning RV is required to generate the murmur .Once RV dysfunction sets the murmur of pulmonary hypertension usually disappear , of course a TR murmur may appear and confuse the picture )
Reference
* Reynolds number is a way to predict under ideal conditions when turbulence will occur. The equation for Reynolds number is:
(Where v = mean velocity, D = vessel diameter, ρ = blood density, and η = blood viscosity )
Dr.S.Venkatesan MD 
Sir why there is pulmonary artery dilation ???
Pls explain the mechanism
Any artery with high distending pressure will tend to dilate beyond certain limit.