Measuring TR peak velocity is the most popular method to assess pulmonary arterial pressure.It is universally believed TR jet predicts the systolic PA pressure fairly accurately. By all means it is a wrong perception.
At best , it has only 40% correlation with cath derived PAP . In other words cardiologist are fooled by TR jet more often than not ! Here is an elegantly done study from American Journal of Respiratoty and critical care medicine in patients who had undergone lung transplantation . It compared systolic PAP derived from Doppler vs cardiac cath.
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Source : http://ajrccm.atsjournals.org/content/167/5/735.full.pdf+html
Important observations about TR jet derived PAP
- Over estimation is the key error.
- Error of under -estimation less common .
- Over estimation often occur in normal persons
- Under estimation more frequent in patients with PAH.
(The above study documents over estimation of 10mmhg in systolic PAP in 50 out of 100 patients )
Final message
Nothing is perfect in science , especially in medical science. In spite of the limitations of TR jet , it will remain the corner stone in the hemodynamic evaluation of right heart pressures . (Forget for the moment . . . the umpteen variables in the modified Bernolui equation , flow acceleration , viscous friction etc )
It is prudent , cardiologists are expected to be aware of this harsh fact and should be meticulous in tracing TR jet and reduce the error.
One controversial but logical suggestion would be to drop the ritual of adding empirical RA pressure 5- 10mmhg over the TR jet while calculating PAP , as there is 60 % error of over-estimation that naturally occur with TR jet.
Reference
http://www.registroep.org/documenti/IPERTENSIONE%20P.%20CRONICA%20TE/06_Sciomer%20ECO.pdf
http://ajrccm.atsjournals.org/content/167/5/735.full.pdf+html
Dr.S.Venkatesan MD 
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