Page 8, Hemodynamics, Dr. D. Penney

You should be aware of some other implications relative to the interconversion of pressure and kinetic energy. For example, if we are measuring the pressure in a catheter placed in a vessel, it is important whether a catheter is downstream or upstream opening is used. With a downstream opening catheter, the pressure measured has already had the kinetic energy portion subtracted so that the pressure you obtain is artificially low. Using an upstream opening catheter the pressure you obtain has had the kinetic energy added to it, so the pressure is artificially high.

In order to obtain an accurate measurement of pressure when the kinetic energy portion is significant, it is necessary to determine its value and then either add or subtract the value from the measured pressure. A side opening catheter will provide a more accurate value.

Summary of the implications of considering total fluid energy.