Your referring, in several posts on Nov 25, to measuring pressure gradient using a catheter has tickled my brain cell since that time.
The quotes were "I also want to measure pressure gradients so i have to catheterize the vein and i need to see the collaterals." and
"Pressure gradient of more than 3-4 mm, visualization of hemiazygous and gonadal collaterals ..."
While I recognize this was in the context of the renal vein, the reason it captured my attention is that by fluid dynamics the velocity of blood flow and its direction of flow are a direct function of pressure gradient. When you put that in the frame of some of Mark Haacke's MRI flow quantification measurements which show, for an obstructed IJV, venous flow away from the heart for part of a pulsatile cycle, followed by substantially increased flow velocity in the correct direction, toward the heart, for the remainder of the cycle. The increase here refers to the greater flow velocity in the obstructed vein than in the other, unobstructed vein. This means the pressure gradient in the obstructed vein is significantly greater than the pressure gradient in the unobstructed vein for at least part of the cycle. This seems to imply hypertension in the obstructed vein. I have the impression that venous hypertension may be a somewhat unaddressed possible medical condition with perhaps some unknown or uncertain consequences.
You mention pressure gradient of more than 3-4 mm, which I take it to mean 3-4 mmHg. However, for the units to be correct this should probably be something like mmHg/cm where the gradient is the slope of pressure drop per unit length. So, is your 3-4 mm perhaps a pressure measurement, not a gradient measurement? Irrespective of the answer if it is a pressure measurement it means, if you can measure pressure at two points in the vein, then you can calculate the gradient. The over-riding question, however, is this: is the measurement an average over a number of pulses, systolic pressure, diastolic or something else? In order to compare your pressure measurement results to Mark Haacke's flow velocity measurement, the pressure measurement will need to be within one pulse cycle, perhaps systolic and diatolic. Is this intrapulse measurement doable with your catheter? Have to talked to Mark about comparing notes on flow vs pressure? If you would like to take this offline drop me a note at firstname.lastname@example.org