Hello all,
I went through slide 18 of the brain physics lectures
http://www.neurosurg.cam.ac.uk/pages/brainphys/18-TCD_Pulsatility_index.pdf
and I have a hard time understanding (from a physiological point of view) a recording (see attached screenshot).
It's not too easy to properly read out the trends from a slide, but it seems that ABP is more or less constant, showing maybe a slight decline.
The ICP recording shows quite a massive plateau wave, and consequently the diastolic and mean flow velocity drop, this makes sense to me. If ABP is constant and ICP is increased, cerebral perfusion pressure (CPP= MAP -ICP) drops and this should reduce blood flow (velocity ...const. MCA diameter).
However, how does the increase in systolic flow velocity come about? Does ABP get narrower with a larger systolic part? What part of the underlying physiology am I missing? Or is it just a pathological case, e.g. stenosis of the vessel?
Thanks a lot
I would strongly recommend that the features you are claiming to see in the slides are truly robust signals and not distortions due to statistical variation (i.e. chance). Just a note about caution for you. Hermann
Yes, that's a good point. I cannot assess this though, since I was not present when the data was recorded.
As blood flow velocity depends not only on perfusion pressure (delta P), blood flow may increase by changes of other variables such as vessel diameter (2*r), viscosity (η), and length (l) of the vessel.
flow velocity = delta P * (π * r⁴ / 8 * η * l)
Comment to Raoul's answer: I take it the formula you wrote in your comment is the relation between pressure gradient and rate of (volume) flow along a straight tube/pipe with constant diameter/radius. Nothing against the formula, but isn't the topic addressed by Frederico somewhat more challenging? As a pulse moves down the pipe (in this case: a blood vessel), the diameter changes. So I doubt Hagen-Poiseuille is applicable. I wonder whether it is even valid as a "reasonable" approximation.
Another problem (at least for me): as the heart compresses, then the pressure gradient wouldn't be a constant along the vessel. Does that imply that volume transport (rate) is constant? After all, the blood vessels change diameter along their length. Any comments?
Cheers, Hermann
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