Last Wednesday I posted a question on Primary LO of the Day “How can you manipulate BP to minimise cerebral blood volume?”
As I mentioned, most registrars initially jump to the incorrect conclusion..
Here are the graphs and equations you should have used to work out you answer, but I’ll leave the final conclusion to you…
Firstly a graph relating cerebral blood flow to MAP
Cerebral blood flow is usually relatively constant between a MAP of 50 – 150mmHg, assuming intact auto regulation and normotension.
Next, which equation relates Flow to Pressure. Think back to Ohm’s Law V=IR and you will be able to derive the equation
Q=Flow, △P = pressure change R= resistance
If flow remains constant if face of a changing pressure, then resistance must also be changing.
In this context we are looking a cerebrovascular resistance. Cerebrovascular resistance controls the diameter of the vessels and hence the volume of blood contained within the cerebral circulation.
Which way will resistance change in each of the high and low MAP situations?
Now you can determine the calibre of the vessels at each end of the flat portion of the curve and determine a MAP to minimise cerebral blood volume.
Remember this only applies with intact autoregulation. If you do the sums for the steep parts of the curve, you will come up with quite different conclusions – give it a go…