The Truth About Plood Pressure, Part 1: Don't Keep Up the Pressure, Just Go With the Flow!

Happy belated New Year everyone!

My New Year's resolution was to be more consistent about blogging on here; major changes to my classes' curricula last semester kept me chained to the lesson plans and LMS (on the plus side, the semester ended on a good note).  So for my first post of 2014, I wanted to make up for the lack of posts by talking about one of the most misunderstood, poorly-taught concepts in EMS education.

Blood pressure.


Many of us were taught that blood pressure is a marker of perfusion, and in our clinical practice, we use blood pressure measurements to make decisions on whether or not the patient is suffering from hypoperfusion.  That's not necessarily a bad thing, but without a good understanding of how the cardiovascular system works, and the physical laws of bloodflow, we're in danger of misinterpreting blood pressure measurements.  Think of your favorite definition of perfusion; it's probably something similar to "Blood flow through an organ or tissue".  Sounds good, but we're making a crucial error when we equate a measurement of vessel pressure with bloodflow through that vessel.

Pressure does not equal flow.

If we're seeking information about perfusion, we should be looking at blood flow, not blood pressure.  Ohm's law applied to hemodynamics as much as electrical flow:

(image source: www.cvphysiology.com)

(sorry about the background of the picture...I can't seem to get it to show up with a white background)

Don't get me wrong; you need pressure in order to have flow (F).  However, you need a change in pressure from one point to another ("triangle"P).  In the setting of tissue perfusion, that change becomes the pressures of the arterioles (Pa) and venules (Pv) of the vascular bed in question.  If we wanted to measure blood flow quantitatively, we'd need a way to measure arterial pressure, venous pressure, and resistance within the vessel or system of vessels.  Blood pressure only provides us with one-third of that!

You can see that resistance (R) plays a big role in determining flow.  In fact, resistance and flow have an inverse relationship; you can dramatically reduce flow by increasing resistance, and vice versa:

(image source: www.cvphysiology.com)

In the next post, we'll talk more about the role resistance plays in tissue perfusion, and how blood pressure doesn't tell us diddly about that either.  Until next time!