Learn how a stethoscope can help determine blood pressure | NCLEX-RN | Khan Academy

Learn how a stethoscope can help determine blood pressure | NCLEX-RN | Khan Academy

So this is a picture
of Doctor Korotkoff. Doctor Korotkoff is a very
famous Russian doctor. He lived about 100 years ago,
and something he came up with, something very clever,
still affects all of us, or most of us today. So he lived in the early 1900s. And around 1905, he
started thinking. He was thinking
about blood vessels. And he was trying
to figure out how to use his stethoscope–
let’s draw the skin here over the blood vessel–
how to use his stethoscope to listen and figure
out the blood pressure. So he was thinking, how
can I get about to do that? So, first thing he decided
to do was inflate a cuff, like a blood pressure cuff. And it would put
a lot of pressure on the outside of the
skin on that blood vessel. So he thought, well, let’s,
for example, let’s do something like 200 millimeters of mercury. And for this example,
let’s just assume that he’s working with me. Let’s assume that he’s
working with my blood vessel. And my blood pressure is about
115 systolic over 75 diastolic. So, if you put 200 millimeters
of pressure on the outside, what’s going to happen? The first thing is that that
vessel is definitely not going to stay open, right? It’s going to collapse down. So it’s going to do this. As it gets all that
pressure in the blood, is going to turn
right around and say, well, we can’t go
through this way. So if he listens with
his stethoscope here, if he’s listening, he would
really hear silence here. He would hear nothing at all. It would be very quiet. So let me put “silence”
because there’s no blood flowing at this point
because the vessels collapsed. OK. So Doctor Korotkoff thought
about this and said, OK, well, what if I go ahead and do
something slightly different? Let’s open this up a
little bit, because what he’s going to do next
is instead of putting 200 millimeters of pressure
as he did the first time, he’s going to put
exactly 115, exactly the systolic pressure
in my blood vessels. And he’s still going to
listen with his stethoscope. So here’s his stethoscope. As before he’s going to listen. And now the pressure on the
inside of the blood vessel actually does reach
115 for just a moment. So it gets there
for just a moment. And a little spurt of
blood gets through. So I’ll say a little
“pulse” or spurt of blood makes it through. And here’s the interesting bit. As that blood is going through,
because that vessel just barely opens up, it actually has
to squeeze its way through. And it creates a
lot of turbulence. Meaning that, that blood
is not flowing smoothly. It’s actually bouncing
off of the walls creating lots of movement. And that actually
translates into sound waves because all the little air
molecules get shaken up. And you can actually hear that. So when you hear that
pulse, when you get a pulse, you actually hear some sound. So for the first
time you actually get a little bit of noise. I’ll put that down too, “noise.” And the reason for
this noise is that you have turbulent flow,
which is actually really, really important. And it’s interesting that he
observed that or figured that out. So when you go from
silence S, to noise N, Doctor Korotkoff
realized that that’s the moment when you’ve
actually figured out someone’s systolic
blood pressure. Really kind of a neat way
to deduce this, right? Systolic blood pressure. And so then he said, OK, what
if I take it one step further? What if I relax that pressure
on the outside even more? So instead of 115, now
let’s say the pressure is, I’m going to say 75. 75 millimeters of mercury. And he is, of course, using
his stethoscope to listen. He’s still listening. And he realizes that
now blood is actually going to flow really
smoothly, very smooth flow through this vessel. And we actually call
that laminar flow. And then you get laminar flow
as opposed to the turbulent flow you had before. And now that you
have laminar flow you don’t have all that
bouncing off of the walls that I was talking
about earlier. And so because of that,
it’s actually quite quiet. It’s very silent again. So again, you go
back to silence, but for a very different reason. The first time there
was no blood flowing, that was when there was
200 millimeters of mercury. But now there’s 75
millimeters of mercury, which is a lot less. And you still get silence
for a different reason. Now there is blood
flowing, but it’s flowing so smoothly–
called laminar flow– that you really don’t hear
anything with your stethoscope. And he realized that now when
you go from a noisy situation back to a silent situation,
now you figured out someone’s diastolic
blood pressure. So every single time we go
to the physician’s office or see the nurse practitioner
to get our blood pressure taken, we owe it to Doctor Korotkoff
and his observations about 100 years ago.


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