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Learn how a stethoscope can help determine blood pressure

Learn how Dr. Korotkoff cleverly used a stethoscope to determine blood pressure, and that we still use his trick today! Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

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  • leafers sapling style avatar for user Peter Collingridge
    Isn't it harmful to stop the flow of blood with such a high pressure?
    (32 votes)
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  • piceratops ultimate style avatar for user ∫∫ Greg Boyle  dG dB
    When a doctor uses a stethoscope on your chest or back, he/she will ask you to breathe deeply then breathe normally. Why the difference? What is the doctor listening for exactly?
    (17 votes)
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    • blobby green style avatar for user jonnyboy221
      @Max101 - I am not a pulmonologist, but after some medical school I have never heard of anyone listening for the blood in the pulmonary arteries and veins. What I listen for in the chest and on the back is the quality of the air moving in the lungs, and to hear any sounds that indicate rubbing, or fluid.
      (11 votes)
  • leaf green style avatar for user Shia
    What if the Dr. measured blood pressure and it made no noise @ 115 HG (using the example mentioned in the video of a blood pressure 115/75) b/c it was measured during a diastolic period? How would he know when to precisely measure for the diastolic or systolic period?
    (9 votes)
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    • leaf blue style avatar for user Ben Fleisher
      The cuff is inflated until no noise is heard...to a pressure higher than the systolic blood pressure. The air is slowly released from the BP cuff and the needle on the gauge (sphygmomanometer) begins to fall. The point at which noise is first heard is the systolic BP. Noise is heard with every heart beat as blood is pushed/pumped though the arteries. So, in a person with a fast heart rate, you would hear a tick, tick, tick type of sound with each beat in rapid succession. While taking the BP of someone with a slower heart rate, the ticking type sounds would be heard maybe once every second as the heart beats. Noise is heard, at every heart beat as blood is "squirted" through the compressed artery, until a certain point. The point that no more noise is heard is the diastolic BP.
      (20 votes)
  • male robot hal style avatar for user tom
    why at a docters office, the nurse gets a lower reading than the doctor, which takes the reading five minutes later?
    (5 votes)
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  • female robot grace style avatar for user trovial
    i undertsand how when the hearing of the heart beat starts means systolic pressure on the vessels is the one being applied to the arm.
    Yet, i dont understand the realtion between the end of the beating and the diastolic.
    Could you elaborate a litle more on that?
    (6 votes)
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    • blobby green style avatar for user rj
      The diastolic number is achieved by the silence because the blood is flowing freely. There is no constriction or resistance on the blood to flow through a smaller space. The beating is essentially you hearing the blood still hitting the artery walls trying to squeeze through.
      (5 votes)
  • spunky sam blue style avatar for user Naomi Wolf
    at how does the doctor know he needs a pressure of 115? In our case someone else has already measured it, but to determine the pressure without knowing it before which pressures do you use?
    (4 votes)
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    • spunky sam blue style avatar for user Lord Of The Sea
      The procedure for checking blood pressure is to inflate the cuff to a point where is cuts off the pressure entirely. The examiner will listen using the stethoscope and slowly release pressure while watching the sphygmomanometer (gauge that tells you mmHg). Once the examiner can hear sounds, this will be your systolic blood pressure, once the sounds go away and continuing to release pressure, this number will be your diastolic.
      (7 votes)
  • purple pi purple style avatar for user Gabby
    How can doctor's tell that your heart is beating with a stethoscope?
    (4 votes)
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    • leaf blue style avatar for user Stian
      When the heart valves slam shut, they produce soundwaves in your chest that the stethoscope picks up. You can hear two sounds following each other, the first one is the mitral and tricuspid valves closing when the systole (heart contracting) starts. The second sound is the aortic and pulmonary valves closing when the diastole (heart relaxing) starts.
      (5 votes)
  • primosaur ultimate style avatar for user Brad English
    I understood everything up until the point where Diastolic is exactly diagnosed.

    Wouldn't just about any amount of pressure applied below the Systolic return it back to laminar flow? What's keeping us from incorrectly diagnosing Diastolic as say 60mmHg, or 105mmHg?

    Is it right at exactly 75mmHg where we begin to see 0 turbulent flow?
    (4 votes)
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    • piceratops ultimate style avatar for user Shane Proctor
      I think part of the answer to this question is revealed once you actually listen to turbulent flow with a deflating BP cuff. You will notice that it slowly fades away as it approaches what we define as the diastolic pressure. As soon as you can no longer hear "thumps" is where you mark the diastolic pressure. You will also notice the needle on the sphygmomanometer continues to bounce for a few seconds after you can no longer hear turbulent flow. So, no, "complete" laminar flow returns gradually not as soon as the pressure is under the systolic pressure. If you think about it, this makes sense given the way the heart contracts and the way BP functions as a wave.

      This also means that someone with very good hearing for low frequency sounds might be able to pick up a sound and get a slightly lower diastolic reading. Or, conversely, someone with hearing loss might get a slightly higher diastolic reading.
      (3 votes)
  • leaf yellow style avatar for user meje427
    Why is it that we use the arm to measure BP as opposed to another body part?
    (2 votes)
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    • blobby green style avatar for user Wesley Willhite
      Actually we can use different parts of the body.
      In Adults the arm is just the easiest way to get a quick and accurate BP.
      However in neonates, infants and toddlers the leg just above the ankle is actually used more often than the arm.
      Sometimes we actually take a person's BP at different places and compare the numbers. Major differences can be a sign of a various serious medical conditions.
      (6 votes)
  • male robot hal style avatar for user Tahsin
    when doing this if the vein 'burst" what will happen?
    (3 votes)
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Video transcript

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.