6
$\begingroup$

As per my understanding a sphygmomanometer when wrapped around the arm and inflated only measures the pressure of the air inside the cuff, doesn't it? How does that translate directly to the pressure value of blood flowing across the artery?

enter image description here

$\endgroup$
5
$\begingroup$

Korotkoff sounds!

The blood pressure measurement process is fairly cool, and goes like this.

  1. Inflate the cuff to well over plausible blood pressures (250mmHg or so).
  2. Slowly deflate the cuff while listening to the artery.
  3. When you start to hear sounds, that's when the systolic blood pressure is higher than the cuff pressure and the heart can squeeze a little blood through the cuff, which makes a little squirty noise.
  4. As cuff pressure continues to drop it stays between the systolic and diastolic blood pressures and therefore bloodflow stops and starts and creates audible turbulence.
  5. When the artery stops making noise, the cuff pressure is below the diastolic blood pressure and the cuff has no effect on the artery, so it goes back to laminar flow that doesn't make noise.

Measuring the pressure of the cuff is just part of controlling the pressure of the cuff. You could conceivably measure blood pressure by submerging their arm in water(mercury would not require a conversion, but vats of mercury are unpopular in doctor's offices) and listening for Korotkoff sounds and measuring the depths where they start and stop.

Edit: The actual question here is how does cuff pressure physically change the arterial surroundings, not how do we link the two clinically.

The body is mostly water, and water is incompressible. For small strains(amount of deformation) flesh isn't that springy and mostly "flows" like water. When you pressurize the cuff around the arm, the flesh of the arm equilibrates to the cuff pressure the way balloons equilibrate to the air pressure, only less dramatically because water is mostly incompressible. For small pressures and large cuffs, therefore, the cuff pressure is the pressure on the artery wall, and by listening for Korotkoff sounds while varying the cuff pressure the blood pressure can be roughly established.

The reason why you don't lose all blood flow to your extremities while diving underwater is because the systemic pressure on all of your body raises your internal blood pressure, making the net external pressure on any part of your body zero. (The cuff works by exerting a relative pressure difference on part of your body.) At high altitude the sphygmanometer is still accurate-ish because the cuff pressure relative to the atmosphere is still the same as the blood pressure, relative to the atmosphere. If you had air for blood(or any compressible fluid) your veins would collapse underwater. This is why your ears hurt while diving deeply if you don't equalize them: your ear is full of compressible air, and at depth the air attempts to shrink more than the flesh around it, pulling things out of place painfully.

$\endgroup$
  • $\begingroup$ Thanks for the answer, but please bear with me as I might be missing something very obvious here because even though I did understand how Korotkoff sounds help in the measurement, yet I'm still having trouble understanding how exactly do these readings on the sphygmomanometer (like 120mmHg for systolic pressure) actually relate to the pressure that's exerted by the blood on the walls of the artery? Doesn't the sphygmomanometer only measure the pressure exerted by the (compressed) air on the walls of the cuff? How does this pressure directly equate to the blood pressure? $\endgroup$ – laggingreflex Jul 6 '15 at 15:14
  • $\begingroup$ When it comes to pressure differentials, the body is essentially incompressible(it's mostly water and water is mostly incompressible). The cuff is long enough and the arm fluid enough so that as long as nothing moves very far, the pressure on a long enough part of the arm is the pressure inside the arm. Your arm is too viscous to "flow" out of the cuff but otherwise it's not unreasonable to treat flesh as a surprisingly stationary liquid. $\endgroup$ – Resonating Jul 6 '15 at 15:35
  • $\begingroup$ okay I understand that, the arm can be treated as stationary liquid, but I'm afraid I still don't see how that answers my question? Actually I'm slightly more confused as to how the arm comes into discussion at all? weren't we just talking about the artery and the cuff? I'm definitely missing something very obvious and I very much appreciate your responses. $\endgroup$ – laggingreflex Jul 6 '15 at 15:44
  • 1
    $\begingroup$ The artery is embedded in the arm, isn't it? When the arm is under pressure, the artery is under pressure. The cuff doesn't pressurize the artery directly (it'd have to be surgically implanted) it just pressurizes your arm and relies on the arm to transmit pressure to the artery. Come to the chat, this is getting too lengthy for comments. $\endgroup$ – Resonating Jul 6 '15 at 15:47
  • 1
    $\begingroup$ (Oh we should totally go to chat these are fascinating questions but they're drifting off-topic.) If you managed to get your arm 2.6m away from your body, that's exactly what would happen(maybe you have very long arms). If you brought your body with you, the body-wide pressure increase would increase your blood pressure systemically and cancel out the increased outside pressure. The closing is a relative pressure effect where the pressure on the arm under the cuff is higher than the pressure on the rest of the body. That's why it still works at altitude. $\endgroup$ – Resonating Jul 6 '15 at 16:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.