I have been wondering. How exactly does sphygmomanometer measure blood pressure in our body? Can someone please explain it to me, it’s wrapped around our hand not even injected in our blood vessels so how does it figure out our BP?
I have been wondering. How exactly does sphygmomanometer measure blood pressure in our body? Can someone please explain it to me, it’s wrapped around our hand not even injected in our blood vessels so how does it figure out our BP?
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If it’s wrapped gently around your arm, it does not impede the flow of blood in your arm at all.
If it’s wrapped tightly around your arm, it squeezes tightly enough to cut off the flow of blood to your hand.
In both of these cases, there will be almost no sensation in your hand, and almost no sound if you listen to your arm with a stethoscope.
If, however, you apply a moderate amount of pressure, when your heart beats, it will squirt blood past the constriction with a characteristic sloshing sound / throbbing sensation when the blood pressure is highest, and the flow will be cut off again between beats, when the pressure is lower.
The cuff applies a variable pressure to your arm, and you measure the highest and lowest pressures at which the throbbing sensation occurs.
The cuff compresses the tissue in your arm such that the vessels carrying blood are closed. There is a gauge that measures the amount of pressure the cuff is exerting (due to the air being pumped into the cuff, which causes it to inflate and exert that pressure), and the medical attendant listens for when the sound of your pulse, which has stopped due to the compression, starts again as they release that pressure. They note the pressure points at which your blood just starts to flow (this is the systolic, or top number in your BP measurement), and when the blood flow is completely unimpeded (this is the diastolic, bottom number in your measurement)which is when the cuff is no longer compressing the artery at all.
The digital cuffs do the same, but they don’t listen, they detect electrical changes as those vibrations start and stop with the change in cuff pressure (which the device is also monitoring).
You are right, these numbers are approximations, and aren’t actually measuring your real arterial pressure. There isn’t a convenient way to do that without slicing open arteries, so we approximate with this external method.
Dang, explanations are huge and over complicated. The cuff has a bladder that fills when you pump it up. When you wrap it around your arm and fill the bladder with air, the bladder can no longer fill easily because your arm is in the way. The meter reads how much pressure is being placed on the bladder itself as you continue to fill it. That’s it. Its also how sleep number beds work, any number will fill the bed all the way unless you lay on it and give it a pressure to read.
If you want to know how you obtain blood pressure from that, once the pressure is above what you guess the blood pressure to be (200 is safe for hypertensive patients, 160 for people with no history), place a stethoscope over the artery in the inner elbow and slowly release the bladder. When you hear the pulse return, the reading the meter is showing is the top number, and when you no longer hear the pulse, the meter is showing the bottom number. Sphygmomanometers are just the cuff and the cuff alone.
Pinch the blood vessels closed and slowly release pressure until you hear a sound from blood passing. That’s the systolic pressure. Let off more pressure until you hear a second sound. That’s the diastolic pressure
Okay so here’s how it works. You’re squeezing the arm and measuring how much pressure is being exerted on that limb. You are then listening for the sound of blood flowing into the artery on the other side of the cuff.
When the pressure of the cuff is equal to the pressure in the artery, the blood stops flowing beyond the cuff. So you pump it up and slowly deflate it. The pressure in the artery is highest during a contraction of the heart so there will be a point where there is enough pressure to cross the cuff during a contraction only. This is the big number on the top, called your systlic that’s normally around 120. You then continue to deflate the cuff. Once the cuff pressure is low enough that blood is freely flowing through the artery you won’t hear that pulsation of the blood going through the cuff only during peak contractions. That number is the diastolic pressure, normally about 80.
So the final number you get is two different numbers. The amount of pressure in the artery when the heart is at maximum contraction, and the lowest pressure in the artery which occurs just before the contraction. There is also a formula to calculate the mean arterial pressure, which is the average amount of pressure in the artery across the cycle.
Let’s say the machine says 120/80.
Think of it like a pressurized plumbing system or a garden hose. A water balloon (your heart) pushing water into a hose (your artery). The cuff squeezes the hose shut, then lets go slowly. The machine listens to how strong the water flows back in.