Eli5: how does the blood pressure cuff determine your blood pressure so accurately?
In: 200
At certain amounts of external pressure, blood vessels are louder because the cells are bouncing off the sides. Less pressure and they can easily flow through, more and they can’t flow fast enough for that effect. Your two blood pressure readings are the endpoints of that “noisy” range, based on how much pressure the cuff is providing.
Fun fact: the device that measures blood pressure is called a sphygmomanometer. Try saying that… even one time slow.
As simple as I can phrase it:
It can sense 2 things: your blood flow and the pressure it’s applying to your arm. If the pressure it applies is higher than your systolic or diastolic blood pressure it cuts blood flow.
So it applies a very high pressure and starts to slowly decrease it. Once it senses the first blood flow (systolic) it knows that your systolic pressure is what it’s measuring at that time. It then keeps lowering the pressure until it can sense your diastolic blood flow.
The old school method is just a doctor checking the pressure value while listening to your bloodflow with a stethoscope.
The cuff has a simple pressure gauge in it that measures the pressure in the cuff. It’s basically the same as a tire pressure gauge.
The cuff is inflated to a pressure greater than your systolic blood pressure (pressure when the heart is pumping). Because the pressure in the cuff is higher than the blood pressure, it squeezes off the blood vessels and the pulse stops.
As air is let out and the pressure in the cuff drops, the person listens through the stethoscope. When the cuff pressure is equal to the blood pressure the vessels will stop being compressed and the pulse will return. The reading on the cuff is the systolic pressure.
Air is contained to be let out of the cuff causing the cuff pressure to continue to drop. But because the cuff pressure is still greater than the blood pressure when the heart is not pumping (diastolic) and the sound through the stethoscope indicates the blood flow is restricted.
The pressure at which the flow sound returns to normal is the lower (diastolic).
It all works because the pressure in the cuff and the pressure in the blood vessels close to the cuff are assumed to equalize. It’s like squeezing off flow in a garden hose. If you apply a pressure greater than the water pressure in the hose, you will stop the flow. As you release the pressure, at exactly an equal pressure, flow will return By reading the pressure in the cuff, you assume you are reading the pressure in the blood vessels.
Have you ever noticed that when you turn on a garden hose, it gurgles and murmurs initially, but then quiets down? The sound is due to the water flowing at different speeds and directions initially before it flows evenly. The “loud” flow is turbulent flow, the “quiet” flow is laminar flow.
When a blood pressure cuff is applied, it gets blown up until it fully stops blood flow in the vessels of your arm. A stethoscope put on the arm will hear nothing.
Then the pressure gets lowered until blood can flow again a little bit every time the heart squeezes. This flow is turbulent, so you can hear it with a stethoscope. The pressure at which that first sound happens is the first number in a blood pressure reading.
Further releasing the pressure gets you to a point when blood starts flowing even when the heart is not squeezing. This flow will NOT be turbulent, but laminar. Laminar flow is quiet. The pressure when the sounds stop is the second (lower) number of a blood pressure reading.
So a blood pressure of 120/80 means that at a pressure of 120 I heard turbulent flow starting and at 80 I stopped hearing turbulent flow and blood is now flowing quietly (laminar).
The other answers are correct only for manual blood pressure reading with a human and a stethoscope, which is famously inaccurate. The automated machine your doctor has uses an oscillometer. It measures variability in the pressure wave amplitude as the cuff pressure drops from above your systolic BP, past your mean pressure, past your diastolic BP. The amplitude is maximal at your mean pressure (very accurate) and exhibits a drastic change as it passes your systolic BP (less accurate, but not bad). It calculates your diastolic using an equation and the two other numbers.
I wondered the same thing myself and started exploring the history of the blood pressure cuff.
Apparently it hasnt been improved upon. Its still as new of an invention as it was 1881.
Sure, theres automated inflators and different sized cuffs (that matters) but the science itself hasnt improved since its inception.
Its not even clear what proof there is that blood pressure is even a proveable scientific tool. Your blood pressure can change drastically even with three different readings in a 15 min period.
Oddly enough, doctors and nurses will continue taking readings until they get a “good” bp reading, i.e. one they like better, if possible.
Imo I dont think its a very good or even an empirical scientific discovery.
I think blood pressure fluctuates naturally. I think that the “high range” guidance is probably good enough advice (its good advice for anyone though) and I am still a little confused as to how blood pressure medication even works. As far as I can see, it just helps you to have less water retention and less salt in your system. Idk if thats necessarily even a good thing. Just because it lowers your bp? I mean, Idk, I think we need to study HBP and see if this is some protective measure the body takes or what, first.
I think that to be a good scientist, we have to recognize that everything we think we know, could be wrong, and be willing to study and review our old ideas and see what else we can learn.
Also, medical journal article in 1880, discussed a lot of creepy and gross widespread ideas like using a womans menstrual period to preserve meat. Or eating a foal to cure epilepsy. So forgive me, but Im just not sold on the idea of blood pressure being especially scientific, or even meaningful, in its nature.
https://books.google.com/books?id=9zNYAAAAMAAJ&newbks=1&newbks_redir=0&printsec=frontcover&pg=PA130&dq=1880s+medical+journals+superstitious&hl=en&source=gb_mobile_entity#v=onepage&q=1880s%20medical%20journals%20superstitious&f=false
There are actually multiple ways of determining blood pressure. Direct and indirect. Direct is when you have a catheter in an artery and it feels the blood pushing. Indirect is with the cuff.
When a human being is taking blood pressure with a cuff and stethoscope, what they’re listening for is turbulence in the artery caused when an artery collapses. So when the blood pressure cuff is inflated to a pneumatic pressure higher than what the heart is pumping, no blood can flow through an artery. Therefore, no sound. Then the operator will slowly release the blood pressure cuff. When the blood pressure coming from the heart matches the pneumatic pressure of the cuff, blood flows again. However, the flow of blood is NOT what you hear. When the blood has been pumped through the artery, the pressure in the blood drops, and the pneumatic cuff pressure is greater than the pressure in the artery again. This causes the artery to collapse against the cuff. This collapse causes a pop that the operator hear. The action of blood overcoming the cuff pressure to collapsing against the cuff happens very fast. Almost instantaneous. The first pop that the operator hear, is the systolic blood pressure. It effectively means the highest pneumatic cuff pressure that the heart can overcome.
Then, the operator keeps releasing the cuff pressure and the cycle repeats. Until, the last pop that the operator hear. The last pop signifies the lowest pneumatic cuff pressure that can still overcome the blood pressure causing the artery to collapse. Any cuff pressure lower than that, the artery is able to stay open, causing no further collapse, causing no further sound. This is the diastolic, or resting blood pressure.
For the machine, it’s actually more complicated than that. The machines don’t listen for the pop. What it does is first inflate the cuff. Then it measures the pressure from the artery pushing against the cuff. It puts it on a graph. The graph is upside down parabola shaped. Then, it deflates the cuff incrementally, then it measures the artery pressure again and charts it. It takes several of these readings at different pneumatic pressures, comparing to see if the graph appears similar. If it does, then it uses integral calculus to determine the area under the curve of each curves, then, average it out. This gives it something called “mean arterial pressure.” With that value, through different proprietary mathematical formula (depending on the manufacturer), calculates the systolic and diastolic blood pressure.
Systolic, or the “big/top number” – you apply so much pressure, no blood can get through. Then you release slowly, watching the pressure on the dial “tick” down, and at the very TOP of its pressure, the blood can spurt past the pressure. This makes a noise that you can hear with a stethoscope. The first “tick” with an associated spurt is the systolic pressured or the peak pressure generated by your hearts contraction. (Or you can cheat and feel their pulse. Don’t do this, young health professionals out there)
Diastolic, or the “bottom/little number” – you watch the pressure spikes “tick” down on the dial. The first “tick” that has no spurt noise associated with it is the residual pressure in the artery when the heart is resting.
I hope this was ELI5 enough.
An automatic blood pressure cuff applies pressure to your arm. Initially, it’s high enough that it stops blood flow. It then releases that pressure in steps until it starts to get a bit of pressure fed back to it with your pulse. Somewhere between where the pressure it can detect back as the pulse at that point and the lowest pressure it couldn’t detect is your systolic blood pressure (the big number).
It then keeps reducing the pressure in steps. This both gets it able to go towards when it won’t detect your pulse with pressure fed back because it isn’t pressing hard enough (which will be the diastolic or lower number) but also tracks how the amount of pressure that gets fed back at each applied pressure shifts and what the pulse looks like at each applied pressure based solely on the pressure fed back.
That middle bit is the big reason that automatic cuffs can get so accurate. There’s a general way to relate the responses they see to more accurate pressures than they can strictly measure quickly without needing an extra set of sensors and really just requires some admittedly complicated math with all the data points to get a good answer (but computers are really good at doing that math).
For old school cuffs with a doctor using a stethoscope, you get a similar start, but you can hear the blood start to move when the applied pressure is the systolic pressure. As the pressure is released, though, you instead listen for the sound of the blood in the vessels as going from constricted to open flow means going from a turbulent/bumpy/very loud flow to a laminar/smooth/more quiet flow, though also when it’s too constricted, you have less total flow, so it’s quieter. It just so happens that at the average of the systolic and diastolic blood pressures, it’s the loudest, and that’s much easier to identify than when it becomes fully laminar, so the diastolic isn’t actually directly measured then usually.
Lastly, if you need *really* accurate info, you can use an automatic blood pressure cuff with a pulse oximeter (the finger light thing) on the same arm sort of like an old school cuff but with the pulse ox instead of the stethoscope and using it to measure blood flow and constriction more accurately than based on volume. Plus, it can once again gather more granular data which can be fed into complicated math equations to give a bit cleaner data because at that point, the act of squeezing the arm might temporarily cause enough pain to throw off the data a bit and shift the value throughout the measurement
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