05-11-2011, 12:45 AM
Medical people used to put the cuff on your arm and manually inflate it and then listen while letting the air out. How does the blood pressure machine know how much to inflate the cuff? How does it decide when to let the air out? Why does it stay inflated so long? What assurance do I have that it's not going to squeeze my arm right off?
I'm asking because the last several times I've had my blood pressure taken by one of those machines, it squeezed so hard my fingers went numb and tingly, and once I had a bruise afterward that showed where the seams of the cuff were. (I was unconscious during surgery then.)
Can they set a maximum pressure? Surely pain is going to affect the reading. Should I buy my own cuff and ask them to do it manually? Does anyone else dread having it done?
Thanks for any information you can provide!
It varies slightly by manufacturer and model, but in general the initial inflate pressure is set by an option on the machine, and is almost always user (ie, doctor, nurse, PA, tech) changeable. I've seen the "defaults" be anywhere from 130 mmHg to 200 mmHg. 200 is def. in the high end, though...I'd say the average default is 140mmHg - 160 mmHg.
There are also 'quick' settings to change it to pediatric or neonatal, since in those cases there is a greater risk of patient injury for over-inflation, and in those departments of the hospital, the machines will often be in those modes at start-up, so there's less of a chance of user error in forgetting to put it into that mode.
An automated NIBP machine works by inflating to the set pressure, and then slowly letting a little bit of air out, and then checking to see if the current inflated pressure of the cuff matches your blood pressure. Doing a quick look in a service manual for a Dinamap V100, it does mention that it looks for two sequential pulses per pressure change to reduce any errors by artifacts from arm (or leg) movement. So that's partly why it takes so long for the pressure to go down.
It uses pressure transducers and complicated algorithms to find the pressure, but the gist of it is that the pressure variations in the cuff have certain predictable profiles depending on whether or not the cuff pressure is higher than your BP, equal to your BP, or lower than you BP. So it inflates, holds for two heart-beats and checks the pressure profile, and then lets out a little bit more. (generally 3-7 mmHg at a time.) It goes through the whole cycle, then looks at the stored data from each pressure step and can then say "ok, based on the profiles at 124 and 75 mmHg, those are the systolic and diastolic pressures."
As for not squeezing your arm off, they all have overpressure safety systems. Said V100 says the overpressure valves open at 300 - 330 mmHg for an adult, and 150 to 166 mmHg for neonatal mode.
If you feel the machines always go too high, don't be afraid to ask the nurse to set the initial inflate pressure a little lower. Some departments set the default a lot higher because they'd rather have it get someone's pressure the first time around if they happen top have high BP than to wait for the machine to go through the whole cycle, fail to get a BP, and then re-inflate at a higher pressure.
05-11-2011, 05:10 PM
Thank you, bouv! That answers my questions!
05-12-2011, 11:26 PM
Could over inflating actually cause someone's blood pressure to increase?
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