Does a blood pressure tester transported from 1000 feet in elevation give accurate readings at 10,000 feet in elevation, or is some calibration or calculation needed?
Would there be a difference between a mercury, dial or digital/automatic one?
I wouldn’t think so. You’re measuring pressure relative to ambient, not absolute pressure. The normal swings in air pressure in a single location are almost as big as blood pressures (barometers usually cover about 28 to 31 inches of Hg, and there are 25.4 millimeters per inch).
That said, I have no practical knowledge about this. By no means should anybody take it as medical advice.
“Gauge” pressure is the pressure referenced to the atmosphere surrounding the gauge. “Absolute” pressure is pressure relative to vacuum. Blood pressure is gauge pressure, from here.
IANAD.
However, I’d opine that it needn’t be calibrated because most commonly, sphygo…sphmy… blood pressure meters don’t measure actual pressure. Rather, they cut off blood flow entirely, then are slowly deflated until the sound of your blood flowing back in the artery can be heard, then further deflated until it can’t be heard anymore. The BP numbers are the time values for those two events, not direct pressure readings. Of course, they are directly related to your actual BP, or rather it can be extrapolated from the time values.
Regardless, ambient air pressure shouldn’t have much influence, if any, on the procedure, much less the instrument itself - the armband acts purely as a mechanical block to blood flow, it could be a rubberband or a piece of string if a method could be devised to loosen it evenly and regularly.
Note however that this is true for the typical sphygmo+stethoscope method - other methods might result in mileage variance.
Never mind the atmosphere, there is a slight change in the strength of the gravitational field at different altitudes. I think I calculated once that it is much bigger than the accuracy of many scales and balances, and would change the weight to mass relationships that are often assumed constant.
Kobal2, are you saying most common sphygmomanometers eliminate the pressure measurement component by precisely metering air flow, knowing the volume and compliance of the cuff, and so on? That seems difficult.
According to the Wikipedia article on Earth’s gravity, there is almost 1/2% change between the Equator and the Poles, and almost 1/3% change with altitude between sea level and the top of Everest.
The same mass weighs 0.4% more in Oslo and Helsinki than in Mexico City.
Firstly, a sphygmomanometer acquired blood pressure is not an exact science. The rate of cuff deflation to interpreting heard sounds, to discerning gauge readings at that precise point in time is all relative to the attention given by the individual taking the reading. Now add in the variables with arm size vs cuff sizes and its easy to understand that a blood pressure is only a survey.
Now those automatic ones that are so convenient and simple to use and work so poorly in the back of an ambulance lumbering down the road might be able to show something remarkable in a laboratory test. but hey its only a survey, IMHO.
I think I get it. Since the air source to blow up the cuff is less dense as well, it shouldn’t matter.
I’m pretty sure that they’re pressure readings. The cup is inflated and gradually deflated. The gauge (or actual mercury) constantly measures the pressure in the cuff. The cuff pressures at which two events occur are noted, giving the two BP numbers. There is no time measurement. “Manometer” means “pressure meter” (I find that this helps me remember that bloody word: sphygmo-manometer).
Re-checked and, big surprise there, you’re right and I was utterly wrong. It *is *the pressure reading at those times, rather than the time value. What’s more, the source I was drawing from to make this statement plainly said so. I have no idea where I got that notion.
Note for self for future reference : do not try and answer GQs when sleep deprived.