[vhfpmr (OP)]

The pressure to which my BP monitor inflates the cuff varies widely. It would seem reasonable to suppose it would inflate until the pressure exceeds systolic by a sufficient margin, but if you look below, the data is grouped into two distinct clusters pretty much independent of systolic bp.


BP mon.png (17.69 kB . 704x385 - viewed 1022 times)

Is it inflating for a preset time? No.
Is it inflating until it exceeds sys bp for a predetermined time? No.
Is it inflating until it exceeds sys bp for a predetermined number of pulses? No.
Is inflation pressure dependent on how tightly the cuff is wrapped? No.

So what are the criteria?

(Monitor: Omron M2 Basic, HEM-7120-E))

[Bored chemist]

A few of us were messing about with a monitor in a coffee break at work ages ago.
I measured my blood pressure and the guy who owned it looked surprised.
When he used it the cuff would inflate a bit, pause, then inflate some more.
My blood pressure was low enough that it could be measured on the first "step".
Presumably the monitor checked on something at that pressure and if it sensed that the blood was still flowing, it inflated some more.
If your monitor does something similar but without the pause an your BP is near the cut off between "about 160" and "about 220" then that would explain the data in your graph.

[vhfpmr (OP)]

I have AF. I don't think it's pausing when the arrhythmia's present, but I'll pay more attention for a bit.

There are those who will tell you that BP monitors are useless with AF, but my monitor doesn't seem fazed by it, nor do the NHS. Some monitors have a function that reports arrhythima, which suggests they're likely to be capable of measuring BP reliably through it, and a nurse with a sphygomanometer was capable of measuring my BP whilst commenting "ooh, I can hear your AF". I have a plot somewhere of of some BP data when I was in Resus, and it's interesting that my BP was rock steady, but high, whist I was in AF, but then started fluctuating wildly as soon as I reverted to NSR.

[paul cotter]

My monitor inflates to a pressure that can be preset by the user. If it encounters a systolic pulse too early on the ramp down it will repressurise to a higher initial pressure and then proceed as normal. My heartbeat tends to be rather irregular at times, however it does not seem to interfere with the measurements.

[vhfpmr (OP)]

From Wikipedia:

The oscillometric method uses a sphygmomanometer cuff, like the auscultatory method, but with an electronic pressure sensor (transducer) to observe cuff pressure oscillations, electronics to automatically interpret them, and automatic inflation and deflation of the cuff. The pressure sensor should be calibrated periodically to maintain accuracy.[15] Oscillometric measurement requires less skill than the auscultatory technique and may be suitable for use by untrained staff and for automated patient home monitoring. As for the auscultatory technique it is important that the cuff size is appropriate for the arm. There are some single cuff devices that may be used for arms of differing sizes, although experience with these is limited.[8]

The cuff is inflated to a pressure initially in excess of the systolic arterial pressure and then reduced to below diastolic pressure over a period of about 30 seconds. When blood flow is nil (cuff pressure exceeding systolic pressure) or unimpeded (cuff pressure below diastolic pressure), cuff pressure will be essentially constant. When blood flow is present, but restricted, the cuff pressure, which is monitored by the pressure sensor, will vary periodically in synchrony with the cyclic expansion and contraction of the brachial artery, i.e., it will oscillate.

Over the deflation period, the recorded pressure waveform forms a signal known as the cuff deflation curve. A bandpass filter is utilized to extract the oscillometric pulses from the cuff deflation curve. Over the deflation period, the extracted oscillometric pulses form a signal known as the oscillometric waveform (OMW). The amplitude of the oscillometric pulses increases to a maximum and then decreases with further deflation. A variety of analysis algorithms can be employed in order to estimate the systolic, diastolic, and mean arterial pressure.

Oscillometric monitors may produce inaccurate readings in patients with heart and circulation problems, which include arteriosclerosis, arrhythmia, preeclampsia, pulsus alternans, and pulsus paradoxus.

So it doesn't really shed any light, but my basic assumption about exceeding systolic bp is on the right track.

[alancalverd]

Suppose the command is "inflate until there is no detectable pulse". Problem is to distinguish between a systolic pulse and noise due to cuff movement, muscle flexion, or random electronic noise, so we also set a maximum around 220 so we don't do any damage. The good news is that it is irrelevant because we only report  sys (pulse appears) and dia (pulse disappears) pressures during deflation.