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The biophysics behind measuring systolic and diastolic blood pressure manually with a cuff and stethoscope is that

  1. Determine the approximate pressure it takes for the cuff to occlude the brachial artery completely (normally around 120mmHg) which is indicated by a lack of pulse in the radial artery.

  2. Deflate then reinflate the cuff to a pressure that is 20-30mmHg above the approximate pressure measured above. Deflate the cuff gradually at a rate of 2-3mmHg/sec and wait to hear the first tapping sound of blood flow, which indicates when blood can finally go through the artery at maximum blood pressure. The pressure of the cuff at the point of the first sound will be the systolic pressure.

  3. Keep deflating the cuff at a rate of 2-3mmHg/sec until no sound can be heard (usually around 80mmHg) as that indicates laminar blood flow, i.e. blood at systolic and diastolic pressure can flow through the artery without disruption. The pressure of the cuff at the point of no sound will be the diastolic pressure

Using these simple principles, regardless of the cuff size, so long the cuff.

  1. Can occlude the artery completely
  2. Can be deflated gradually at a rate of 2-3mmHg/sec
  3. Can un-occlude the artery completely

Then the results of cuffs of different sizes will be the same, i.e. same accuracy.

I believe a cuff size that is too large or small may fail to occlude the artery completely or un-occlude the artery completely respectively, which would fail to meet the three crtieria above. However, in that case, by listening to the blood flow, one could tell systolic and diastolic pressure cannot be measured as the three criteria cannot be fully met, and thus one could not collect data on the blood pressure (thus there will be no inaccurate results to begin with).

So my question is, why can incorrect cuff size for measuring blood pressure manually or automatically lead to inaccurate results?

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  • $\begingroup$ Are you stating that incorrect cuff size gives inaccurate results and asking why it is so? Or are you asking whether cuff of different sizes can give variable results? $\endgroup$
    – Deep
    Commented Oct 12, 2020 at 3:30
  • $\begingroup$ @Deep Yes, the former. But I suppose the latter question very much answers the former, as variable results suggest numerous inaccurate results since only one result can be the 'correct' one $\endgroup$
    – Bøbby Leung
    Commented Oct 12, 2020 at 9:44
  • $\begingroup$ Isn't simply that too large cuff can't inflate to push on you, or too little already push at start? $\endgroup$
    – Alchimista
    Commented Oct 13, 2020 at 13:09

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