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How Do Automatic Blood Pressure Cuffs Work?

One of the most common mistakes I’ve heard on rounds, in the ICU, on the floor, or just in general conversation is that the “patient’s blood pressure is XXX over XXX.” The overwhelming majority of clinics and hospitals rely on automatic noninvasive blood pressure (NIBP) cuffs – monitors which do NOT measure systolic or diastolic blood pressures.

Instead, the automatic NIBP cuffs only measure the mean arterial pressure (MAP) through an oscillometric technique. Let’s assume we’re taking a NIBP on on the arm. Initially, the cuff inflates to occlude the brachial artery (no flow). As the cuff pressure drops, turbulent flow is generated through the vessel creating oscillations against the arterial wall. As the pressure keeps dropping, these oscillations reach a point of maximal amplitude. The cuff pressure at this point is the MAP. The cuff then fully deflates opening up the artery, promoting more laminar flow, and reducing oscillations.

When we’re first taught to perform manual blood pressure with auscultation and Korotkoff sounds, we can discern the systolic and diastolic pressures based on when oscillations begin and end. Keep in mind that automated cuffs don’t auscultate. They can only feel for oscillations, and therefore, only measure a MAP.

This is extremely important to know since most patients in the hospital do NOT have invasive arterial lines for blood pressure monitoring. If you’re writing a antihypertensive, make sure you place appropriate hold/treatment parameters knowing that the NIBP cuff only measures a MAP.

For example, it’s not appropriate to write labetalol 10 mg IV q4 hour PRN for SBP > 160 mmHg in a patient who has an automatic NIBP cuff. Instead, consider the treatment parameter as a MAP > 100 mmHg. πŸ™‚

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12 Comments

  1. I worry that people will read this and believe that the only “real” number on the screen is the MAP. While its true that the monitor uses an algorithm to determine SBP/DBP, the commercial (expensive) devices used in inpatient settings have been shown to be relatively accurate when compared with manual BPs, especially the SBP. They use the first/last detected oscillations combined with MAP measurement to determine numbers. Otherwise for any given MAP you could plug in widely varying values into the formula and still come up with the same MAP.

    Anecdotally, in my own practice I have found this to be true. Our very experienced techs take manual BPs on all trauma patients and all critical patients on their arrival to the ED, they almost always strongly correlate with the subsequent automated BP (In my experience usually within 10 mmHg)

    1. Your points are well taken, but the point of the post was to emphasize something I hear rampant throughout the intensive care units, perioperative settings, etc. That is, the lack of understanding of oscillometric blood pressure cuffs. It’s a similar pet peeve when people titrate blood pressure medications to systolic blood pressures on an arterial line without understanding that the location of the A-line will make a difference (ie, dorsalis pedis SBP usually exceeds radial artery SBP even though the MAPs should be comparable).

  2. Manual BP measuring devices are more accurate than automatic machines??
    And since MAP is calculated from sbp and dbp, how can we be sure that the value obtained from automatic BP monitors is correct vs. from manual ones??

    1. Manual and automatic BP cuffs are measuring different things. Manual cuffs directly measure systolic and diastolic pressures via Korotkoff sounds which you can use to back-calculate the MAP whereas automated cuffs directly measure the MAP and use proprietary algorithms to generate systolic and diastolic pressures. Just depends on what you care about measuring.

  3. Hi can you elaborate more:

    How does the manual cuff feel the osscillations ? And why does the oscillation peak at the MAP. I guess these are beyond your scope and enter realm of engineering and physics. If you are aware I would love to know.

    Also how does an invasive lines in the ICU measure BP?

    Thanks for writing this blog, as a student it’s an easier way for us to ask β€œdumb” questions instead of embarrass ourselves during rounds in the SICU.

    1. I’m not sure exactly HOW they feel oscillations (I’d imagine some sort of compression of a diaphragm or something), but invasive lines directly measure systolic and diastolic pressures, traditionally, through a Wheatstone bridge circuit and rely on the strain gauge principle. I’ll blog more about this in the coming days/weeks. πŸ™‚

  4. During auscultation of blood pressures would the maximal sound heard be a rough approximation of the MAP? Eliminate math! Ha
    Thanks for all you post and write ups!

    1. Typically if you’re using Korotkoff sounds (auscultation) to determine a manual cuff pressure, you’d determine the systolic and diastolic pressures and calculate the MAP as 1/3 systolic + 2/3 diastolic. From an auscultation standpoint, it would sound like a crisp, swishing sound (whatever that means). πŸ˜€

  5. This is the reason I check manual blood pressures in my outpatient practice. I’m sure I scarred several administrators previously when I was “surprised” with new BP machines and I promptly got rid of them.

      1. For an automated cuff, there is no significance. Yes, the diastolic and systolic pressures might be in the ballpark, but they aren’t directly measured.

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