The Curious Case of the Unreliable Partner

NIBP or non-invasive blood pressure. As EMS providers we all use it on every single call. But I'll go out on a limb and say the majority of us actually have no idea how our friendly robot partner comes up with those numbers, and we probably should have an idea of how the tools work that we use to do our job.

"I know I've made some very poor decisions recently, but I can give you my complete assurance that my work will be back to normal. I've still got the greatest enthusiasm and confidence in the mission. And I want to help you." ~ HAL 9000

Lately through various social media banter and even some in person discussions it would appear that many prehospital providers have a misunderstanding of how their own machines operate. While perhaps not the most exciting topic in the world - possibly the most boring I've authored on - I think it's an important one. It's critical we understand how the tools we use to do our job work, and also understand their limitations and flaws. I'm talking about NIBP or non-invasive blood pressure cuffs/monitoring.

Here's some quotes I've either heard in person or read online over the last few months:

"I don't know why our base hospital (medical oversight) insists on a manual BP"

"Why is the result sometimes spot on and other times it's not even in the ballpark?"

"How can an NIBP pick up a pressure when I can't hear or feel one?"

"Why is NIBP worse in shock states?"

To answer some of these questions we need to first examine how NIBP machines operate because what most providers don't realize is that they function very different than our sentient human being partners.

We all know and learned how to measure blood pressure via auscultation, and that's by listening for Korotkoff sounds. Remember that in school? Those sounds are blood flow sounds that providers observe while taking blood pressure with a sphygmomanometer over the brachial artery in the antecubital fossa. These sounds appear and disappear as the blood pressure cuff is inflated and deflated and give you a systolic and diastolic number. It's relatively reliable assuming you know what you're doing.

What most don't understand is that prehospital cardiac monitors that have NIBP functions do not take a blood pressure this way. It's not HAL. They don't listen in* like humans do to obtain systolic and diastolic values. They work in a completely different fashion and essentially in reverse of how humans do things. (*There are some NIBP machines that do use a auscultatory technique however this is not the case on prehospital cardiac monitors).

We've all been there at some point before. We press the NIBP button and it returns a value that doesn't look anything close to how the patient is presenting, we see two numbers that are ridiculously close together, or very rare occasions see a bottom number higher than the top... and then we get frustrated and straighten an arm and press it again...

"Look Dave, I can see you're really upset about this. I honestly think you ought to sit down calmly, take a stress pill, and think things over." ~ HAL9000

So how does it work? You press your NIBP button on your Zoll-X, LifePak, whatever the case may be and the cuff inflates to occlude the brachial artery (a state of no flow). As the cuff pressure drops a 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 patients MAP or Mean Arterial Pressure. I could literally end the article here because that's it. Your NIBP does not measure systolic or diastolic values. It actually uses the oscillations to obtain the patient's MAP. From there is uses proprietary mathematical algorithms to determine the patients systolic and diastolic values.

What this means is that your NIBP may swing and a miss occasionally on those top and bottom numbers, but what it's *really good* at is MAP. PS that's that number in brackets next to your calculated value. And let's prove it: A 2013 ICU study(1) looked at primarily acute kidney injury and mortality but it also looked at the difference between arterial line pressure and NIBP pressures (both systolic and MAP). What they found was that the lower blood pressure the patient had, the more off their NIBP systolic blood pressure was. MAP however was almost spot on at both normal and hypotensive pressures compared to the art line values. What does this mean? It shows that while NIBP cuffs tend to significantly overestimate systolic BP in shock states, the MAP it obtains is almost identical to the MAP obtained through invasive arterial monitoring and is very reliable. One positive aspect to this is that your NIBP will be able to pick up really low blood pressures via MAP that you may not be able to hear via auscultation.

So let's get back to the oscillation technique and how it then figures out systolic and diastolic pressures. As I said previously, the algorithms are proprietary to the manufacturer however they primarily fall under two different categories: height based and slope based algorithms.

In height based algorithms, the peak pulse amplitude is treated as MAP and normalized to a value of 100%. The cuff pressure at MAP is the MAP pressure. Systole and diastole are fixed percentages based on MAP. The cuff pressure under diastole is the diastolic pressure and the cuff pressure under systole is the systolic pressure. In slope based algorithms, slopes are drawn and their intersections are used as the values. As shown below, the cuff pressure under the intersection of the slopes is treated as the systolic and diastolic pressures. There is no industry standard for height or slope based algorithms and may differ from manufacturer to manufacturer, and thus some may be considered "better" than others.

And that's why obtaining at least one human auscultated blood pressure on a patient is quite valuable. Assuming you know what the hell you're doing you probably have a better chance at getting an accurate systolic/diastolic reading by listening. But your HAL9000 has a much better shot at accurately measuring the MAP for you as opposed to our standard MAP calculation - which can have some margin of error depending on the calculation method used and the current heart rate. Both of these values are important to guide your clinical decision making and one could easily argue MAP is actually more valuable. This isn't meant to be a primer on MAP and why it's important but if you want to segue into a fantastic read however you can venture over here to FOAMFrat on "The MAP to Clarity". But the unfortunate reality is many of our protocols are driven by systolic and diastolic numbers - here's hoping that changes one day.

And take it easy on HAL. He has feelings too.

"I am putting myself to the fullest possible use, which is all I think that any conscious entity can ever hope to do." ~ HAL9000

1. Methods of blood pressure measurement in the ICU, Crit Care Med. 2013 Jan;41(1):34-40. doi: 10.1097/CCM.0b013e318265ea46.