“If there’s a place you want to go, I can get you there I know, I’m the map. If there’s a place you need to get, I can get you there I bet, I’m the map. I’m the map, I’m the map, I’m the MAP!” – Dora the Explorer
A lot of the millennial medics were probably happily singing along when they read that, while their parents and older colleagues with children are probably still using a cheese grater on their forebrain to remove the memory.
Don’t worry, Baby Boomer medics, I’m not going to start singing the “Barney” theme next, but I do want to show a little love to that one vital sign parameter that too often gets ignored, the mean arterial pressure (MAP). We need to pay more attention to that little number in parentheses next to our NIBP reading, because it’s one of the more reliable measures of perfusion we can assess.
NIBP inaccuracies
Back in 2011, I wrote a column entitled “Blood pressure reading tips and tricks for EMS.” That column, along with Mike McEvoy’s “Five errors that are giving you incorrect blood pressure readings,” pops up quite frequently in social media and the EMS blogs, whenever a new EMT asks for help on how to obtain an accurate blood pressure in a moving ambulance.
And invariably, in those comment threads, some sage old paramedic points out that NIBP readings are notoriously inaccurate, and that you should “treat the patient, not the monitor.” Often, he’ll suggest that, if they don’t ditch the NIBP machine altogether, they should, at the very least, obtain the first pressure manually.
And he’s right, NIBP machines are notoriously inaccurate. Even the device manufacturers say so. Read the operator’s manual of your cardiac monitor, and buried somewhere in there will be the disclaimer that, in the patients where we most need an accurate BP – the ones on either end of the scale – the NIBP reading on your screen may not be, well, the actual blood pressure.
In fact, they are so inaccurate, they might even be as bad as human ears.
If I had a dollar for every EMT who confidently announced a BP that ends in a zero – 150/90, 120/80, 110/70, every single time – or the one who pulls the stethoscope from his ears and hesitantly stammers a BP ending with odd numbers read on a gauge with even-numbered hash marks … well, I’d have a lot of dollars. Maybe not enough to buy that swanky double-wide I’ve have my eye on for so long, but it happens often enough that I call a manual sphygmomanometer gauge the Paramedic Lie Detector.
Obtaining mean arterial pressure through invasive arterial monitoring
A lot goes into auscultating a blood pressure;
- Differences in hearing acuity
- Road noise
- Ambient noise
- Cuff sizing
- Even holding the arm too high or too low
If it were easy to auscultate a blood pressure, we wouldn’t be posting tips and tricks on how to do it every year or so.
But 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.
In a study of 4,957 adult ICU patients at a tertiary care center, over 27,000 simultaneous blood pressures were obtained by NIBP and arterial line transducers. Both acute kidney injury and mortality rates were higher in the group with systolic NIBP measurement than those with arterial systolic BP in the same range (< 70 mmHg).1
When comparing acute kidney injury prevalence and ICU mortality between arterial MAP and non-invasive MAP, there was very little difference, leading study authors to conclude that NIBP overestimates systolic BP in shock states, but the MAP obtained by non-invasive blood pressure cuffs correlates very closely to arterial MAP (Figures 1 and 2).
So why is it that NIBP machines are very accurate at measuring MAP, and very unreliable at measuring systolic blood pressure? The answer is that what NIBP machines actually measure is the MAP; the blood pressure they derive by calculation.
That is exactly the opposite of the way we do it manually; we auscultate Korotkoff sounds to determine systolic and diastolic BP, and then mathematically derive the mean arterial pressure via the following equation:
MAP = SBP + 2 (DBP)
3
Many of us learned that calculation in paramedic school, and then promptly forgot it because most of our treatment parameters were keyed to systolic blood pressure, and because we hate math. As it turns out, NIBP machines deriving the blood pressure from MAP and pulse rate aren’t much better at the math than we are.
Vital organ perfusion thresholds
Most of us who have been in the back of an ambulance more than five minutes realize that a smiling, pink granny who weighs ninety pounds with a BP of 84/60 is a different critter entirely than a 250-pound man with a BP of 90/40. For grandma, that might be her blood pressure every day, and she’s perfusing her vital organs just fine. For the burly truck driver with gray skin and diaphoresis, he’s hypoperfusing, even though his systolic blood pressure is higher.
Most sources consider a MAP of 65 mmHg to be the minimum threshold for vital organ perfusion, with normal MAP ranging from 70-110 mmHg. While some animal studies have shown that hemorrhaging rats (did you know that rats have the same MAP as humans?) can tolerate a MAP as low as 50 mmHg for up to 90 minutes, human kidneys are far more sensitive to hypoperfusion. Acute kidney injury starts in about 20 minutes with a MAP below 60 mmHg.
So the next time you’re wondering if your patient is a quart low but the blood pressure isn’t all that bad, watch the plethysmography waveform on your pulse oximeter, and if you’re torn between believing the NIBP blood pressure that doesn’t really match the patient’s clinical picture, or your partner’s lying ears, pay more attention to that little number in parentheses next to your NIBP reading. The MAP won’t lead you down the wrong path.
