Invasive and Noninvasive BP, and What They Should Mean

Blood pressure (BP) is a crucial vital sign. It is used by every medical clinician in determining the severity of their patient’s condition and treatment modalities. Having an accurate BP is paramount in determining the correct and optimal treatment for our patients. The gold standard in monitoring BP in critically ill patients is via an invasive method. This is accomplished by placing a catheter into an artery and directly measuring those pressures. In the prehospital setting, this in usually not available unless you are a critical care transport provider. The next best method is auscultation, though this is not always practical in the back of a noisy ambulance or aircraft. Cardiac monitors with automated BP capabilities are what is commonly used. These automatic BP monitors extract and measure oscillation waves produced by cardiac frequency, and they use complex formulas and algorithms to determine the systolic pressure, diastolic pressure, and mean arterial pressure (MAP). This method is often how treatments are determined in the prehospital environment. In patients with severe hypotension or hypertension, the result can sometimes be questionable, resulting in repeated BP’s being taken. In patients with severe hypotension, delay in treatment can be detrimental. How accurate are these automatic BP results compared to invasive BP monitoring? And if they are not as accurate, is there another method we can use that is as close to the “gold standard” as possible?

I came across a study titled, “Methods of Blood Pressure Measurement in the ICU”. The authors looked at patients admitted in the ICU from 2001 to 2007. They looked at 27,022 simultaneously measured invasive arterial BP’s and noninvasive BP’s. They found there to be clinically significant differences; especially in the hypotensive patient. As the patient becomes more hypotensive, the greater the discrepancy is between the invasive systolic and the noninvasive systolic BP. In hypotensive patients at a systolic BP of 60, there was an average offset of 10.05mmHg. As the systolic BP reaches ~95mmHg, the pendulum swings the other way, and the invasive systolic BP starts to become higher than the systolic noninvasive BP. What is interesting is that no matter what the noninvasive BP or invasive BP was, the MAP difference for both was only ~3.90. These findings were true whether patients were on vasopressors or not. The authors also looked at acute kidney injury (AKI), and the mortality rates associated with hypotensive readings for noninvasive and invasive BP’s. ICU mortality rates for patients with a minimum BP <70mmHg was 33.7% for invasive monitoring, and 43.4% for noninvasive. Acute kidney injury (AKI) in patients whose noninvasive BP’s were 70mmHg or less was 51.3%, and 34.7% in the invasive group. In comparison, “AKI prevalence based on MAP was not significantly different between invasive vs. noninvasive techniques (e.g., p=0.28 when MAP <60mm Hg).”

So what do the results of this study mean for us in the prehospital setting? Should we be determining our treatments based on MAP, especially in hypotensive patients? I would say “yes” based on current evidence.

If you are interested in knowing more about MAP, I would suggest visiting and reading Sam Ireland’s article entitled “The Map to Clarity” (2017).

Babbs, C.F. (2012).Oscillometric measurement of systolic and diastolic blood pressures validated in a physiologic mathematical model. BioMedical Engineering OnLine, 11, 56.

Haddad, S. H., & Arabi, Y. M. (2012). Critical care management of severe traumatic brain injury in adults. Scandinavian Journal of Trauma, Resuscitation and Emergency Medicine, 20, 12.

Lehman, L. H., Saeed, M., Talmor, D., Mark, R., & Malhotra, A. (2013). Methods of Blood Pressure Measurement in the ICU. Critical Care Medicine, 41(1), 34–40.

Romagnoli, S., Ricci, Z., Quattrone, D., Tofani, L., Tujjar, O., Villa, G., … De Gaudio, A. R. (2014). Accuracy of invasive arterial pressure monitoring in cardiovascular patients: an observational study. Critical Care, 18(6), 644.

2 thoughts on “Invasive and Noninvasive BP, and What They Should Mean

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  1. Good posts. Its funny how critical care providers (medics, nurses, doctors) often cite MAP as guidance for resuscitation, yet in the heat of the moment you always get a quick reaction pertaining to the SPB. I am not immune to this as i have found myself doing the knee-jerk WTH when the SBP shows hypotensive only to see a good MAP. Some of this is due to the MAP being proportionally smaller on the cardiac monitors but another is that we base protocols off of SBP. Especially a patient under anesthesia, which can very much have a SBP of 80’s-100’s yet still show no signs of shock (cap refill, diaphoresis, increased HR) due to the patient having better blood FLOW rather than solely pressure. Good blood flow at an appropriate MAP is what matter.

    One question I believe the cited study raises is, should retrieval teams (HEMS, ground CCT) have A-lines in place for resuscitation purposes? or be placing them?

    We are entering a time where field blood products administration, Severe sepsis transfers are becoming more common, often times needing the retrieval team to either start, continue, or significantly modify the resuscitation. Can they effectively do this with NIBP?

    1. You bring up excellent points. It is our opinion that any patient with hemodynamic instability should have invasive monitoring in place. In our current HEMS program, our protocols allow us to insert radial art lines for this very reason.

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