COMPARING NIBP AND ABP

A recent article in Anesthesiology is interesting. The researchers explore how noninvasive blood pressure (NIBP) correlate to radial arterial blood pressure (ABP). They demonstrate how there are significant differences between NIBP and ABP in hypotensive or hypertensive patients. Using NIBP also makes us less likely to intervene against blood pressure extremes.

The study
Thanks to computerised data collection systems the group could obtain data from a huge group of anaesthetised patients where NIBP and/or ABP was recorded. In a group of 24225 patients 63% had recordings of NIBP and ABP. In the remaining 37% only ABP was recorded.

The first group allowed for plotting of NIBP against ABP. Then the second group, with ABP only, was compared with the NIPB+ABP-group regarding the frequency of interventions against hypertension or hypotension.

Results
Results are displayed in the curves. ABP on the x-axis, the difference between NIBP-ABP on the y-axis. The further you look towards the extremes the larger the discrepancy between NIBP and ABP becomes. Click image to enlarge.

NIBP are significantly higher when ABP-measurements indicate hypotension. And vice versa, when ABP indicate hypertension, NIBP is likely to be lower.

Interventions
In the group where NIBP and ABP were measured simultaneously, intervention against hypertension or hypotension was less likely than in the group where only ABP was measured.

27% of the hypotensive NIBP+ABP patients were transfused
43% of the hypotensive ABP patients were transfused

7% of the hypotensive NIBP+ABP-patients were given pressors
18% of the hypotensive ABP-patients were given pressors

12% of the hypertensive NIBP+ABP-patients were given antihypertensive medications
44% of the hypertensive ABP-patients were given antihypertensive medications

Why?
So, clearly measuring NIABP and ABP has a big impact on how we manage intraoperative hypo- and hypertension. How can this be explained? In hypotension and hypertension NIBP is likely to display a better pressure. The authors hypothesise that the anaesthetist is more likely to trust the better value.

If a patient is hypotensive according to the artery line measurement, the physician will find comfort in the NIBP, that likely shows a more tolerable value, and refrain from giving fluids, blood or pressors.

On the other hand, if reading ABP alone, then the anaesthetist only has that one alarmingly low blood pressure measurement to go by and is more likely to intervene.

Is this good or bad?
Hard to say. The authors, and others, suggest NIBP is more representative of the central (important) circulation than pressures in the radial arterial catheter – the most common site for measuring ABP. And giving transfusions and/or pressors/inotropes are not without complications. So seeking refuge in NIBP when arterial blood pressure readings show extremes might not be a bad idea.

Future think tank
The article authors hypothesize that femoral or axillary ABP might better reflect central BP, altough numbers in this study were too low to make that conclusion. But you might want to consider placing a more central arterial line for unstable patients?

That’s what’s being done for some patient groups where blood pressure control is critical. And the femoral ABP is considered the most reliable reading. And there’s a small study in Transplantation Proceedings on liver transplants comparing femoral ABP (fABP) to radial ABP (rABP) and NIBP, concluding that both NIBP and rABP would be needed in concert to be a reliable alternative to fABP.

Take home message
The BP cuff is not dead. Do not rely entirely on the arterial line. Especially in patients who are severely hypo- or hypertensive.

Anesthesiology. 2011 Nov;115(5):973-8.
Invasive and concomitant noninvasive intraoperative blood pressure monitoring: observed differences in measurements and associated therapeutic interventions.
Wax DB, Lin HM, Leibowitz AB.

Transplantation Proceedings. 2007 Jun; 39(5):1326-8.
Comparison of Femoral Arterial Blood Pressure With Radial Arterial Blood Pressure and Noninvasive Upper Arm Blood Pressure in the Reperfusion Period During Liver Transplantation.
Shin BS et al
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