Advice from the BP Measurement Experts
It’s one of the occasional issues we hear about BP monitors and automated BP technology, ours included. And while it’s one of the reasons that we blog about the 10 Steps to Accurate Manual Blood Pressure Measurement and the 10 Factors That Can Affect Blood Pressure Readings, sometimes it takes more guidance to diagnose if a monitor is mis-reading. Comparing BP measurements manually taken by an observer to those from a monitor is a common way of examining the accuracy of a monitor. Clinical standards1 that determine a monitor's accuracy provide detailed methods for making these comparisons. Unfortunately, there are so many things done to control the environment and patient in these standards tests that it is not realistically possible to replicate in clinical practice. However, here are a few of the more important issues that Richard Prowse, our OEM sales manager for Europe, Middle East, and Africa, shares with his customers when they are investigating a monitor that might be reading high.
The most striking thing that will exaggerate the difference between manual and automated readings is the technique being used by the clinicians doing the comparison. The act of removing the cuff of the automated monitor and then applying the manual cuff changes the set up every time readings are taken. In SunTech's experience, this can induce greater differences, especially if the cuff/bladder sizes are not the same. The time this takes to swap the cuffs out also introduces error. The sequential readings (i.e. taken one right after the other) should be made within 30 seconds to reduce error.
When taking a manual measurement the cuff deflation speed should be 3mmHg per second to give optimum results. If the deflate rate being applied is significantly higher, manual measurements will underestimate the patient's BP, making the automated monitor seem as if it's reading high.
Environment noise can also be a big issue here. Even with optimum conditions (i.e. a quiet room, no talking and using proper technique), hearing the systolic and especially the diastolic point is often difficult. Auscultatory gap is another feature that can give higher systolic values, and it is also something that is difficult to pick up even as a trained observer.
In comparing measurements sequentially taken, BP variability can be quite an important factor. All clinical guidelines, including the American Heart Association’s, acknowledge variability in BP by recommending that patients “be instructed to relax as much as possible and to not talk” for at least 5 minutes or more. This needs special attention in hypertensive patients where BP variability can be more pronounced and more likely. There can be significant differences in systolic pressures between readings in this kind of patient even if the manual reading is taken promptly after the automatic one.
Furthermore, if the pulse volume is low and there is movement (by the patient or induced by the environment like in the back of an ambulance), it is possible that the reading taken by the monitor and the auscultatory reference will not agree closely. So what’s one to do? The experts can’t even agree except to accept the fact that this can happen. Arrhythmia can also be problematic, and it may be that the manual observer is better suited to taking this kind of measurement.
These are just a few of the factors that need to be considered when examining a monitor and its accuracy. Have you had any similar problems with a BP monitor? Did any of the issues above arise? Were there any others that you encountered?
1ISO 81060-2 is the clinical standard that many regulatory organizations like the FDA will be placing more focus on in the future. Historically, there have been others from the Association for the Advancement of Medical Instrumentation (AAMI), the British Hypertension Society (BHS), and the European Society of Hypertension (ESH).