How do blood pressure monitors work?

Oscillatory blood pressure monitoring devices became popular over the last few years in hospitals as ambulatory blood pressure (BP) monitor as well as in private households as home blood pressure monitors. These devices measure the vibrations in the arterial wall caused by the blood, which flows through an artery between systolic and diastolic pressures.

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The electrical signal resulting from it is shown on a digital display. The systolic pressure is the measurement of the pressure in the blood vessels during the heartbeat. The blood vessel pressure, which is measured while the heart is resting, is called diastolic pressure. The newer devices determine how much inflation is necessary to reach 20mm Hg above the systolic pressure.

The blood flow stops as soon as this pressure is reached, and the cuff deflates below the systolic pressure allowing the blood to flow again and causing a vibration of the artery wall. After the cuff pressure falls below the patient's diastolic pressure, normal blood flow continues.

The vibrations of the artery wall can occur at any time when the cuff pressure is high enough for the blood to push the arterial wall open in order to flow through it. Digital devices deflate around 4 mm Hg per second. Although they seem to work slower than non-digital devices, some are more accurate.

What is hypertension?

Hypertension affects around 43 to 56 million adults and is a major health problem in some developing countries. It occurs when the blood flows through the arteries at higher pressures than normal. The pressure at which the blood pushes against artery walls as the heart pumps blood is known as the blood pressure. Too much of this force causes hypertension.

The condition contributes towards major cardiovascular diseases, and therefore monitoring the blood pressure is vital for some patients. For many, home blood pressures are the easiest way to monitor any changes in blood pressure. Therefore, it is important that these monitors are accurate. However, the accuracy of home blood pressure monitors varies and may be affected by the specific monitor and how it is used.

Are blood pressure monitors accurate?

The common criticism of home BP monitoring is the uncertainty about whether the data are accurate. The reported accuracy of HBP monitors varies. The BP inaccuracy may come from the operator of the device or from the device itself.

A recent cross-sectional study by Ruzicka M. et al. in 2016 included 210 patients who used home blood pressure monitors. For all patients, three measurements with the mercury sphygmomanometer and the three measurements with the home blood pressure monitor were taken.

The study has reported that in many of these home blood pressure monitors, differences of 5 mm Hg were detected compared to the mercury sphygmomanometer. These differences can be dangerous for some patients who require constant blood pressure monitoring due to their condition. Therefore, it is suggested to reassess home blood pressure monitors.

Previous studies by Hahn et al. and Merrick et al. had reported similar outcomes. The study by Merrick et al. in 1997 stated that the BP reading of 34% of patients was inaccurate using home blood pressure monitors. Campbell et al. observed that 35% of home blood pressure monitors were inaccurate using 4mm Hg as a threshold.

Protocols by the Association for the Advancement of Medical Instrumentation (AAMI), the British Hypertension Society (BHS), and the European Society of Hypertension are in place for pre-licensing of commercial BP devices. BP measurement devices already on the market are difficult to re-evaluate, as the process is costly and time-consuming.

However, accuracy studies for ambulatory BP monitors have also been done. Goodwin J. conducted a study in 2007 on 114 patients, which compared two different oscillometric ambulatory blood pressure monitors for accuracy. High accuracy was reported with both devices. The accuracy of these devices has also been confirmed in a study by Ronald M.D. in 1991.

Nasothimiou E. G. conducted a further study to evaluate the accuracy differences between home BP and ambulatory BP measurements. The study reported consistency between HBP and ABP reading for the diagnosis of hypertension. Furthermore, home BP monitors had high sensitivity and specificity compared to ambulatory monitoring for diagnosing hypertension.

Real-world use

In addition to considering how BP monitors function under ideal conditions (i.e., use by experienced professionals or in clinical studies), real-world use should be examined in determining how accurate these machines can be. There are proscribed methods for the proper evaluation of blood pressure that should be used in health settings and in the home.

Using consistent conditions minimizes the risk of error in comparing blood pressures over time. For example, patients should be seated quietly for 5 minutes with their legs uncrossed and feet on the floor prior to a blood pressure measurement. Blood pressure should also be checked in both arms. Patients may not realize that these conditions are important for the validity of results.

Healthcare professionals should also be reminded that it is not appropriate to ask the patient questions during the evaluation and to allow patients to rest comfortably for several minutes before taking the measurement.

Take home message

Several studies have revealed that some blood pressure monitors are accurate whilst others are not. Further investigation is necessary to determine the general accuracy of these devices. Additionally, the accuracy depends on the operator and the device itself, and therefore it is recommended to gather information about a monitor before purchase.

A general conclusion about whether BP monitors are accurate or inaccurate cannot be made. Patients who own blood pressure monitors are advised to bring them to an appointment so that the healthcare provider can observe how the patient uses the device and offer recommendations on proper use.

Source

Goodwin, J., 2007. Validation of the Oscar 2 oscillometric 24-h ambulatory blood pressure monitor according to the British Hypertension Society protocol. https://www.ncbi.nlm.nih.gov/pubmed/17353655.

Hahn, L.P. et al., 1987. Prevalence and accuracy of home sphygmomanometers in an urban population. Am J Public Health. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1647089/.

Merrick, R.D. et al., 1997. Factors influencing the accuracy of home blood pressure measurement. South Med J. https://europepmc.org/abstract/med/9386053.

Campbell, N.R., 2001. Self-measurement of blood pressure: accuracy, patient preparation for readings, technique and equipment. Blood Pressure Monitor. journals.lww.com/.../...easurement_of_blood_pressure__accuracy,.3.aspx.

Ruzicka, M., 2016. How Accurate Are Home Blood Pressure Devices in Use? A Cross-Sectional Study. www.ncbi.nlm.nih.gov/pmc/articles/PMC4889144/#pone.0155677.ref025.

Ronald, M.D. et al., 2007. Efficacy of 24-hour ambulatory blood pressure monitoring in children. The Journal of Pediatrics. https://doi.org/10.1016/S0022-3476(05)82193-6

Steven, A. et al., 2000. Home Blood Pressure Monitoring. JAMA Internal medicine. jamanetwork.com/journals/jamainternalmedicine/article-abstract/485316