Monitoring of Blood Pressure: Issues in Self-Measurement of Blood Pressure
Why has home blood pressure monitoring not become as widely accepted by healthcare providers as home blood glucose monitoring? A number of issues in self-measurement of blood pressure have limited its clinical utility, including difficulties defining “normal” home blood pressure, lack of clinical data justifying its place in clinical practice, questionable accuracy of the values that are obtained, inaccurate reporting of home blood pressures by patients, and lack of reimbursement by third-party payors.
‘Normal’ Home Values
Home blood pressures are typically lower than office blood pressures. On average, office blood pressure is 8.1/5.6 mmHg higher than home blood pressure; however, this effect may be attenuated by long-term antihypertensive treatment. There is no universally agreed-on upper limit of normal home blood pressure, but data suggest it should be 125/80 to 135/85 mmHg.These values are derived from both statistical assumptions and morbidity and mortality data.
Clinical Justification
Most studies have shown that home blood pressure monitoring is a better determinant of target-organ damage than office blood pressure. This has been shown for the degree of left ventricular hypertrophy and the progression of diabetes-related nephropathy. Only limited data, derived from a population in Japan, are available about the predictive value of home blood pressures and hypertension-related morbidity and mortality. Prospective studies are needed to determine whether home blood pressure monitoring offers advantages over office blood pressure monitoring for the prediction of morbidity and mortality.Home blood pressure monitoring has not been shown definitively to improve blood pressure control.
Accuracy
A common criticism of home blood pressure monitoring is the uncertainty about whether the data obtained are accurate. Inaccuracy may stem from the operator of the device or the device itself. Occasional differences of less than 5 mmHg are rarely clinically important, especially if many measurements are taken. However, consistent differences of 5 mmHg or greater may result in the false diagnosis of new or uncontrolled hypertension. There is a long list of factors that can affect the accuracy of blood pressure measurements (see Table 3).
Before accuracy standards were developed, home blood pressure monitoring was often found to be inaccurate. Today, the Association for the Advancement of Medical Instrumentation (AAMI) standards for sphygmomanometers have been recognized as the American national standard, and are used by the Food and Drug Administration, which regulates the home blood pressure monitor industry.Manufacturers must comply with the AAMI standards or other recognized standards to distribute and sell home blood pressure monitors in the U.S.; however, monitors do not have to pass AAMI standards to be marketed. A monitor that fails AAMI may be marketed but cannot bear labeling that it has been found accurate. Monitors that pass manufacturers’ AAMI tests are allowed to have an accuracy claim.
Under independent evaluation many models do not pass validation testing. To date, only a fraction of the devices available worldwide have been independently validated. This may be partly due to the expense of conducting validation studies using complex protocols. Recently, however, international organizations of specialists in hypertension have unanimously recommended that all devices for measuring blood pressure be independently validated. The Working Group on Blood Pressure Monitoring of the European Society of Hypertension (ESH) is reviewing blood pressure devices to guide medical and lay purchasers of all types of blood pressure measuring devices, including home blood pressure monitors. The ESH Working Group is using two of the most widely used protocols for evaluating accuracy of BP measuring devices, the AAMI standards and the British Hypertension Society (BHS) protocol. The criteria for fulfilling the AAMI protocol are that the test device must not differ from the mercury standard by a mean difference >5 mmHg or a standard deviation of >8 mmHg. The criteria for fulfilling the BHS protocol are that devices must achieve at least grade B (where A denotes greatest agreement with mercury standard and D denotes least agreement) for systolic and for diastolic pressures based on the summation of the percentage of readings with differences of ±5, ±10, and ±15 mmHg. The ESH Working Group classifies devices as “recommended,” “questionable recommendation,” and “not recommended.”
| Table 3. Errors in Measurement |
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To date, 23 home blood pressure devices have been evaluated by the ESH Working Group; tests are ongoing. Of the 23 devices evaluated, only 5 are recommended by the Working Group and all 5 measure blood pressure in the arm (see TABLE 1). The models available in the U.S. that are equivalent to those recommended include Omron HEM-705CP, HEM-711AC, HEM-712C, and HEM-739AC. The EHS Working Group is “reluctant” to recommend wrist monitors regardless of accuracy because of the dependency on the correct positioning of the device.
False Reporting
Substantial discrepancies, primarily in underreporting of blood pressures, have been found in home blood pressure monitoring studies comparing self-reported values and digitally stored values. False reporting has the potential to misguide treatment. If false reporting is suspected, first validate the accuracy of the monitor by calibrating it against a standard mercury sphygmomanometer. Once accuracy is confirmed, then ambulatory BP monitoring or home BP monitoring with a device that digitally stores results could be considered.
Other
Healthcare providers may be reluctant to recommend home BP monitoring because it may be perceived as an activity that induces anxiety or causes the patient to take an obsessive interest in blood pressure. Also, a factor that likely dissuades patients, and perhaps some healthcare providers, from home monitoring is that monitors are often not covered by patients’ insurance. Currently, Medicare does not cover home blood pressure monitoring.
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