Home Monitoring of Blood Pressure
The number of patients monitoring their blood pressure at home is growing. This can be attributed to a population that is aging, and increasingly obese and health-conscious. Of the estimated 50 million adults in the United States with hypertension, nearly a third are unaware that they have hypertension and approximately 70% do no have their blood pressure under control. The Healthy People 2010 initiative aims to have hypertension controlled in 50% of patients with hypertension by the year 2010.
Hypertension Morbidity and Mortality
Hypertension in adults traditionally has been defined as having a systolic blood pressure greater than or equal to 140mmHg and/or a diastolic blood pressure greater than or equal to 90 mmHg, or taking antihypertensive medication. Heart disease is the leading cause of death in the U.S. and stroke is the third. In patients with type 2 diabetes mellitus, in whoom the prevalence of hypertension is twice as high as in patients without diabetes, the risk of cardiovascular disease is doubled when hypertension is also present. Home blood pressure monitors vary in their accuract, ease of use, and cost. Digital monitors with automatic cuff inflation are the most popular.
Types of Self-Monitors
Mercury: Although the mercury sphygmomanometer is still considered the most accurate device for clinical use, it is generally not practical for home use. The mercury sphygmomanometer works by gravity to give consistent and accurate readings. It has a long, tubular gauge made of glass or plastic connected to a reservoir or mercury.
Aneroid: These dial-type monitors have been widely used by patients but also require skill because blood pressure is auscultated with a stethosope and inflation of the cuff is done manually.
Digital: Available with automatic inflation or manual inflation cuffs, nearly all digital devices deflate automatically. Automatic-inflation devices are especially useful for patients with arthritis.
Nearly all digital self-monitors measure blood pressure oscillometrically rather than by auscultation. Oscillometric measurement uses small oscillations, or changes, in cuff pressure to identify the systolic, mean and diastolic pressures.
Alternative Site Measurement
Manufacturers have developed blood pressure monitors that work on the wrist and the finger. However, devices that work on the upper arm are still preferable to wrist and finger monitors because of superior accuracy.
Wrist: Wrist measurement of blood pressure using the oscillometric technique has shown generally good agreement compared with the upper arm measurement using auscultatory or oscillometric methods. However, wrist measurement has shown high variability in individual cases and the correlation coefficients generally are not as high as those using the upper arm methods. The wrist measurements are very dependent on position of the device in relation to heart level.
Finger: It is widely agreed that finger blood pressure monitors are not accurate for home use. These devices measure BP by occluding a digital artery in the finger. Limb position is even more critical with finger monitors, and there is the additional problem of peripheral vasoconstriction affecting accuracy.
Benefits of Home Monitoring
White-coat Hypertension: White-coat hypertension, which may be more appropriately termed “isolated office hypertension,” is a condition in patients whose blood pressure is consistently elevated in the clinician’s office but normal at other times. A considerable portion of patients thought to have resistant or uncontrolled hypertension on the basis of office readings actually have white-coat hypertension and may not need ore aggressive therapy.
Assessing Response to Therapy: Home BP monitoring can be helpful in assessing response to antihypertensive therapy. It can be used to ensure adequate blood pressure control across the drug-dosing interval during awake hours, to evaluate effects of increasing or decreasing amounts of therapy, to ascertain whether poor office blood pressure response to increasing treatment represents overtreatment or true resistance, and to evaluate the relation of blood pressure levels to presumed side effects of therapy.
Adherence: Hypertension is a “silent disease,” often without signs or symptoms. Home blood pressure monitoring provides tangible signs of disease control. It may increase patients’ participation in their own care, and possibly help them copy with the disease. In general, adherence may be improved.
Costs: Home monitoring may provide a simple and cost-effective means for obtaining a large number of blood pressure readings. A study of 200 patients randomized either to usual care or to home monitoring in a closed model HMO found that self-measurement of blood pressure may be cost-effective. Potential savings from the use of home blood pressure monitoring, which have not yet been evaluated fully in clinical trials, include reductions in costs of medication for patients with white-coat hypertesion, lessneed for clinic visits, and possible reductions in costs associated with cardiovascular morbidity.
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 the 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 payers.
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. 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.
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. Manufacturers must comply with the AAMI (Advancement of Medical Instrumentation) standards or other recognized standards to distribute and sell home blood pressure monitors in the U.S.. Monitors that pass AAMI tests are allowed to have an accuracy claim.
Patient Counselling Devices
Accuracy is the most important factor in selecting a device. A device that passes independent validation testing is preferred. The limitation of wrist and finger monitors should be explained to the patient. Ease of use, display readability, machine portability, and cost hsould be considered as well as the need or desire for special features.
Cuffs: The patient should purchase an appropirately sized cuff. An inappropriately sized cuff can lead to incorrect blood pressure measurements. If the size of the bladder is too narrow or too short, overestimation of the blood pressure may occur.
Conversely, if the bladder is too wide or too long, the blood pressure may be underestimated. Generally, standard-size cuffs are for arms 9 to 13 inches in circumference. Most large cuffs are designed for arms 13 to 17 inches.
Self-Measurement Procedure: A validated blood pressure monitoring device should be used. Arm monitors are preferred. Finger monitors should not be used.