The reality of blood pressure measurement is that human beings do not do it very well. The time has come to delegate this job to machines that can do it better.
Several automatic devices are available. Used in physicians’ offices and in patients’ homes, they can eliminate some types of observer error as well as the “white-coat effect,” ie, the tendency of some patients to have higher blood pressure when medical personnel are present than in their natural environment. Since a difference of only a few millimeters of mercury can affect the physician’s decision to start or to modify treatment, measurements that more accurately reflect a person’s average blood pressure are to be desired.
In the following pages, we review the problems that plague blood pressure measurement by human observers, and we describe the advantages of automatic devices.
SHORTCOMINGS OF OFFICE BLOOD PRESSURE MEASUREMENT
For decades, large surveys have provided invaluable information on the prevalence of hypertension, its relationship to cardiovascular disease, and the benefits of treating it.1–3 Unfortunately, the percentage of hypertensive patients whose blood pressure is under control has remained low despite our increased knowledge about hypertension’s diagnosis and therapy.4
In these surveys, blood pressure was measured by auscultation by human observers using mercury or aneroid sphygmomanometers, and most physicians still use this method in clinical practice. But in spite of multiple guidelines for accurate measurement of blood pressure in the office, the overall accuracy and reproducibility of office blood pressure measurements remain poor.5–7
The accuracy of blood pressure measurement with aneroid and mercury manometers is affected by a number of observer errors and patient factors.8,9
Failure to prepare the patient. National guidelines5 state that before having their blood pressure taken, patients should be allowed to sit quietly for at least 5 minutes, which often does not happen. Another error is that clinicians rarely discourage patients from smoking cigarettes or drinking coffee in the 30 minutes prior to measurement.
Equipment and layout problems. Equipment should be properly calibrated and validated. 5 However, even if the sphygmomanometer is periodically calibrated, too often it is mounted on the wall adjacent to the examination table in the examination room, making it difficult to provide a comfortable seat with back and arm support during the reading. The measurement should be done with the patient sitting in a chair (not on an examination table), with feet on the floor and the arm supported at the level of the heart. If the forearm is not supported in the horizontal position and with the cuff at heart level, the blood pressure and heart rate tend to be higher.10 Further, the diastolic blood pressure and heart rate may be misleadingly low with the patient supine rather than seated,11,12 so readings should be taken with the patient sitting.
Miscuffing, ie, the use of a blood pressure cuff that is too large or, more often, too small for the patient’s arm, is a common source of error. The cuff bladder should encircle at least 80% of the arm.5 However, some offices do not have a large blood pressure cuff for overweight patients or a pediatric cuff for children or adults with arms of small circumference. It is recommended that a large blood pressure cuff be used routinely in adults, since a smaller cuff gives falsely high readings in people with large upper arms (circumference > 29 cm).13,14
Digit preference. Many physicians round off the blood pressure to the nearest 5 or 10 mm Hg. This problem may go along with:
Deflating the cuff too rapidly.
Talking to the patient while taking the blood pressure can contribute to higher readings.9
Not taking enough readings. Ideally, at the initial visit, blood pressure should be measured in both arms with the patient seated, and another reading should be taken with the patient standing. The arm with the higher pressure should be used for subsequent readings. Physicians should not make any treatment decisions based on blood pressure during an initial clinic visit, and at least two readings should be taken even on subsequent visits. However, owing to time constraints in busy clinical practices, treatment decisions are often based on single readings or on multiple readings on a single visit.
Discrepancies between observers. The blood pressure readings obtained by the nurse or medical assistant may differ significantly from those obtained by the physician. These differences can be large enough to affect treatment decisions,15,16 and they can be partially corrected by adequate training of all medical personnel who take blood pressure, doctors as well as nurses.
Given that time is tight in busy clinical practices and a trained blood pressure nurse or technician is usually not available, we will probably not see any significant improvement in the accuracy of blood pressure measurement using older technology and current physician practices.
The white-coat effect
Most patients have a higher level of anxiety, and therefore higher blood pressure, in the physician’s office or clinic than in their normal environment (as revealed by ambulatory monitoring or home blood pressure measurements), a phenomenon commonly called the white-coat effect.
Several factors can increase this effect, such as observer-patient interaction during the measurement. The effect tends to be greatest in the initial measurement, but can persist through multiple readings by the doctor or nurse during the same visit.
Whether the white-coat effect is due purely to patient anxiety about an office visit or to a conditioned response has been a point of interest in clinical studies. Regardless, it may result in the misdiagnosis of hypertension or in overestimation of the severity of hypertension and may lead to overly aggressive therapy. Antihypertensive treatment may be unnecessary in the absence of concurrent cardiovascular risk factors.17
“White-coat hypertension” or “isolated office hypertension” is the condition in which a patient who is not on antihypertensive drug therapy has persistently elevated blood pressure in the clinic or office (> 140/90 mm Hg) but normal daytime ambulatory blood pressure (< 135/85 mm Hg).18 Since patients may have an elevated reading when seen for a first office visit, at least several visits are required to establish the diagnosis. Multiple studies have suggested that white-coat hypertension may account for 20% to 25% of the hypertensive population, particularly in older patients, mainly women.19,20
Both white-coat hypertension and the white-coat effect can be avoided by using an automatic and programmable device that can take multiple readings after the clinician leaves the examination room (more about this below).21