Taking blood pressure: Too important to trust to humans?
ABSTRACTThe measurement of blood pressure in the physician’s office is subject to a number of observer errors and also to the “white-coat effect.” Automatic devices that measure blood pressure without a human observer in the room can eliminate many of these problems. We argue for greater use of these devices in the physician’s office.
KEY POINTS
- The white-coat effect, ie, the tendency of many patients to have higher blood pressure in the presence of medical personnel than in their own environment, can lead to inappropriate diagnosis of hypertension and unnecessary treatment.
- Out-of-office blood pressure correlates better with cardiovascular risk than does the blood pressure in the physician’s office, but ambulatory monitoring is costly and not widely available, and few physicians recommend self-measurement at home.
- Several available devices can take a series of blood pressure measurements at preset intervals while the patient sits alone in the examination room, eliminating the white-coat effect.
- The mean of five automatic readings taken at intervals of 1 or 2 minutes correlates well with the mean value while awake on ambulatory monitoring.
MEASURING BLOOD PRESSURE OUTSIDE THE OFFICE
Recent studies have reported that the information obtained by 24-hour ambulatory blood pressure monitoring and by self-measurement of blood pressure in the home more accurately reflects the patient’s risk of future cardiovascular events than do conventional blood pressure measurements taken in the physician’s office. 22–24 Current national guidelines recognize this pattern and the frequent measurement inaccuracies observed in clinical practice, and they are recommending including out-of-office measurements in the diagnosis of hypertension. 25,26
Ambulatory monitoring provides the most accurate measurement of out-of-office blood pressure. With ambulatory monitoring, the normal mean daytime pressure is considered to be lower than 135/85 mm Hg, in contrast to the 140/90 mm Hg cutoff used in the physician’s office with standard aneroid or mercury devices.
Self-monitoring of blood pressure at home has now become widely available with single-measurement oscillometric devices. (Oscillometric means that these devices measure the blood pressure by sensing the oscillations in pressure in the cuff induced by the pulsation of the brachial artery, as opposed to auscultating the Korotkoff sounds.) Blood pressures lower than 135/85 mm Hg outside the clinician’s office are considered normal with these devices.
However, despite its proven value, ambulatory monitoring is neither widely available nor cost-effective for the long-term management of hypertension. Furthermore, few physicians recommend that patients take their blood pressure at home, although the information obtained can be of significant value in the patient’s long-term management.
AUTOMATED MEASUREMENT IN THE OFFICE
In recent years, several automated oscillometric sphygmomanometers have been developed for measuring blood pressure in the office, and more are on the way. These devices can be programmed to take multiple readings without a clinician observer in the examination room, thus reducing the white-coat response.
Omron (Kyoto, Japan) makes several devices, including the HEM-907 and the HEM-705, that have been used in the clinical setting. 21,27–29 They can be programmed to take two or three readings at intervals of 1 to 2 minutes, with up to 5 minutes before the first reading. Unfortunately, data were not recorded with the patient alone in the room in many studies of the Omron devices, even though the devices meet national and international standards for accuracy.
The Microlife Watch BP Office (Microlife, Widnau, Switzerland) is currently undergoing development.30
The BpTRU (BpTRU Medical Devices, Coquitlam, BC, Canada) has enjoyed greater clinical acceptance, since it can take up to five blood pressure readings at intervals of 1 to 5 minutes, and calculates the mean of all five readings, taken with the patient resting comfortably in a quiet room without a clinician present.
The accuracy and durability of the device has been well established. Since the BpTRU self-calibrates between every blood pressure measurement, periodic calibration has not been required. The device can be placed on a table, mounted on the wall, or mounted on a cart if used in several locations in the office.
At Cleveland Clinic, several departments are using the BpTRU on a daily basis. Soon, we will be able to transfer data directly from the BpTRU to our electronic medical record system.
Studies of the BpTRU device
To date, most of the studies of automated office blood pressure measurement have used the BpTRU with the recording interval set at 1 to 2 minutes.
Myers31 used the BpTRU device in 50 hypertensive patients. The physician took the patient’s blood pressure with a mercury sphygmomanometer while the BpTRU device made the first reading, and then he left the room. The next five readings were taken at 2-minute intervals with the patient alone in the room. The mean initial reading by the machine was 162/85 mm Hg; the reading by the physician was 163/86 mm Hg. The third automatic reading was the lowest (averaging 140/84 mm Hg), and the mean of the five automated readings was 142/80 mm Hg, which was significantly lower than the initial reading obtained by the physician (P < .001).
In another study, Myers et al32 compared the measurements obtained by 24-hour ambulatory monitoring and by the BpTRU device (the mean of five readings obtained at 1-minute or 2-minute intervals) in 309 hypertensive patients. The mean blood pressure with the Bp-TRU was 132/75 mm Hg, which correlated well with the mean awake ambulatory blood pressure (134/77 mm Hg; r = 0.62 for the systolic pressure and 0.72 for the diastolic pressure).
We recently reviewed the records of 278 patients seen in our preventive medicine clinic (D.G. Vidt, MD, unpublished data, November 2009). The group included patients with and without established hypertension, and among the hypertensive group, both treated and untreated individuals. We had initially set the device to take readings at 3-minute intervals following the initial nurse-initiated reading. But in view of the recent data on the Bp-TRU using shorter intervals, we also obtained readings in 51 patients with the device set to record at 2-minute intervals, and then in 72 additional patients at 1-minute intervals. In all three groups, blood pressure had stabilized by the third reading after the clinician had left the room. These observations support those reported by Myers et al.31,32 Of particular importance is the observation that the white-coat effect dissipates within 2 to 3 minutes after the clinician leaves the room.33
The shorter measurement intervals can add up in a busy office practice, in which the time relegated to taking blood pressure is often limited.
In fact, waiting 5 minutes between measurements may allow the patient to become too relaxed and the blood pressure to drop too low vis-a-vis the gold standard, ambulatory monitoring. Culleton and colleagues34 compared the blood pressure in 107 hypertensive patients as measured four ways: by the referring physician, by a nurse who was trained to adhere to the protocol of the Canadian Hypertension Education Program, by 24-hour ambulatory monitoring, and by the BpTRU (the mean of five readings obtained at 5-minute intervals). The mean measured values were:
- 150/90 mm Hg by the referring physician
- 139/86 mm Hg by the nurse
- 142/85 mm Hg by ambulatory monitoring
- 132/82 mm Hg by the BpTRU device.
Although the BpTRU reduced the white-coat effect and white-coat hypertension, it underestimated the blood pressure, leading to misclassification of hypertension. Using 140/90 mm Hg as the cutoff for whether the patient was hypertensive and using ambulatory monitoring as the gold standard, the BpTRU misclassified more than half of the patients, agreeing with the classification of hypertensive or not hypertensive by ambulatory monitoring in only 48%. The authors recommended that the BpTRU not be set at 5-minute measurement intervals.34
