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How to Measure Blood Pressure Again
Abstract & Commentary
By Michael H. Crawford, MD, Editor
Sources: Powers BJ, et al. Measuring blood pressure for decision making and quality reporting: Where and how many measures? Ann Intern Med 2011;154:781-788; Appel LJ, et al. Improving the measurement of blood pressure: Is it time for regulated standards? Ann Intern Med 2011;154:838-839.
New emphasis on optimal medical therapy in atherosclerotic cardiovascular (CV) disease has focused attention on the short-term variability of blood pressure (BP) measurements and the difficulty this poses for the diagnosis and treatment of hypertension. The availability of home BP monitors adds complexity to the problem, as no guidelines exist for their use. Thus, these investigators from the Durham Veterans Affairs Medical Center compared home, clinic, and research systolic blood pressure (SBP) measurements in 444 patients with hypertension who were part of a study to test the efficacy of BP management by nurses over the phone using home BP telemetry.
In this study, BP was measured by three methods: nurse measurements at routine clinic visits; two research clinic measurements 6 months apart for 18 months; and at least three home measurements a week for 18 months. For each measurement method, the mean within patient coefficient of variation in SBP (SD/mean SBP) was calculated. The patients were almost all men (92%), half were black, mean age was 64 years, and most had hypertension for > 10 years. More than 111,000 BP measurements were analyzed: 7121 clinic, 3218 research, and 100,842 home. SBP control was defined as < 140 mmHg for clinic and research measurements and < 135 mmHg for home measurements. Control varied considerably: 28% clinic, 47% home, and 68% research. Short-term variability (30 days clinic, 90 days home) was similar for all three measurements at about 10%. Comparing measurement methods, 52% of patients had a mean clinic BP ≥ 10 mmHg higher than their mean home BP. Within patient variance decreased with the number of measurements using all three methods. The greatest reduction in variance occurred with going from one to two measurements and decreased rapidly thereafter plateauing at four to six measurements. The number of measurements required by all three methods depends on the measured SBP. For example, a patient with one clinic value of 132 mmHg has a 40% probability of having a true SBP of > 140 mmHg; whereas a patient with a single measurement of 150 mmHg has a 70% probability of a true SBP > 140 mmHg. The authors concluded that the average of several measurements are more accurate than one clinic measurement of SBP.
Managing hypertension in many of our CV disease population is a challenge in many patients. One source of the difficulty is the variability in BP measurements in individual patients. The other major source is the difference between home and clinic BPs. The uncertainty about BP measurements is a common reason physicians are reluctant to change a patient's medicines. Adding more medication for high clinic pressures which are spuriously high may lead to hypotension, syncope, renal failure, myocardial ischemia, and a vicious cycle to hospital admission or death in the worst case scenario. A better way to determine a patient's true BP would be welcome. This study addresses this issue.
They document a 10 mmHg difference between clinic BPs and home BPs, which also has been shown in previous studies. The editorial accompanying this paper suggests that this may be due in part to the casual approach to BP measurement techniques in many clinics. In this study, SBP control was found in 68% of the patients on the research clinic measurements, 47% of the home measurements, and 28% of the clinic measurements. Although some of these discrepancies may be due to white coat effect in clinic, the difference in home vs research clinic suggests that poor technique probably plays a role. As the editorial points out, the American Heart Association had published guides for measuring BP properly since 1939, apparently to no avail. This is truly a sad state of affairs and should be addressed immediately by us all.
Variability within the patient occurs over hours to days, so multiple measurements of BP in clinic will not solve this problem. Also, the problem plagued all three measurement techniques to the same extent. This study suggests that the average of repeated measures on different days is the best way to improve accuracy. The JNC recommends two separate BP measurements on different days. This study shows that two measures will give you the biggest increment in SBP accuracy but 5-6 measures increase accuracy maximally unless SBP is near 140 mmHg, then 10 measurements are necessary. These data suggest that home BPs are the best way to achieve accuracy, since it is impractical to have the patient return to clinic 5-10 times in many situations. Unfortunately, the instruments are expensive and some patients do not have the wherewithal to measure their own BP. Accommodating these patients is difficult, but could be achieved by home visits, or multiple visits to a non-physician provider, or a home telemetry device that does not require a high level of patient interaction.