What is the Truth? Differences in Ambulatory Blood Pressure Data Between Monitors

For treating patients with hypertension, organizations such as the UK’s National Institute for Health and Clinical Excellence (NICE) state that ambulatory blood pressure monitoring (ABPM) should be offered to patients with suspected hypertension. ABPM has been shown to provide a better predictive value for target organ damage and vascular events compared with clinic blood pressure (BP) measurement although the reproducibility of nocturnal BP fall by ABPM seems limited. In this month’s issue, Kallem and colleagues reported their comparison of ambulatory BP (ABP) readings from two monitors (Mobil-O-Graph [I.E.M., Stolberg, Germany] and Spacelabs 90207 [Spacelabs Medical, Issequah, WA]) worn at the same time by 12 untreated adults. The Spacelabs monitor recorded a 7.9-mm Hg higher systolic pressure compared with the Mobil-OGraph in the randomized condition outside of the confines of a clinical setting. In 1989, Santucci and colleagues compared the data obtained with two different ABP monitors from 17 normotensive and 28 hypertensive patients. Both devices were satisfactory and showed good agreement in a clinical setting. Although these two studies used different conditions for the BP measurements, we recognize that the results reported by Kallem and associates bring up an important issue regarding the accuracy of ABPM devices. Several points should be taken into account when interpreting the results of the Kallem study. The first point is the manner in which the number of patients was determined. The study population (12 adults in each nonrandomized or randomized condition) was small, and thus the results of the study might be overinterpreted. The second point is that no data regarding the mercury standard or intra-arterial BP were reported in the study. It is thus not possible to discern which of the two ABPM devices was more accurate. Few studies have compared an ABP monitoring sequence using two monitors worn simultaneously by adult patients in a randomized study design. The data presented by Kallem and colleagues thus make an important contribution, provided they are considered within the context of the study’s limitations. Validation studies have been conducted for Mobil-OGraph and Spacelabs 90207, but it has not been confirmed whether those validations were accurate. For example, most validation protocols that have been introduced for automated BP measuring devices recommend procedures carried out at rest only. O’Brien and colleagues indicated that many factors could influence the ABPM validation procedure, including compliance to the protocol being employed; the accuracy of the standard; establishing precisely the model being validated; the influences of BP level, age, and exercise on device accuracy; the provisions necessary for special populations, such as pregnant women, the elderly, and children; the influence of oscillometric vs Korotkoff sound detection and electrocardiographic gating on comparative measurements; the assessment of performance as distinct from accuracy; and the relevance of general factors such as the algorithm being employed and computer compatibility. In fact, Czarkowski and coworkers determined that venous blood redistribution affects the accuracy of the oscillometric method of BP measurement (Spacelabs 90207) using lower-body negative pressure to simulate changes of body position. They found that although the Spacelabs 90207 remained in the A class according to British Hypertension Society (BHS) protocol, the device’s accuracy was altered by venous blood redistribution, invoking the question of whether such an alternation constitutes an additional limitation of oscillometric ABPM. In addition, Rossen and colleagues pointed out an important error in the Spacelabs Report Management System 92506 software. The numerical differences between the printout and on-screen hourly averages were considerable, ranging from 37 mm Hg to 18 mm Hg systolic BP and from 16 mm Hg to 10 mm Hg diastolic BP. There are now numerous ABPM devices available, and it is more important than ever that the devices for measuring ABPM are accurately validated. In a recent systematic review, Hodgkinson and coworkers assessed methodological quality by determining whether validation protocols had been used and followed correctly. From 5420 journal articles identified, 108 met the inclusion criteria for the review. Excluding studies that assessed monitors that are no longer in use, 40 relevant studies of 21 different monitors were reviewed. Thirty-eight (95%) studies used a validation protocol, and 28 of these studies assessed a monitor in the general population. The protocols “passed” the validation in 24 of the 28 studies, but 12 of the 24 (50%) reported a difference of at least 5 mm Hg Address for correspondence: Kazuomi Kario, MD, Division of Cardiovascular Medicine, Department of Medicine, Jichi Medical University School of Medicine, Yakushiji 3311-1, Shimotsuke, Tochigi 329-0498, Japan E-mail: kkario@jichi.ac. jp