Indirect ambulatory blood pressure monitoring devices

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1 AJH 1992;5: Comparison of Ambulatory and Clinic Blood Pressure and Heart Rate in Older Persons With Isolated Systolic Hypertension Gale H. Rutan, Robert H. McDonald, and Lewis H. Kuller We compared the blood pressure (BP) measurements obtained with a random-zero sphygmomanometer and an ambulatory BP monitor in older persons with isolated systolic hypertension at one site of the multicenter, randomized, double-blind clinical trial, the Systolic Hypertension in the Elderly Program (SHEP) randomized clinical trial. The subjects were community-dwelling elderly participants with isolated systolic hypertension enrolled in the SHEP study and already receiving stable doses of double-blind medication (n = 35 for active treatment group; η = 32 for placebo group). We measured seated (clinic) BP obtained with a random-zero sphygmomanometer, pulse rate, and BP and heart rate measurements obtained with an ambulatory BP monitor (average 24 h, daytime, and nighttime BP and heart rate). Across treatment groups clinic and ambulatory systolic BPs were not significantly different, but the placebo group had higher ambulatory, but not clinic, diastolic BPs. Within each treatment group (active treatment and placebo) there were no significant differences between clinic and average 24-h, daytime, or nighttime ambulatory systolic BPs. There were also no significant differences between clinic diastolic BP and average 24 h, daytime, or nighttime ambulatory diastolic BPs in the active treatment group, but in the placebo group average 24 h diastolic BP obtained by the ambulatory monitor was 4.6 mm Hg higher than clinic diastolic BP (P = ). The average 24 h heart rate was 6 to 7 beats/min higher as measured by the ambulatory monitor compared to clinic pulse (P <.01). In the placebo group of this study, average 24 h ambulatory diastolic BPs were consistently higher than clinic diastolic BPs. Except for this subgroup, ambulatory BP measurements closely approximated clinic BPs. Am J Hypertens 1992;5: KEY WORDS: Blood pressure, ambulatory blood pressure monitoring, random-zero sphygmomanometer, randomized clinical trial, Systolic Hypertension in the Elderly Program. Received February 27, Accepted August 18, From the Department of Veteran Affairs and the Departments of Medicine and Preventive Medicine, University of Tennessee (GHR), Memphis, Tennessee, and Departments of Medicine (RHM) and Epidemiology (RHM, LHK), University of Pittsburgh, Pittsburgh, Pennsylvania. This study was supported in part by National Institutes of Health Grant No. HL Presented in part at the 32nd Annual Conference on Cardiovascular Disease Epidemiology, American Heart Association, Memphis, Tennessee, March 20, The opinions expressed in this paper are those of the authors and do not necessarily reflect those of the Department of Veterans Affairs. Address correspondence and reprint requests to Gale H. Rutan, Ambulatory Care (11C), Memphis VA Medical Center, 1030 Jefferson Ave., Memphis, TN Indirect ambulatory blood pressure monitoring devices are being used more frequently for research in hypertension and for the assessment of the effects of antihypertensive treatment. Studies have occasionally shown differences between clinic blood pressures (BPs) and those obtained by these new instruments. 1,2 Consensus criteria for therapeutic intervention, based on the results of longitudinal observational studies and randomized clinical trials, have used clinic BPs (taken with either a standard mercury sphygmomanometer or a random-zero sphygmomanometer) and precise protocols of rest prior to the seated BP measurement. However, even an individual with "normal" BP 2992 by the American Journal of Hypertension, Inc /92/$5.00

2 AJH-DECEMBER 1992-VOL 5, NO. 12, PART 1 AMBULATORY BLOOD PRESSURE IN THE ELDERLY 881 exhibits wide fluctuations in arterial pressure throughout the day. 3 ' 4 Several studies have suggested that ambulatory BP measurements are superior to casual BP for monitoring the effects of antihypertensive treatment and for the assessment of risk of subsequent hypertensive sequelae. 5,6 However, little data now exist on the distribution curves of ambulatory BP readings, and even less data exist on ambulatory BP readings in the elderly, especially those with isolated systolic hypertension. In the Pittsburgh center of the Systolic Hypertension in the Elderly Program (SHEP) randomized clinical trial, we compared the clinic BP with BPs obtained using an ambulatory BP monitoring device. METHODS The Systolic Hypertension in the Elderly Program (SHEP) The relationship of ambulatory BPs with clinic BPs was examined in a subset of participants at the Pittsburgh SHEP center. The SHEP was a multicenter, randomized, double-blind, placebo-controlled clinical trial assessing the ability of antihypertensive drug treatment to reduce the risk of fatal and nonfatal stroke in persons aged 60 years and older with isolated systolic hypertension. 7 In summary, 4736 persons aged 60 years and older with an average systolic BP of 160 to 219 mm Hg and a diastolic BP < 90 mm Hg were randomized in approximately equal numbers to either placebo or active treatment. Blood pressure measurements were obtained at an initial screening and at two baseline visits prior to randomization. Treatment consisted of placebo or the use of step therapy with 12.5 and 25 mg/day chlorthalidone and 25 and 50 mg/day atenolol. The main results of the trial, described elsewhere, showed significant reductions in stroke and major cardiac and cardiovascular events with active therapy. 7 Ambulatory BP monitoring was done in a subset of participants on both placebo and active treatment at the Pittsburgh site of SHEP. Participants for the ambulatory BP study were recruited while already enrolled in SHEP and receiving a stable dose of SHEP medication. Criteria for study inclusion were fully ambulatory participants without any handicap (eg, blindness or requiring ambulatory assistive devices), which may have hampered full participation in the project, and residence within close proximity to the SHEP center to avoid inconvenience when returning the ambulatory BP monitor the following day. Blood Pressure Measurements The technique for obtaining BP and pulse rate measurements in SHEP followed a standardized protocol and was based upon American Heart Association recommendations. 8,9 For the initial screening visit, BP was obtained with a standard mercury sphygmomanometer; a random-zero sphygmomanometer (Hawksley and Sons, Ltd., Lancing, Sussex, England) was used for baseline (prerandomization) and all clinic visits. In summary, BP and pulse rate were measured after the participant was seated quietly for at least 5 min, in a comfortable posture, with feet flat on the floor. The radial pulse was palpated for 30 sec, and two seated BP measurements were taken with a 30 sec interval between measurements. The appropriate size cuff was deflated at a constant rate of 2 mm/sec until 10 mm Hg below the level of the diastolic BP. Blood pressures were recorded to the nearest even digit as the first (systolic) and fifth (diastolic) phase Korotkoff sound. Averages of these two systolic and diastolic measurements were used as the recorded values for both baseline and all clinic visits. Quality assurance for BP technique began with standardized training followed by periodic recertification of all observers, using both written and videotape testing of BP technique. Ambulatory BP Measurement Ambulatory BP monitoring was done once on each participant who entered this study, when the dose of either placebo or active medication was stabilized to reach either goal clinic BP or maximal dose of medication/placebo. The SpaceLabs 5200 BP monitor was used (SpaceLabs, Redmond, WA). Previously this model exhibited acceptable accuracy compared to five consecutive standard BPs done simultaneously with the ambulatory recordings. 10 The 5200 monitor records systolic BP 70 to 270 mm Hg and diastolic BP 40 to 155 mm Hg, and it meets the specifications of the Association for the Advancement of Medical Instrumentation for ambulatory BP monitoring. The auscultatory method is used unless the unit cannot detect Korotkoff sounds, then it automatically selects oscillometric. The monitor was attached to the left arm, according to the manufacturer's directions, at the completion of the second or third annual SHEP clinic visit, towards the end of the morning. Clinic BP had been taken approximately 1 h before the monitor was attached. Simultaneous ambulatory BPs and clinic BPs were not done. The participant was given oral and written instructions on the instrument's use. Blood pressure was monitored every half hour during the day and every hour from midnight to 6 AM. The device was turned off the next day and returned to the clinic. Daytime was considered 6 AM to 6 PM; evening and nighttime was 6 PM to 6 AM. The ambulatory readings were automatically deleted if the systolic BP < 80 or > 220 mm Hg (there was only one reading once > 220 mm Hg); the diastolic BP < 40 or > 135 mm Hg; or the heart rate < 40 or > 175 beats/min. Minimum inclusion criteria for daynight analysis were at least two technically adequate BPs per time period. Statistical Analyses Descriptive statistics (means, standard deviations, frequencies, and percentages) were generated for baseline characteristics and separately for subsequent treatment group and then compared. Differences between the treatment groups were tested with 11 appropriate parametric tests. All tests were two-tailed 12 at the Ρ =.05 significance level. Between group differ-

3 882 RUTAN ET AL AJH-DECEMBER 1992-VOL 5, NO. 12, PART 1 TABLE 1. DEMOGRAPHIC CHARACTERISTICS OF SHEP PARTICIPANTS UNDERGOING AMBULATORY BP MONITORING Active Treatment Placebo Ρ Ν = 35 Ν = 32 (Two-Sample t Test) Sex Female 18 (51%) 15 (47%).72 Male 17 (49%) 17(53%) Age (year) 71.5 ± ± Weight (kg) 74.4 ± ± Screening BP (mm Hg) Systolic ± ± Diastolic 81.7 ± ± First baseline BP (mm Hg) Systolic ± ± Diastolic 80.7 ± ± Second baseline BP (mm Hg) Systolic ± ± Diastolic 80.1 ± ± ences (active therapy ν placebo) were compared with the two-sample t test. Differences between clinic BP and ambulatory BP (average 24 h, daytime, nighttime, and between daytime and nighttime) for each participant were compared using the matched pairs t test. Because the objective of this analysis was to compare BPs using both the standard technique and ambulatory BP monitoring, participants were compared within separate treatment groups by actual treatment status (placebo ν active therapy) rather than by an intention-to-treat analysis. Thus, of the 67 participants who underwent ambulatory BP monitoring, there were two who had been randomized to the placebo group, but who were known to be taking antihypertensive medication during ambulatory BP monitoring, and they were analyzed in the active treatment group. There were no participants who were randomized to the active treatment group and who had not taken antihypertensive medication on the day of their clinic visit. RESULTS For this study 67 SHEP participants underwent 24 h ambulatory BP monitoring. Of these, 35 were receiving active therapy and 32 were on placebo. Of the 35 participants receiving antihypertensive medication, 23 were receiving 12.5 mg chlorthalidone only, eight were receiving 25 mg chlorthalidone only, one was receiving atenolol only, and three were receiving both chlorthalidone and atenolol. Their demographic characteristics, screening, and baseline BPs are listed in Table 1. There were no significant differences between the two groups in age, gender, weight, screening, or baseline BPs (Table 1). A total of 1640 technically adequate ambulatory BP measurements were recorded for the 67 participants (average = 24.5 readings/participant/24 h period; range = 4 to 41). Treatment ν Placebo Group Mean Findings With Clinic and Ambulatory BPs Between treatment groups there were significant differences for clinic systolic BP, average 24 h systolic BP, daytime systolic BP, and nighttime systolic BP (all Ρ < ; Table 2). Average clinic systolic BP was mm Hg for the active treatment group and mm Hg for the placebo group (P < ). There were also significant group differences in ambulatory systolic BPs for all time periods (all Ρ <). TABLE 2. COMPARISON OF CLINIC AND AMBULATORY SYSTOLIC BP (mm Hg) READINGS IN SHEP PARTICIPANTS Active Treatment Placebo ρ N = 3 5 Ν = 32 (Two-Sample t Test) Clinic systolic BP ± ± 16.5 < 001 Average 24 h ambulatory BP ± ± 16.4 < Ρ (paired t test; clinic ν 24 h BP) Daytime ambulatory systolic BP ± ± 17.2 < Ρ (paired t test; clinic ν daytime BP) Nighttime ambulatory systolic BP ± ± 18.2 < Ρ (paired t test; clinic ν nighttime BP).06.22

4 AJH-DECEMBER 1992-VOL 5, NO. 12, PART 1 AMBULATORY BLOOD PRESSURE IN THE ELDERLY 883 TABLE 3. COMPARISON OF CLINIC AND AMBULATORY DIASTOLIC BP READINGS (mm Hg) IN SHEP PARTICIPANTS Active Treatment Ν = 35 Placebo Ν = 32 (Two-Sample t Test) Clinic diastolic BP Average 24 h ambulatory BP Ρ (paired t test; clinic ν 24 h BP) Daytime ambulatory diastolic BP Ρ (paired t test; clinic ν daytime BP) Nighttime ambulatory diastolic BP Ρ (paired t test; clinic ν nighttime BP) 72.0 ± ± ± ± ± ± ± ± Average clinic diastolic BP was not significantly different between treatment groups (Table 3). The between-group mean difference in clinic BP was only 2.5 mm Hg (P =.20). However, the placebo group had higher diastolic BPs based on ambulatory monitoring compared to the active treatment group, and these BPs were consistently higher for the 24 h average (mean group difference = 6.9 mm Hg), daytime (difference = 8.0 mm Hg), and nighttime (difference = 7.3 mm Hg) diastolic BPs (all Ρ <.01). There were no between group differences in pulse at clinic visit or heart rate on ambulatory BP monitor. Average clinic pulse (beats/minute) was 69.5 for the active treatment group and 71.4 for the placebo group (P =.49; Table 4). There were also no between-group differences in 24 h, mean daytime, or mean nighttime heart rate. Only four participants in the active treatment group were receiving therapy with the ^-antagonist atenolol). Within-Group Mean Findings With Clinic and Ambulatory BPs Within each treatment group, there were no significant differences between any of the mean systolic BPs obtained by ambulatory monitoring and clinic systolic BP (Table 2). Intraindividual differences in systolic BP (clinic BP minus average 24 h ambulatory BP) are illustrated in Figure 1. Although there was a wide range of intraindividual clinic ν 24 h average systolic BP differences, they were not statistically significant for either treatment group (P =.62 for active; Ρ =.74 for placebo; Table 2). Within each treatment group correlation coefficients between clinic systolic and both average 24 h systolic and daytime systolic BP ranged from 0.41 to 0.57 (P <.01). Nighttime systolic BP was not significantly correlated with clinic systolic BP in either treatment group (active treatment: correlation coefficient = 0.18, Ρ =.15; placebo: correlation coefficient = 0.28, Ρ =.06). The within-group differences for diastolic BP are seen in Table 3. For the active treatment group there are no differences between clinic diastolic BP and ambulatory diastolic BP (average 24 h, daytime, or nighttime BPs). However, for the placebo group, there are significant differences between clinic diastolic BP and both average 24 h and daytime ambulatory BP, with higher average ambulatory values (Table 3; Ρ <.01). The intraindividual differences in diastolic BP (clinic BP minus average 24 h ambulatory BP) are illustrated in Figure 2. As can be seen in this figure, there are small intraindividual clinic ν 24 h average diastolic BP differences within the active treatment group (P =.83; Table 3). However, within the placebo group, the graph is skewed, reflecting higher 24 h ambulatory diastolic BPs compared to clinic BPs (P = ; Table 3). In both the active treat- Clinic pulse Average 24 h heart rate Ρ (paired t test; pulse ν 24 h heart rate) Daytime ambulatory heart rate Ρ (paired t test; pulse ν daytime heart rate) Nighttime ambulatory heart rate Ρ (paired t test; pulse ν nighttime heart rate) TABLE 4. COMPARISON OF CLINIC PULSE AND AMBULATORY HEART RATE READINGS (beats/min) IN SHEP PARTICIPANTS Active Treatment Ν = ± ± ±11.0 < 73.8 ± Placebo Ν = ± ± 8.6 < 79.8 ± 10.3 < 75.8 ± (Two-Sample t Test)

5 884 RUTAN ET AL AJH-DECEMBER 1992-VOL 5, NO. 12, PART 1 FIGURE 1. Intraindividual differences in systolic pressure. Values in mm Hg. CO c 03 QL 8 "o 7 t 6 CL 5 ο 4 d ζ ment and the placebo groups, clinic diastolic BP was significantly correlated with both average 24 h diastolic BP and daytime diastolic BP, with correlation coefficients ranging from 0.48 to 0.65 (P <.01). For the active treatment group the correlation between the clinic diastolic BP and nighttime diastolic BP was 0.31 (P =.04); for the placebo group the correlation coefficient was 0.62 (P < ). Within each treatment group average 24 h heart rate and daytime heart rate was 6 to 7 beats/min higher than seated clinic pulse (P <.01; Table 4). Clinic pulse and heart rate for the 24 h period, the daytime period, and the nighttime period were all significantly correlated, with correlation coefficients ranging from 0.36 to 0.59 (P <.05). Within the active treatment group 31 of 35 participants and within the placebo group 30 of 32 participants had sufficient nighttime ambulatory BP recordings for Active Placebo -12 to-8-7 to -3 2 to to to +12 > +13 Clinic BP minus Avg. 24 hr Ambulatory BP analysis of day-night differences. As noted previously, //nighttime,, is defined as 6 PM to 6 AM, and therefore reflects both evening leisure activity and sleep BPs. In both the active treatment group and the placebo group, average daytime systolic and diastolic BPs were higher than nighttime BPs (Tables 2 and 3). These day-night systolic and diastolic BP differences were significant within each group (P <.01 by paired t tests; data not shown) and were independent of treatment. DISCUSSION Results of this study show that in a group of elderly participants with isolated systolic hypertension, systolic BPs obtained with a random-zero sphygmomanometer closely approximated the systolic BPs obtained using an ambulatory BP monitor. Group estimates of BP were especially well-approximated. The intraindividual differences in systolic BPs illustrated in Figure 1 were, for FIGURE 2. Intraindividual differences in diastolic pressure. Values in mm Hg. Z to-8-7 to -3-2 to to to +12 > +13 Clinic BP minus Avg. 24 hr Ambulatory BP

6 AJH-DECEMBER 1992-VOL 5, NO. 12, PART 1 AMBULATORY BLOOD PRESSURE IN THE ELDERLY 885 the most part, insignificant (see Table 2 for statistical analysis). However, there were discrepancies in diastolic BP between treatment groups based upon ambulatory BP measurements, with the placebo group having higher ambulatory diastolic BPs compared to active treatment. It is not surprising that clinic BP and pulse would be correlated and not significantly different from the BP and heart rate measurements obtained with an ambulatory BP monitoring instrument, since these data represent repeated measures. 13 Other investigators also show small group differences between clinic and ambulatory BPs. One study comparing the SpaceLabs 5300 monitor (Redmond, WA) with intraarterial readings demonstrated discrepancies, often large, between these two methodologies in individual subjects, although group estimates were similar. 14 Casadei and colleagues concluded that the device was better suited to estimation of group BPs, but only because errors were "smoothed by the summation of individual errors of opposing signs." 14 Using an Accutracker (Suntech Medical Instruments, Raleigh, NC) instrument, Appel and colleagues demonstrated a mean difference (pairwise comparison) between ambulatory and clinic systolic and diastolic BPs of less than 3 mm Hg in 120 subjects. 15 On the other hand, these investigators also found that the ambulatory monitor tended to underestimate high systolic and diastolic BPs, and that increased age was significantly associated with underestimation of systolic BP. Miller and colleagues also concluded that the Accutracker instrument systematically underestimated systolic BPs, with a 0.15 mm Hg discrepancy in systolic BP per year of age. 16 The Accutracker instrument uses the auscultatory method, as does the SpaceLabs 5200 model. Hla et al. also demonstrated lower systolic ambulatory BPs (oscillometric device) compared to standard BPs in an elderly cohort. 17 However, these BPs were performed in-hospital during stressful procedures (intraarterial cannulation), which may partially account for this difference. Ruddy and colleagues found that the SpaceLabs 5200 monitor obtained lower awake systolic BPs, but higher awake diastolic BPs compared to clinic BPs in 81 elderly participants with untreated isolated systolic hypertension. 18 Average clinic systolic BP (mean ± standard deviation) was 169 ± 8 mm Hg, while average awake ambulatory systolic BP was 145 ± 14 mm Hg (P < ). Average clinic diastolic BP in Ruddy's study was 79 ± 8 mm Hg, while average awake ambulatory diastolic BP was 83 ± 8 mmhg(p <.05 by intragroup comparison). We also obtained higher daytime ambulatory diastolic BPs than clinic diastolic BPs in the placebo group (Table 3). Unlike Ruddy's results, however, there were no significant differences between clinic and ambulatory systolic BPs in our cohort. In another study using a Remler (Brisbane, CA) M2000 ambulatory BP recorder with a deflation rate of 10 mm Hg per 1.5 seconds, Dupont and colleagues 19 demonstrated lower ambulatory BPs compared to office systolic and diastolic BPs for the elderly subgroup (age > 60 years; η = 26). However, this difference between office and ambulatory BPs may partially reflect regression to the mean, since the disparity for both systolic and diastolic BP increased with increasing office BP, which was obtained at one sitting on the same day the ambulatory BP monitor was attached. 19 For purposes of randomized clinical trials ambulatory BPs may give additional meaningful information compared to clinic BPs alone. Although we noted similar ambulatory and clinic systolic BPs, there were discrepancies between ambulatory diastolic BPs within and between treatment groups. In addition to these group differences, for individual cases the ambulatory BP measurement may reflect a variability throughout the 24 h period that cannot be obtained by resting clinic BP. Reviewing the literature, the Working Group on ambulatory BP monitoring concluded that BP variability tended to be as great within subjects as it was between subjects. 20 More recently Bottini and colleagues examined 50 research subjects with repeat office and ambulatory BP measurements. 21 They found that the variability in mean 24 h automated BPs was less than that for casual BPs. The results of our study were based on a small sample of older adults with isolated systolic hypertension, by definition a truncated sample, and ambulatory monitoring was performed only once in this cohort. In addition, our ability to compare evening and nighttime BP and heart rate data within and between treatment groups was limited. As defined previously, the ambulatory monitor recorded these data only hourly between midnight and 6 AM, SO that participants were not unduely disturbed, and compliance with the main SHEP study objectives was not compromised. However, clinic BPs were obtained using a precise protocol within a multicenter randomized trial, with the associated quality control practices utilized in BP research. This strict adherence to protocol defining a clinic BP only after 5 min of seated rest, taken by trained nurses who had previously established rapport with the study participants during the first few years of the SHEP study, probably minimized the possibility of "white coat" hypertension as the cause of significantly elevated clinic BPs compared to ambulatory levels. Despite recent concern regarding possible inaccuracy of the random-zero sphygmomanometer, 22 the SpaceLabs 5200 monitor closely approximated the systolic BP obtained using a random-zero sphygmomanometer in older adults with isolated systolic hypertension. Although ambulatory BP devices eliminate digit preference and expectation bias, 17 they impose more of a burden on the individual participant. The great similarity of ambulatory and clinic systolic BPs and the dissimilarity of diastolic BPs

7 886 RUTAN ET AL AJH-DECEMBER 1992-VOL 5, NO. 12, PART 1 suggest that the utility of ambulatory BP monitoring in estimating group BPs in randomized controlled trials remains unresolved. These findings did not address the utility of such measurements in the management of individual patients. ACKNOWLEDGMENTS The authors thank Ms. Barbara Kaczmarek for statistical analysis, Ms. Sharon L. Quarles for secretarial assistance, and Dr. Jeremiah Stamler for his thoughtful comments. REFERENCES 1. Pickering TG, James GD, Boddie C, et al: How common is white coat hypertension? JAMA 1988;259: White WB: Assessment of patients with office hypertension by 24-hour noninvasive ambulatory blood pressure monitoring. Arch Intern Med 1986;146: Armitage P, Rose GA: The variability of measurements of casual blood pressure. I. A laboratory study. Clin Sci 1966;30: Armitage P, Fox W, Rose GA, Tinker CM: The variability of measurements of casual blood pressure. II. Survey experience. Clin Sci 1966;30: Perloff D, Sokolow M, Cowan RM, Juster RP: Prognostic value of ambulatory blood pressure measurements: further analysis. J Hypertens 1989;7(suppl 3):S3-S Sokolow M, Werdegar D, Kain HK, Hinman AT: Relationship between level of blood pressure measured casually and by portable recorders and severity of complications in essential hypertension. Circulation 1966;34: SHEP Cooperative Research Group: Prevention of stroke by antihypertensive drug treatment in older persons with systolic hypertension: final results of the Systolic Hypertension in the Elderly Program (SHEP). JAMA 1991;265: Frohlich ED, Grim C, Labarthe DR, et al: Recommendations for human blood pressure determination by sphygmomanometers: report of a special task force appointed by the steering committee, American Heart Association. Hypertension 1988;11:209A-222A. 9. Labarthe DR, Blaufox MD, Smith WM, et al: Systolic Hypertension in the Elderly Program (SHEP). Part 5: Baseline blood pressure and pulse rate measurements. Hypertension 1991; 17: Harshfield GA, Pickering TG, Blank S, et al: Ambulatory blood pressure monitoring: records, applications and analyses, in Weber MA, Drayer JIM (eds): Ambulatory Blood Pressure Monitoring. New York, Springer-Verlag, 1984, p SPSS-X User's Guide, 3rd ed. Chicago IL, SPSS, Inc, Fleiss JL: Statistical Methods for Rates and Proportions. John Wiley & Sons, New York, Olham PD: A note on the analysis of repeated measurements of the same subjects. J Chron Dis 1962;15: Casadei R, Parati G, Pomidossi G, et al: 24-Hour blood pressure monitoring: evaluation of SpaceLabs 5300 monitor by comparison with intra-arterial blood pressure recording in ambulant patients. J Hypertens 1988;6: Appel LJ, Whelton PK, Seidler AJ, et al: The accuracy and precision of the Accutracker ambulatory blood pressure monitor. Am J Epidemiol 1990;132: Miller ST, Elam JT, Graney MJ, Applegate WB: Discrepancies in recording systolic blood pressure of elderly persons by ambulatory blood pressure monitor. Am J Hypertens 1992;5: Hla KM, Vokaty KA, Feussner JR: Observer error in systolic blood pressure measurement in the elderly. Arch Intern Med 1986;146: Ruddy MC, Bialy GB, Malka ES, et al: The relationship of plasma renin activity to clinic and ambulatory blood pressure in elderly people with isolated systolic hypertension. J Hypertens 1988;6(suppl 4):S412-S Dupont AG, Vanderniepen P, Volckaert A, et al: Noninvasive ambulatory monitoring of blood pressure in essential hypertension: effect of age on variability and disparity. J Clin Hypertens 1986;3: National High Blood Pressure Education Program Coordinating Committee: National High Blood Pressure Education Program Working Group Report on ambulatory blood pressure monitoring. Arch Intern Med 1990; 150: Bottini PB, Carr AA, Rhoades RB, Prisant M: Variability of indirect methods used to determine blood pressure: office vs mean 24-hour automated blood pressures. Arch Intern Med 1992,152: Kronmal RA, Rutan GH, Borhani NO, et al: Potential problems with random-zero sphygmomanometer (letter). Lancet 1990;i:360.