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Continuous vs Intermittent Blood Pressure Measurements in Estimating 24-Hour Average Blood Pressure MARCO DI RIENZO, P H . D . , GUIDO GRASSI, M.D., ANTONIO PEDOTTI, P H . D . , AND GIUSEPPE MANCIA, M.D. Downloaded from http://hyper.ahajournals.org/ by guest on June 12, 2017 SUMMARY In the past few years noninvasive automatic blood pressure (BP) recorders have been increasingly used to estimate patients' 24-hour BP more accurately than by one or few isolated measurements. However, these recorders only allow BP to be intermittently measured at intervals between 5 to 30 minutes, which means that the number of values collected over 24 hours (10 to 100) remains a tiny fraction of the thousands of values that occur during the same period. To determine whether this represents a limitation to this approach, BP was recorded intraarterially for 24 hours (Oxford method) in 20 ambulant hypertensive patients. A beat-to-beat analysis of the BP recording was provided by a computer, and the average 24-hour systolic, diastolic, and mean BP values were compared with those obtained by analyzing single BP waves of the same recording at intervals of 5,10, 15, 30, and 60 minutes. In each subject the average 24-hour BP values obtained by the beat-to-beat analysis closely corresponded to those obtained by the analysis performed at 5-, 10-, 15-, or 30-minute intervals. In most subjects, this was the case also when the analysis was performed at 60-minute intervals. Thesefindingsdemonstrate that intermittency of measurements does not limit the accurate assessment of true average BP. Indeed, accurate assessment can be achieved at intervals as much as 30 or 60 minutes apart. (Hypertension 5: 264-269, 1983) KEY WORDS • blood pressure variability intraarterial blood pressure recording automatic blood pressure recording I TABLE 1. Age, Sex, 24-Hour Mean Arterial Pressure and Standard Deviation for MAP (SD) in 20 Subjects N recent years noninvasive automatic blood pressure recorders have been developed in an attempt to record blood pressure more accurately than has been possible with instruments affording isolated measurements1"5 often collected under psychologically unfavorable conditions. 4 ' 6 ^ Most of these new recorders still present limitations with regard to accuracy.9"13 Also, there are problems with the intermittency of the readings, usually at intervals of 5 to 30 minutes, which means a maximum of a few hundred values in 24 hours. This number is a minute fraction of the many thousand values that make up a 24-hour blood pressure profile. Our study investigates whether this limitation affects the accuracy of the results. No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Methods Our study was performed on 20 hospitalized subjects all with uncomplicated untreated essential hypertension of mild or moderate degree. In each subject arterial blood pressure was invasively recorded for 24 hours with the Oxford technique,14-15 the arterial cath- Istituto di Clinica Medica IV, Universita di Milano, Centre di Rsiologia Clinica e Ipertensione, Ospedale Maggiore, CNR, and Centre di Bioingegneria, Fondazione Don Gnocchi, Milano, Italy. Address for reprints: Professor Giuseppe Mancia, Clinica Medica IV, Policlinico, Via F. Sforza 35, 20122 Milano, Italy. Received April 5, 1982; revision accepted October 7, 1982. Age (yrs) 45 52 41 29 51 36 Sex M M M F M F 24 M 48 43 63 50 33 43 42 38 47 54 38 45 68 F F M M M F M F M M F M F MAP (mm Hg) 110.7 144.5 105.2 108.3 100.5 103.9 100.3 113.9 119.3 97.5 130.4 92.0 117.5 188.5 105.6 130.5 100.3 76.7 83.9 140.5 (MAP), SD (mm Hg) 9.9 17.2 12.8 19.4 15.4 14.5 11.5 18.5 18.3 12.9 18.4 16.2 15.7 15.3 13.9 17.1 10.8 12.3 12.4 10.7 MAP refers to the average 24-hour values obtained in each subject by continuous analysis of the direct blood pressure recording. SD also refers to data obtained by continuous analysis. 264 INTERMITTENT VS CONTINUOUS BLOOD PRESSURE ASSESSMENTS/ Rienzo et al. eter being implanted percutaneously in a radial artery. During the recording the subjects were free to move within the hospital area and to engage in recreational and social activities as inpatients not confined to bed. Each recording was analyzed by a computer (PDP 11/34) that sampled the blood pressure signal every 60 msec and calculated for each pressure wave the systolic, diastolic, and mean arterial pressures. These values were stored on a magnetic disk to be later analyzed by two different methods. First, the average systolic, diastolic, and mean arterial pressures were calculated for the whole 24-hour recording period by making use 265 of all available values, i.e., the values of all pressure waves. Second, the average systolic, diastolic, and mean arterial pressures over 24 hours were also calculated by making use of the data of single pressure waves regularly taken at intervals of 5, 10, 15, 30, or 60 minutes. The results obtained by this latter method, which simulated intermittent blood pressure measurements, were compared with those obtained by the former method in which measurements were virtually continuous. In each subject, calculation of average 24-hour systolic, diastolic, and mean arterial pressures was ac- Downloaded from http://hyper.ahajournals.org/ by guest on June 12, 2017 A SBP mmHg 0 i-^-^V- - •-- - 15 + 10 *" m vr -10 + 10 0--»=- =j, - 10 + 1O ]0 0 -10- - 10J 5 10 15 30 60 5 10 15 30 60 periodical 0 — 24 h SBP by continuous 5 10 15 30 60 10 15 30 60mm measurements recording FIGURE I. Evaluation of the average 24-hour systolic blood pressure (SBP) value obtained by taking into account all SBP values in the 24 hours and by considering SBP values at intervals of 5, 10, 15, 30, and 60 minutes throughout the same period of time. The results obtained via the second method of analysis (indicated by the points) are shown as differences from the result obtained via the former method of analysis (indicated by the dashed horizontal line intersecting the 0 point). Data from the 20 subjects are separately shown. 266 HYPERTENSION companied by calculation of their respective 24-hour standard deviations. Also, in this instance a comparison was made of the results obtained through continuous and variably intermittent analyses of the data. In this way it was possible to assess whether intermittent blood pressure measurements could evaluate blood pressure variability precisely, a phenomenon that is important for understanding cardiovascular regulation16"18 and which may also represent an independent risk factor in hypertension. The results were separately analyzed for each patient and also calculated as means ( ± SE) for the whole group of patients. Before performing the 24-hour blood pressure recording, all patients gave free and informed consent to the procedure. VOL 5, No 2, MARCH-APRIL 1983 Results The average 24-hour mean arterial pressure obtained in each subject by continuous analysis of the 24hour recording is shown in table 1. Comparison of the data obtained by the continuous and intermittent measurements of the blood pressure tracing are shown in figures 1,2, and 3. In each patient the intermittent measurements provided average values for systolic, diastolic, and mean arterial pressure that were close (and often identical) to those obtained by the continuous measurements. This occurred when the intervals between the intermittent measurements were set at 5, 10, or 15 minutes, and also (with a slightly greater approximation) when the intervals were set at 30 minutes. Even prolongation of the intervals to 60 minutes Downloaded from http://hyper.ahajournals.org/ by guest on June 12, 2017 A DBP mm Hg 10 lO r Or - « -10- 0-10+ O-i 10- 1O 1© - 1OJ 0 -10- 5 0 10 15 30 60 5 10 15 30 60 5 10 15 30 p e r i o d i c a l measurements — 24 h DBP by continuous 60 10 15 30 60min recording FIGURE 2. Evaluation of the average 24-hour diastolic blood pressure (DBP) value obtained by taking into account all DBP values in the 24 hours and by considering DBP values at intervals of 5, 10, 15, 30, and 60 minutes throughout the same period of time. Explanations as for figure I. Data from the same subjects as in figure 1. INTERMITTENT VS CONTINUOUS BLOOD PRESSURE ASSESSMENTS; Rienzo el al. caused only small differences to appear in only few subjects. When expressed as average data for all patients (fig. 4), the 24-hour readings provided by the intermittent measurements at 60-minute intervals differed from those obtained through continuous measurements by no more than 2-3 mm Hg. For the intermittent measurements at shorter intervals, these differences were 1 mm Hg or less. The results obtained for mean arterial pressure variability are shown in table 1 and figure 5, which compare the data obtained by continuous and intermittent measurements. The 24-hour standard deviations of mean arterial pressure that were obtained by intermittent and continuous measurements differed much more than the average 24-hour mean arterial pressure values. When the intervals between the intermittent meas- 267 urements were 5, 10, or 15 minutes, the differences with the data obtained by continuous measurement were in the range of ± 10%. When the intervals were of 30 or 60 minutes, the differences remained within a similar range in a number of subjects, but when the intervals were 24 hours, the differences increased in a number of patients, to produce erratic results. Similar results were obtained for standard deviations of systolic and diastolic blood pressure. Discussion Our observations demonstrate that over a 24-hour period the average blood pressure values from measurements at intervals from 5 to 30 or 60 minutes do not Downloaded from http://hyper.ahajournals.org/ by guest on June 12, 2017 A MAP mmHg 10 lO 0 -10 + 10-1 ( 0 -10 + 10 0 - 1OJ +1O 1© - 1OJ + 10-1 - 10 J 5 0 10 15 30 60 5 10 15 30 60 5 10 15 30 6 0 periodical measurements 24 h MAP by continuous 10 15 30 60min recording FIGURE 3. Evaluation of average 24-hour mean arterial pressure (MAP) values obtained by taking into account all pressure values in the 24 hours, and by considering MAP values of the pressure waves occurring at intervals of 5, 10, 15, 30, and 60 minutes throughout the same period of time. Explanations as for figure I. Data from the same subjects as in figure I. 268 HYPERTENSION DBP SBP Downloaded from http://hyper.ahajournals.org/ by guest on June 12, 2017 0 15 30 No 2, MARCH-APRIL MAP 5 10 15 30 60 periodical measurements — 24 h by continuous recording 10 VOL 5, 60 10 15 30 FIGURE 4. Mean(± SEM) data for SBP, DBP, and MAP obtained from the individual data shown in figures 1, 2, and 3. Explanations as in the preceding figures; 24-hour average (± SE) values for systolic, diastolic, and mean arterial pressure obtained by analysis of all values were, respectively, 174.5 ± 6.7, 101.2 ± 3.4 and 125.4 ± 4.3 mm Hg. + 24 + 200 20 180 16 z ^ 160 E E c o - 140 c o • .? 6 a •o c 60 •o •o J 40 •o c " < 2 0 - 20 0 20 - - 40 4 10 0 15 30 60 periodical 5 10 measurements 15 30 60min 24 h SD by continuous recording FIGURE 5. Evaluation of 24-hour standard deviation for MAP obtained by taking into account all pressure values in the 24 hours, and by considering only the pressure waves occurring at intervals of 5, 10, 15, 30, and 60 minutes. The results obtained via the second method of analysis are shown for each subject as absolute (left panel) and percent (right panel) differences from those obtained via the former method of analysis (for each subject represented by the dashed horizontal line intersecting the 0 point). The 24-hour average (± SE) standard deviation for MAP was 15.1 ± 1.1 mm Hg. 1983 INTERMITTENT VS CONTINUOUS BLOOD PRESSURE ASSESSMENT/^ Rienzo et al. Downloaded from http://hyper.ahajournals.org/ by guest on June 12, 2017 differ substantially from mean values obtained by continuous measurement. This finding implies that a large number of blood pressure values are not necessary to achieve accurate mean values, a factor that may considerably reduce the complexity and cost of computer programs and facilities. Also, and more important, these results reinforce the use of noninvasive techniques that enable blood pressure levels to be automatically assessed at selected intervals. The accuracy of the single blood pressure readings provided by these devices is still under scrutiny. However, it is important that intermittency of blood pressure assessment does not itself represent a limitation in estimating the true average blood pressure of patients. It is also important that such an assessment can be obtained even by increasing the interval of the sampling up to 30 or 60 minutes. This may justify future adoption of automatic blood pressure measurements more widely spaced, with less disturbance to the patient. It may also simplify the construction of the recorders and avoid some technical difficulties. With regard to the second issue of our study, the standard deviations of the 24-hour blood pressure values were also not too different when derived by intermittent blood pressure measurements or by continuous blood pressure analysis. However, for intervals of measurements of 5 to 15 minutes, these differences encompassed a range of ± 10%, and this was much more than the ± 1 % to 2% difference found for the mean blood pressure values. Furthermore, in a number of subjects (about 30%) the differences in the standard deviation for blood pressure became larger, and often much so, for intermittencies of 30 or 60 minutes. This may be explained simply in statistical terms: according to the inference formulas, when the sampling size is reduced the potential error in estimating the true variance of the population under study progressively increases, and the rate of this increase largely exceeds that which characterizes the error in estimating the mean of the same population. At any rate, it is clear that intermittent blood pressure measurements may assess blood pressure variability less precisely than is the case with mean blood pressure values. In particular serious errors may occur if blood pressure assessments are too far apart from each other. 269 References 1. Richardson DW, Honour AJ, Fenton GW, Stott FH, Pickering GW: Variation in arterial pressure throughout the day and night. Clin Sci 26: 445, 1964 2. Athanassiadis D, Draper GJ, Honour AJ, Cranston WI: Variability of automatic blood pressure measurements over 24 hour periods. Clin Sci 36: 147, 1969 3. IrvinJB, EwingDJ, Kirby BJ: Value of prolonged recording of blood pressure in assessment of hypertension. Br Heart J 37: 859, 1974 4. Perloff D, Sokolow M: The representative blood pressure: usefulness of office, basal, home and ambulatory readings. Cardiovasc Med 3: 655, 1978 5. Sokolow M, Perloff D, Cowan R: Contribution of ambulatory blood pressure to the assessment of patients with mild to moderate elevation of office blood pressure. Cardiovasc Reviews and Reports 1: 295, 1980 6. Pickering G: The nature of essential hypertension. London: Churchill LTD, 1961, pp 1-151 7. Gould BA, Mann S, Davies AB, Altman DG, Raftery EB: Does placebo lower blood pressure? Lancet 2: 1377, 1981 8. Mancia G, Ferrari A, Bertinieri G, Grassi G, Zanchetti A: Blood pressure rises associated with cuff-measurement procedure in hypertension. Clin Sci 63: 387S, 1982 9. Labarthe DR, Hawkins CM, Renningham RD: Evaluation of performance of selected devices for measuring blood pressure. Am J Cardiol 32: 546, 1973 10. Sokolow M: Validation and limitations of the ambulatory blood pressure recorder. In Blood Pressure Variability, edited by Clement DL. Lancaster: MTP Press Limited, 1979, p 13 11. Edwards RC, Bannister R, Goldberg AD, Raftery EB: The infrasound blood pressure recorder. A clinical evaluation. Lancet 2: 398, 1976 12. O'Brien ET, O'Malley K: ABC of blood pressure measurements: future trends. Br Med J 2: 1124, 1979 13. Pickering GW, Stott FD: Ambulatory blood pressure. A review. In Proceedings of 4th Int Symp Ambulatory Monitoring, edited by Stott FD, Raftery EB, Goulding L. New York: Academic Press, 1981, p 135 14. Goldberg AD, Raftery EB, Green HL: The Oxford continuous blood pressure recorder. Postgrad Med J 52 (suppl 7): 104, 1976 15. Stott FD, Teny VG, Honour AJ: Factors determining the design and construction of a portable pressure transducer system. Postgrad Med J 52 (suppl 7): 97, 1976 16. Mancia G, Zanchetti A: Arterial blood pressure recording in human hypertension. In Atherosclerosis Reviews, vol 7, edited by Hegyeli R. New York: Raven Press, 1980, p 247 17. Watson RDS, Stallard TJ, Flinn RM, Littler WA: Factors determining direct arterial pressure and its variability in hypertensive man. Hypertension 2: 333, 1980 18. Mancia G, Ferrari A, Gregroini L, Parati G, Pomidossi G, Bertinieri G, Grassi G, Zanchetti A: Blood pressure variability in man: its relation to high blood pressure, age and baroreflex sensitivity. Clin Sci 59: 401s, 1980 Continuous vs intermittent blood pressure measurements in estimating 24-hour average blood pressure. M di Rienzo, G Grassi, A Pedotti and G Mancia Hypertension. 1983;5:264-269 doi: 10.1161/01.HYP.5.2.264 Downloaded from http://hyper.ahajournals.org/ by guest on June 12, 2017 Hypertension is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231 Copyright © 1983 American Heart Association, Inc. 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