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Variability and reliability of diastolic blood pressure during adolescence: The Philadelphia blood pressure project
PREVENTIVE
MEDICINE
10,
292-300
(1981)
Variability and Reliability of Diastolic Blood Pressure during Adolescence: The Philadelphia Blood Pressure Project’ MARY L. HEDIGER,*,~ JOAN I. SCHALL,* EVELYNJ.BOWERS,* ALAN B. GRUSKIN,~
WILLIAM AND
F. BARKER,* H. KATZ*
SOLOMON
*The W. M. Krogmnn Center for Research in Child Growth and Developwwnt, The Children’s Hospital of Philadelphia, I Children’s Cetzter, Philadelphia, Pennsylvania 19104, and tLkpartmrnt of Nephrology, St. Christopher’s Hospital for Children, 5th and Lehigh Avenue, Philadelphia, Pennsylvania 19133
This paper reports results of diastolic blood pressure studies done as part of the Philadelphia Blood Pressure Project (PBPP), a 3-year longitudinal study of black adolescents, initially ages 11- lS, who were enrolled originally in the Philadelphia Collaborative Perinatal Project (CPP). Three samples were studied by the PBPP: a representative sample of over 500 black adolescents stratified by age and sex, a smaller sample of adolescents who had blood pressures elevated beyond one standard deviation of the CPP population mean at age 7, and a sample of adolescents born of toxemic pregnancies. We found, for the representative sample, consistent differences in the means of diastolic blood pressure phases IV and V and between diastolic blood pressures taken in supine and seated positions. Supine diastolic blood pressures were more variable at every age and were lower than the comparable seated diastolic blood pressures. In addition, we estimated on a random subsample from all three samples diastolic blood pressure reliability for supine readings taken sequentially by the same (N = 96) and different (N = 55) examiners. We also estimated measurement reliability simultaneously (N = 91 supine, N = 96 seated) using a double-listening stethoscope. We found that although diastolic phase IV blood pressure was more reproducible or less variable over short periods of time (5- 15 min) when reliability measures were taken sequentially, two examiners simultaneously could not measure phase IV with significantly greater accuracy than phase V. We concluded that the fourth and fifth Korotkoff sounds should not be used interchangeably during adolescence and that postural position is an important contributor to diastolic blood pressure variance during adolescence. Also, since it has not yet been established which is more statistically predictive of adult blood pressure levels, whenever possible, both phase IV and V should be recorded for adolescents. INTRODUCTION
There is evidence that childhood and adolescent blood pressure status may be predictive of adult blood pressure (6, 10, 11). However, rapid changes in body size and maturational status increase variation in blood pressure during adolescence (2, 8, 16). It is therefore necessary for blood pressure to be assessedas precisely and accurately as possible during this period, but there is still some disagreement over which Korotkoff sound, phase IV (muffling) or phase V (disappearance), is the more reliable measure of diastolic blood pressure in adolescence, Most blood pressure studies do not present reliability estimates as part of their methods; notable exceptions have been the Framingham Study (5) and the 1 This work 2 To whom
was supported reprint requests
by NHLBI Grants HL19869 should be addressed. 292
0091-7435/81/030292-09$02.00/O Co~~righlO 1981 by Academic Press. Inc. All rights of reproduction in any form reserved.
and HL23511.
ADOLESCENT
BLOOD
PRESSURE
293
Bogalusa Heart Study (18). In addition, methods of taking blood pressure, both in terms of position and which Korotkoff phase is recorded, have varied from study to study making comparisons difficult. For example, Londe (12) and the Health and Nutrition Examination Survey (15) both report blood pressures on adolescent populations which were taken supine using phase V to represent diastolic blood pressure. The Tecumseh Study (7) and study by Morrison et al. (13) also used the fifth phase, but took blood pressures seated. On the other hand, the most recent standards (2) and the Bogalusa Heart Study (17) report seated blood pressures and phase IV. Recently, Voors et al. (18) concluded that as a seated measure phase IV is the preferable measure of diastolic blood pressure in children, ages 5- 14, because it was more reliable. They estimated reliability using blood pressures measured sequentially-that is, from approximately 5 min to 1 hr apart. This way of assessing reliability is better termed reproducibility and is complicated by blood pressure lability. An equally important consideration in choosing which phase to record in adolescence is which can be measured more reliably. In this paper, we present analyses of three different types of reliability estimates. These are (a) sequential intrareliability, the same examiner later remeasuring an adolescent; (b) sequential interreliability, a different examiner later remeasuring an adolescent; and (c) simultaneous interreliability, two examiners reading the same blood pressure using a double-listening stethoscope. The first two of these, similar to the reliability estimates presented by Voors et al. (18), involve both blood pressure lability and examiner perception, that is, which phase is more reproducible over time or varies less when later remeasured by the same or different examiners. The third is a check on examiner perception alone, that is, which phase can be measured more accurately by two examiners at any given point in time. MATERIALS
AND METHODS
Population sampling. Our subjects were members of the Philadelphia Blood Pressure Project (PBPP), a 3-year longitudinal study on a population of black adolescents, initially ages 11- IS, who were enrolled originally in the Philadelphia Collaborative Perinatal Project (CPP). The Philadelphia CPP prospectively gathered comprehensive obstetric, medical, psychological, and social information prenatally through age 7 on the course and products of almost 10,000 pregnancies (14). Registration into the CPP began in January 1959 and ended in December 1965. The mothers were from urban Philadelphia; 87% are black, 10% white, and 3% Puerto Rican. In general, the families were of relatively low socioeconomic status. For the PBPP, three samples were selected from the Philadelphia CPP cohorts registering between 1961 and 1965. Over 1,000 adolescents were seen at least once; most subjects were seen once a year for 3 consecutive years, beginning in February 1977. The three samples are: (a) a sample of over 500 black adolescents, representative of the CPP, stratified by age and sex; (b) a sample (N > 250) of black adolescents whose blood pressure (systolic and/or diastolic) at age 7 exceeded one standard deviation of the CPP population mean; and (c) a sample (N >
294
HEDIGER
ET
AL.
250) of black adolescents who were the products of toxemic pregnancies. The latter two samples were studied because we hypothesized that such individuals would be at special risk for elevated blood pressure in adolescence. The representative sample consists of five age cohorts, initially ages ll- 15, with each cohort containing about 100 adolescents, 50 of each sex. Data collected on the representative sample are analyzed here using mixed longitudinal approaches. Examination protocol. An average of 12 adolescents were examined per day during the latter half of each school year from 1977 to 1979. As soon as they arrived at the examination center, subjects rested recumbent for at least 10 min after which two supine blood pressures were taken. They then sat up, and two seated measurements were made. The minimum time seated prior to measurement was 5 min. This basic protocol was followed in all 3 years, with the exception that seated blood pressures were added midway through the first year so that we would have blood pressure data collected similarly to that of the most recent standards (2). At first, we elected to measure blood pressure supine to follow up on the protocol used by the CPP. A more thorough description of the representative sample, the use of mixed longitudinal designs, the data collection protocol, and our methods is given in Katz et af. (9). Blood pressure methods. Blood pressure was measured using a mercury sphygmomanometer (Baumanometer), with an inflatable cuff of appropriate width and length based on upper arm circumference. Two supine. measures were taken on the right arm, a minute apart with an intervening 30-set pulse. The two seated measures were taken similarly. Systolic, diastolic phase IV, and diastolic phase V pressures were recorded at each reading. In the first year, about 10% of the readings were taken using a (Gelman-Hawksley) random-zero sphygmomanometer (19). The random-zero was assigned each day to a different examiner to control for digit preference. However, since it was determined during the first year that the random-zero introduced another source of error (1) and that no significant digit preference was evident, the use of this device was discontinued. We use the average of two readings for each position to represent blood pressure. It has been suggested that more than two serial readings should be taken and averaged (4, 15, 18). In the second year, we took three blood pressure readings, each 1 min apart, on 71 subjects supine and 68 seated. Analyses of variance comparing the three serial readings indicated that there were no significant differences among them for any phase (Table 1). We concluded, therefore, that no additional information could be gained, under the conditions of our study, by taking three readings at such close intervals. Blood pressure reliabilities were taken routinely as part of the protocol to estimate examiner reliability. We found no significant differences for the correlation coefficients among pairs of examiners. There were three principal blood pressure examiners each year, a total of six in all. During the first year, reliability was determined using sequential readings. About 5- 15 min after the original reading, either the same or a different examiner would remeasure, on the average of one per day, the blood pressure of an adolescent selected at random. In the second and third years, reliability was assessed mostly using a double-listening stethoscope. Again, approximately one adolescent per day was measured jointly by two examiners .
ADOLESCENT
TABLE MEANS
OF SERIAL
BLOOD
295
BLOOD PRESSURE 1
PRESSURES
TAKEN
MINUTE
ONE
APART
Diastolic
Diastolic
N
Systolic
phase IV
phase V
1 2 3
71 71 71
114.5 114.9 114.4
68.5 70.2 70.1
57.1 56.3 57.6
1 2 3
68 68 68
112.8 112.8 112.8
74.0 74.6 75.4
67.6 68.8 70.1
Reading No.” Supine
Seated
o No differences among readings are significant by analysis of variance.
RESULTS Differences between Diastolic Phases IV and V-The
Representative Sample
Mean diastolic phase IV and V blood pressures, for readings taken both supine and seated, are given in Table 2. Readings from the representative sample for all 3 years were analyzed together; this means that most individuals are represented three times, once in each of three successive age groups. Five supine fifth-phase 2
TABLE MEANS
AND
STANDARD
IN BLACK
DEVIATIONS
OF DIASTOLIC
ADOLESCENTS-REPRESENTATIVE
BLOOD
Males Phase IV A&
PRESSURES’
SAMPLE
Females Phase V
Phase IV
Phase V
SD
N
I
SD
N
?
SD
60.2 57.6 58.3 58.6 57.7 60.0
Supine 11.5 12.3 13.3 11.6 12.4 11.3
101 148 161 148 128 75
71.0 70.9 71.8 71.1 70.3 71.2
9.2 9.1 9.3 8.3 9.1 9.5
100 150 156 148 127 75
60.0 60.2 62.1 61.0 59.7 62.0
11.9 11.1 10.7 9.4 11.9 11.4
66.8 68.6 67.9 69.4 69.2 69.3
Seated 9.5 9.4 10.4 10.3 10.6 10.6
73 129 142 132 122 75
75.2 74.9 76.4 75.1 75.4 75.5
8.2 7.5 7.4 8.0 8.1 8.0
73 128 143 132 121 75
69.2 70.3 71.5 70.0 69.8 70.7
11.0 8.6 9.2 9.6 9.0 8.1
N
x
SD
N
E
12 13 14 15 16 17
127 191 166 147 120 56
71.3 70.4 70.0 70.8 69.5 70.7
8.7 9.4 12.0 9.6 10.9 10.5
125 187 160 143 117 54
12 13 14 15 16 17
93 178 149 126 118 56
73.1 74.3 74.2 75.7 75.5 76.2
7.7 7.6 8.6 8.4 8.4 7.7
93 179 149 125 118 56
” Readings less than 20 mm Hg were not included in the calculations. Values are in mm Hg. * Chronological age groups represent kO.5 of the integer age, i.e., 12 = 11.5- 12.4 years. The data on 11-year-olds were not included in these analyses because the age distribution of the children representing 11-year-olds was skewed toward the upper end of the interval.
296
HEDIGER
ET AL.
readings, out of a total of 1,547, were excluded for being less than 20 mm Hg in accordance with the results on phase V distributions presented by Voors et al. (18). Eliminating the 5 fifth-phase readings changed no mean by more than 0.5 mm Hg and did not significantly alter the standard deviations. Supine diastolic blood pressure variances are greater than those for seated diastolic blood pressure at every age. There is no significant trend over this age range, 12- 17 years, toward higher diastolic readings for females either supine or seated, or for males when supine. A slight increase in phase IV readings, about 3 mm Hg, is evidenced for males when seated; similarly, for males there is a slight increase in seated phase V. In contrast, we have found a rather large rise among males in mean systolic blood pressure, from 107.7 + 9.4 mm Hg at age 12 to 118.4 + 9.9 at age 17. A comparable rise over the 12- 17 age span was found for male supine systolic blood pressure. There is not much rise, from about 109 to 111 mm Hg seated and 111.5 to 113 mm Hg supine, in systolic blood pressure among females in this age range. The means of the differences between diastolic phase IV and V also do not reveal any age trends. The overall mean difference, however, both supine and seated, is significantly smaller (P < 0.001) for females than for males (Tables 3,4). For supine readings, the mean difference overall for males is 12.5 as opposed to 10.5 mm Hg for females (Table 3). For readings taken when the subject was seated (Table 4), the difference between fourth and fifth phases is about half of the supine differences, with the overall mean for males being 6.2, and for females, 5.2 mm Hg. Sequential
Reliabilities
Analyses are presented on estimates of diastolic blood pressure reliability using readings taken sequentially in the first year of our study. More supine than seated TABLE MEANS
OF THE DIFFERENCE” BETWEEN DIASTOLIC REPRESENTATIVE SAMPLE,
Chronological as@ 12 13 14 15 16 17 Overall Range
3 PHASE SUPINE
IV AND PHASE V BLOOD
PRESSURE,
READINGS
Males
Females
N
P
SD
N
x
SD
125 186 160 143 117 54
11.2 13.1 12.7 12.8 12.6 11.3
7.5 9.0 8.6 8.0 8.3 7.8
100 148 156 146 127 75
11.2 10.6 10.3 10.5 10.6 9.1
7.0 6.9 6.1 6.4 8.4 5.8
785
12.5 O-46 mm Hg
8.4
752
10.5c O-51 mm Hg
7.0
’ Values are in mm Hg. b Chronological age groups represent 20.5 of the integer age, i.e., 12 = 11.5- 12.4 years. ’ Significantly different from the overall mean for males at P < 0.001. No age trends were significant by analysis of variance.
ADOLESCENT MEANS
OF THE
12 13 14 15 16 17 Overall Range
297
PRESSURE
TABLE 4 DIFFERENCE” BETWEEN DIASTOLIC PHASE IV AND PHASE V REPRESENTATIVE
Chronological ageb
BLOOD
SAMPLE,
SEATED
BLOOD
PRESSURE,
READINGS
Males
Females
N
x
SD
N
x
SD
93 178 149 125 118 56
6.3 5.7 6.3 6.4 6.3 6.9
5.9 5.4 5.7 5.7 5.4 6.1
73 128 142 132 121 75
6.0 4.6 5.0 5.1 5.6 4.8
6.3 4.1 5.3 4.0 4.4 3.3
719
6.2 O-33 mm Hg
5.6
671
5.2’ O-40 mm Hg
4.6
n Values are in mm Hg. b Chronological age groups represent kO.5 of the integer age, i.e., 12 = 11.5- 12.4 years. c Significantly different from the overall mean for males at P < 0.001. No age trends were significant by analysis of variance.
reliability readings were taken since seated readings were added only midway through the first year. Sequential reliabilities were measured in two ways: (a) the same examiner remeasuring an adolescent (intrareliability), and (b) a different examiner remeasuring an individual (interreliability). Since reliability readings were taken on subjects chosen randomly, data from all three PBPP samples are pooled. Means of the absolute differences between the initial or original and the reliability readings have been calculated to estimate the magnitude of the variation between the two. Means of the actual differences were also computed to determine if there was an effect of time. In addition, original and reliability readings were correlated to provide an indication of the extent to which the two covaried. For the supine sequential intrareliabilities (N = 96), the variance (SD2) of the absolute differences for the fifth phase (Table 5) was significantly greater (P < 0.01) than that for the fourth phase. The mean of the absolute differences was also greater (P < 0.05) for diastolic phase V. On the other hand, the correlation coefficients relating original and reliability readings were of about the same magnitude. Although, according to the mean blood pressure values, there was a tendency for the reliability readings to be slightly higher, no mean of the actual differences (original minus reliability reading) was significantly different than zero at CY= 0.05. Similar results were obtained on a smaller sample of seated sequential intrareliabilities (N = 21). For the supine sequential interreliabilities (N = 55), there was likewise a trend toward increased variance for the fifth-phase absolute differences, although the variances were not significantly different than those for phase IV (Table 5). The mean of the absolute differences for phase V supine was significantly greater (P < 0.05). As in the case of the intrareliabilities, no mean of the actual differences was significantly different than zero at a! = 0.05. For the supine readings, the correlation coefficients were much smaller than those for the intrareliability readings.
298
HEDIGER
ET AL.
TABLE 5 SEQUENTIAL RELIABILITIES Absolute difference 1original-reliability) N
x
Original reading
SD
Intrareliabilities Supine readings Phase IV Phase V Seated readings Phase IV Phase V
SD
x
SD
r
(Same Examiner)
96 94
4.93 6.44”
3.80 5.58
72.0 59.2
9.2 11.0
72.1 60.4
9.7 11.2
0.78” 0.71”
21 21
4.33 7.95”
4.20 7.98
75.4 68.2
8.7 10.9
77.1 69.3
8.2 13.8
0.7@ 0.60”
Interreliabilities Supine readings Phase IV Phase V Seated readings Phase IV Phase V
x
Reliability
(Different Examiners)
55 54
7.06 9.26
6.29 6.51
72.2 59.7
8.8 11.5
70.5 57.9
9.5 11.9
0.48” 0.54a
11 11
5.00 7.00
4.02 4.92
76.0 65.4
9.3 11.6
73.5 65.6
12.2 12.4
0.87” 0.77
n Correlation coeffkient significant at P < 0.005. b Significant at P < 0.01. r Variance significantly greater than that for Phase IV at P < 0.01. d Mean significantly greater than that for Phase IV at P < 0.05.
Again, on a smaller sample (N = 11) of seated sequential inter-reliabilities, the results seem to be confirmed. From the pattern of these results on diastolic blood pressures taken sequentially, it appears that the fourth phase varies less than the fifth phase over short periods of time. Simultaneous
Measures
of Diastolic
Reliability
Since it was clear after the first year that the sequential readings indicated lability as well as measurement accuracy, in the second and third years of our study, blood pressure measurement reliability was assessed principally using a double-listening stethoscope. Results from analyses of the simultaneous interreliabilities differed from those of the sequential reliabilities in several ways. As can be seen on Table 6, the mean absolute differences were smaller than those for the sequential reliabilities. The mean absolute differences between supine measures of phase V also were nearly identical to those of phase IV. However, the variance of the absolute differences for seated measures of phase IV was significantly greater (P < 0.01) than that for the fifth phase. The means of the original and reliability readings were within 1 mm Hg of each other, but the correlation between seated original and reliability readings of phase IV (r = 0.83) was significantly smaller (P < 0.01) than the correlation for phase V (r = 0.92). These results indicate that, for our sample in the seated position, we measured phase V blood pressure more reliably than phase IV, while there was no difference in measurement reliability for phases IV and V in the supine position.
ADOLESCENT
BLOOD PRESSURE
TABLE 6 SIMULTANEOUS INTER-RELIABILITIES(DOUBLE-LISTENING Absolute difference (original-reliability 1
Original reading
299 STETHOSCOPE) Reliability
N
x
SD
P
SD
Y
SD
Supine readings Phase IV Phase V
91 88
2.96 3.09
2.38 2.86
71.0 61.1
10.5 11.5
70.9 60.9
9.8 11.1
0 93” a:930
Seated readings Phase IV Phase V
96 96
2.61 2.40
3.57’ 2.40
75.7 70.9
7.4 8.4
75.9 71.0
7.7 8.2
0 83”,* 0:s.z
o Correlation coefficient significant at P < 0.0001. * Significantly smaller than correlation coefficient for Phase V at P < 0.01 c Variance significantly greater than that of Phase V at P < 0.01. Comment
For our sample, we found consistent mean differences among adolescents (12- 17 years) between supine and seated diastolic blood pressures. Mean seated fourth and fifth phase readings were higher than the comparable supine readings at every age. The mean differences between diastolic IV and V also differed by position; we found fourth and fifth phases to be about 5-6 mm Hg apart in the seated position, as did Johnson et al. (7). The mean differences for supine readings were twice as large. In addition, supine diastolic blood pressures were more variable at every age. Given these differences between phases and by position, it is clear that diastolic fourth and fifth phases should not be used or compared interchangeably during adolescence. Further, only diastolic blood pressure readings taken in the same position should be compared since postural position appears to contribute to diastolic blood pressure variance. The data presented on diastolic reliability readings taken sequentially and simultaneously suggest that phase IV is less variable and more reproducible over periods of a few minutes or hours. This is true of readings taken both by the same and by different examiners. This does not mean that we measured the fourth phase more reliably. On the contrary, if reliability is defined as the replicability of the same reading by two examiners at the same time, then both phase IV and V are equally reliably measured for adolescents in the supine position, and we measured phase V more reliably than phase IV in the seated position. However, the most important issue to be considered ultimately in determining which diastolic Korotkoff phase to measure in adolescence is which phase is more statistically predictive of future blood pressure levels. The data presented here on diastolic blood pressure reliability do not answer that question. Phase IV may be the most stable over short periods of time; future research may show, on the other hand, that the more labile or variable pressure may actually be statistically more informative for long-term predictive purposes. Hence, until it has been determined conclusively which diastolic Korotkoff phase is better associated with adult levels, we suggest that whenever possible both phase IV and V should be recorded
300
HEDIGER
for adolescents, as well as postural position, diastolic blood pressure variance.
ET
AL.
since it is an important
contributor
to
ACKNOWLEDGMENTS We would like to acknowledge the support and cooperation of the Division of School Health Services of the School District of Philadelphia and the many participants in the Philadelphia Blood Pressure Project. In addition, we are indebted to Steven Aurand, Ruth Barr, Jonathan Caldwell, Louis Carreras, James Coleman, Jr., Elizabeth Dickie, Lila Philson, Caroline Stuckert, and Linda Valleroy for their technical help. Ms. Susan Peterson assisted in the preparation of the manuscript. Finally, we would like to thank Drs. Robert Mack and James Prebis for their medical support.
REFERENCES 1. Barker, W. F., Hediger, M. L., Bowers, E. J., et al. Blood pressure determination: Concurrent validity studies. Prev. Med. 8, 128 (1979). 2. Blumenthal, S., ef a/. Report of the task force on blood pressure control in children. Pediatrics 59, 797-820 (1977). 3. Clarke, W. R., Schrott, H. G., Leaverton, P. E., et al. Tracking ofblood lipids and blood pressure in school age children: The Muscatine Study. Circularion 8, 626-634 (1978). 4. Epstein, F. H., Francis, T. Jr., Hayner, N. S., ef al. Prevalence of chronic diseases and distribution of selected physiologic variables in a total community, Tecumseh, Michigan. Amer. J. Epidemiol. 81, 307-322 (1965). 5. Gordon, T., Sorlie, P., and Kannel, W. B. Problems in the assessment of blood pressure: The Framingham Study. ht. .I. Epidemiol. 5, 327-334 (1976). 6. Heyden, S., Bartel, A. C., Hames, C. G., and McDonough, J. Elevated blood pressure levels in adolescents, Evans County, Georgia, 7 year follow-up of 30 patients and 30 controls. JAMA 209, 1682- 1689 (1969). 7. Johnson, B. C., Epstein, F. H., and Kjelsberg, M. 0. Distributions and familial studies on blood pressure and serum cholesterol in a total community-Tecumseh, Michigan. J. Chron. Dis. 18, 147- 160 (1965). 8. Johnson, B. C., Karunas, T. M., and Epstein, F. H. Longitudinal changes in blood pressure in individuals, families and social groups. Clin. Sci. Mol. Med. 45, 35s-45s (1973). 9. Katz, S. H., Hediger, M. L., Schall, J. I., et al. Blood pressure, growth and maturation from childhood through adolescence: Mixed longitudinal analyses of the Philadelphia Blood Pressure Project. Hypertension 2 (Suppl. I), 1-55-I-69 (1980). 10. Kilcoyne, M. M., Richter, R. W., and Alsup, P. Adolescent hypertension I. Detection and prevalence. Circulation 50, 758-764 (1974). 11. Lauer, R. M., Clarke, W. R., and Rames, L. K. Blood pressure and its significance in childhood. Postgrad. Med. J. 54, 206-210 (1978). 12. Londe, S. Blood pressure in children as determined under office conditions. Clin. Pediar. 5, 71-78 (1966). 13. Morrison, J. A., Khoury, P., Kelly, K., et a/. Studies of blood pressure in school children (ages 6- 19) and their parents in an integrated suburban school district. Amer. J. Epidemiol. 111, 156- 165 (1980). 14. Niswander, K. R., and Gordon, M. “The Women and Their Pregnancies.” Sanders, Philadelphia, 1972. 15. Roberts, J., and Maurer, K. “Blood Pressure of Youths 12- 17 Years.” Vital and Health Statistics, Series 11, No. 163, Washington, D.C., 1977. 16. Rosner, B., Hennekens, C. H., Kass, E. H., and Miall, W. E. Age-specific correlational analysis of longitudinal blood pressure data. Amer. J. Epidemiol. 106, 306-313 (1977). 17. Voors, A. W., Foster, T. A., Frerichs, R. R., et al. Studies on blood pressures in children ages 5- 14 years, in a total biracial community. Circulation 54, 319-327 (1976). 18. Voors, A. W., Webber, L. S., and Berenson, G. S. A choice of diastolic Korotkoff phases in mercury sphygmomanometry of children. Prev. Med. 8, 492-499 (1979). 19. Wright, B. M., and Dore, C. F. A random-zero sphygmomanometer. Lancer 1, 337-338 (1970).
MEDICINE
10,
292-300
(1981)
Variability and Reliability of Diastolic Blood Pressure during Adolescence: The Philadelphia Blood Pressure Project’ MARY L. HEDIGER,*,~ JOAN I. SCHALL,* EVELYNJ.BOWERS,* ALAN B. GRUSKIN,~
WILLIAM AND
F. BARKER,* H. KATZ*
SOLOMON
*The W. M. Krogmnn Center for Research in Child Growth and Developwwnt, The Children’s Hospital of Philadelphia, I Children’s Cetzter, Philadelphia, Pennsylvania 19104, and tLkpartmrnt of Nephrology, St. Christopher’s Hospital for Children, 5th and Lehigh Avenue, Philadelphia, Pennsylvania 19133
This paper reports results of diastolic blood pressure studies done as part of the Philadelphia Blood Pressure Project (PBPP), a 3-year longitudinal study of black adolescents, initially ages 11- lS, who were enrolled originally in the Philadelphia Collaborative Perinatal Project (CPP). Three samples were studied by the PBPP: a representative sample of over 500 black adolescents stratified by age and sex, a smaller sample of adolescents who had blood pressures elevated beyond one standard deviation of the CPP population mean at age 7, and a sample of adolescents born of toxemic pregnancies. We found, for the representative sample, consistent differences in the means of diastolic blood pressure phases IV and V and between diastolic blood pressures taken in supine and seated positions. Supine diastolic blood pressures were more variable at every age and were lower than the comparable seated diastolic blood pressures. In addition, we estimated on a random subsample from all three samples diastolic blood pressure reliability for supine readings taken sequentially by the same (N = 96) and different (N = 55) examiners. We also estimated measurement reliability simultaneously (N = 91 supine, N = 96 seated) using a double-listening stethoscope. We found that although diastolic phase IV blood pressure was more reproducible or less variable over short periods of time (5- 15 min) when reliability measures were taken sequentially, two examiners simultaneously could not measure phase IV with significantly greater accuracy than phase V. We concluded that the fourth and fifth Korotkoff sounds should not be used interchangeably during adolescence and that postural position is an important contributor to diastolic blood pressure variance during adolescence. Also, since it has not yet been established which is more statistically predictive of adult blood pressure levels, whenever possible, both phase IV and V should be recorded for adolescents. INTRODUCTION
There is evidence that childhood and adolescent blood pressure status may be predictive of adult blood pressure (6, 10, 11). However, rapid changes in body size and maturational status increase variation in blood pressure during adolescence (2, 8, 16). It is therefore necessary for blood pressure to be assessedas precisely and accurately as possible during this period, but there is still some disagreement over which Korotkoff sound, phase IV (muffling) or phase V (disappearance), is the more reliable measure of diastolic blood pressure in adolescence, Most blood pressure studies do not present reliability estimates as part of their methods; notable exceptions have been the Framingham Study (5) and the 1 This work 2 To whom
was supported reprint requests
by NHLBI Grants HL19869 should be addressed. 292
0091-7435/81/030292-09$02.00/O Co~~righlO 1981 by Academic Press. Inc. All rights of reproduction in any form reserved.
and HL23511.
ADOLESCENT
BLOOD
PRESSURE
293
Bogalusa Heart Study (18). In addition, methods of taking blood pressure, both in terms of position and which Korotkoff phase is recorded, have varied from study to study making comparisons difficult. For example, Londe (12) and the Health and Nutrition Examination Survey (15) both report blood pressures on adolescent populations which were taken supine using phase V to represent diastolic blood pressure. The Tecumseh Study (7) and study by Morrison et al. (13) also used the fifth phase, but took blood pressures seated. On the other hand, the most recent standards (2) and the Bogalusa Heart Study (17) report seated blood pressures and phase IV. Recently, Voors et al. (18) concluded that as a seated measure phase IV is the preferable measure of diastolic blood pressure in children, ages 5- 14, because it was more reliable. They estimated reliability using blood pressures measured sequentially-that is, from approximately 5 min to 1 hr apart. This way of assessing reliability is better termed reproducibility and is complicated by blood pressure lability. An equally important consideration in choosing which phase to record in adolescence is which can be measured more reliably. In this paper, we present analyses of three different types of reliability estimates. These are (a) sequential intrareliability, the same examiner later remeasuring an adolescent; (b) sequential interreliability, a different examiner later remeasuring an adolescent; and (c) simultaneous interreliability, two examiners reading the same blood pressure using a double-listening stethoscope. The first two of these, similar to the reliability estimates presented by Voors et al. (18), involve both blood pressure lability and examiner perception, that is, which phase is more reproducible over time or varies less when later remeasured by the same or different examiners. The third is a check on examiner perception alone, that is, which phase can be measured more accurately by two examiners at any given point in time. MATERIALS
AND METHODS
Population sampling. Our subjects were members of the Philadelphia Blood Pressure Project (PBPP), a 3-year longitudinal study on a population of black adolescents, initially ages 11- IS, who were enrolled originally in the Philadelphia Collaborative Perinatal Project (CPP). The Philadelphia CPP prospectively gathered comprehensive obstetric, medical, psychological, and social information prenatally through age 7 on the course and products of almost 10,000 pregnancies (14). Registration into the CPP began in January 1959 and ended in December 1965. The mothers were from urban Philadelphia; 87% are black, 10% white, and 3% Puerto Rican. In general, the families were of relatively low socioeconomic status. For the PBPP, three samples were selected from the Philadelphia CPP cohorts registering between 1961 and 1965. Over 1,000 adolescents were seen at least once; most subjects were seen once a year for 3 consecutive years, beginning in February 1977. The three samples are: (a) a sample of over 500 black adolescents, representative of the CPP, stratified by age and sex; (b) a sample (N > 250) of black adolescents whose blood pressure (systolic and/or diastolic) at age 7 exceeded one standard deviation of the CPP population mean; and (c) a sample (N >
294
HEDIGER
ET
AL.
250) of black adolescents who were the products of toxemic pregnancies. The latter two samples were studied because we hypothesized that such individuals would be at special risk for elevated blood pressure in adolescence. The representative sample consists of five age cohorts, initially ages ll- 15, with each cohort containing about 100 adolescents, 50 of each sex. Data collected on the representative sample are analyzed here using mixed longitudinal approaches. Examination protocol. An average of 12 adolescents were examined per day during the latter half of each school year from 1977 to 1979. As soon as they arrived at the examination center, subjects rested recumbent for at least 10 min after which two supine blood pressures were taken. They then sat up, and two seated measurements were made. The minimum time seated prior to measurement was 5 min. This basic protocol was followed in all 3 years, with the exception that seated blood pressures were added midway through the first year so that we would have blood pressure data collected similarly to that of the most recent standards (2). At first, we elected to measure blood pressure supine to follow up on the protocol used by the CPP. A more thorough description of the representative sample, the use of mixed longitudinal designs, the data collection protocol, and our methods is given in Katz et af. (9). Blood pressure methods. Blood pressure was measured using a mercury sphygmomanometer (Baumanometer), with an inflatable cuff of appropriate width and length based on upper arm circumference. Two supine. measures were taken on the right arm, a minute apart with an intervening 30-set pulse. The two seated measures were taken similarly. Systolic, diastolic phase IV, and diastolic phase V pressures were recorded at each reading. In the first year, about 10% of the readings were taken using a (Gelman-Hawksley) random-zero sphygmomanometer (19). The random-zero was assigned each day to a different examiner to control for digit preference. However, since it was determined during the first year that the random-zero introduced another source of error (1) and that no significant digit preference was evident, the use of this device was discontinued. We use the average of two readings for each position to represent blood pressure. It has been suggested that more than two serial readings should be taken and averaged (4, 15, 18). In the second year, we took three blood pressure readings, each 1 min apart, on 71 subjects supine and 68 seated. Analyses of variance comparing the three serial readings indicated that there were no significant differences among them for any phase (Table 1). We concluded, therefore, that no additional information could be gained, under the conditions of our study, by taking three readings at such close intervals. Blood pressure reliabilities were taken routinely as part of the protocol to estimate examiner reliability. We found no significant differences for the correlation coefficients among pairs of examiners. There were three principal blood pressure examiners each year, a total of six in all. During the first year, reliability was determined using sequential readings. About 5- 15 min after the original reading, either the same or a different examiner would remeasure, on the average of one per day, the blood pressure of an adolescent selected at random. In the second and third years, reliability was assessed mostly using a double-listening stethoscope. Again, approximately one adolescent per day was measured jointly by two examiners .
ADOLESCENT
TABLE MEANS
OF SERIAL
BLOOD
295
BLOOD PRESSURE 1
PRESSURES
TAKEN
MINUTE
ONE
APART
Diastolic
Diastolic
N
Systolic
phase IV
phase V
1 2 3
71 71 71
114.5 114.9 114.4
68.5 70.2 70.1
57.1 56.3 57.6
1 2 3
68 68 68
112.8 112.8 112.8
74.0 74.6 75.4
67.6 68.8 70.1
Reading No.” Supine
Seated
o No differences among readings are significant by analysis of variance.
RESULTS Differences between Diastolic Phases IV and V-The
Representative Sample
Mean diastolic phase IV and V blood pressures, for readings taken both supine and seated, are given in Table 2. Readings from the representative sample for all 3 years were analyzed together; this means that most individuals are represented three times, once in each of three successive age groups. Five supine fifth-phase 2
TABLE MEANS
AND
STANDARD
IN BLACK
DEVIATIONS
OF DIASTOLIC
ADOLESCENTS-REPRESENTATIVE
BLOOD
Males Phase IV A&
PRESSURES’
SAMPLE
Females Phase V
Phase IV
Phase V
SD
N
I
SD
N
?
SD
60.2 57.6 58.3 58.6 57.7 60.0
Supine 11.5 12.3 13.3 11.6 12.4 11.3
101 148 161 148 128 75
71.0 70.9 71.8 71.1 70.3 71.2
9.2 9.1 9.3 8.3 9.1 9.5
100 150 156 148 127 75
60.0 60.2 62.1 61.0 59.7 62.0
11.9 11.1 10.7 9.4 11.9 11.4
66.8 68.6 67.9 69.4 69.2 69.3
Seated 9.5 9.4 10.4 10.3 10.6 10.6
73 129 142 132 122 75
75.2 74.9 76.4 75.1 75.4 75.5
8.2 7.5 7.4 8.0 8.1 8.0
73 128 143 132 121 75
69.2 70.3 71.5 70.0 69.8 70.7
11.0 8.6 9.2 9.6 9.0 8.1
N
x
SD
N
E
12 13 14 15 16 17
127 191 166 147 120 56
71.3 70.4 70.0 70.8 69.5 70.7
8.7 9.4 12.0 9.6 10.9 10.5
125 187 160 143 117 54
12 13 14 15 16 17
93 178 149 126 118 56
73.1 74.3 74.2 75.7 75.5 76.2
7.7 7.6 8.6 8.4 8.4 7.7
93 179 149 125 118 56
” Readings less than 20 mm Hg were not included in the calculations. Values are in mm Hg. * Chronological age groups represent kO.5 of the integer age, i.e., 12 = 11.5- 12.4 years. The data on 11-year-olds were not included in these analyses because the age distribution of the children representing 11-year-olds was skewed toward the upper end of the interval.
296
HEDIGER
ET AL.
readings, out of a total of 1,547, were excluded for being less than 20 mm Hg in accordance with the results on phase V distributions presented by Voors et al. (18). Eliminating the 5 fifth-phase readings changed no mean by more than 0.5 mm Hg and did not significantly alter the standard deviations. Supine diastolic blood pressure variances are greater than those for seated diastolic blood pressure at every age. There is no significant trend over this age range, 12- 17 years, toward higher diastolic readings for females either supine or seated, or for males when supine. A slight increase in phase IV readings, about 3 mm Hg, is evidenced for males when seated; similarly, for males there is a slight increase in seated phase V. In contrast, we have found a rather large rise among males in mean systolic blood pressure, from 107.7 + 9.4 mm Hg at age 12 to 118.4 + 9.9 at age 17. A comparable rise over the 12- 17 age span was found for male supine systolic blood pressure. There is not much rise, from about 109 to 111 mm Hg seated and 111.5 to 113 mm Hg supine, in systolic blood pressure among females in this age range. The means of the differences between diastolic phase IV and V also do not reveal any age trends. The overall mean difference, however, both supine and seated, is significantly smaller (P < 0.001) for females than for males (Tables 3,4). For supine readings, the mean difference overall for males is 12.5 as opposed to 10.5 mm Hg for females (Table 3). For readings taken when the subject was seated (Table 4), the difference between fourth and fifth phases is about half of the supine differences, with the overall mean for males being 6.2, and for females, 5.2 mm Hg. Sequential
Reliabilities
Analyses are presented on estimates of diastolic blood pressure reliability using readings taken sequentially in the first year of our study. More supine than seated TABLE MEANS
OF THE DIFFERENCE” BETWEEN DIASTOLIC REPRESENTATIVE SAMPLE,
Chronological as@ 12 13 14 15 16 17 Overall Range
3 PHASE SUPINE
IV AND PHASE V BLOOD
PRESSURE,
READINGS
Males
Females
N
P
SD
N
x
SD
125 186 160 143 117 54
11.2 13.1 12.7 12.8 12.6 11.3
7.5 9.0 8.6 8.0 8.3 7.8
100 148 156 146 127 75
11.2 10.6 10.3 10.5 10.6 9.1
7.0 6.9 6.1 6.4 8.4 5.8
785
12.5 O-46 mm Hg
8.4
752
10.5c O-51 mm Hg
7.0
’ Values are in mm Hg. b Chronological age groups represent 20.5 of the integer age, i.e., 12 = 11.5- 12.4 years. ’ Significantly different from the overall mean for males at P < 0.001. No age trends were significant by analysis of variance.
ADOLESCENT MEANS
OF THE
12 13 14 15 16 17 Overall Range
297
PRESSURE
TABLE 4 DIFFERENCE” BETWEEN DIASTOLIC PHASE IV AND PHASE V REPRESENTATIVE
Chronological ageb
BLOOD
SAMPLE,
SEATED
BLOOD
PRESSURE,
READINGS
Males
Females
N
x
SD
N
x
SD
93 178 149 125 118 56
6.3 5.7 6.3 6.4 6.3 6.9
5.9 5.4 5.7 5.7 5.4 6.1
73 128 142 132 121 75
6.0 4.6 5.0 5.1 5.6 4.8
6.3 4.1 5.3 4.0 4.4 3.3
719
6.2 O-33 mm Hg
5.6
671
5.2’ O-40 mm Hg
4.6
n Values are in mm Hg. b Chronological age groups represent kO.5 of the integer age, i.e., 12 = 11.5- 12.4 years. c Significantly different from the overall mean for males at P < 0.001. No age trends were significant by analysis of variance.
reliability readings were taken since seated readings were added only midway through the first year. Sequential reliabilities were measured in two ways: (a) the same examiner remeasuring an adolescent (intrareliability), and (b) a different examiner remeasuring an individual (interreliability). Since reliability readings were taken on subjects chosen randomly, data from all three PBPP samples are pooled. Means of the absolute differences between the initial or original and the reliability readings have been calculated to estimate the magnitude of the variation between the two. Means of the actual differences were also computed to determine if there was an effect of time. In addition, original and reliability readings were correlated to provide an indication of the extent to which the two covaried. For the supine sequential intrareliabilities (N = 96), the variance (SD2) of the absolute differences for the fifth phase (Table 5) was significantly greater (P < 0.01) than that for the fourth phase. The mean of the absolute differences was also greater (P < 0.05) for diastolic phase V. On the other hand, the correlation coefficients relating original and reliability readings were of about the same magnitude. Although, according to the mean blood pressure values, there was a tendency for the reliability readings to be slightly higher, no mean of the actual differences (original minus reliability reading) was significantly different than zero at CY= 0.05. Similar results were obtained on a smaller sample of seated sequential intrareliabilities (N = 21). For the supine sequential interreliabilities (N = 55), there was likewise a trend toward increased variance for the fifth-phase absolute differences, although the variances were not significantly different than those for phase IV (Table 5). The mean of the absolute differences for phase V supine was significantly greater (P < 0.05). As in the case of the intrareliabilities, no mean of the actual differences was significantly different than zero at a! = 0.05. For the supine readings, the correlation coefficients were much smaller than those for the intrareliability readings.
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HEDIGER
ET AL.
TABLE 5 SEQUENTIAL RELIABILITIES Absolute difference 1original-reliability) N
x
Original reading
SD
Intrareliabilities Supine readings Phase IV Phase V Seated readings Phase IV Phase V
SD
x
SD
r
(Same Examiner)
96 94
4.93 6.44”
3.80 5.58
72.0 59.2
9.2 11.0
72.1 60.4
9.7 11.2
0.78” 0.71”
21 21
4.33 7.95”
4.20 7.98
75.4 68.2
8.7 10.9
77.1 69.3
8.2 13.8
0.7@ 0.60”
Interreliabilities Supine readings Phase IV Phase V Seated readings Phase IV Phase V
x
Reliability
(Different Examiners)
55 54
7.06 9.26
6.29 6.51
72.2 59.7
8.8 11.5
70.5 57.9
9.5 11.9
0.48” 0.54a
11 11
5.00 7.00
4.02 4.92
76.0 65.4
9.3 11.6
73.5 65.6
12.2 12.4
0.87” 0.77
n Correlation coeffkient significant at P < 0.005. b Significant at P < 0.01. r Variance significantly greater than that for Phase IV at P < 0.01. d Mean significantly greater than that for Phase IV at P < 0.05.
Again, on a smaller sample (N = 11) of seated sequential inter-reliabilities, the results seem to be confirmed. From the pattern of these results on diastolic blood pressures taken sequentially, it appears that the fourth phase varies less than the fifth phase over short periods of time. Simultaneous
Measures
of Diastolic
Reliability
Since it was clear after the first year that the sequential readings indicated lability as well as measurement accuracy, in the second and third years of our study, blood pressure measurement reliability was assessed principally using a double-listening stethoscope. Results from analyses of the simultaneous interreliabilities differed from those of the sequential reliabilities in several ways. As can be seen on Table 6, the mean absolute differences were smaller than those for the sequential reliabilities. The mean absolute differences between supine measures of phase V also were nearly identical to those of phase IV. However, the variance of the absolute differences for seated measures of phase IV was significantly greater (P < 0.01) than that for the fifth phase. The means of the original and reliability readings were within 1 mm Hg of each other, but the correlation between seated original and reliability readings of phase IV (r = 0.83) was significantly smaller (P < 0.01) than the correlation for phase V (r = 0.92). These results indicate that, for our sample in the seated position, we measured phase V blood pressure more reliably than phase IV, while there was no difference in measurement reliability for phases IV and V in the supine position.
ADOLESCENT
BLOOD PRESSURE
TABLE 6 SIMULTANEOUS INTER-RELIABILITIES(DOUBLE-LISTENING Absolute difference (original-reliability 1
Original reading
299 STETHOSCOPE) Reliability
N
x
SD
P
SD
Y
SD
Supine readings Phase IV Phase V
91 88
2.96 3.09
2.38 2.86
71.0 61.1
10.5 11.5
70.9 60.9
9.8 11.1
0 93” a:930
Seated readings Phase IV Phase V
96 96
2.61 2.40
3.57’ 2.40
75.7 70.9
7.4 8.4
75.9 71.0
7.7 8.2
0 83”,* 0:s.z
o Correlation coefficient significant at P < 0.0001. * Significantly smaller than correlation coefficient for Phase V at P < 0.01 c Variance significantly greater than that of Phase V at P < 0.01. Comment
For our sample, we found consistent mean differences among adolescents (12- 17 years) between supine and seated diastolic blood pressures. Mean seated fourth and fifth phase readings were higher than the comparable supine readings at every age. The mean differences between diastolic IV and V also differed by position; we found fourth and fifth phases to be about 5-6 mm Hg apart in the seated position, as did Johnson et al. (7). The mean differences for supine readings were twice as large. In addition, supine diastolic blood pressures were more variable at every age. Given these differences between phases and by position, it is clear that diastolic fourth and fifth phases should not be used or compared interchangeably during adolescence. Further, only diastolic blood pressure readings taken in the same position should be compared since postural position appears to contribute to diastolic blood pressure variance. The data presented on diastolic reliability readings taken sequentially and simultaneously suggest that phase IV is less variable and more reproducible over periods of a few minutes or hours. This is true of readings taken both by the same and by different examiners. This does not mean that we measured the fourth phase more reliably. On the contrary, if reliability is defined as the replicability of the same reading by two examiners at the same time, then both phase IV and V are equally reliably measured for adolescents in the supine position, and we measured phase V more reliably than phase IV in the seated position. However, the most important issue to be considered ultimately in determining which diastolic Korotkoff phase to measure in adolescence is which phase is more statistically predictive of future blood pressure levels. The data presented here on diastolic blood pressure reliability do not answer that question. Phase IV may be the most stable over short periods of time; future research may show, on the other hand, that the more labile or variable pressure may actually be statistically more informative for long-term predictive purposes. Hence, until it has been determined conclusively which diastolic Korotkoff phase is better associated with adult levels, we suggest that whenever possible both phase IV and V should be recorded
300
HEDIGER
for adolescents, as well as postural position, diastolic blood pressure variance.
ET
AL.
since it is an important
contributor
to
ACKNOWLEDGMENTS We would like to acknowledge the support and cooperation of the Division of School Health Services of the School District of Philadelphia and the many participants in the Philadelphia Blood Pressure Project. In addition, we are indebted to Steven Aurand, Ruth Barr, Jonathan Caldwell, Louis Carreras, James Coleman, Jr., Elizabeth Dickie, Lila Philson, Caroline Stuckert, and Linda Valleroy for their technical help. Ms. Susan Peterson assisted in the preparation of the manuscript. Finally, we would like to thank Drs. Robert Mack and James Prebis for their medical support.
REFERENCES 1. Barker, W. F., Hediger, M. L., Bowers, E. J., et al. Blood pressure determination: Concurrent validity studies. Prev. Med. 8, 128 (1979). 2. Blumenthal, S., ef a/. Report of the task force on blood pressure control in children. Pediatrics 59, 797-820 (1977). 3. Clarke, W. R., Schrott, H. G., Leaverton, P. E., et al. Tracking ofblood lipids and blood pressure in school age children: The Muscatine Study. Circularion 8, 626-634 (1978). 4. Epstein, F. H., Francis, T. Jr., Hayner, N. S., ef al. Prevalence of chronic diseases and distribution of selected physiologic variables in a total community, Tecumseh, Michigan. Amer. J. Epidemiol. 81, 307-322 (1965). 5. Gordon, T., Sorlie, P., and Kannel, W. B. Problems in the assessment of blood pressure: The Framingham Study. ht. .I. Epidemiol. 5, 327-334 (1976). 6. Heyden, S., Bartel, A. C., Hames, C. G., and McDonough, J. Elevated blood pressure levels in adolescents, Evans County, Georgia, 7 year follow-up of 30 patients and 30 controls. JAMA 209, 1682- 1689 (1969). 7. Johnson, B. C., Epstein, F. H., and Kjelsberg, M. 0. Distributions and familial studies on blood pressure and serum cholesterol in a total community-Tecumseh, Michigan. J. Chron. Dis. 18, 147- 160 (1965). 8. Johnson, B. C., Karunas, T. M., and Epstein, F. H. Longitudinal changes in blood pressure in individuals, families and social groups. Clin. Sci. Mol. Med. 45, 35s-45s (1973). 9. Katz, S. H., Hediger, M. L., Schall, J. I., et al. Blood pressure, growth and maturation from childhood through adolescence: Mixed longitudinal analyses of the Philadelphia Blood Pressure Project. Hypertension 2 (Suppl. I), 1-55-I-69 (1980). 10. Kilcoyne, M. M., Richter, R. W., and Alsup, P. Adolescent hypertension I. Detection and prevalence. Circulation 50, 758-764 (1974). 11. Lauer, R. M., Clarke, W. R., and Rames, L. K. Blood pressure and its significance in childhood. Postgrad. Med. J. 54, 206-210 (1978). 12. Londe, S. Blood pressure in children as determined under office conditions. Clin. Pediar. 5, 71-78 (1966). 13. Morrison, J. A., Khoury, P., Kelly, K., et a/. Studies of blood pressure in school children (ages 6- 19) and their parents in an integrated suburban school district. Amer. J. Epidemiol. 111, 156- 165 (1980). 14. Niswander, K. R., and Gordon, M. “The Women and Their Pregnancies.” Sanders, Philadelphia, 1972. 15. Roberts, J., and Maurer, K. “Blood Pressure of Youths 12- 17 Years.” Vital and Health Statistics, Series 11, No. 163, Washington, D.C., 1977. 16. Rosner, B., Hennekens, C. H., Kass, E. H., and Miall, W. E. Age-specific correlational analysis of longitudinal blood pressure data. Amer. J. Epidemiol. 106, 306-313 (1977). 17. Voors, A. W., Foster, T. A., Frerichs, R. R., et al. Studies on blood pressures in children ages 5- 14 years, in a total biracial community. Circulation 54, 319-327 (1976). 18. Voors, A. W., Webber, L. S., and Berenson, G. S. A choice of diastolic Korotkoff phases in mercury sphygmomanometry of children. Prev. Med. 8, 492-499 (1979). 19. Wright, B. M., and Dore, C. F. A random-zero sphygmomanometer. Lancer 1, 337-338 (1970).