810
Aerobic Fitness and the Diurnal Rhythm of
Blood Pressure in Adolescents
Gregory A. Harshfield, Lynn M. Dupaul, Bruce S. Alpert, John V. Christman,
Elaine S. Willey, Joseph K. Murphy, and Grant W. Somes
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We examined the effects of aerobic fitness and race on the diurnal rhythm of blood pressure of
175 healthy adolescents who performed a cycle ergometer maximal exercise test while oxygen
consumption was measured. A median split of maximum oxygen consumption for boys and girls
separately classified them as either "more-fit" or "less-fit" subjects. Ambulatory blood pressure
recordings were also performed, and the data were analyzed for means while subjects were
awake and asleep. Less-fit black boys had higher systolic pressures than more-fit black boys
while awake (124 vs. 115 mm Hg;/><0.009) and asleep (117 vs. 108;p<0.001). Less-fit black
boys also had higher systolic pressures than less-fit white boys while awake (114;/?< 0.002) and
asleep (105; /?< 0.001), and they had higher systolic pressures than more-fit white boys while
asleep (105;p<0.01). Less-fit black girls had higher systolic pressures than more-fit black girls
while awake (116 vs. 109;p<0.004) and asleep (109 vs. 100;p<0.001). Less-fit black girls also
had higher diastolic pressures than more-fit black girls while awake (71 vs. 66; p < 0.002) and
asleep (66 vs. 61;p<0.001). In addition, less-fit black girls had higher systolic pressures than
more-fit white girls while asleep (104; p< 0.05), higher diastolic pressures than more-fit white
girls while awake (68;p<0.05) and asleep (60; p < 0.006), and higher diastolic pressures than
less-fit white girls while asleep (61;/?< 0.001). These findings indicate that fitness influences
ambulatory blood pressure profiles, particularly in black adolescents. {Hypertension
1990;15:810-814)
P
revious studies using ambulatory blood pressure monitoring have demonstrated racial
differences in the diurnal rhythm of blood
pressure in adolescents1 and adults.2-3 Specifically,
black individuals and white individuals have similar
levels of blood pressure while awake, but black
individuals have higher levels of blood pressure while
asleep. Aerobic fitness may be a second factor that
influences the diurnal variation of blood pressure.
Increased physical activity and aerobic fitness are
associated with lower casual blood pressure,4-11 as
well as a reduced risk for the development of hypertension and coronary heart disease.12-15 The purpose
of the present study was to examine the effects of
aerobic fitness and the interaction between aerobic
fitness and race on the diurnal variation of blood
pressure in healthy, normotensive adolescents.
From the Department of Pediatrics, University of Tennessee,
Memphis and the Clinical Research Center, LeBonheur Children's Medical Center (G.A.H., L.M.D., B.S.A., J.V.C., J.K.M.)
and the Department of Biostatistics and Epidemiology, University
of Tennessee (E.S.W., G.W.S.), Memphis, Tennessee.
Supported by National Institutes of Health grants HL-35788,
HL-42645, and GCRC M01-RR0021.
Address for correspondence: Gregory A. Harshfield, PhD,
University of Tennessee, Memphis, 848 Adams Avenue, Memphis,
TN 38103.
Methods
Subjects
A biracial sample of 226 healthy, normotensive
children between the ages of 10 and 18 years was
recruited as part of an ongoing study evaluating
hypertensive risk factors in children. A total of 175 of
the 226 children (77%) completed both the cycle
ergometer maximal exercise test and the ambulatory
blood pressure recording. Parental consent was
obtained before testing.
The sample included 80 boys and 95 girls, who
were divided into "more-fit" and "less-fit" categories
based on a median split of maximum oxygen consumption (Vc^max) performed separately for boys
and girls. The more-fit male group was composed of
23 white and 18 black boys with a mean (±SD) age of
14 ±2 years, a mean casual blood pressure of 111/
60±ll/13 mmHg, and a mean VOjmax of 48±5
ml/kg/min. The less-fit male group was composed of
21 white and 18 black boys with a mean age of 13±2
years, a mean casual blood pressure of 108/59 ±13/10
mm Hg, and a mean Vc^max of 37 ±4 ml/kg/min.
Less-fit boys were heavier than more-fit boys (55 ±15
vs. 63±18 kg;/j<0.01). The more-fit female group
was composed of 19 white and 27 black girls with a
Harshfield et al
1301
.002
Boys
0
Fitness and Ambulatory Blood Pressure
811
Fit White
Unfit White
Fit Black
Unfit Black
FIGURE 1. Bar graph showing least-square
means of systolic blood pressure for more-fit
(Fit) and less-fit (Unfit) white and black boys
while awake and asleep.
100
Awake
Asleep
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mean age of 13 ±2 years, a mean casual blood pressure of 99/57 ±11/13 mm Hg, and a mean Vc^max of
38±5 ml/kg/min. The less-fit female group was composed of 18 white and 31 black girls with a mean age
of 13±2 years, a mean casual blood pressure of
102/65±9/10 mm Hg, and a mean Vchmax of 27±3
ml/kg/min. Less-fit girls were heavier than more-fit
girls (45±9 vs. 62±15 kg;/?<0.01). The groups did
not differ by parental history of hypertension, alcohol
intake, caffeine intake, drug use, or smoking history.
Casual measurements were the average of three
readings taken in the seated position.
Voluntary Maximal Exercise Testing
Maximal exercise testing was performed on an
electronically braked cycle ergometer (Ergotest, Jaeger, Rockford, Illinois), as described previously.16
The mask and headgear used for gas analysis were
explained and fitted to the subject. The crank length
of the ergometer was adjusted for the child's leg
length, and the seat height was adjusted to maintain
the same optimal biomechanical knee position for all
subjects. Analysis of gas transport was performed
from rest through the fifth minute of recovery (Ergopneumotest, Jaeger). Gas analysis was printed every
15 seconds, and Vo2max, which was measured as
milliliters per kilogram per minute, was used for
analysis.
The continuous, graded maximal exercise test was
performed as follows: 1) Predetermined workloads
were selected based on gender, body surface area, and
race.16 2) Three-minute stages were designed to produce 33% of maximum during stage one, 67% during
stage 2, and 100% during stage 3. 3) Children were
encouraged during the test to complete at least three
stages. 4) Additional stages (increasing workloads of
one half of the increment used in previous stages)
were used until the child reached voluntary exhaustion. Maximal effort was defined as a respiratory
quotient of 1.05 or greater and subjective exhaustion.
Ambulatory Blood Pressure and
Heart Rate Recordings
The subject was seated, and the ambulatory blood
pressure recorder (model 5200, SpaceLabs Inc., Redmond, Washington) was applied and calibrated as
described previously.1 The subject was instructed to
relax the arm for a blood pressure determination and
to provide diary information for each hour the monitor was worn, including the time of sleep and
awakening. The child then resumed normal daily
activities and returned the following day for the
removal of the recorder. The recorder was set to take
a reading at 20-minute intervals between the hours of
6:00 AM and midnight and 60-minute intervals
between the hours of midnight and 6:00 AM. The
following day the child returned to the hospital, and
the recorder was removed. A successful recording
was one in which less than 25% of the readings
during the recording were artifacts and subjects had
data for at least 20 hours of the day.
The ambulatory blood pressure data were retrieved from the recorder and entered into a VAX
1170 computer for data editing and analyses as
described previously.1 Hourly averages were computed for each individual and coded based on the
subject's diary according to whether the subject was
awake or asleep during the hour.
The Statistical Analysis System's general linear
model analysis of variance procedure was used to
determine the effects of fitness level, race, and the
interactions among the variables. Planned comparisons were performed with t tests. Separate
models were run for awake and asleep readings and
for boys and girls.
Results
Boys
Systolic blood pressure. The least-square means for
awake and asleep systolic blood pressure for more-fit
and less-fit white boys and black boys are shown in
Figure 1. The interaction between fitness and race
812
Hypertension
Vol 15, No 6, Part 2, June 1990
Girls
F3 Fit White
Unfit White
Fit Black
Unfit Black
FIGURE 2. Bar graph showing least-square means
of systolic blood pressure for more-fit (Fit) and
less-fit (Unfit) white and black girls while awake and
asleep.
Awake
Asleep
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was significant while awake (/?<0.002) and asleep
(p<0.0l). More-fit and less-fit white boys did not
differ from each other while awake or asleep. Less-fit
black boys had higher systolic blood pressures than
more-fit black boys while awake (p< 0.009) and
asleep (/?<0.001). Less-fit black boys also had higher
systolic blood pressures than less-fit white boys while
awake (p<0.Q02) and asleep (p<0.001) and higher
systolic blood pressures than more-fit white boys
while asleep (/?<0.001).
Diastolic blood pressure. Fitness and the interaction
between fitness and race were not significant for
diastolic blood pressure while awake or asleep. Race
was not a significant factor during waking hours;
however, it was significant while the boys were
asleep. Black boys had higher diastolic blood pressures than white boys (66 vs. 60 mm Hg, /?<0.001).
Girls
Systolic blood pressure. The least-square means for
awake and asleep systolic blood pressure for more-fit
and less-fit white girls and black girls are shown in
Figure 2. Fitness was significant while awake
(/?<0.03) and asleep (p<0.005). Planned comparisons showed that less-fit black girls had higher systolic blood pressures than more-fit black girls while
awake O<0.004) and asleep (p<0.001). Less-fit
Girls
E
black girls also had higher systolic blood pressures
than more-fit white girls while asleep (p<0.05).
Less-fit white girls had higher systolic blood pressures than more-fit black girls while awake (p<0.04)
and asleep (p<0.04).
Diastolic blood pressure. The least-square means
for awake and asleep diastolic blood pressure for
more-fit and less-fit white girls and black girls are
shown in Figure 3. Fitness was significant while
awake (p<0.003). Planned comparisons showed that
less-fit black girls had higher awake diastolic blood
pressures than more-fit black girls (p<0.002) and
more-fit white girls (p<0.05). Less-fit white girls also
had higher diastolic blood pressures while awake
than more-fit black girls (p<0.03). The interaction
between race and fitness was significant for asleep
diastolic blood pressure (/?<0.05). Less-fit black girls
had higher diastolic blood pressures while asleep
than more-fit black girls (p<0.001), less-fit white
girls (p< 0.002), and more-fit white girls (p< 0.006).
Influence of Body Size on Blood Pressure
A stepwise regression was run to determine the
effects of Quetelet index, body surface area, height,
and weight on blood pressure while awake and asleep
in boys and girls separately. Quetelet index showed
the most consistent associations. Separate analysis of
Fit White
Unfit White
Fit Black
Unfit Black
FIGURE 3. Bar graph showing least-square means
of diastolic blood pressure for more-fit (Fit) and
less-fit (Unfit) white and black girls while awake and
asleep.
Awake
Asleep
Harshfield et al Fitness and Ambulatory Blood Pressure
variance models, including Quetelet index, were then
run, and the resulting least-square means were compared with the previous means. Inclusion of Quetelet
index into the model led to the following modifications: no change in the awake or asleep least-square
means of less-fit white boys or girls, a decrease of
less-fit black boys by 2 mm Hg while awake and 1
mm Hg while asleep, a decrease in less-fit black girls
by 2 mm Hg while awake and an increase by 2 mm Hg
while asleep, an increase in more-fit white boys by 1
mm Hg while awake, an increase in more-fit white
girls by 1 mm Hg while awake and 2 mm Hg while
asleep, an increase of more-fit black boys and girls by
2 mm Hg while awake, and an increase of more-fit
black boys and girls by 1 mm Hg while asleep. Similar
results were found for diastolic blood pressure.
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Discussion
Black Americans between the ages of 18 and 74
have a prevalence of essential hypertension 1.4 times
greater than white Americans.17 Preliminary research
suggests that racial differences may, in part, be
related to differences in prehypertensive 24-hour
ambulatory blood pressure profiles. In a previous
study, we demonstrated that the blood pressures of
black and white children were similar while the
children were awake but that black children had
higher blood pressures than white children during
sleep.1 In a second study, Harshfield et al2 extended
these findings to healthy, normotensive black adults
without a history of disease. In contrast to previous
reports in white adults,18 they reported that black
adults did not show a significant decline in blood
pressure during sleep. Finally, Murphy et al3 examined ambulatory blood pressure patterns in black
adults and white adults under evaluation for borderline hypertension. The average daytime blood pressure of the black adults and white adults was equivalent, but the black adults had a significantly smaller
decline in blood pressure during sleep. In each of
these studies, the black individuals displayed either
elevated levels of blood pressure throughout the day
or the lack of a nocturnal decline in blood pressure.
Either of these patterns would result in additional
strain on cardiovascular, renal, and central regulatory systems, which could lead to the development of
hypertension and associated target organ damage.
The results of the current study suggest that aerobic fitness is a factor that influences ambulatory
blood pressure patterns, particularly in black children. In white children, there were no differences in
the blood pressures of less-fit and more-fit boys or
girls while the children were either awake or asleep.
However, in black children, less-fit black children
had consistently elevated levels of blood pressure
relative to more-fit black children and to less-fit and
more-fit white children. Specifically, less-fit black
boys had higher awake and asleep systolic blood
pressure than more-fit black boys. In addition, less-fit
black boys had higher levels of systolic blood pressure
than less-fit white boys while awake and asleep and
813
higher systolic blood pressures than more-fit white
boys while asleep. A similar pattern was found for
girls. Less-fit black girls had higher systolic and
diastolic blood pressure than more-fit black girls
while awake and asleep. In addition, less-fit black
girls had higher systolic blood pressures than more-fit
white girls while asleep, higher diastolic blood pressures than more-fit white girls while awake and
asleep, and higher diastolic blood pressures than
less-fit white girls while asleep. Additional analyses
indicated that differences in body size between lessfit and more-fit children did not account for the
differences in blood pressure.
The results are in part consistent with previous
studies4-7-9-10 that have demonstrated an inverse relation between physical activity or aerobic fitness and
casual blood pressure in children and adolescents.
The importance of the study of fitness has been
demonstrated by previous studies12-15 that have indicated a significant relation between physical activity
or fitness and future morbid events.
References
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L: Race and gender influence ambulatory blood pressure
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KEY WORDS
fitness
.*. V^
! e ? I U C *te™™
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pressure • adolescents • diurnal patterns • ambulatory blood
pressure
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Aerobic fitness and the diurnal rhythm of blood pressure in adolescents.
G A Harshfield, L M Dupaul, B S Alpert, J V Christman, E S Willey, J K Murphy and G W
Somes
Hypertension. 1990;15:810-814
doi: 10.1161/01.HYP.15.6.810
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