(Hypertension. 1997;30:782-787.)
© 1997 American Heart Association, Inc.
Articles |
Cardiovascular Reactivity to Stress and Left Ventricular Mass in Youth
From the Department of Psychiatry (M.T.A., K.A.M.) and Departments of Pediatrics and Obstetrics, Gynecology, and Reproductive Sciences (F.S.S.), University of Pittsburgh (Pa).
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Abstract We studied the relationships of cardiovascular reactivity during mental stress with left ventricular mass index in a group of prepubertal children 8 to 10 years old and in a group of peripubertal or postpubertal adolescents 15 to 17 years old. One hundred fifteen participants, varying in age group, sex, and race (black and white), took part in a laboratory stress protocol consisting of a reaction-time task, a mirror tracing task, a cold forehead challenge, and a stress interview. Cardiovascular measures included blood pressure and heart rate, as well as cardiac output, stroke volume, total peripheral resistance, and preejection period obtained noninvasively with impedance cardiography. Measures of left ventricular mass were made by echocardiography. Results indicated that across all participants, left ventricular mass index was associated with cardiovascular responses during the mirror tracing and cold forehead tasks, especially with those responses reflecting increased vasoconstriction. Subgroup analyses showed that these associations were significant for males and sometimes adolescents but not for females and children. As mirror tracing and cold forehead tasks most consistently produce
-adrenergic activation, the
results suggest a model in which vasoconstriction due to mental stress
is related to increased left ventricular mass in
susceptible individuals, even at a young age.
Key Words: left ventricular mass • cardiovascular reactivity • children • adolescents • stress • ethnicity
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Introduction |
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A large LVM is a significant predictor of cardiovascular morbidity in adults, independent of other established risk factors such as smoking, SBP, and age.1 2 Although left ventricular hypertrophy may be a consequence of elevated arterial blood pressure due to increased workload on the heart with resultant myocardial cell growth,3 the risk for complications in patients at any level of blood pressure is higher in those individuals with left ventricular hypertrophy than in those with normal heart size.1 4
Race and sex may affect the relationships between LVM and cardiovascular disease in adults. Hypertensive blacks develop more left ventricular hypertrophy than do hypertensive whites at similar levels of blood pressure, which may be partially responsible for the greater morbidity and mortality in hypertensive blacks.5 In addition, men have greater LVM than women, even when corrections are made for body size2 6 7 and physical activity.8 Because estradiol has been found to inhibit and testosterone to facilitate LVM growth in rats,9 there has been speculation that sex hormones in premenopausal women provide some degree of protection for development of left ventricular hypertrophy.7
Other evidence suggests that cardiac changes leading to increased LVM may begin early in life in susceptible individuals and that these changes precede the development of elevated blood pressure. Radice et al10 reported that LVM was significantly greater in young male adolescents with a family history of hypertension than in those with no family history, even though blood pressure levels were not different for the two groups. In a study of children aged 6 to 15 years who were followed up over a mean period of 3.4 years, LVM was not predicted by initial resting blood pressure, but follow-up SBP was predicted by LVM, among other variables.11 It was concluded that increased LVM in childhood may be an important predictor of subsequent blood pressure increases.
Excessive cardiovascular responses to challenging or stressful situations, called cardiovascular reactivity, may be a risk factor for cardiovascular disease.12 13 A cornerstone of the reactivity hypothesis is that the large responses shown by susceptible individuals represent a stable, individual difference over time. Indeed, the majority of studies report moderate stability in SBP and heart rate response to laboratory challenges over periods ranging from months to many years in adults14 15 16 and children.17 18 19 20 Additional support for the reactivity hypothesis is given by recent findings showing that cardiovascular reactivity during various physical and mental challenges predicts future resting blood pressure among black and white children.20 21 22 23 24 Thus, cardiovascular reactivity to physical and mental challenges predicts future blood pressure levels in youth, although sex and race differences in the strength of the relationships are reported.24 25
The blood pressure response during the cold pressor task predicted the
development of later hypertension in two large longitudinal studies of
adults.26 27 This task produces -adrenergic activation,
which results in widespread vasoconstriction with subsequent increases
in TPR. Susceptible individuals who exhibit exaggerated -activation
during relevant eliciting challenges occurring in daily activities may
elevate the workload on the heart chronically, which could lead to
greater increases in LVM. Alternatively or additionally, the increased
LVM in susceptible prehypertensive individuals may also be related to
increased cardiovascular reactivity through
mechanism(s) as yet unknown.
In support of the relationship between reactivity and LVM is a recent study examining reactivity during a public speaking task and LVMI (LVM indexed by body surface area) in a group of Nigerian civil servants.28 Hypertensive men who exhibited SBP increases >40 mm Hg during the speech task had significantly greater LVMI than did those hypertensive men with smaller SBP increases. Of even greater relevance for the present study are other cross-sectional and longitudinal studies with youths that find relationships between cardiovascular reactivity and LVM.11 27 29 30 31 The findings of Papavassiliou et al31 and Treiber et al32 are especially important in that the participant pool included black and white normotensive youths. LVMI levels in these studies were correlated with cardiovascular reactivity during both postural change and the forehead cold pressor.
The present study examined the relationships of LVMI with
cardiovascular reactivity to laboratory stressors,
especially to stressors that elicit an -adrenergic response, in a
group of children 8 to 10 years old and a group of adolescents 15 to 17
years old. Because it is unclear how the pubertal transition and the
corresponding changes in reproductive hormone levels affect
cardiovascular reactivity and LVMI, the younger group
was restricted to prepubertal individuals, whereas individuals in the
adolescent group were required to be at mid to late puberty. Because
sex and race affect the -adrenergic response to
stress,20 25 33 34 35 equal numbers of female and male as
well as black and white children and adolescents participated.
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Methods |
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Participants
A total of 159 children and adolescents were recruited from school districts in an eastern urban area to participate in a study of cardiovascular risk factors. A second session to have an echocardiographic examination for the purpose of measuring LVM was optional; a total of 115 children and adolescents chose to participate in both sessions. Our final distribution contained 13 female black children, 12 male black children, 18 female white children, 18 male white children, 10 female black adolescents, 10 male black adolescents, 18 female white adolescents, and 16 male white adolescents.
Eligibility requirements were no history of cardiovascular disease or any condition that would require medication that might affect the cardiovascular system (eg, high blood pressure, asthma, oral contraceptive use); no drug or alcohol abuse, history of mental illness, or any professional psychiatric counseling within the past year; no smoking within 12 hours before the session; and <80% above their ideal weight according to the Metropolitan height and weight tables. Note that few children or adolescents were 30% or more above their ideal body weight: 2 male and 2 female white children, 3 male and 4 female white adolescents, and 3 male and 2 female black adolescents.
All children were 8 to 10 years old and were required to show no signs of pubertal development according to the Petersen Pubertal Development Scale.36 All adolescents were 15 to 17 years old, and signs of mid to late puberty were required for their involvement.
Physiological Recording
Apparatus for Cardiovascular Reactivity
Impedance cardiography and the ECG were used for the measurement
of stroke volume, PEP, and heart rate. A Minnesota Impedance
Cardiograph model 304B (Instrumentation for Medicine) was used with a
tetrapolar band electrode configuration.37 The ECG signal
was transduced with two disposable silver/silver chloride electrodes,
and it was filtered and amplified by a Coulbourn S75-11
amplifier/coupler (Coulbourn Instruments).
The impedance signals and ECG were processed with the COP (Microtronics Corp.). Basal impedance, the first derivative of the pulsatile impedance signal (dZ/dt), and the ECG were sampled at 500 Hz per channel. The output of the COP program included stroke volume from the Kubicek equation,38 heart rate, CO (calculated as the product of mean stroke volume and heart rate for a given period), and PEP. Details of the calculations of the various physiological measures from impedance cardiography can be found in Sherwood et al.39
SBP and DBP were monitored with an IBS model SD-700A automated blood pressure monitor (Industrial and Biomedical Sensors Corp.) with the cuff placed on the participant's nondominant arm. The monitor uses a low-frequency sensor mounted on the cuff to detect arterial wall motion and Korotkoff vibrations. Detection of Korotkoff vibrations, in addition to sounds, allows measurements of low levels of blood pressure. The device has automatic inflation and deflation, which can be present for any rate ranging from 1 to 6 mm Hg, and indicates invalid readings due to movement artifacts, noise, etc. Pediatric, adult, and obese cuffs were used according to the arm size of the participant. The device can be interfaced with a Baumanometer mercury column to permit blood pressure readings simultaneous with the automated device. Validation studies in our laboratory conducted by experimenters certified by the Multiple Risk Factor Intervention Trial protocol have shown correlations between the two methods of .94 to .99 for SBP and .90 to .96 for fifth-phase DBP.
The pressure readings were entered into the COP program after the
experimental session was over and the program automatically computed
TPR by the formula
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Determination of Left Ventricular Mass From
Echocardiography
Calculations of left ventricular mass were done
echocardiographically by a pediatric cardiologist
(F.S.) and an ultrasound technician using an Acuson 128XP ultrasound
instrument with a 3.5-MHz phased array transducer. Standard M-mode
measurements were made, and the correlation for measures of LVM between
two different readers on 15 randomly chosen examinations was .98. The
LVM was calculated on-line from the formula designated as the standard
of the American Society of Echocardiography and
derived from Devereux et al.40 This formula is LVM=0.8
{1.04[(IVST+LVIDd+PWT)3-
LVID3]}+0.6 g, where LVIDd is the left
ventricular internal diameter in diastole, IVST
the interventricular septal thickness, and PWT the
posterior wall thickness. LVMI was computed by dividing LVM by body
surface area. (Although calculating LVMI by dividing LVM by body
surface area has been widely used, there has been much debate about the
most appropriate metric to use as the divisor. A recent article by de
Simone et al41 on a study involving children reported
concern that the use of body surface area as the divisor may
overcompensate the normalization of LVM in obese individuals. They
suggested using height2.7 as the divisor. We calculated
LVMI using the de Simone et al recommendation, and we computed the
regressions and correlations using these alternative LVMI values. The
results of LVMI and cardiovascular reactivity were
essentially identical to those using body surface area as the divisor.
We decided to report the results using body surface area so as to
report LVMI values that could be compared with the large number of past
studies using body surface area as a normalization criterion.) Body
surface area was computed from the DuBois formula.42
Experimental Tasks
All tasks were presented while the participant was
sitting upright in a lounge chair.
Reaction Time
A 3-minute computerized choice reaction-time task required the
participant to respond as quickly as possible to randomly
presented 1000-Hz tones by pressing a joystick button but to
refrain from responding to 2000 Hz tones. This task was chosen on the
basis of past studies indicating that the task elicits a ß-adrenergic
pattern in many participants.43
Mirror Tracing
Participants were required to trace around a copper star with a
metal stylus for 3 minutes while being allowed to see only the mirror
image of the star. This task has been described previously as producing
increases in vascular resistance due to increased -adrenergic
activation.44
Cold Forehead
A 2-quart bag of two parts crushed ice and one part water was
placed on the participant's head for 1 minute. Only one participant
was unable to complete the entire minute. This task has been used
frequently in studies of racial differences in
reactivity,33 35 and it elicits increased -adrenergic
activation.
Social Competency Interview
Participants were given the interview developed by Ewart and
Kolodner45 by an experimenter trained in its
administration. The SCI elicits cardiovascular changes
while participants describe an interpersonal source of emotional
distress. Children and adolescents were encouraged to discuss for at
least 10 minutes (but not longer than 15 minutes) a stressful life
situation involving another person while the interviewer attempted to
promote detailed recall of the event. Physiological
measurements were collected for the first 10 minutes of the task
only.
Experimental Protocol
Participants were recruited through school districts by letters
describing the study to the parents of children and adolescents.
Parents could either call or return a postcard for a screening
interview. Adolescents and their parents were required to sign an
informed consent form before participation in the protocol; the younger
children signed an assent form and their parents a separate informed
consent form before their participation. All consent and assent forms
were approved by the Psychosocial Institutional Review Board of the
University of Pittsburgh Medical Center.
Participants arrived at the laboratory at about 8:30 AM after an overnight fast and fluid restriction. Height, weight, and skinfolds were measured after the participant changed into a hospital gown. Venous blood was then drawn for a variety of biochemical assays that are not reported here. After the blood draw, the children were fed a light breakfast, followed by the application of electrodes for impedance cardiography and the ECG. The blood pressure cuff was placed on the upper aspect of the nondominant arm, and the microphone was placed above an area where the brachial artery could be palpated. Children were then given instructions for an initial 10-minute rest period.
The reaction-time, mirror tracing, and cold forehead tasks were given in a counterbalanced order with 8-minute intertask rest periods. The SCI was always given last because of the length of the task. A final 10-minute rest period followed the SCI. Sensors were removed after the last rest period, and participants were then given psychosocial questionnaires. Participants were paid $75 for completing the protocol, along with money earned on the reaction time task.
Appointments for resting echocardiographic examinations generally were scheduled at the close of the laboratory protocol. These examinations usually occurred within a few weeks of the reactivity session. Participants were paid $25 for completion of the echocardiographic examination.
Data Reduction
Data for heart rate and impedance-derived variables were
collected on a minute-by-minute basis during the last 3 minutes of the
initial and final rest periods, during the last minute of the intertask
rest periods, during all 3 minutes of reaction time and mirror tracing,
and during the first 10 minutes of the SCI. These minute-by-minute
values were averaged to form means for each period. Data were collected
in 10-second blocks during the minute of cold forehead. The six
10-second blocks during the cold stimulus were averaged to form a mean
for that task.
Blood pressures were taken at the 5-, 7-, and 9-minute marks of the initial and final rest periods, and the last two readings were averaged to form SBP and DBP means for those periods. Three readings were taken during the reaction-time and mirror tracing tasks, and these readings were averaged to form task means. Readings were taken every other minute during the SCI, and the five readings taken during the first 10 minutes of the interview were averaged. Finally, one blood pressure reading was taken during the cold forehead task.
Change scores during tasks for heart rate, blood pressure, and PEP were computed by subtracting the initial baseline means from the task mean. For CO, stroke volume, and TPR, percent change from baseline was calculated because of questions concerning the interpretation of absolute levels of these impedance cardiography–derived measures.39
Data Analyses
To examine whether subject groups differed on various
anthropometric and psychosocial measures, a series of 2x2x2 (age
group by sex by race group) ANOVAs were computed. Repeated-measures
ANOVAs with task as the repeated-measures factor and age group, sex,
and race group as the between-subjects factors were performed on
cardiovascular change scores to examine group
differences for cardiovascular reactivity. Simple
effects analyses were used to follow up significant
interactions. A significance level of .05 was adopted for all
analyses.
To examine relationships between LVMI and reactivity variables, partial Pearson correlation coefficients were computed for the entire sample, for which age group, race group, and sex were partialled out. To evaluate possible differences in relationships by age, race, or sex, stepwise regression analyses were computed for each reactivity variable to predict LVMI. Age group, sex, and race group were added to the model at step 1, followed by the interaction of the reactivity variable with either age group, sex, or race group at step 2. A significant interaction of the variable with a group status factor suggested that the correlation of that variable with LVMI differed for the two levels of the grouping factor. We did not enter more than two-way interactions because of the resultant small sample sizes and potentially unstable correlations.
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Results |
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Sample Characteristics
Sample characteristics in Table 1
show that the males were 0.3 year older than the females. Weight,
height, body surface area, body mass index, and uncorrected LVM were
all larger among the adolescents than the children. Larger values were
also found among male than female adolescents for weight, height, and
body surface area. Sex differences were not present in the
children. When body surface area was controlled for, adolescents still
had larger values of LVMI than children, and males had larger values
than females. No race main effects or other interactions were found
with LVMI.
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Adolescents had higher resting SBP levels than did children, and males had higher levels than did females. However, the age group difference was significant for males but not for females. In addition, black adolescents had higher resting SBP levels than black children; white adolescents also had higher resting SBP levels than white children, but the difference was not as pronounced as in blacks.
Male children had higher resting DBP levels than male adolescents, whereas no age-group differences were found for females. No other significant effects were found for resting DBP. Finally, females exhibited higher heart rate levels than did males, and children had higher heart rate levels than did adolescents. No effects involving race were found.
With one exception, no significant associations of resting blood pressure and LVMI were found after age group, sex, and race were controlled for. The exception was the interaction of resting DBP and age group (standardized beta coefficient (B)=-1.38, P=.04). Partial correlations of resting DBP and LVMI, controlling for race and sex, were not significantly different from zero for either children (r=.132) or adolescents (r=-.227); the correlations were significantly different from each other. Despite this latter finding, resting blood pressure did not appear to be related to LVMI to any appreciable degree in these young individuals.
Cardiovascular Reactivity During Laboratory
Tasks
As in the full sample of 159 participants,46
adolescents in this sample of 116 (Table 2) who participated in the
echocardiographic examination exhibited greater changes
during stressors than did children on those variables reflecting
ß-adrenergic activation (ie, greater heart rate, SBP, and CO response
and greater decreases in PEP and TPR), especially during the reaction
time and SCI tasks. On the other hand, response patterns during the
cold-forehead challenge were almost identical for children and
adolescents.
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Adolescent females showed greater CO responses across tasks than did the males (8.4% versus 7.0%), whereas the males exhibited greater TPR responses (8.9% versus 5.8%). These sex differences were not observed consistently across stressors among children. Finally, the only racial difference was greater blood pressure responses by white adolescents during the SCI compared with black adolescents (SBP: blacks, 9.6 mm Hg; whites, 14.5 mm Hg; DBP: blacks, 11.5 mm Hg; whites, 15.5 mm Hg). These patterns were upheld in the full group of participants reported elsewhere.46
Relationships of LVMI With Cardiovascular
Reactivity Variables
Significant partial correlations, adjusted for race, age,
and sex, between LVMI and cardiovascular reactivity
measures were found for the two laboratory tasks thought to elicit an
-adrenergic response (Table 3). For
the mirror tracing task, the greater the LVMI the greater the SBP and
TPR change and the smaller the stroke volume change. For the cold
forehead task, the greater the LVMI the greater the TPR and PEP change.
No significant partial correlations of LVMI and
cardiovascular reactivity were found for either the SCI
or reaction time tasks, although two marginally significant
correlations with LVMI (SBP and stroke volume change) were found for
the latter task.
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We next examined whether the overall correlations of LVMI and reactivity during the mirror tracing and cold forehead tasks were due to stronger relationships among a particular subgroup. Four interactions of reactivity and a grouping factor were significant or showed a trend toward significance: SBP change during mirror tracing and sex (B=.340, P=.035), DBP change during mirror tracing and age group (B=.909, P<.01), DBP change during mirror tracing and sex (B=.387, P=.015), and percent TPR change during the mirror tracing task and sex (B=.264, P=.075). Significant positive partial correlations controlling for age group and race were present for LVMI with both SBP and TPR change during the mirror tracing task for males (rs=.361 and .404, respectively) but not for females (rs=.021 and .067, respectively). Regarding DBP change during mirror tracing and LVMI, the correlations for males (r=.225) and females (r=-.166) were not significantly different from zero, although they were significantly different from each other. Additionally, the partial correlation of DBP change with LVMI, controlling for sex and race, was significant for adolescents (r=.283) but not for children (r=-.199).
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Discussion |
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The present study examined the relationships of LVMI with measures of cardiovascular reactivity during four laboratory stressors in groups of female and male, black and white, prepubertal children and peripubertal or postpubertal adolescents. A number of findings emerged from the results that have implications for the natural history of the relationships of cardiovascular reactivity to stress and LVM.
First, increased LVMI was associated with increased TPR and blood
pressure and decreased CO or stroke volume during the tasks that most
consistently produce increased -adrenergic activation with
resultant increases in vascular resistance, that is, the mirror tracing
and cold forehead tasks. In contrast, the reaction-time task, which
elicits primarily a ß-adrenergic response, produced only one
marginally significant correlation with LVMI. The SCI task produced
substantial reactivity, especially in the adolescent group, but the
pattern of responses suggests a more mixed adrenergic activation. This
mixed pattern during the SCI may be the result of a less
consistent response to the task across individuals, because the
task was more open ended than the other tasks. The results of the
partial correlations between reactivity and LVMI suggest that tasks
eliciting -adrenergic activation may be more effective probes for
studying the role of stress responses in the pathogenesis of essential
hypertension and the relationship to LVMI than tasks eliciting other
autonomic patterns.
Second, the relationships between cardiovascular reactivity and LVMI varied as a function of sex and age. The associations between LVMI and vascular responses during the mirror tracing task were characteristic of males and adolescents, the two groups that have greater LVMI levels compared with females and prepubertal children, even when body size is adjusted. The mirror tracing task is often described as "frustrating" by subjects, and a significant amount of irritation is often observed. Thus, one speculation is that increases in vascular reactivity during the mirror tracing task are associated with increased LVMI due to the ability of the task to elicit frustration and irritation, at least in males. We hypothesize that increased vascular responses during a physical stressor like the cold forehead challenge is associated with greater LVMI but that a task requiring the elicitation of frustration or irritation may be necessary to discriminate between males and females.
The results suggest a model whereby large vasoconstrictive responses to eliciting mental (ie, mirror tracing) and physical (ie, cold forehead) stress are related to increased LVM in children and adolescents. To date, most studies that have found relationships between LVMI and cardiovascular reactivity have used exercise or cold-stimulation tasks. Yet the traditional view of the reactivity hypothesis is that cardiovascular reactivity due to more psychological stressors may be involved in the pathogenesis of cardiovascular disease. It may be that the ability of a stimulus, whether physical or psychological, to elicit a vasoconstrictive response may be paramount in the relationship of reactivity with future blood pressure status or LVMI. Frequent large increases in vascular resistance in response to manifold daily challenges due to early pathological changes in the vasculature may lead to increased myocardial growth due to greater workload on the heart. Ongoing studies in our laboratory are examining these participating children longitudinally as they cross the pubertal transition to help further clarify the time course of LVMI and cardiovascular reactivity relationships.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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This research was supported by NIH grant HL-25767. The authors acknowledge the contributions of Deborah Flotta, Diana Block Buck, Karen Kenyon, and Renee Rhodes.
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Footnotes |
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Reprint requests to Karen A. Matthews, PhD, Department of Psychiatry, University of Pittsburgh, 3811 O'Hara St, Pittsburgh, PA 15213.
Received August 19, 1996; first decision September 25, 1996; accepted March 5, 1997.
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References |
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