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Age and Ageing 2002; 31: 131–135
2002, British Geriatrics Society
Short-term heart rate variability
during a cognitive challenge in
young and older adults
R OBERT W OOD, B RIAN M ARAJ1 , C. M ATTHEW L EE, R AFAEL R EYES
Louisiana State University, Department of Kinesiology, Baton Rouge, Louisiana, USA
1
University of Alberta, Department of Physical Education and Recreation, Alber ta, Calgary, Canada
Address correspondence to: R. Wood. 112 Long Field House, Baton Rouge, L A 70803, USA. Fax: (q1) 225 578 3680.
Email: [email protected]
Abstract
Background: attention-demanding tasks cause changes in the autonomic modulation of cardiac function. Heart
rate variability, an index of autonomic modulation of heart rate, decreases with age.
Objective: to examine heart rate variability in elderly and young participants at rest and during an attentiondemanding task.
Methods: we assessed 16 old participants (ages 72–91) and 16 college-age (ages 20–25) participants for shortterm (5 min) heart rate variability at rest and during a simple-reaction time task. We report heart rate variability
as the standard deviation of all interbeat intervals, and as the relative contribution of changes occurring at low- and
high-frequencies.
Results: there were no group differences in resting heart rate. A 262 mixed model ANOVA suggested a main
effect of age on standard deviation of all interbeat intervals (P-0.05) which was significantly lower for the older group
than their younger counterparts. There was also a significant effect of the test condition on standard deviation
of all interbeat intervals and spectral measures of heart rate variability (P-0.05) in that standard deviation of all
interbeat intervals dropped during the simple reaction time as did high-frequencies, while normalized low frequency
power increased.
Conclusion: cardiac autonomic modulation during provocative stress show similar physiologic responses in young
and older adults.
Keywords: ageing, cardiovascular, heart, autonomic, stress
Introduction
Chronological age is inversely related to autonomic
modulation of heart rate: older people have reduced
heart rate variability (HRV) [1–3]. Frequency domain
analyses reveal that the lower variance in heart rate
observed in older adults is marked by global reductions
across the entire power spectrum [2], suggesting that
sympathetic and parasympathetic modulation of heart
rate are inversely related to age, but that sympathovagal balance is not age-dependent. Although the
mechanism(s) responsible for the age-related changes
in autonomic control of the heart are not completely
understood, considerable evidence points towards impairment at the level of the receptor or post-receptor (e.g.
cyclic AMP activity) and not necessarily the autonomic
nervous system itself [4, 5]. Whatever the physiological
mechanisms, age-related changes in HRV are clinically
important as low HRV is associated with risk of
cardiovascular and all-cause death [6, 7].
Acute periods of mental effort also result in reduced
HRV [8–10]. This phenomenon suggests a mechanism
through which psychological stress might exacerbate
cardiac rhythm disturbances. This may be of particular
concern among older adults who, having low baseline
HRV, appear particularly susceptible to such events.
Therefore we examined the effect of cognitive effort on
HRV in healthy young and elderly adults.
Data from previous investigations indicate that
older subjects typically present exaggerated elevations
of plasma catecholamines and exaggerated parasympathetic withdrawal during stressful tasks [4, 11]. However, reports of end-organ responsiveness are equivocal,
varying from exaggerated [12], to no difference in [13]
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R. Wood et al.
or, in some cases, smaller cardiovascular responses
[11, 14, 15] among older adults. With respect to HRV,
the few available studies are also somewhat disparate.
Ferrari et al. [16] observed a lower cardiac responsiveness
to graded electrical stimulation of the vagus nerve in
older as compared to younger rats. Consistent with this
are the recent findings of Uchino et al. [12] who reported
an inverse relationship between age and parasympathetic withdrawal during psychological stress. However,
Boutcher and Stocker [17] reported healthy young and
old men to have similar stress-induced changes in HRV.
In contrast to these previous studies, we have
examined apparently healthy elderly adults (over 70).
We have also examined the spectral measures of HRV
to provide indices of sympathovagal balance. We hypothesized that the older participants would have low
resting overall HRV, setting the stage for a blunted
response to cognitive challenge. We also hypothesized
that the change in sympathovagal balance during the
cognitive challenge would not be age-dependent, as
both sympathetic and parasympathetic modulation of
the heart appear to deteriorate at a similar rate with
age [2, 12].
Methods
Participants
Forty-two independent residents of a retirement community responded to an invitation to participate in this
study. We obtained current medical records (i.e. within
one year, or since the onset of any symptoms within
the last year) to screen for the presence or history of
diagnosed chronic heart failure, myocardial infarction,
hypertension, and neurological conditions. Other exclusion criteria included the use of drugs that could affect
cardiovascular function. All medications were discontinued for 24 h before data collection. Nineteen
respondents met the inclusion criteria and completed
the study (15 women and 4 men). The age range of
this group of participants was 72–93 years. Twentyeight younger participants (ages 19–23) also responded
to an invitation to participate. Nineteen were assigned
to the young group to sex-match the elderly and young
groups.
Procedures
The institutional review board of the host site approved
all procedures. On arriving at the testing site, the
participant provided written informed consent and
completed a standard medical history questionnaire.
Following the administration of these items, visual acuity
was assessed using a Snellen eye chart that was read from
20 feet. The participant was allowed to use corrective
lenses throughout the study. For auditory problems, we
relied only on self-reporting and medical histories.
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Electrocardiograph (ECG), pnuemotachograph, and
blood pressure data were then collected during 5 min
of spontaneous breathing and during 5 min of the
simple reaction time task. During spontaneous breathing, the participant sat motionless in a chair, without
engaging in conversation. During the reaction time task,
the participant responded to visual and auditory cues by
depressing a keypad device as quickly as possible. The
visual stimulus was a light encased in a small circular
frame that was illuminated on a panel. The auditory
stimulus was a tone-generated device: as soon as subjects
heard the sound, they depressed the switch. There was
no choice of keys involved in this task. The participant
responded to two sets of 50 auditory cues and two sets
of 50 visual cues, the order of which was counterbalanced. A Biopac [18] data acquisition board was used
to collect ECG and pnuemotachograph data throughout,
and blood pressure was assessed twice by mercury
sphygmomanometry during each 5-minute period. We
report the averages of the two observations for systolic
and diastolic pressure.
Data analysis
We collected the 5-minute ECG and pnuemotachograph
data at 200 Hz and converted them to tachograms
of consecutive interbeat intervals and respiratory frequency, respectively. We used the tachogram of respiratory frequency to assess mean respiratory rate. We
analyzed the interbeat interval data for the standard
deviation of all normal interbeat intervals (SDNN), an
estimate of overall HRV which may reflect vagal
modulation of the heart [19]. The tachogram was then
resampled at 4.0 Hz and subjected to a fast Fourier
transformation using a Hamming window. We then
analyzed the squared modulation of the resultant
frequency spectrum (power spectrum) for normalized
low- and high-frequency power (LFnu = 0.04–0.15 Hz,
and HFnu = 0.15–0.40 Hz, respectively) in accordance
with established guidelines [19].
Statistical analysis
We used a 262 (age group6test condition) mixedmodel ANOVA, with repeated measures on the test
condition, to evaluate the dependent measures for differences according to test condition or age group, and age
group by test condition interactions. The GreenhouseGeisser method of controlling for sphericity of values
was employed (sphericity of values, or circularity, refers
to the General Linear Modal assumption of equal
variances in the context of repeated-measures designs
[20]). Alpha was set at 0.05.
Results
The descriptive statistics for the dependent measures are
presented in Table 1.
Age, stress, and heart rate variability
Table 1. Descriptive statistics
Participants
Age-range
Test condition
Young
20–25 years
Spontaneous breathing
SRT
Elderly
72–91 years
Spontaneous breathing
SRT
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Visual-SRT (msec)
Auditory-SRT (msec)
RR (msec)
SDNN (msec)
HFnu
LFnu
SBP (mmHg)
DBP (mmHg)
Respiratory Rate (breaths/min)
–
–
887"52.3
60.7"4.7
0.63"0.19
0.37"0.19
116.4"10.7
78.2"6.2
14.2"2.2
287.0"35.3
213.1"33.8
763"48.6b
50.6"4.6b
0.53"0.15b
0.47"0.15b
132.6"12.5b
81.4"7.1
14.6"2.8
–
–
859"60.1
29.3"4.3a
0.63"0.16
0.37"0.16
131.9"13.1a
80.2"8.9
14.8"2.7
401.6"61.9a
295.8"72.2a
723"52.3b
22.9"3.7a,b
0.57"0.22b
0.43"0.22b
141.0"10.2a,b
84.4"7.9
15.2"3.3
a
Main effect of age group (P-0.05).
Main effect of test condition (P-0.05).
Values are mean"standard deviation.
SRT, simple reaction time task; RR, heart period (RR interval); SDNN, standard deviation of all normal interbeat (RR) intervals; HFnu, normalized
high-frequency power [HF/(HFqLF)]; LFnu, normalized low-frequency power [LF/(HFqLF)].
b
Age-related differences
The ANOVA revealed (P-0.05) a main effect of age:
the young participants demonstrated a higher SDNN
and lower systolic blood pressure than their elderly
counterparts at rest and during the reaction time task
(Table 1). There were no effects of age under either test
condition on heart period, spectral measures of HRV,
diastolic blood pressure, or respiratory frequency.
Effects of the simple reaction time task
The performance of the reaction time task resulted in
(P-0.05) a shortening of the heart period, an increase in
systolic blood pressure, and a reduction in SDNN and
HFnu (and by definition, an increase in LFnu) (Table 1)
in both the young and elderly participants. The simple
reaction time task did not evoke changes in diastolic
blood pressure. There were no significant interactions
between age and the test condition.
Discussion
These results are consistent with an age-related decrease
in overall short-term HRV (SDNN), but no age-related
differences in spectral measures [1–3]. The global
reduction across all power bands may reflect a downregulation of cardiac receptor sensitivity that appears
with advanced age [4, 5]. As is often the case in cross
sectional studies, the assignment of group differences to
the effects of age is limited. Despite screening for disease
in the study sample, the possibility of undetected cardiac
and metabolic disease cannot be ruled out completely,
nor can the long-term effects of drug actions and
interactions. However, in support of the notion that
the study sample is relatively healthy, there was a lack of
age-related difference in resting heart period. Heart
period changes very little with age in the absence of
disease (see [21] for a review).
These data indicate a main effect of the simple
reaction time task on HRV (SDNN and spectral
measures) and systolic blood pressure. These findings
suggest a withdrawal of vagal modulation of the heart
and a shift towards greater sympathovagal balance (i.e.
a relative increase in the proportion of power that is
sympathetic in origin). Previous investigations indicate
that cognitive effort reduces overall HRV in different
populations [8–10, 16, 17]. The findings of the present study suggest that such an effect is also apparent
among elderly subjects, but do not support the hypothesis that an interaction between age and the stressor
would exist. Thus, these data extend the findings of
Boutcher and Stocker to elderly subjects, and contrast
with the findings of Uchino et al. [12]. The main effects
of age and mental effort can be visualised in the Poincare
plots shown in Figure 1, depicting heart rate data for
one elderly and one young adult. These plots illustrate
an intact end-organ response to mental effort in elderly
participants.
With respect to the spectral components of HRV,
our findings are in agreement with Pagani et al. [9]
who observed augmented sympathovagal balance during
the cognitive challenge. Wood et al. [10] did not observe
such changes, despite a decrement in overall HRV (i.e.
SDNN) during the cognitive task. The lack of agreement
among these investigations may reflect the controversy
surrounding the use of low-frequency power to quantify
sympathetic modulation of the heart. Autonomic blockade studies [22] indicate that vagal modulation of the
heart can show up in the low-frequency power band.
This makes it difficult to suggest that our findings are
truly indicative of a shift towards greater sympathovagal
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R. Wood et al.
Poincare Plots of Heart Rate
a
b
c
d
Figure 1. Poincare plots of heart rate by age and test condition. The plots in (c) represent the heart rate defined by one R–R
interval (HRn) plotted against the heart rate from the next successive R–R interval (HRnq1). These plots are for one young
participant (age = 21 years) and one elderly (age = 85 years) participant during the 5-minute paced breathing and SRT test
conditions. Narrow or ‘torpedo’ shaped distributions of data points (as with the elderly participant, or as observed during
the SRT) have been associated with greater sympathetic modulation of the heart.
balance, or if the heightened LFnu is merely reflecting
a greater proportion of vagal modulation occurring at
a low frequency.
In conclusion, these data suggest that despite the
age-related differences in HRV, the central cardiovascular response to mental effort in healthy elderly
adults remains intact. Clinicians are beginning to use
psychological stress testing for quantifying cardiovascular
risk and disease severity [22], particularly among individuals for whom traditional measures (such as graded
exercise tolerance tests) are not appropriate. Thus the
continued development of psychological stress testing
may be of particular benefit to elderly people. To this
end, it is important to characterize the cardiovascular
responses in healthy and diseased older individuals,
and to include in particular the quantification of
HRV, as it may provide unique information on risk of
arrhythmogenesis and cardiac and all-cause mortality.
Key points
. Ageing
is associated with reduced heart rate
variability.
. Exposure to cognitive challenges further reduces
heart rate variability.
. The magnitude of change in heart rate variability
during a cognitive challenge does not appear to be
a function of age.
134
Acknowledgements
We would like gratefully to acknowledge the St James
Place Continuing Care Retirement Community in Baton
Rouge, Louisiana for their co-operation in this study.
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Received 27 April 2001; accepted in revised form 25 October
2001
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