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TABLE I. A
GES AND
G
ENDERS OF
P
ARTICIPANTS
Gender
Age
Female Male
Total
20-29 72
33
105
30-39 43
80
123
40-49 45
45
90
50-69 22
20
42
Total
182
178
360
Dependence of Heart Rate Variability on Stress
Factors of Stress Response Inventory
Lizawati Salahuddin, Myeong Gi Jeong, Desok Kim
School of Engineering
Information and Communication University
Daejeon, Korea
lizawati, mgjeong, kimdesok@icu.ac.kr
Seong-Kyeon Lim
1
, Kim Won
2
, Jong-Min Woo
2
1
Department of Neuropsychiatry and
2
Stress Research
Institute, Seoul Paik Hospital
Inje University School of Medicine
Seoul, Korea
healthpsy@naver.comphrenie@dreamwiz.com,
menfi@naver.com
Abstract—Heart rate variability (HRV) analysis is commonly
used as a quantitative marker depicting the activity of autonomic
nervous system (ANS) related to mental stress. Stress Response
Inventory (SRI) has been devised to score mental and physical
symptoms occurred during the past two weeks. SRI is composed
of seven stress factors that may influence the status of mental
stress levels. In this study, we investigated the relationships of
physiological measures and HRV features on ages and stress
factors. Physiological measures and HRV features in low (SRI
scores: 7.1 ± 4.1, n=225) and high stress group (22.5 ± 7.4, n=135)
were compared with age as the covariate (ANCOVA). Age was
reconfirmed as a significant factor influencing physiological
measures and most of HRV features. Age was also inversely
correlated to stress factor scores. Systolic blood pressure, glucose
level, and normalized HF were significantly lower, whereas body
temperature, LF/HF, and normalized LF were significantly
higher in high stress group. Our results showed that stress levels
were associated with ages, physiological measures, and HRV
features.
Keywords-Heart rate variability; autonomic nervous system;
physiological measure; stress; Stress Response Inventory
I.
I
NTRODUCTION
Stress involves alteration in behavior, autonomic function
and the secretion of several hormones such as cortisol,
corticosterone, and adrenal catecholamines [1]. Higher blood
pressure and heart rates during stress reflected the
predominance of sympathetic nervous system activity [2].
Mental stress decreased high frequency of heart rate variability
(HRV) and increased low frequency of HRV [3]. HRV
decreased in subjects with depression, higher hostility and
anxiety [4]. Stress increases susceptibility to negative health
outcomes [5]. Autonomic nervous system (ANS) contributes
to the physiological adaptive process in short durations but
may be damaged if the releases of the mediators such as
adrenaline from the adrenal medulla are not terminated when
they are no longer needed.
Numerous stress questionnaires have been used in clinical
practice and psychiatric researches such as Perceived Stress
Scale (PSS) [6], Life Events and Coping Inventory (LECI) [7],
and Stress Response Inventory (SRI) [8]. PSS measures the
degree to which situations in one’s life are considered as
stressful. A number of questions may be included to scale
feelings, thoughts, and current levels of experienced stress
during the last month [6]. LECI is composed of 125 life event
questions that assess the experience of life stress and 42
questions that ask the use of coping behaviors in 12-14 years
old children [7]. Recently, SRI questionnaire has been devised
to score mental and physical symptoms occurred during the
past two weeks that may influence the status of mental stress
levels. SRI consists of 39 items that focus on the emotional,
somatic, cognitive, and behavioral stress responses. SRI scores
could be categorized into seven stress factors: tension,
aggression, somatization, anger, depression, fatigue, and
frustration [8]. Both PSS and SRI were designed to measure
stress severity in adults. PSS was designed to assess how
unpredictable, uncontrollable, and overloaded respondents find
their lives. Unlike PSS, SRI assesses the stress severity based
on the stress symptoms or the effects of stressors. However,
the relationships of ages, stress factors, physiological measures
and HRV features have not been investigated thoroughly. In
this study, we compared physiological and HRV features in
subjects with high and low stress factors to investigate stress-
related symptoms and their influence on HRV features.
II. M
ETHODS
A. Subjects and Data Acquisition during Baseline Stage
The experiment was carried out in Department of
Neuropsychiatry and Stress Research Institute, Seoul Paik
Hospital, Seoul, Korea and Information and Communications
This research was supported by the grant from Samsung Electronics, Inc.,
Suwon, Korea.
TABLE II. A
S
IMPLIFIED
S
TRESS
R
ESPONSE
I
NVENTORY
I
TEMS
C
ATEGORIZED INTO
S
EVEN
S
TRESS
F
ACTORS
Stress Factor
Questions
My body trembles.
I feel tense.
Tension
My head hurts or it feels heavy.
Aggression
I act violently (such as reckless driving, cursing,
fighting).
I suffer from indigestion.
My stomach hurts.
Somatization
I feel dizzy.
My voice is louder than it usually is.
Anger
I easily get impatient.
I have lost my self-confident.
I have lost incentive to do anything.
I have no future in my current work.
I often stare blankly.
I feel bored.
I am useless (or unworthy).
Depression
I don’t like moving any part of my body.
I am easily fatigued.
Fatigue
I feel totally exhausted.
My chest feels tight.
Everything bothers me.
I feel on edge.
Frustration
My heart throbs.
University, Daejeon, Korea. This study was approved by the
Institutional Review Board (IRB) and all subjects signed the
consent form that allowed the use of data. All subjects visited
the clinic or laboratory for physical check up. Subjects who
had psychopathic treatment history were excluded from the
experiment. Four hundred two normal subjects aged from 21
to 69 years (37.3 ± 10.7) participated in the study originally
(Table I).
A simplified version of original SRI questionnaire was
devised by one of the authors (JMW) and used in this study.
The simplified version of SRI questionnaire was composed of
22 questions (Table II) that have been categorized into seven
stress factors as in the original SRI [8]. Each question was
scored in a five-point Likert scale format: ‘Not at all’ (0),
‘Somewhat’ (1), ‘Moderately’ (2), ‘Very much’ (3), or
‘Absolutely’ (4). SRI questionnaires were filled up before the
physiological and heartbeat measurements.
Height, body fat, body temperature (at the forehead), blood
pressure and blood glucose levels were measured. Subjects
were seated in the comfortable chair and rested for five minutes
prior to the heartbeat measurement. Three minute records of
heartbeat were then recorded right after the resting stage.
Finger photoplethysmography sensor (Freeze-Framer®,
Institute of HeartMath LLC, Boulder Creek, CA) was used to
produce heartbeat (R peak) interval records.
B. Data Processing and Feature Calculation
Thirty eight R peak interval records containing motion
artifacts were discarded from further analysis. Four records
were removed from the analysis due to the lack of blood
pressure and blood glucose data. This reduced the usable
records to 360 (Table I). In each record, the following HRV
metrics were calculated: mean heart rate (Mean HR), mean R
peak intervals (Mean RR), standard deviation of R peak
intervals (SDNN), root mean square of standard deviation
(RMSSD), and % R peak intervals with difference in
successive R peak intervals greater than 50 ms (PNN50) as
time domain features; HRV index (HRV Index, bin width of
8.0 ms), triangular interpolation of R peak intervals histogram
(TINN), and stress index (SI) [9] as geometrical analysis
features; and very low frequency (VLF), low frequency (LF),
high frequency (HF), the ratio of LF to HF (LF/HF),
normalized LF (LFnu), and normalized HF (HFnu) as
frequency domain features [10].
C. Data Analysis
To assess the association with stress factors, individual SRI
scores were grouped into their corresponding stress factors
(Table II) to calculate stress factor scores. Dependence on ages
of stress factor scores as well as physiological measures and
HRV features was evaluated using Pearson’s correlation
analysis (StatGraphics Plus V4.1, Manugistics, Inc., Rockville,
MD). Dependence on stress factors of physiological measures
and HRV features was evaluated using multiple regression
analysis (StatGraphics Plus V4.1).
The subjects were divided into low and high stress group
using k-means cluster analysis (based on square Euclidean)
with stress factor scores as the variables (StatGraphics Plus
V4.1). Physiological measures and HRV features in these two
groups were compared, with age as the covariate using analysis
of covariance (ANCOVA) (Minitab V15, Minitab Inc., State
College, PA).
III. R
ESULTS
A. Relationships of Physiological Measures, HRV Features,
and Stress Factors with Ages
Table
III summarizes the relationships of physiological
measures, HRV features, and stress factors with ages. Body fat
content, blood pressures, blood glucose level, mean RR, and SI
were positively correlated, whereas body temperature, mean
HR, SDNN, CV, RMSSD, PNN50, HRV Index, TINN, VLF,
LF, and HF were inversely correlated with ages (Pearson’s
correlation, p < 0.05). All stress factors scores were negatively
correlated with ages (p < 0.001).
Tension, depression and frustration were the stress factors
that were frequently associated with body fat, body
temperature, and HRV features (data not shown). Depression
factors were positively associated with body fat, body
temperature, SI, and HFnu; negatively associated with CV,
VLF, LF, LF/HF, and LFnu (multiple regression, p < 0.05).
Tension factors were positively associated with LF, LF/HF,
and LFnu; negatively associated with body fat, SDNN, and
HFnu (multiple regression, p < 0.05). Frustration factors were
positively associated with SI, LF/HF, and LFnu; negatively
associated with RMSSD, HRV Index, TINN, HF, and HFnu.
None of the individual SRI scores were correlated with
diastolic blood pressure, mean HR, and mean RR (multiple
regression, p > 0.05).
Cluster analysis classified 225 subjects as low stress factor
(total SRI scores: 7.1 ± 4.1) and 135 as high stress factor group
(22.5 ± 7.4).
TABLE III. S
TATISTICALLY
S
IGNIFICANT
F
EATURES THAT
D
ISTINGUISH
T
WO
G
ROUPS WITH
L
OW VERSUS
H
IGH
S
TRESS
F
ACTORS
(ANCOVA
WITH AGE AS COVARIATE
,
P
<0.05)
Measures
Subject with Low
Stress Factor
Scores (n=225)
Subjects with High
Stress Factor
Scores (n=135)
Body Temperature 36.36 ± 0.352
36.43 ± 0.629
Systolic blood pressure
120.0 ± 14.12
115.5 ± 13.39
Glucose level 98.38 ± 17.20
91.70 ± 12.10
LF/HF 1.751 ± 1.715
1.862 ± 1.526
LFnu 53.43 ± 18.74
56.94 ± 16.94
HFnu 45.82 ± 18.93
42.31 ± 17.15
TABLE IV. R
ELATIONSHIPS
OF
P
HYSIOLOGICAL
M
EASURES
,
HRV
F
EATURES
,
AND
S
TRESS
F
ACTORS WITH
A
GES
(S
IMPLE REGRESSION
,
N
=360)
Measurements/Stress Factors
Correlation with age (ρ
ρρ
ρ)
Body fat 0.255**
Body temperature -0.304**
Systolic blood pressure 0.322**
Diastolic blood pressure 0.244**
Glucose level 0.392**
Mean HR -0.138**
Mean RR 0.139**
SDNN -0.332**
CV -0.432**
RMSSD -0.241**
PNN50 -0.253**
HRV Index -0.402**
TINN -0.398**
SI 0.421**
VLF -0.233**
LF -0.270**
HF -0.155**
LF/HF -0.023**
LFnu -0.069**
HFnu 0.063**
Tension -0.236**
Aggression -0.234**
Somatization -0.190**
Anger -0.232**
Depression -0.259**
Fatigue -0.245**
Frustration -0.246**
*p < 0.05
**p < 0.005
B. Physiological Measures and HRV Features in Subjects
with Low and High Stress Factors Group
Using ANCOVA with age as covariate, several
physiological measures and HRV features were found to be
significantly different in the low and high stress factor group
(Table IV). Systolic blood pressure, glucose level, and HFnu
were significantly lower, whereas body temperature, LF/HF,
and LFnu were significantly higher in high stress factor group.
IV. C
ONCLUSIONS AND
D
ISCUSSION
Previous studies have found that HRV declines with ages
[11, 12]. In the present study, age was newly found to be
correlated with geometrical features such as HRV index, TINN,
and SI at moderate levels (-0.398 ~ 0.421) (Table III). In
addition, all physiological measures were found to be
dependent on ages although at low levels in our subjects
(0.244~0.392). Normalized HRV features such as LF/HF,
LFnu, and HFnu did not show significant dependence on ages
(Table III).
Physiological measures and HRV features were correlated
with the stress factor scores in the SRI questionnaire (data not
shown). In brief, tension, depression and frustration were the
main stress factors associated with HRV features. Tension and
frustration factors were positively associated with the
sympathetic activity (LF and LFnu) and negatively associated
with parasympathetic activity (HFnu). Conversely, depression
factors were negatively associated with sympathetic activity
and positively with parasympathetic activity. During the
experience of negative emotions such as anger, frustration, or
anxiety, heart rhythms are known to become disordered,
indicating less synchronization in the reciprocal action between
the parasympathetic and sympathetic branches of the ANS [13].
Since age was a strong factor influencing HRV features,
we sough to rule out age-dependent features and seek out the
relationship between stress factors and HRV features. Cluster
analysis using stress factor scores was useful to identify high
stress group in our case. High stress group showed higher
LF/HF (reflects the predominance of sympathetic over
parasympathetic activity) and LFnu (mainly influenced by
sympathetic activity), whereas lower HFnu (mainly influenced
by parasympathetic activity) compared to the low stress factors
group (Table IV). Significant association of stress factors with
HRV features suggested that the questionnaire items in our
simplified version of SRI are useful to classify subjects into
high and low stress group. In addition, our results indicated
that further investigation is warranted for stress factors and
their relationships with body temperature systolic blood
pressure, and blood glucose level.
A
CKNOWLEDGMENT
We would like to thank all participants and students at ICU
who participated in the experiment. All correspondence should
be addressed to Desok Kim, PhD, School of Engineering,
Information and Communications University, 103-6, Munji
Dong, Yuseong Gu, Daejeon, 305-732, Korea; phone: 82-42-
866-6156; fax: 82-42-866-6110 and Jong-Min Woo, MD, PhD,
Inje University Paik Hospital, Sanggye 7 Dong, Nowon Gu,
Seoul, Korea 139-707; phone: 82-2-2270-0063; fax: 82-2-
2270-0334.
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