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Aviation, Space, and Environmental Medicine x Vol. 79, No. 9 x September 2008 1
RESEARCH ARTICLE
M ÄKINEN TM, M ÄNTYSAARI M, P ÄÄKKÖNEN T, J OKELAINEN J,
P
ALINKAS LA, H ASSI J, L EPPÄLUOTO J, T AHVANAINEN K, R INTAMÄKI H.
Autonomic nervous function during whole-body cold exposure before
and after cold acclimation. Aviat Space Environ Med 2008; 79: 1 – 8 .
Introduction: Cold habituation could affect sympatho-vagal balance,
which modulates cold stress responses. The study examined cardiovas-
cular autonomic function at the sinus node level during controlled
breathing and while undertaking isometric exercise during whole-body
cold exposure before and after cold acclimation. Methods: There were
10 male subjects who were exposed to control (25°C) and cold (10°C)
environments for 2 h on 10 successive days in a laboratory. Time and
frequency domain heart rate variability (HRV) in terms of root mean
square of successive differences in RR intervals, total, high, and low
frequency power were determined from controlled breathing at the be-
ginning and end of cold acclimation. Heart rate and blood pressure
during an isometric handgrip test (30% MVC for 3 min) were recorded
at the beginning and end of cold acclimation. Catecholamines (NE
and E), mean skin (T
¯
sk ), and rectal temperatures (T
rect ) were measured.
Results: Acute cold exposure increased total (36%), low (16%), and high
frequency power (25%) and RMSSD (34%). Cold acclimation resulted
in higher T
¯
sk (0.6°C) and lower NE (24%) response in cold. The cold-in-
duced elevation in high frequency power became signifi cant after cold
acclimation, while other HRV parameters remained unchanged. A
smaller increase in heart rate and blood pressure occurred at 10°C dur-
ing the handgrip test after cold acclimation. Discussion: Cold exposure
increased sympathetic activity, which was blunted after cold acclima-
tion. Parasympathetic activity showed a minor increase in cold, which
was enhanced after cold acclimation. In conclusion, cold habituation
lowers sympathetic activation and causes a shift toward increased para-
sympathetic activity.
Keywords: controlled breathing , habituation , heart rate variability , iso-
metric handgrip .
C OLD EXPOSURE IS an environmental stressor that
potentially leads to an increase in the loss of body
heat. This is counteracted by physiological adjustments
that improve thermal insulation from the environment
(peripheral vasoconstriction and centralization of circu-
lation) and increase heat production (shivering). The
primary role of the sympathetic nervous system during
cold exposure is to stimulate peripheral vasoconstric-
tion. This is refl ected by an increase in plasma norepi-
nephrine concentrations and blood pressure ( 30 ).
It is claimed that chronic and repeated cold exposures
causing marked whole-body cooling result in more pro-
nounced physiological responses, like enhanced vaso-
constriction and metabolic rate ( 30 ). However, repeated
brief exposures to cold not involving marked whole-
body cooling are suggested to result in habituation. Cold
habituation is a form of cold adaptation that denotes the
reduction of responses to, or perception of, a repeated
stimulation ( 8 ). This is suggested to be the most common
form of cold adaptation ( 30 ). Habituation in humans can
develop after only a few repeated brief ( , 2 h) exposures
to cold air or water ( 19,21 ). The observed responses
are shivering habituation (e.g., delayed onset and low-
ered metabolic rate), higher skin temperatures (due to
dampened vasoconstriction), less intense cold sensations,
and a lowered blood pressure and norepinephrine (NE)
response in cold ( 13,19,30 ). The diminished rise in blood
pressure and plasma NE concentrations suggests that
sympathetic activity is affected ( 18,30 ).
Cardiovascular responses, such as heart rate and
blood pressure, have been measured in studies examin-
ing cold acclimation due to repeated cold water immer-
sions ( 14,24,29 ), exposures to cold air ( 12 ), or employing
local cooling ( 13 ). However, to our knowledge, auto-
nomic nervous system responsiveness measured at the
sinus node level by assessing heart rate variability (HRV)
while being exposed to cold has not been examined in
controlled laboratory conditions. Furthermore, the ef-
fects of cold acclimation on HRV are not known. Changes
in autonomic nervous system activity assessing HRV
and hormonal secretion among over-wintering person-
nel in Antarctica have been followed previously. Those
From the Institute of Health Sciences, University of Oulu, Oulu, Fin-
land; the Finnish Defence Forces, Centre of Military Medicine, Re-
search and Development Unit, Lahti, Finland; the Finnish Defence
Forces, Centre of Military Medicine, Aeromedical Centre, Helsinki,
Finland; the Department of Physiology, University of Oulu, Oulu, Fin-
land; the Unit of General Practice, Oulu University Hospital, Oulu,
Finland; the School of Social Work, University of Southern California,
Los Angeles, CA; New Technologies and Risks, Finnish Institute of
Occupational Health, Helsinki, Finland; and Physical Work Capacity,
Finnish Institute of Occupational Health, Oulu, Finland.
This manuscript was received for review in November 2007 . It was
accepted for publication in June 2008 .
Address requests for reprints to: Tiina M. Mäkinen, Ph.D., Institute
of Health Sciences, University of Oulu, P.O.Box 5000, FI-90014 Oulu,
Finland; tiina.makinen@oulu.fi .
Reprint & Copyright © by the Aerospace Medical Association, Alex-
andria, VA.
DOI: 10.3357/ASEM.2235.2008
Autonomic Nervous Function During Whole-Body
Cold Exposure Before and After Cold Acclimation
Tiina M. Mäkinen , Matti Mäntysaari , Tiina Pääkkönen ,
Jari Jokelainen , Lawrence A. Palinkas , Juhani Hassi ,
Juhani Leppäluoto , Kari Tahvanainen , and
Hannu Rintamäki
2 Aviation, Space, and Environmental Medicine x Vol. 79, No. 9 x September 2008
COLD & AUTONOMIC NERVOUS FUNCTION — MÄKINEN ET AL.
studies detected decreased sympathetic and increased
parasympathetic activity during the residence, followed
by a reduction in anterior pituitary and adrenal hor-
monal secretion ( 4,11 ). However, the presence of several
concurrent stressors in these isolated environments
makes it diffi cult to distinguish the effects of cold expo-
sure. HRV has also been examined in experiments em-
ploying local cooling to the face ( 16 ) or using cold pres-
sor tests for evaluating cardiac autonomic function ( 28 ).
One previous study examined the isolated effects of core
and peripheral cooling on HRV and showed that the
mode of cooling caused a specifi c thermoregulatory in-
fl uence on the very-low frequency component of HRV
related to core hypothermia, while skin cooling was as-
sociated with less specifi c HRV changes ( 5 ). We consider
that studying HRV provides a reliable noninvasive
method for the assessment of autonomic cardiovascular
regulation and sympatho-vagal interaction.
In the present study we also undertook a static iso-
metric contraction test to further examine sympathetic
nervous system activity and how it is affected by cold
habituation. Both cold exposure and isometric exercise
separately involve the activation of the sympathetic ner-
vous system. However, the cardiovascular responses to
isometric exercise during whole-body cold exposure are
not known. Furthermore, it is not known how cold ac-
climation affects sympathetic responses during isomet-
ric exercise in cold.
The objective of the present study was to examine
how repeated short cold exposures, mainly causing su-
perfi cial cooling, are manifested in the cardiovascular
regulation by the autonomic nervous system. The study
used a cold acclimation protocol known to elicit cold
habituation responses ( 19 ). Our hypothesis was that cold
acclimation would result in cold habituation and changes
in HRV, demonstrating a reduction in sympathetic and
an increase in parasympathetic activity.
METHODS
Participants
Following approval of the experimental protocol by
the University of Oulu and Northern Ostrobothnia
Hospital research ethics committee and obtaining writ-
ten informed consent, 10 young healthy, non-smoking,
normotensive male participants volunteered for the
study. Mean characteristics of these participants were:
age, 22.5 yr (SD 1.6); height, 180.8 m (SD 7.2); weight,
72.4 kg (SD 7.3); body fat, 17.1% (SD 1.9); body mass
index, 22.3 kg z m 2 2 (SD 1.6); and peak oxygen con-
sumption (
Vo
2max ), 53.1 ml z min 2 1 z kg 2 1 (SD 6.1). For
determining body fat percent, skin fold thickness was
measured from triceps, biceps, subscapularis, and su-
prailiaca and calculated according to Durnin and
Rahaman 1967 ( 3 ). Maximal oxygen consumption was
measured with a bicycle ergometer. The test was ended
when the subject could no longer maintain the physi-
cal activity level and/or when RQ exceeded 1.15. The
Vo
2max value represented the peak value from the last
30 s of the test.
Experimental Protocol
The tests were performed in Oulu, Northern Finland
(65 °N 25 °E), during September-November. During this
period the mean monthly temperatures ranged between
14.3°C and 2 0.5°C. The subjects participated in the tests
at least 2 h after a meal and were instructed not to use
coffee or alcohol the evening before the measurements
( , 12 h). They were also instructed to avoid strenuous
exercise and to sleep normally (e.g., 8 h) during the night
preceding each test. The autonomic tests were per-
formed in a quiet and environmentally controlled labo-
ratory environment. During the experiments the subject
were exposed to cold (10°C) for 2 h z d 2 1 on ten succes-
sive days. They were lightly clad in shorts, socks, and
athletic shoes. The experiments started at the same time
each day for each subject. Autonomic nervous function
was assessed at the beginning (Day 1) and end (Day 10)
of the cold acclimation protocol for both control and
cold conditions. Control measurements were performed
in a temperature control chamber regulated to ambient
air temperature of 25°C. This environment was thermo-
neutral for the subjects as judged by the measured skin
temperatures and thermal sensations. Subjects were
then transferred to a second temperature controlled
chamber maintained at 10°C for 2 h each day. In both of
these climatic chambers the relative humidity was 50 6
3% and air velocity less than 0.2 m z s 2 1 .
Instrumentation
Skin temperatures were measured from 10 sites: fore-
head, upper back, chest, abdomen, upper arm, lower
arm, back of the hand, anterior thigh, dorsal side of the
foot, and calf (NTC DC 95, Digi Key, Thief River Falls,
MN ). The thermistors were attached to the skin with
adhesive material. Rectal temperature (T
rect ) was mea-
sured 10 cm beyond the anal sphincter with an YSI 401
probe (Yellow Springs Instrument Co., Yellow Springs,
OH). Skin and rectal temperature values were recorded
at 1-min intervals with a datalogger (SmartReader 8 1 ,
ACR Systems, Surrey, BC, Canada ) throughout the
control and cold exposure. Mean skin temperature
(T
¯
sk ) was calculated as an area-weighted average accord-
ing to the following formula: 0.07 z (T forehead ) 1 0.35 z
mean(T
chest , T scapula, T abdomen ) 1 0.14 z mean(T upper arm ,
T
lower arm ) 1 0.05 z (T dorsal hand ) 1 0.19 z (T thigh ), 1 0.13 z
(T
calf ) 1 0.07 z (T dorsal side of foot ) ( 10 ). The presented T
¯
sk and
T
rect results represent values recorded during the auto-
nomic nervous system tests.
Systolic and diastolic blood pressure was measured
from sitting subjects with the arm cuff method immedi-
ately before the controlled breathing test using an am-
bulatory blood pressure monitoring device (Meditech
ABPM-04, Meditech Ltd., Budapest , Hungary). Blood
pressure was also measured during the sustained hand-
grip test before the test and in the fi nal minute of the 3
min of sustained exercise.
Blood samples (10 ml) were obtained from nine sub-
jects by antecubital venipuncture into EDTA-containing
glass tubes (Terumo Venoject, Terumo Corp., Leuven,
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Aviation, Space, and Environmental Medicine x Vol. 79, No. 9 x September 2008 3
COLD & AUTONOMIC NERVOUS FUNCTION — MÄKINEN ET AL.
Belgium) before and after the cold exposure. For techni-
cal reasons, no blood sample was available for one sub-
ject. Within 10 min, the samples were centrifuged (1500 3
g, 10 min) to harvest the plasma. The plasma was im-
mediately transferred to polypropylene tubes (Eppen-
dorf AG, Hamburg, Germany) and stored in a 2 80°C
freezer until analyzed. The catecholamine analyses
consisted of assessing plasma epinephrine (E) and NE
concentrations and were performed using the high-
performance liquid chromatography method.
Autonomic Nervous Function
Power spectral analysis allows the quantifi cation of
HRV and autonomic responsiveness at different fre-
quency ranges ( 7 ). Overall HRV, termed total power
(variance of normal-to-normal RR intervals over the se-
lected time segment), is recommended to be divided
into three spectral components. These components are
integrals from the very low frequency (0 – 0.04 Hz), low
frequency (0.04 – 0.15 Hz), and high frequency (0.15 – 0.40
Hz) spectral bands. The distribution of the power and
the central frequency of low and high frequency bands
are not fi xed, but may vary in relation to changes in au-
tonomic modulations of the heart period ( 26 ). HRV in
the high frequency range is so rapid that it can only be
mediated by the parasympathetic nervous system.
However, part of the high frequency power seems to be
caused by respiration-induced changes in the intratho-
racic pressure and blood volume ( 17 ). In the low fre-
quency range, both the sympathetic and the parasympa-
thetic nervous system can affect HRV. It seems possible
that sympathetic and parasympathetic nervous systems
infl uence each other, and that humoral regulatory sys-
tems can also affect the autonomic nervous infl uences in
the HRV. The physiological mechanisms altering HRV
in the very low frequency range are not well known. It
has been suggested that humoral regulatory mecha-
nisms, thermoregulation, changes in physical activity,
and even diurnal variation can infl uence very low fre-
quency power of HRV ( 26 ).
HRV during controlled breathing refl ects autonomic
nervous system regulation at the sinus node level, pro-
viding information of sympathetic and parasympathetic
activity. This test was performed after 10 min of exposure
to 25°C and after 60 min of exposure to 10°C. During the
tests the subjects were in supine position and the test was
started after a 5-min stabilizing period. Blood pressure
was measured before the test using the arm cuff method
at the brachialis level. The breathing rate was controlled
(0.25 Hz, i.e., 2 s for inspiration and 2 s for expiration)
during the study with the use of audio feedback from the
data acquisition computer. The duration of the controlled
breathing test was 5 min. Electrocardiogram was re-
corded using a bipolar precordial lead. The recorded sig-
nals were digitized with a 12-bit resolution at a sampling
rate of 200 Hz (WinAcq-F, Absolute Aliens Oy, Turku,
Finland). The analyses were performed offl ine from sta-
tionary regions free of ectopic beats and technical arti-
facts with WinCPRS software (Absolute Aliens Oy).
Cardiac autonomic function was assessed during the
controlled breathing test with 1) time domain analysis of
RR interval and 2) power spectral analysis of RR inter-
val. Time domain analysis included the following indi-
ces: mean heart rate, root mean square of successive dif-
ferences (RMSSD) of RR intervals, and the percentage of
successive interbeat intervals with over 50-ms differ-
ences in duration. In power spectral analysis of HRV,
based on Fast Fourier transformation, we determined
total power, low-frequency power (0.04 – 0.15 Hz), and
high-frequency power (0.15 – 0.40 Hz), and low-to-high
frequency ratio.
Isometric Handgrip Test
Static isometric contraction evokes the exercise pres-
sor refl ex, which results in vagal withdrawal and sym-
pathetic activation, increasing heart rate, cardiac output,
and peripheral vascular resistance during static exercise
( 1,2,20 ). This response partially originates in the isomet-
rically contracting muscles, and afferentation from the
active muscles to the central nervous system (CNS ) is
necessary for the development of this peripherally in-
duced response. However, part of this response seems
to originate in the CNS without any need for the afferent
information from the contracting muscles. This effect is
suggested to be related to the CNS activation needed to
initiate and maintain the voluntary isometric muscular
contraction, the “ central command ” ( 1,9 ). The isometric
handgrip test was performed after 20 min of exposure to
control (25°C) and after 70 min of exposure to cold (10°C)
after the controlled breathing test. During the test the
subjects were sitting in a chair in a relaxed position with
their arms supported. At fi rst a brief (~3 s) maximal con-
traction was performed with a dynamometer (Newtest
Ltd., Oulu , Finland). After a suffi cient recovery period
( . 10 min), arm cuff blood pressure and heart rate at rest
were recorded. During the test static handgrip was per-
formed with the dominant hand at 30% of maximal vol-
untary contraction for 3 min. The appropriate level of
exercise was monitored from a digital display by the ex-
perimenter and subject. The blood pressure and heart
rate of the subject were recorded during the last minute
of exercise.
Statistical Analyses
Heart rate, cardiovascular, and temperature parameters
between 25°C and 10°C were compared to paired t -tests.
When necessary, logarithmic transformations were per-
formed to normalize the data. Data that failed to normal-
ize after logarithmic transformation were analyzed by the
nonparametric Wilcoxon sign rank test. Furthermore, the
difference in heart rate parameters between cold and
warm was calculated, and this difference was compared
between Day 1 and Day 10 using the paired t -test (or
Wilcoxon sign rank test). For some of the parameters one-
tailed testing was used. Statistical tests were performed
using the SPSS (SPSS 15.0, SPSS Inc., Chicago, IL) software.
Data are presented as means 6 SD and the signifi cance
was set at P , 0.05.
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4 Aviation, Space, and Environmental Medicine x Vol. 79, No. 9 x September 2008
COLD & AUTONOMIC NERVOUS FUNCTION — MÄKINEN ET AL.
RESULTS
At the end of the cold acclimation period the systolic
( P 5 0.010) and diastolic blood pressure at 25°C ( P 5
0.003) was signifi cantly lower and mean skin tempera-
ture ( P 5 0.015) was signifi cantly higher than on Day 1.
By the end of the cold acclimation period, at 25°C blood
pressure was signifi cantly lower ( P 5 0.003) than on Day
1 ( Table I ). There were no signifi cant differences in any
of the other parameters (heart rate, NE, E).
Mean skin temperature decreased by 6.3 – 6.4°C ( P 5
0.000) and rectal temperature by 0.1 – 0.2°C during the
fi rst hour of exposure to 10°C ( P 5 0.044) compared to
25°C. Systolic blood pressure increased on average by 16
mmHg ( P 5 0.000-0.001) and diastolic blood pressure by
11 mmHg at 10°C ( P 5 0.000-0.022). Plasma NE concen-
trations were signifi cantly higher ( P 5 0.000) and E con-
centration lower ( P 5 0.018) at 10°C compared to 25°C
( Table I ).
Mean skin temperature was 0.3°C higher at 25°C (n.s.)
and 0.6°C higher at 10°C ( P 5 0.015) at the end of the
cold acclimation period ( Table I ). Systolic blood pres-
sure was 5 mmHg (4%) ( P 5 0.010) and diastolic blood
pressure 8 mmHg (11%) ( P 5 0.003) lower at 25°C on
Day 10 compared to Day 1. In addition, the cold accli-
mation resulted in NE concentrations that were on aver-
age 24% lower ( P 5 0.043) at 10°C. E was not affected by
the cold acclimation period.
Exposure to cold increased total power by 36% ( P 5
0.028), which indicates a higher overall HRV in cold.
Low frequency power was also 16% ( P 5 0.033) higher
at 10°C. High frequency power was 25% higher at 10°C
during Day 1 compared to 25°C (n.s.). RMSSD was 34%
( P 5 0.020) longer when measured at 10°C and com-
pared with 25°C ( Table II ) .
By the end of the cold acclimation (Day 10), cold ex-
posure increased total power by 51% ( P 5 0.040), high
frequency power by 54% ( P 5 0.043), and RMSSD by
36% ( P 5 0.036) in the cold compared to 25°C ( Table II ).
Low-to-high frequency ratio was 10 – 12% lower at both
25°C and 10°C at the end of the cold acclimation period
compared to Day 1 (n.s.). When comparing the change
in HRV indices between the end and beginning of the
cold acclimation period, no signifi cant differences were
observed.
Fig. 1 provides an example of the power spectral den-
sity of HRV in one subject during controlled breathing
(0.25 Hz) in warm and cold before and after cold accli-
mation. The fi gure shows decreased low frequency
power (sympathetic activity) both in warm and cold
temperature after the cold acclimation period. In addi-
tion, in this particular subject the high frequency power
is decreased in warm conditions by the end of the cold
acclimation.
The cardiovascular responses during the 3-min iso-
metric handgrip tests are presented in Table III and
Fig. 2 . On average, at 25°C heart rate increased by 9 – 15
bpm (Day 1, P 5 0.001; Day 10, P 5 0.018), systolic blood
pressure by 19 – 23 mmHg (Day 1, P 5 0.000; Day 10, P 5
0.000), and diastolic blood pressure 22 – 23 mmHg (Day
1, P 5 0.000; Day 10, P 5 0000) during the isometric
handgrip. At 10°C heart rate increased by 2 – 9 bpm (Day
10, n.s.; Day 1, P 5 0.002) systolic blood pressure by 14 –
17 mmHg (Day 1, P 5 0.001; Day 10, P 5 0.001) and dia-
stolic blood pressure 12 – 19 mmHg (Day 10, P 5 0.004;
Day 1, P 5 0.000) during the isometric handgrip. By the
end of the cold acclimation period the rise in heart rate
during the handgrip test was signifi cantly smaller at
10°C compared to Day 1 ( P 5 0.015) ( Table III ). Further-
more, diastolic blood pressure increased less at 10°C on
Day 10 compared to Day 1 ( P 5 0.035) and also com-
pared to the end of the handgrip test at 25°C ( P 5 0.004)
( Table III ).
DISCUSSION
Autonomic nervous system responsiveness at the si-
nus node level (by assessing HRV) in response to cold
acclimation has not been previously examined in con-
trolled laboratory conditions. The results of our study
confi rm earlier research using hormonal and cardiovas-
cular measurements that repeated cold exposure results
in a decrease in sympathetic response to acute cold ex-
posure. This is demonstrated by a smaller increase in to-
tal HRV, blood pressure, and plasma norepinephrine.
The study also demonstrated for the fi rst time an in-
crease in parasympathetic activity following a short pe-
riod of cold acclimation. This is characterized by a con-
siderable increase in high frequency power after cold
acclimation.
TABLE I. RECTAL AND SKIN TEMPERATURES, BLOOD PRESSURE, AND CATECHOLAMINE RESPONSES DURING THE AUTONOMIC TESTS IN
WARM (25°C) AND COLD (10°C) ON DAYS 1 AND 10.
Day 1 Day 10
25°C 10°C P 25°C 10°C P
Heart rate (bpm) 69 6 6 65 6 11 ns 68 6 11 66 6 11 ns
SBP (mmHg) 128 6 13 144 6 14 0.001 123 6 11* 138 6 12 0.000
DBP (mmHg) 74 6 11 85 6 7 0.022 66 6 8
†
84 6 5 0.000
NE (pmol z ml
2
1 ) 4.6 6 1.9 9.8 6 2.5 0.000 4.1 6 1.0 7.4 6 2.3 ‡ 0.000
E (pmol z ml
2
1 ) 1.5 6 0.9 1.1 6 0.8 0.018 1.2 6 1.1 1.4 6 1.2 ns
Rectal temperature (°C) 37.1 6 0.2 37.0 6 0.2 0.044 37.1 6 0.2 36.9 6 0.2 ns
Skin temperature (°C) 33.1 6 0.6 26.6 6 1.2 0.000 33.4 6 0.3 27.2 6 0.7
§ 0.000
The values represent means 6 SD ( N 5 10). SBP 5 systolic blood pressure, DBP 5 diastolic blood pressure, NE 5 norepinephrine, E 5 epinephrine.
Cold acclimation: signifi cantly different from same exposure during Day 1, * P 5 0.010 (one-tailed),
†
P 5 0.003 (one-tailed),
‡
P 5 0.043 (one-tailed),
§ P 5 0.015.
Aviation, Space, and Environmental Medicine x Vol. 79, No. 9 x September 2008 5
COLD & AUTONOMIC NERVOUS FUNCTION — MÄKINEN ET AL.
Fig. 1 . Power spectral density of HRV in one subject during controlled breathing in control (left) and cold (right) conditions during Day 1 (upper
panel) and Day 10 (lower panel).
TABLE II. HEART RATE BASELINE PARAMETERS DURING CONTROLLED BREATHING IN WARM (25°C) AND COLD (10°C) AT THE
BEGINNING (DAY 1) AND END (DAY 10) OF THE COLD ACCLIMATION PERIOD.
Day 1 Day 10
25°C 10°C P 25°C 10°C P
Total power ms
2 5893 6 9144 9210 6 5307 0.028 5414 6 5420 11,011 6 8081 0.040
LF ms
2 1518 6 2448 1813 6 1103 0.033 1326 6 1376 2668 6 2575 0.063
HF ms
2 2998 6 5527 3983 6 3786 0.443 1826 6 1835 4003 6 3993 0.043
LF/HF (%) 101 6 89 87 6 76 0.501 91 6 48 77 6 47 0.491
RMSSD (ms) 77 6 70 117 6 66 0.020 70 6 48 111 6 63 0.036
pNN50 (%) 31 6 25 50 6 26 0.052 35 6 28 50 6 20 0.104
RRI (ms) 882 6 76 962 6 161 0.072 910 6 161 958 6 205 0.168
The values represent means 6 SD ( N 5 10). LF 5 low frequency; HF 5 high frequency; LF/HF 5 low-to-high frequency ratio; RMSSD 5 root mean
square of successive differences; pNN50 5 successive interbeat intervals with over 50-ms differences in duration; RRI 5 RR interval.
Consistent with previous research ( 5 ), cold exposure
signifi cantly increased total and low frequency power of
HRV, whereas high frequency power showed an insig-
nifi cant elevation. The increased low frequency power
in cold could indicate increased sympathetic activation,
though it is also partially infl uenced by the parasympa-
thetic activity. Other parameters refl ecting sympathetic
activation were the increased NE concentrations and
higher blood pressure in cold. The observed elevation in
the RMSSD in cold suggests that parasympathetic co-
activation also occurs, which response was signifi cantly
increased on Day 10, and accompanied by higher HRV
in terms of total power in cold. Another indicator sup-
porting increased parasympathetic activity in the cold
was the observed slight, but not signifi cant, reduction in
heart rate.
The observation that both sympathetic and parasym-
pathetic activity is increased during whole-body cooling
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6 Aviation, Space, and Environmental Medicine x Vol. 79, No. 9 x September 2008
COLD & AUTONOMIC NERVOUS FUNCTION — MÄKINEN ET AL.
is interesting, as this has not been documented before. It
has been suggested that the cooling pattern infl uences
the cardiovascular and autonomic nervous system re-
sponses ( 5,6 ). Local cooling experiments, e.g., the cold
pressor test, caused an increase in sympathetic and a re-
duction in parasympathetic activity ( 28 ), although it
must be remembered that the cold pressor responses are
mainly related to pain and differ from exposure to cold
air. However, on the sinus node level cold-water face
immersion elicits increases in cardiac parasympathetic
activity (increase in high frequency power), but not in
sympathetic activity ( 16 ). The study of Fleischer et al. ( 5 )
is one of the few examining the relationship between
HRV and thermoregulation in response to whole-body
cooling in man, although in their study core and periph-
eral cooling were examined separately. It was observed
that core cooling increased the very low frequency
power, while very low frequency and low frequency
power increased due to skin surface cooling ( 5 ). We ob-
served an increase in low frequency power in response
to skin cooling, but we also detected an increase in high
frequency power at the same time.
In the present study, the mean skin temperature was
higher and the elevation in plasma NE smaller in the cold
after the cold acclimation period. At the same time, a re-
duction in metabolic rate was observed ( 22 ), together with
less intense cold sensations than those presented else-
where ( 23 ). These observations are consistent with earlier
studies demonstrating habituation to repeated cold stim-
uli ( 19,30 ). Therefore, it was considered that the cold accli-
mation protocol was successful in eliciting the desired re-
sponses. We also observed a reduced blood pressure and
increased mean skin temperature in a warm environment
after the cold acclimation, which could represent adapta-
tion to the experimental situation with lower sympathetic
activity at the beginning of the control test session.
A shift in autonomic nervous system activity between
the beginning and end of the cold acclimation period
was also detected in the present study. This change from
the cold-induced elevation in low frequency power on
Day 1 to the elevation in high frequency power on Day
10 suggests that at the sinus node level the sympathetic
activation due to cold exposure is blunted during cold
acclimation, and replaced to some extent by increased
parasympathetic infl uence. This shift in autonomic ner-
vous system activity resembles the results obtained from
studies of over-wintering personnel in Antarctica ( 4,11 ).
The increase in parasympathetic effects at the sinus node
level may be due to a true increase in parasympathetic
activity. Alternatively, it may also result from reduced
sympathetic activity, allowing a better detection of the
parasympathetic infl uence ( 27 ). Theoretically, the higher
high frequency power by the end of the cold acclimation
period could also be due to increased respiratory tidal
volume ( 26 ). However, during the controlled breathing
test the respiratory rate was strictly timed, allowing no
signifi cant changes in the tidal volume. Furthermore,
tidal volume measurements at rest (results not pre-
sented) did not show changes during cold acclimation,
supporting the validity of the HRV results during the
controlled breathing test.
The isometric handgrip test was employed in this study
to assess the responsiveness of the autonomic nervous sys-
tem and its ability to affect the blood pressure responses
during the combined effects of cold and isometric exercise.
Furthermore, the objective was to examine how a cold ac-
climation period affects sympathetic responsiveness. Cold
exposure and isometric exercise both involve the activa-
tion of the sympathetic nervous system. Previous studies
examining the effects of local cooling on cardiovascular re-
sponses during isometric exercise have recorded additional
effects of cold and exercise on the cardiovascular responses
( 25 ), whereas others have failed to detect them ( 15 ).
This is the fi rst study to examine the effects of whole-
body cooling on cardiovascular responses during a
handgrip test. According to our results, similar eleva-
tions in heart rate and blood pressure occurred in both a
cold and warm environment before the cold acclimation
period. However, cold habituation resulted in a signifi -
cantly smaller increase in heart rate and diastolic blood
pressure during the handgrip test in the cold. No signifi -
cant differences were observed in the heart rate and
blood pressure responses during cold acclimation at
25°C, suggesting that the daily repetitions per se did
not infl uence the responses to isometric handgrip. The
lower heart rate and elevations in diastolic blood pres-
sure during isometric handgrip after repeated cold ex-
posures suggest that the sympathetic activation due to
TABLE III. MEAN ( 6 SD) HEART RATE (HR), SYSTOLIC (SBP), AND DIASTOLIC (DBP) BLOOD PRESSURE (MMHG) IN THE BEGINNING AND
AFTER A 3-MIN ISOMETRIC HANDGRIP TEST (30% OF MAXIMAL VOLUNTARY CONTRACTION) AT 25°C AND 10°C DURING DAYS 1 AND 10.
25°C 10°C
Start End P Start End P
HR (bpm)
Day 1 70 6 8 85 6 11 0.001 68 6 8 77 6 11* 0.002
Day 10 78 6 6 87 6 10 0.018 72 6 2 74 6 7
†
0.389
SBP (mmHg)
Day 1 131 6 5 150 6 5 0.000 152 6 13
†
166 6 12
‡
0.001
Day 10 122 6 9 145 6 13 0.000 138 6 10** 155 6 10
† †
0.001
DBP (mmHg)
Day 1 79 6 6 101 6 8 0.000 92 6 6
†
111 6 9
‡ ‡
0.000
Day 10 73 6 9 96 6 9 0.000 89 6 9
†
101 6 6 0.004
Warm (25°C) vs. cold (10°C): * P 5 0.022,
†
P 5 0.001,
‡
P 5 0.007, ** P 5 0.000,
† †
P 5 0.019,
‡ ‡
P 5 0.013.
Aviation, Space, and Environmental Medicine x Vol. 79, No. 9 x September 2008 7
COLD & AUTONOMIC NERVOUS FUNCTION — MÄKINEN ET AL.
the handgrip exercise also becomes blunted after cold
habituation. Our data do not allow evaluating the role of
peripheral and central mechanisms in this change of the
isometric handgrip reaction. Further studies are thus
needed to assess, e.g., peripheral vascular resistance re-
sponses due to isometric exercise in the cold, and would
also provide further insight into cardiovascular control
mechanisms.
The signifi cance of the results of the present study is
that it provides novel information about how repeated
cooling and the associated thermal responses refl ect the
modulation of the autonomic nervous system at the
level of the heart. Previous studies have examined the
aspect of cold adaptation using classical measures such
as hormonal and blood pressure responses ( 30 ). In con-
junction with these traditional measures, our study ex-
amined the independent effects of cold habituation on
the sympathetic-vagal balance, using classic tests for as-
sessing autonomic nervous system function. In accor-
dance with our initial hypothesis, this study indicates
that cold habituation not only involves a reduction in
sympathetic activity, but also increases in parasympa-
thetic activity. We also found that the cardiovascular ef-
fects of sympathetic activation due to isometric hand-
grip were blunted in cold after cold acclimation. This
observation emphasizes the possibility that combining
training of specifi c tasks with cold acclimation can lead
to physiological changes that are not observed when ex-
amined separately.
One of the study limitations is that the most marked
effects of cold acclimation on the thermal and other
physiological responses (e.g., temperature, thermal sen-
sation, blood pressure) could be detected during the ini-
tial cooling period (fi rst 30 min of cold exposure). There-
fore, performing the tests while also assessing autonomic
nervous system function at the beginning of the cooling
period might have provided additional information on
the effects of cold acclimation. On the other hand, our
aim was to assess autonomic activity during a plateau
phase after the skin temperatures had stabilized in the
cold. It is also recognized that HRV shows day-to-day
variation ( 26 ), and repeated daily measurements might,
therefore, have brought additional information about
the successive changes occurring during cold habitua-
tion. Furthermore, it cannot be completely excluded that
the changing outdoor temperatures during the study
period might have partially infl uenced the physiological
responses. Finally, from a statistical perspective, the rel-
atively small sample size may have precluded our abil-
ity to have suffi cient power to rule out a type II error for
some of the responses.
In conclusion, whole-body cold exposure involving
mainly surface cooling signifi cantly increased blood
pressure, plasma NE, total and low frequency power,
and RMSSD. Following cold acclimation, mean skin
temperature was higher and the increase in plasma NE
smaller than before the cold acclimation period. In addi-
tion, the increase in high frequency power due to cold
exposure became signifi cant after cold acclimation. Cold
habituation resulted in diminished sympathetic activity
during isometric exercise, as judged by reduced heart
rate and diastolic blood pressure elevations during the
handgrip test. Our results suggest that cold habituation
not only blunts the sympathetic activation due to cold
exposure, but also increases the parasympathetic infl u-
ence, at least at the sinus node level. Further studies ex-
Fig. 2 . Mean difference between start-end of 3-min isometric hand-
grip test in A) heart rate; B) systolic blood pressure; and C) diastolic
blood pressure at 25°C and 10°C. Values represent means 6 SD ( N 5
10). Change (start-end) signifi cantly different from Day 1:
‡
P 5 0.015,
†
P 5 0.0325; signifi cantly different from 25°C: * P 5 0.004.
8 Aviation, Space, and Environmental Medicine x Vol. 79, No. 9 x September 2008
COLD & AUTONOMIC NERVOUS FUNCTION — MÄKINEN ET AL.
amining the successive changes during cold acclimation
are warranted.
ACKNOWLEDGMENTS
This study was partially supported by the Graduate School of
Circumpolar Wellbeing, Health and Adaptation coordinated by the
Centre for Arctic Medicine at the University of Oulu. Dr. Tiina Mäkinen
acknowledges the Finnish Defence Forces for salary support when
fi nalizing this manuscript. We would like to thank the test subjects for
their dedication to this study. The experiments performed during this
study comply with the current laws of Finland. There are no confl icts
of interest in the present study.
Authors and affi liations: Tiina M. Mäkinen, Ph.D., Jari Jokelainen,
M.Sc., Juhani Hassi, M.D., Ph.D., Institute of Health Sciences, University
of Oulu, Oulu, Finland; Tiina M. Mäkinen, Ph.D., Finnish Defence
Forces, Centre of Military Medicine, Research and Development
Unit, Lahti, Finland; Matti Mäntysaari, M.D., Ph.D., Finnish Defence
Forces, Centre of Military Medicine, Aeromedical Centre, Helsinki,
Finland; Tiina Pääkkönen, M.Sc., Juhani Leppäluoto, M.D., Ph.D.,
Hannu Rintamäki, Ph.D., Department of Physiology, University of
Oulu, Oulu, Finland; Jari Jokelainen, M.Sc., Unit of General Practice,
Oulu University Hospital, Oulu, Finland; Lawrence A. Palinkas,
Ph.D., School of Social Work, University of Southern California, Los
Angeles, CA; Kari Tahvanainen, M.Sc., New Technologies and Risks,
Finnish Institute of Occupational Health, Helsinki, Finland; and
Hannu Rintamäki, Ph.D., Physical Work Capacity, Finnish Institute of
Occupational Health, Oulu, Finland.
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