Effects of posture and sleep deprivation on heart rate variability

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					        Effects of Posture and Sleep Deprivation on Heart Rate Variability
         Ki Chang Nam1, Young Woo Shim2, Jae Lim Choi2, and Deok Won Kim2*
                        Korea Electrotechnology Research Institute, Ansan, Korea
         Department of Medical Engineering, Yonsei University College of Medicine, Seoul, Korea
                                Corresponding author e-mail:

    Heart rate variability (HRV) has been analyzed to assess activity of autonomic nervous
system in either sitting or supine posture in many electromagnetic field (EMF) provocation
studies. Drowsiness has been observed while inquiring subjective symptoms and EMF
perception in our previous study [1]. It was necessary to wake one up when drowsiness
occurred, and drowsiness was found to be related with posture. However, there were no
explicit reasons why sitting or supine posture was chosen in many studies. Because it was
reported that sleep deprivation could activate sympathetic nerve, resulting in increased low
frequency power/high frequency power (LFP/HFP) [2], we investigated which posture
resulted in less variation of the LFP/HFP.

     A total of 52 healthy subjects participated in this 30 min experiment (25 males and 27
females; 24.5 ± 2.7 years). They were recruited by advertisements at the Yonsei University
Hospital System and told not to do excessive exercise, use mobile phone for more than three
minutes, drink, or take any medicine at least for one day before the experiment. As it was
reported that HRV can be influenced by age and obesity, subjects were limited to 20-29 years
old with body mass index (BMI) between 18.5 and 22.9 [3].
    Before the experiment, subjects were invited to rest in sitting and supine postion with an
angle of 30 degrees on the experimental bed for at least 10 minutes. Each subject was tested
for sitting position on the first day and for supine position on the second day, or vice versa.
No matter which came first, sitting or supine position, the second session was always given at
approximately the same time of the day as the first day in order to maintain the subject’s
physiological rhythm. The order of sitting and supine position for a subject was randomly
assigned to minimize experiment bias.
    The duration of each test session was 40 minutes including 10 minutes of resting and six
5-minute stages. Electrocardiogram (ECG) were collected from stages 1 through 6 using
PolyG-I (Laxtha, Daejeon, Korea) and Ag-AgCl electrodes (2223, 3M, St. Paul, MN) placed
on both arms and the right leg. The collected ECG data were divided into six 5-minute stages
for HRV analysis.
     For HRV, the R-R intervals were acquired from the measured 5 minutes of ECG data, and
its power spectrum was obtained using software (TeleScan Ver.2.8, Laxtha). LFP/HFP was
calculated with HRV power spectrum to analyze changes in the ANS. To analyze the relative
change in LFP/HFP, the LFP/HFP of stage 1 for sitting and supine positions was set at 100%.
χ2 test of crosstabs was applied to determine the independence between the number of sleep
deprivation (drowsiness) and posture. A repeated one-way ANOVA test was performed to
examine significant effects of posture on LFP/HFP using SPSS software (SPSS 10, SPSS,
Chicago, IL) with a significance level of 0.05. A sub-group analysis was applied to examine
the variation of LFP/HFP in each stage for sitting and supine positions.

  Sleep deprivation was significantly dependent on posture (p=-0.002). LFP/HFP in sitting
posture was significantly different from that in supine posture (p=0.033). Applying the sub-
group analysis, differences in LFP/HFP between the first and the other stages for sitting and
supine postures are shown in Figs. 1 and 2, respectively. Comparison between Figs. 1 and 2
also confirms that LFP/HFP in sitting posture is less varying than that in supine posture.

          Fig. 1: Changes of LFP/HFP in each stage for the sitting (A) and supine postures (B) (n=52)

   As expected, fewer subjects experienced drowsiness in sitting posture than in supine
posture, and thus fewer wake-up, resulting in fewer sleep deprivation. Fewer sleep
deprivation means less sympathetic stimulation, resulting in smaller LFP/HFP. Therefore, it
would be more appropriate to use a chair that is less comfortable than a bed to avoid
drowsiness in future EMF studies. Usage of heart rate or HRV with unwanted drowsiness
may falsely indicate the effects of EMF on the autonomic nervous system.

    This work has been supported by KESRI (08123), which is funded by Ministry of
Knowledge & Economy of Korea and National Research Foundation (NRF) grant funded by
the Korea government (MEST) (No. 2009-0083613).

[1] K. C. Nam, J. H. Lee, H. W. Noh, E. J. Cha, N. H. Kim, and D. W. Kim. Hypersensitivity to RF fields
emitted from CDMA cellular phones: A provocation study. Bioelectromagnetics, vol. 30: 641-650, 2009.
[2] X. Zhong, H. J. Hilton, G. J. Gates, S. Jelic, Y. Stearn, M. N. Bartels, R. E. Demeersman, and R. C. Basner.
Increased sympathetic and decreased parasympathetic cardiovascular modulation in normal humans with acute
sleep deprivation. J. Appl. Physiol., vol. 98(6): 2024-2032, 2005.
[3] I. Antelmi, R. S. de Paula, A. R. Shinzato, C. A. Peres, A. J. Mansur, and C. J. Grupi. Influence of age,
gender, body mass index, and functional capacity on heart rate variability in a cohort of subjects without heart
disease. Am. J. Cardiol., vol. 93(3): 381-385, 2004.

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