Original paper Graded response of heart rate variability, associated with an alteration of geomagnetic activity in a subarctic area S. Oinuma a , Y. Kubo a , K. Otsuka a, *, T. Yamanaka a , S. Murakami a , O. Matsuoka a , S. Ohkawa a , G. Cornélissen b , A. Weydahl c , B. Holmeslet d , C. Hall d , F. Halberg b on behalf of the “ICEHRV” Working Group a Department of Medicine, Tokyo Women’s Medical University, Daini Hospital, Tokyo, Japan b Chronobiology Laboratories, University of Minnesota, Minneapolis, Minnesota, USA c Finnmark University College, Alta, Norway d Auroral Observatory, University of Tromso, Tromso, Norway Abstract It is becoming recognized that geomagnetic activity may influence biological processes, including the incidence of various human diseases. There is evidence that heart rate variability (HRV) may serve not only as an index of autonomic coordination of the circulation, but also as a powerful predictor of risk in apparently healthy subjects. This study focuses on the effects of geomagnetic disturbance on HRV, by comparing different indices of HRV of young, healthy men living in a subarctic area on days of low (Ap; 0-7), middle (Ap; 7-20), and high (Ap; 20-45) geomagnetic activity. The effect of geomagnetic disturbance on HRV is examined on the basis of 7-day records by Holter ECG, obtained longitudinally on 5 clinically healthy men, 21-31 years of age, in Alta, Norway (70 degree N). Frequency- and time-domain measures of HRV were analyzed for each subject on separate 24-hour spans. A graded alteration of HRV endpoints was found in association with increased geomagnetic activity. As time-domain measures of HRV, SDNNIDX and the 90% length of the Lorenz plot decreased statistically significantly on days with increased geomagnetic disturbance (p=0.0144 and p=0.0102, respectively). A graded decrease in frequency-domain HRV measures was also validated statistically for the total spectral power (decrease of 18.1% and 31.6% on days when 7< Ap<20 and 20< Ap<45 versus days when Ap<7; p=0.0013). The decrease in spectral power was mainly found at frequencies below 0.04 Hz, in the “ultra-low-frequency” (0.0001-0.003 Hz; 18.1% and 27.5% decrease, respectively; p=0.0102) and “very-low-frequency” (0.003-0.04 Hz; 12.9% and 28.6% decrease, respectively; p=0.0209) regions of the spectrum. The decrease in spectral power was much less pronounced around 10.5 sec (“low frequency”; N.S.) and around 3.6 sec (“high frequency”; N.S.). Evidence is provided here that HRV decreases on magnetically disturbed days, and that it does so in a dose-dependent fashion, HRV being depressed more on days when 20< Ap<45 than on days when 7< Ap<20, by comparison with days when Ap<7. This graded response of HRV to geomagnetic activity should encourage us to search for human magnetoreceptors and for a better understanding of putative mechanisms of magnetoreception. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Geomagnetic activity; Heart rate variability; Graded response; Magnetoreception; Chronoastrobiology; Subarctic area 1. Introduction Influences of geomagnetic activity on biological pro- cesses [1-4] and on the incidence of various human diseases [5-10] have been reported. Putative associations between geomagnetic storms and myocardial infarction or stroke are of particular interest and constitute active areas of current research [11,12]. In Minnesota, mortality from myocardial infarction increases by 5% at times of maximal solar activity [12], when geomagnetic disturbances occur more frequently. Susceptible individuals may then be at an increased cardiovascular disease risk. Heart rate variability (HRV) is known to be a powerful predictor of vascular disease risk in apparently healthy people (as shown in an elderly cohort [13]) as well as in * Corresponding autor. E-mail address: frtotk99@ba2.so-net.ne.jp (K. Otsuka). Biomed Pharmacother 56 (2002) 284s–288s www.elsevier.com/locate/biopha © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 0 7 5 3 - 3 3 2 2 ( 0 2 ) 0 0 3 0 3 - 7