Bioelectromagnetics 28:76^79 (2007)
Brief Communication Changes in Heart Rate Variability among RF Plastic Sealer Operators Jonna Wile¤n,1* Urban Wiklund,2 Rolf Ho«rnsten,3 and Monica Sandstro«m1 1
2
National Institute for Working Life, Ume, Sweden Department of Biomedical Engineering & Informatics, University Hospital, Ume, Sweden 3 Department of Clinical Physiology, University Hospital, Ume, Sweden
In a previous study, we showed that operators of radiofrequency (RF) plastic sealers, RF operators (n ¼ 35) had a lower heart rate during nighttime compared to a control group (n ¼ 37). We have analyzed the heart rate variability (HRV) on the same group of people to better understand the possible underlying rhythm disturbances. We found a significantly increased total HRVand very low frequency (VLF) power during nighttime among the RF operators compared to a control group. Together with our previous finding of a significantly lower heart rate during nighttime among the RF operators compared to the controls, this finding indicates a relative increase in parasympathetic cardiac modulation in RF operators. This could in turn be due to an adaptation of the thermoregulatory system and the cardiac autonomic modulation to a long-term low-level thermal exposure in the RF operators. Bioelectromagnetics 28:76–79, 2007. 2006 Wiley-Liss, Inc. Key words: occupational exposure; radiofrequency fields; cardiovascular effects; thermoregulation; HRV; autonomic nervous system
In a previous study [Wile´n et al., 2004], we showed that operators of radiofrequency (RF) plastic sealers (RF operators) had a significant lower heart rate measured during 24 h, more episodes of bradycardia and arrhythmia compared to a control group. In this study, we hypothesized that our previous findings in RF operators are due to a dysregulation in the cardiac autonomic modulation and analyzed their heart rate variability (HRV). Detailed information about the selection procedure is found in Wile´n et al. [2004]; below only a short description of the methods are given. We established contact with a number of RF welding industries using RF plastic sealers on a daily basis. We used a cross-sectional study design where both the RF operators and controls were selected with the criteria that they did not have a diagnosed heart disease, diabetes, or be on heart medication. The controls were selected from the same company as the RF operators or at least from companies in the same neighborhood. They also were of the same age and gender distribution as the RF operators. In total, nine companies were selected, five with both RF operators and controls, two with RF operators, and two with controls only. The mean value (and standard deviation (SD)) of the spatially averaged electric field strength was 88 (102) V/m for the RF operators. The corresponding values for the magnetic field strength was .19 (.19) A/m. The electrocardiogram (ECG) was 2006 Wiley-Liss, Inc.
recorded during a 24-h period during an ordinary working day for the RF operators as well as controls, using a ambulatory Holter recorder (Braemar DL700, Braemar, Inc., Eagan, MN). The system was checked in our laboratory during similar RF exposure conditions to those in the RF plastic welding industry, but was not affected by the exposure. The recording started in the morning and ended approximately at the same time the next day. The analysis of the HRV [Camm et al., 1996] was performed using commercially available software (Danica Holter Replay Unit, Danica Biomedical, Sweden). R–R interval data were calculated from accepted beats, which were manually edited and confirmed. Power spectral analysis was done by means of the Fast Fourier Transformation algorithm, using the Welch method. Total power, as well as the power of the very low frequency ————— — Grant sponsor: Swedish Council for Work Life Research (RALF). *Correspondence to: Jonna Wile´n, National Institute for Working Life, PO Box 7654, S-907 13 Umea˚, Sweden. E-mail:
[email protected] Received for review 8 November 2005; Final revision received 23 May 2006 DOI 10.1002/bem.20276 Published online 26 September 2006 in Wiley InterScience (www.interscience.wiley. com).
HRVamong RF Operators
(VLF; .003–.05 Hz), low frequency (LF; .05–.15 Hz), and high frequency (HF; .15–.40 Hz) components were calculated for each hour (e.g., from 08:00–09:00) of the 24-h recording. The normalized high frequency power HFN ¼ HF/(total powerVLF) value was also calculated. Hourly averages were considered ‘‘missing’’ if they were based on less than 30 min of data with acceptable technical quality. The statistical analysis was made on data from 3 P.M. to 6 A.M., the time period when data were available for all subjects in the study. All frequency domain HRV indices were log-transformed. Group differences in hourly averages were analyzed by repeated measures analyses of variance (ANOVA) with GROUP, AGE, and TIME as factors. Post-hoc comparisons were performed by Student’s t-tests (two-sided) for variables where there was a statistically significant interaction between the GROUP and TIME factors. The post-hoc tests were based on means and SDs after adjustments for age. Associations between exposure variables and HRV were analyzed using multiple linear regression. Statistical significance was defined as a P
