Morning surge in sympathetic nervous activity in the indoor environment during the cold winter season

Hypertension Research https://doi.org/10.1038/s41440-022-01020-0 BRIEF COMMUNICATION Fast track – ISH2022 KYOTO Morning surge in sympathetic nervous activity in the indoor environment during the cold winter season Hironori Nakagami1 Hitoshi Akiyama2 Hiroki Otsuka2 Sho Adnas Takahashi3 Katsutoshi Sawada3 Nobutoshi Kobayashi3 Atsushi Iwamae4 Hidekazu Yamada5 ● ● ● ● ● ● ● 1234567890();,: 1234567890();,: Received: 13 August 2022 / Revised: 24 August 2022 / Accepted: 30 August 2022 © The Author(s), under exclusive licence to The Japanese Society of Hypertension 2022 Abstract We addressed to the sympathetic nervous activation of the same people in both their houses and a highly insulated and airtight model house (model house) during the cold winter season. Eight subjects (4 males and 4 females) stayed two nights at each house and were continuously monitored for sympathetic nerve system by calculating LF (low frequency)/HF (high frequency) in the analysis of heart rate variability using a wearable electrocardiography equipment. The room temperatures were kept constant at 20 °C or more in model house, but much lower in their houses. In all subjects, the sleeping duration is longer in model house compared with that in the participants’ houses. Four subjects showed a morning surge in sympathetic activity that were more intense at their houses. This morning surge in sympathetic activity in a residential setting suggests the importance of the indoor environment in the management of early morning hypertension. Keywords Morning surge Indoor environment Winter season ● ● Introduction Blood pressure (BP) fluctuations including morning surges can be triggered by combination of several stressors, such as physical exertion, and severe emotional and environmental stress. Morning BP surge are augmented in cold seasons, and cold temperature has an impact on the BP surge. We and Supplementary information The online version contains supplementary material available at https://doi.org/10.1038/s41440022-01020-0. * Hironori Nakagami 1 Department of Health Development and Medicine, Graduate School of Medicine, Osaka University, 2-2 Yamada-oka, Suita, Osaka, Japan 2 Asahi Kasei Construction Materials Corporation, 1-105 Kanda Jimbocho, Chiyoda Ward, Tokyo, Japan 3 JSR corporation, 1-9-2 Higashi-Shimbashi, Minato Ward, Tokyo, Japan 4 Department of Architecture, Kindai University Faculty of Architecture, Higashi Osaka, Osaka, Japan 5 Department of Dermatology, Nara Hospital, Kindai University Faculty of Medicine, Nara, Japan others have reported that indoor temperature showed increased blood pressure with fluctuation, which might lead to high cardiovascular mortality in the winter season [1–4]. Activation of the sympathetic nervous system in response to cold temperature increases vascular tone in resistance arteries, and bedtime administration of an α-adrenergic blocker suppresses morning BP surge by reducing resistance [5]. The rhythmic components of heart rate variability can be separated and quantitatively assessed by means of power spectral analysis, which can be associated with autonomic nervous system activity [6, 7]. In this study, we focused on the real time analysis of sympathetic nervous system by frequency domain analysis of heart rate variability signals using wearable electrocardiography devices, when the same subjects spent 2 days each at their home and in a highly insulated, airtight model house (model house). We also measured the accurate and continuous indoor temperature using fixed thermos sensor at living room, bedroom and dressing room. Methods Study protocol This study protocol was approved by the Faculty of Architecture, Kindai University Ethics Committee H. Nakagami et al. Graphical Abstract Morning surge and indoor environment highly insulated and airtight model house morning surge (2020–1). All subjects were enrolled in this study from Dec 2021 to Feb 2022 during winter season. Four subjects (1–4) stayed at their homes for 2 days and then stayed at the model house for 2 days, and the other four subjects (5–8) did the opposite. Basically, they stayed in the indoor environment all day. Because the participants could not be relaxed in a new environment on the first day, we focused on the analysis from the second day in both houses, except for the case of Subject 8 at their house. Subject 8 did not sleep in their home at all on the second day. The highly insulated and airtight model house (model house) and temperature measurements The model house is a two-story wooden detached house with a total floor area of 130.08 m2 located in Sakaimachi, Sashima-gun, Ibaraki Prefecture in Japan (Asahi Kasei Construction Materials Corporation). The house has a UA value of 0.20 W/(m2-K) and a C-value of 0.12 cm2/m2. The UA value indicates the exterior skin average heat transfer coefficient, a standard that indicates the heat insulation performance of a building, with smaller values indicating higher performance; the C-value is the equivalent clearance area, an indicator of the airtightness of a building: the smaller the value is, the higher the performance, which is an index of the degree of high heat insulation and airtightness. This house achieves a comfortable temperature environment by combining a performance that far exceeds the national heat insulation standard with energy-saving equipment. The indoor environments in the subjects’ homes and the model house were recorded by installing data loggers equipped with temperature and humidity sensors in various rooms and taking continuous measurements at 10 min intervals. comfortable temperature Sympathetic nervous activity by frequency domain analysis of heart rate variability The rhythmic components of heart rate variability can be separated and quantitatively assessed by means of power spectral analysis. The powers of high frequency (HF) and low frequency (LF) components of heart rate variability have been shown to estimate cardiac vagal and sympathetic activities, and LF/HF ratio is a marker of sympathetic nervous system activity [6, 7]. Eight subjects (four males and four females) carried portable electrocardiography (Heartnote, JSR corporation, Tokyo) devices that were capable of simultaneously measuring heart rate, CVRR (coefficient of variation of R-R interval), LF, HF and physical activity throughout the day. Sleeping duration was determined by the monitoring of continuous physical activity. Sympathetic activation was defined as showing more than +2 SDs for distribution within an individual in terms of the LF/HF ratio calculated every 10 s. The sympathetic activity analysis of Subject 7 and 8 at first day (model house) was incomplete due to accidental reasons. Results and discussion Regarding indoor temperature measurements, room temperatures were kept constant at 20 °C or more in the model house. However, in the participants’ houses, temperatures were low with large temperature differences between rooms, and (...truncated)