Do night and around-the-clock firefighters’ shift schedules induce deviation in tau from 24 hours of systolic and diastolic blood pressure circadian rhythms?

ABSTRACT

Systolic (S) and diastolic (D) blood pressures (BP) [SBP and DBP] are circadian rhythmic with period (τ) in healthy persons assumed to be maintained at 24.0h. We tested this assumption in a sample of 30 healthy career (mean >12 yrs) 30-to-46 yr-old male Caucasian French firefighters (FFs) categorized into three groups according to work schedule and duties: Group A – 12 FFs working 12h day, 12h night, and occasionally 24h shifts and whose primary duties are firefighting plus paramedical and road rescue services; Group B – 9 FFs working mostly 12h day and 12h night shifts and whose duties are answering incoming emergency calls and coordinating service vehicle dispatch from fire stations with Group A personnel; Group C – 9 day shift (09:00–17:00h) FFs charged with administrative tasks. SBP and DBP, both in winter and in summer studies of the same FFs, were sampled by ambulatory BP monitoring every 1h between 06:00–23:00h and every 2h between 23:01–05:59h, respectively, their approximate off-duty wake and sleep spans, for 7 consecutive days. Activity (wrist actigraphy) was also sampled at 1-min intervals. Prominent τ of each variable was derived by a power spectrum program written for unequal-interval time series data, and between-group differences in incidence of τ≠24h of FFs were assessed by chi square test. Circadian rhythm disruption (τ≠24h) of either the SBP or DBP rhythm occurred almost exclusively in night and 24h shift FFs of Group A and B, but almost never in day shift FFs of Group C, and it was not associated with altered τ from 24.0h of the circadian activity rhythm. In summer, occurrence of τ≠24 for FFs of Group A and B differed from that for FFs of Group C in SBP (p=0.042) and DBP (p=0.015); no such differences were found in winter (p>0.10). Overall, manifestation of prominent τ≠24h of SBP or DBP time series was greater in summer than winter, 27.6% versus 16.7%, when workload of Group B FFs, i.e. number of incoming emergency telephone calls, and of Group A FFs, i.e. number of dispatches for provision of emergency services, was, respectively, two and fourfold greater and number of 12h night shifts worked by Group B FFs and number of 24h shifts worked by Group A FFs was, respectively, 92% and 25% greater. FFs of the three groups exhibited no winter-summer difference in τ≠24h of SBP or SDP; however, τ≠24h of DBP in Group B FFs was more frequent in summer than winter (p=0.046). Sleep/wake cycle disruption, sleep deprivation, emotional and physical stress, artificial light-at-night, and altered nutrient timings are hypothesized causes of τ≠24h for BP rhythms of affected Groups A and B FFs, but with unknown future health effects.

KEYWORDS:

• Firefighters
• circadian disruption
• circadian τ desynchronization
• diastolic blood pressure
• shift work schedule
• systolic blood pressure
• workload

Acknowledgments

We acknowledge Colonel Michel Marlot of the SDIS-71 who passed away early in 2017 for facilitating the conduct of this biological rhythm investigation and the 30 dedicated French firefighters under his command for their dedicated and compliant participation. Special thanks are due also to Mohamed Mechkouri for his technical assistance with data processing, Germaine Cornelissen for her advice, assistance, and review of the statistical analyses and manuscript content, and Linda Sackett-Lundeen for her expert proof reading of the manuscript and references for accuracy.

Funding

This research was supported by a grant-in-aid from the “Fond National de Prévention” (CNRACL) and Thérèse Tremel – Pontremoli Donation at the Fondation Adolphe de Rothschild for research in chronobiology.

Additional information

Funding

This research was supported by a grant-in-aid from the “Fond National de Prévention” (CNRACL) and Thérèse Tremel – Pontremoli Donation at the Fondation Adolphe de Rothschild for research in chronobiology.