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

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.

Twenty-four-hour pattern in French firemen of lag time response to out-of-hospital cardiac arrest and work-related injury.

Circadian cognitive and physical rhythms plus 24 h patterns of accidents and work-related injuries (WRI) have been verified in numerous studies. However, rarely, if ever, have 24 h temporal differences in both work performance and risk of WRI been assessed in the same group of workers. We explored in a homogenous group of French firemen (FM) 24 h patterns of both lag time (LT) response duration to emergency calls for medical help (ECFM) for life-threatening out-of-hospital cardiac arrests (OHCA), used as a non-specific index of work performance, and WRI. Our studies demonstrate rather high amplitude statistically significant 24 h patterns of the two variables. The LT response duration was twice as long -0500 h (slowest response) than -1600 h (fastest response). In the same group of FM, the actual number WRI/h was greatest -1600 h and lowest in the early morning hours. However, the 24 h pattern of the relative risk (RR) of WRIs, i.e., per clock hour number of WRI/(total number of responses to emergency calls x number of FM at risk per response), was very different, the RR being greatest -0200 h and lowest in the afternoon. The 24 h pattern in LT response duration to ECMH for OHCA and RR of WRI was strongly correlated (r = +0.85, P < 0.01), with the nocturnal trough (slowest response) in LT response duration coinciding with the nocturnal peak RR of WRI. These findings indicate the requirement for circadian rhythm-based interventions to improve the nocturnal compromised work performance and elevated risk of WRI of shift-working FM.

Circadian time organization of professional firemen: desynchronization-tau differing from 24.0 hours-documented by longitudinal self-assessment of 16 variables.

We investigated the circadian synchronization/desynchronization (by field-study assessment of differences in period, τ, of 16 coexisting and well-documented rhythms) of 30 healthy firemen (FM) exposed to irregular, difficult, and stressful nocturnal work hours who demonstrated excellent clinical tolerance (allochronism). Three groups of FM were studied (A = 12 FM on 24-h duty at the fire station; B = 9 FM on 24-h duty at the emergency call center; C = 9 day-shift administrative FM) of mostly comparable average age, body mass index, career duration, chronotype-morningness/eveningness, and trait of field dependence/independence. The self-assessed 16 circadian rhythms were (i) physiological ones of sleep-wake (sleep log), activity-rest (actography), body temperature (internal transmitter pill probe), right- and left-hand grip strength (hand dynamometer), systolic and diastolic blood pressure (BP) plus heart rate (ambulatory BP monitoring device); (ii) psychological ones (visual analog self-rating scales) of sleepiness, fatigue, fitness for work, and capacity to cope with aggressive social behavior; and (iii) cognitive ones of eye-hand skill and letter cancellation, entailing performance speed (tasks completed/unit time) and accuracy (errors). Data (4-6 time points/24 h; 2 591 480 values in total) were gathered continuously throughout two 8-d spans, one in winter 2010-2011 and one in summer 2011. Each of the resulting 938 unequal-interval time series was analyzed by a special power spectrum analysis to objectively determine the prominent τ. The desynchronization ratio (DR: number of study variables with τ = 24.0 h/number of study variables × 100) served to ascertain the strength/weakness of each rhythm per individual, group, and season. The field study confirmed, independent of group and season, coexistence of rather strong and weak circadian oscillators. Interindividual differences in DR were detected between groups and seasons (χ(2), correlation tests, analysis of variance [ANOVA]). Moreover, in each group, both in winter and summer, a normal distribution was observed in the number of FM with rhythms with τ = 24.0 h, e.g., ranging from 5/16 (large desynchronization) to 16/16 (no desynchronization). Such a normal distribution with intraindividual stability over time (i.e., seasons) is consistent with the hypothesis of an inherited origin of a differential propensity to circadian desynchronization and which is supported by the distribution of τs in winter and summer following the Dian-Circadian Genetic Model, i.e., with τ = 24.0 h, τ = 24.0 h + n(0.8 h), and τ = 24.0 h - n(0.8 h).

24-hour pattern of work-related injury risk of French firemen: nocturnal peak time.

The first aim of the study was to assess clock-time patterning of work-related injuries (WRIs) of firemen (FM) of Saône et Loire-71 (France) during the 4-yr span of 1 January 2004 to 31 December 2007. FM of this service are legally required to log every WRI and seek its evaluation by the medical service, whether the WRI was the result of worksite duties or exercise/sport activities at the station. WRI was defined specifically as a (nonexercise, nonsport, and nonemotional/stress) work-associated trauma, verified both by log book and medical records. For the corresponding years, the 24-h pattern of emergency calls (Calls) plus road traffic (Traffic) on the main roads of the service area was also assessed. Relative risk (R) of WRI was calculated as the quantity of WRIs/h divided by the quantity of Call responses/h × 1000, which takes into account the number of at-risk FM/unit time, since each dispatched emergency vehicle is staffed with 4 FM. Comparably trained regular (RFM) and volunteer (VFM) FM experienced a total of 187 WRIs. The 24-h WRI curve patterns of RFM and VFM were correlated (r = 0.4, p < .05), with no histogram difference (p > .05). Analysis of variance (ANOVA) validated comparable clock-time patterns in WRIs of RFM and VFM each year and each season (all p < .0001). Thus, time series of the RFM and VFM were pooled, revealing a statistical significant 24-h variation in WRIs (ANOVA, p > .0006; Cosinor analysis, p < .0001), with peak at 16:00 h and trough at 04:00 h. The 24-h pattern in Traffic, which mirrors that of human activity, with peak ∼18:00 h and trough ∼03:00 h, was also verified (ANOVA, p < .0001; Cosinor, p < .0001). Calls (n = 112,059) resulting in FM responses also exhibited statistically significant 24-h variation, with peak at ∼20:00 h and trough at ∼06:00 h. The 24-h pattern of R showed a nocturnal peak at 02:00 h (R = 2.87 ± 0.46; mean ± SEM) and diurnal trough 14:00 h (R = 1.30 ± 0.05) (t test, p < .02); clock-time-related changes in R were further validated by ANOVA (p = .0001) and Cosinor (p < .0001), with acrophase (peak time, Ø) of 02:43 h ± 68 min (SD). The second aim of the study was to evaluate the relationship between the 24-h patterns of WRIs and lag-time (LT) response (used as a measure of work performance) of FM of the same service to urgent medical calls for out-of-hospital cardiac arrests. Highest R of WRI at 02:00 h corresponded closely to longest LT (raw data at ∼02:00 h and Cosinor derived Ø of 02.54 h ± 71 min [SD]), thereby supporting the hypothesis of a common mechanism underlying the two 24-h profiles. A third aim was to determine the relevance of a new concept in work safety, "chronoprevention," for future FM training programs.

24-hour pattern in lag time of response by firemen to calls for urgent medical aid.

The aim of the study was to assess the group 24-h pattern of lag time (LT) in response by regular and volunteer firemen (RFM and VFM) to calls for medical help (CFMH), specifically calls for out-of-hospital cardiac arrest (OHCA). LT, duration in min between a CFMH and departure of service vehicle equipped with a semiautomated defibrillator and generally staffed with four well-trained and ready-to-go FM, represents the integrated duration of several processes, each with separate reaction and decision-making times. The exact time of each CFHM (in min, h, day, month, yr) was recorded electronically, and the exact departure time from the station of the responding FM vehicle was recorded by an on-duty FM. Overall, CFMH made up 53 ± 9% (SEM) of all emergencies calls for aid. To standardize the study methods, the reported findings are based on 568 CFMH specifically regarding OHCA that occurred during the 4-yr study span (January 2005 to December 2008). CFMH exhibited a 24-h pattern with a major peak at 10:00 h (mean ± SEM: n = 9.5 ± 1.6) and major trough at 01:00 h (n = 1.3 ± 0.3; t test, p < .001). From year to year and season to season, a 24-h pattern was detected in the total of CFMH/h with two peaks (∼10:00 and ∼17:00 h) and two troughs (∼01:00 and ∼15:00 h) (analysis of variance [ANOVA], p < .01; Cosinor, p < .05 to < .003), with neither season- nor year-related differences (χ(2), p > .05). In CFMH/h pooled time series, ANOVA-detected differences between the hourly means (p < .01), and Cosinor analysis validated a 24-h rhythm (p < .002). In raw data, the longest LT, indicative of poorest performance, occurred at 05:00 h (8.8 ± 0.7 min) and the trough of LT, indicative of best performance, at 16:00 h (4.3 ± 0.8 min (t test, p < .02). 24-h patterning in LT was validated both by ANOVA of hourly means (p < .0006) and Cosinor analysis (p < .05), with longest LT ∼05:00 h and shortest LT ∼16.00 h for data of the individual yearly time-series data. The 24-h LT rhythm was also validated in the pooled time series by Cosinor (p < .0001), with the 24-h mean ± SEM = 6 ± 0.17 min and acrophase (peak) of 03:00 h ± 88 min (SD). Curve patterns of CFMH/h and LT/h differed widely. As a group phenomenon, the LT 24-h rhythm mimics the 24-h pattern of performance, as demonstrated by many laboratory and field investigations. The stability of the LT rhythm between years and seasons and its weak relationship with the CFMH 24-h pattern favors the hypothesis of an endogenous component or origin. The nighttime trough of performance is presumably linked to the elevated risk of work accidents in the same population of FM.