Effects of Strain on Cardiovascular System Activity in Human-Operators at Different Times of Day and Working Week under Round-the-Clock Industry

Bobko, Natalia

Institute for Occupational Health / Saksagansky St., 75 / 01033 Kiev Ukraine

+380 44 2894605 / Natalia@ioh.freenet.kiev.ua

ABSTRACT

Blood pressure and heart rate were measured in electricity distribution network controllers working 12-hour shifts under 2-day rotation (3050 subject-observations). Perceived strain was self-estimated using a 5-anchor scale. Strain at the day shifts was found to be maintaining mainly by vascular part of bloodcirculation selfregulation, at the night shifts – by cardiac part, probably, owing to the attenuation of vascular regulation at night. Paradoxical character of blood pressure reaction to strain increase was revealed at the second consecutive shifts evidencing the attenuation of vascular regulation under remained fatigue after the first shifts. Start level of cardiovascular system functioning plays basic role to maintain the requested working strain at night shifts, while at the day shifts its current functional reserves are in active use as well. Hence, the necessity of complete rest of human-operators on the eve of the night and second consecutive 12-hour shifts has to be of a special importance.

Keywords

cardiovascular system, shift work, time of day, time of working week, strain

INTRODUCTION

Working strain leads to an increase in cardiovascular system (CVS) activity that is necessary to maintain doing the professional duties [12]. However, human being is a daytime species and his/her nighttime activity contradicts the natural going of a body clock. After night shifts, the predominance of the parasympathetic activity over the sympathetic one has been revealed [13] that is linked in literature to a strain increase in CVS and sympathetic-adrenal system activities [16], with a decrease in adaptive reserves of CVS and deterioration in nervous system state [18]. During sleep, after night working, cardiac sympathetic activity increases while parasympathetic activity decreases delaying circadian phase in a cumulative manner the more significantly the more number of consecutive night shifts were worked [6,7]. Shiftwork could lead to the smoothing or inverse of circadian rhythms in CVS activity parameters as well as other body systems [3,4,9,14,17] that is an unfavourable index of human health as a prognosis [11,19]. Increased functional strain in shiftworkers has been accompanied with the increased morbidity including CVS pathology development [5,10,15].

The purpose was to reveal the differences in the effects of strain on cardiovascular system activity in human-operators at different times of 24-hour day and working week under round-the-clock industry.

METHODS

Heart rate (HR) and both systolic and diastolic blood pressures (BPS and BPD) were measured in electricity distribution network controllers at their work places at the beginning and end of 12-hour shifts. Subjects worked over 8-day week periods under 2-day shift rotation: D12, R12, D12, R48, N12, R12, N12, R72, where D – day shift 8:00-20:00h, N – night shift 20:00-8:00h, R – rest; the figure means the number of hours. Altogether 1826 subject-observations were completed in 19 controllers (by 2 times per 913 subject-shifts).

The same measures were conducted every 2 hours during 3 working weeks in each of 17 controllers. Altogether 1224 subject-observations were completed (17 controllers by 6 times during 12 working shifts ((2 night shifts + 2 day shifts) * 3 weeks) ).

Perceived strain was estimated by controllers using 5-anchor scale in the end of each working shift.

Eight haemodynamic parameters were calculated for each observation: Pulse Pressure PP=BPS-BPD; Middle-Dynamic Pressure MDP=0.42PP+BPD; Heart Volume HV=100+0.5PP-0.6BPD-0.6*Age; Circulatory Minute Volume CMV=HV*HR; Periphery Vascular Resistance PVR=(MDP*1333*60)/CMV; Insufficient Bloodcirculation Index IBI=BPS/HR; Kerdo's Vegetative Index KVI=(1-BPD/HR)*100% [1,8]; Index of difference ID between CMV and PVR indices.

All studied parameters were recalculated as a percentage of overall means of the individual data. Effects of strain on CVS work parameters in the end of the shifts, their differences between the beginning and the end of the shifts as well as on within- shift variations in parameters were analysed using MANOVA (multivariate Pillai’s test) and ANOVA at a p-value of no less than 0.05.

RESULTS AND DISCUSSION

Within the series of observations where measurements were acquired at the beginning and end of working shifts, there were found 90 working shifts of relatively low strain, 289 shifts of less than average strain, 417 shifts of average strain, 98 shifts of higher than average strain and 13 shifts of very high working strain. Amongst the 1st day shifts respectively: 22, 68, 120, 31 and 3 shifts (altogether 244 shifts),amongst the 2nd consecutive day shifts – 15, 62, 90, 35 and 6 shifts (altogether 208shifts), amongst the 1st night shifts – 32, 79, 107, 23 and 3 shifts (altogether 244shifts), amongst the 2nd consecutive night shifts – 21, 80, 100, 9 and 1 shift(altogether 211 shifts).

Schedule of labour process of electricity distribution network controllers provides for taking and transmitting of a working shift that is accompanied with an increased strain of psychophysiological functions in workers at the beginning and end of a shift. In this, BPS and BPD at these periods were higher compared to the middle period of a shift, while HR was higher only at the beginning of a shift [2]. Comparison of data received under taking-transmitting of a shift showed that BPS, BPD, MDP, PRV, IBI and ID in the end of a shift were higher compared to the beginning of it while HR, HV, CMV and KVI – lower (df=1,1817;p<0.0001).

Interactive changes in CVS activity were found to be coupled with level of perceived strain for both indices – shift-end score and difference between the beginning and the end of the shifts in CVS work parameters (Pillai’s test: (1) shift-end changes in all 11 parameters F(44,3576)=2.46, p<0.0001; in 3 registered parameters

F(12,2706)=3.08, p<0.0005; in 8 calculated haemodynamic parameters F(32,3588)=2.72, p<0.0001; (2) difference between the beginning and the end of the shifts in all 11 parameters F(44,3580)=1.99, p=0.0003; in 3 registered parameters F(12,2700)=2.61, p<0.003; in 8 calculated haemodynamic parameters F(32,3584)=2.38, p<0.01).

Increased level of working strain was related to shift-end increase in BPS (p<0.0001), BPD (p<0.004), HR (p<0.040), PP (p<0.009) and MDP (p<0.0001). Fromthe beginning till the end of the shifts increased the rise in BPS (p<0.005), PP (p<0.05), MDP (p<0.004), BPD (only under higher than average strain, p<0.03) and decreased the fall in HR (p<0.040). These findings could reflect the activation in CVS activity and could evidence the increase in physiological cost of the work done.

Interactive changes in CVS activity and also significant (p<0.05) changes in 2-3 parameters of CVS activity were found in the end of both day shifts as a reply to working strain variations (table 1). In the 2nd day shifts these changes were pronounced to the less extent, obviously, owing to the masking effect of the remained fatigue after the 1st day shift. No interactive changes in CVS activity were found in the end of night shifts. However, 2 parameters changed significantly. Probably, functioning of central regulatory mechanisms to maintain working strain in the end of night shifts becomes complicated to a considerable extent, maybe, as a result of superposition of fatigue and morning activation within a body.

Table 1. Effect of strain on CVS parameters in the end of the shifts

“U” – univariate test, “M” – multivariate test, “*” – effect of strain at p<0.05.

Interactive changes in CVS activity as well as significant changes in 3-4 parameters of it from the beginning till the end of the shifts were found to be coupled with variations in strain level at the 1st day shifts and both night shifts (table 2). No effect of strain on CVS parameters from the beginning till the end of the 2nd day shifts was found. Probably, at the 1st day shifts and both night shifts, the start level of CVS functioning is of importance to maintain the requested increased strain (in particular – during taking-transmitting of a shift) and working of central regulatory mechanisms. The masking effect of the remained fatigue after the 1st day shift could smooth such effect in the 2nd day shifts.

Table 2. Effect of strain on CVS parameters difference in the beginning and in the end of the shifts

Designations see in the table 1.

Furthermore, it is worth to note, that to maintain the requested working strain in the day shifts, mainly BP and related calculated haemodynamics parameters were found to be involved (table 1 and 2) while in the night shifts – HR and related parameters. Hence, maintenance of the working strain in the day shifts from CVS is provided mainly by vascular part of bloodcirculation selfregulation while in the night shifts – by heart part. Evidently, remained fatigue following the 1st 12-hour shift leads to the attenuation of the resources in the vascular part of bloodcirculation selfregulation to maintain working strain that considerably tells on the scheme of CVS reply to the working strain changes in the 2nd consecutive 12-hour day shifts and little tells – in the 2nd night shifts.

Within the series of observations where measurements were acquired each 2 hours during working shifts, there were found 15 working shifts of relatively low strain,79 shifts of less than average strain, 88 shifts of average strain, 16 shifts of higher than average strain and 5 shifts of very high working strain. Amongst the 1st day shifts respectively: 3, 22, 22, 3 and 1 shift (altogether 51 shifts), amongst the 2ndconsecutive day shifts – 5, 19, 20, 4 and 2 shifts (altogether 50 shifts), amongst the1st night shifts – 5, 18, 19, 7 and 2 shifts (altogether 51 shifts), amongst the 2ndconsecutive night shifts – 2, 20, 27, 2 and 0 shifts (altogether 51 shifts).

Perceived strain variations were found to be accompanied with the most pronounced changes in CVS activity in the day shifts – both interactive changes and changes in near each parameter, while in the night shifts only some interactive changes in CVS activity were found, and also in the 2nd night shifts – in 3 parameters (table 3).

Probably, in the day shifts along with start level of CVS functioning (in the 1st day shifts, table 2) the current body possibilities at each real time period also are the basis to maintain working strain (in both the 1st and 2nd day shifts, table 3) that is biorhythmically provided by the resources of body functioning during active phase of 24-hour cycle. Also, in the day shifts both central (due to the multivariate test) and

peripheral (due to the univariate test) mechanisms in CVS work regulation play important role. However, during the night shifts the working strain is maintained mainly by the central mechanisms in haemodynamic selfregulation. In this, start level of CVS functioning is of special importance for working strain maintenance at the night shifts (effect of strain on CVS functioning at the night shifts was found to be more pronounced than at the day shifts in the table 2 only). Probably, current functional body possibilities at night are weak to maintain the requested level of the working strain.

Table 3. Effect of strain on changes in CVS parameters during the shifts

Designations see in the table 1.

Visual data analysis showed that an increase in a working strain at the 1st day shifts led to the increase in the minimum reached scores of BPS and BPD during the shifts, but did not influenced HR. [The working shifts of low and less than average strain were summed up because of small number of low strain shifts; the working shifts of higher than average and very high working strain were expulsed from analysis because of small number of them even as a sum]. This evidences the adequate increase in CVS activation to maintain the professional working under increase in production demands – mainly for the account of vascular part of bloodcirculation selfregulation. In the 2nd consecutive day shifts under low working strain, BPS and BPD were found at the same levels as under average strain in the 1st day shifts. Increase in strain to the average level led to the decrease in BP, that was inadequate reaction and showed the difficulties of such conditions for physiological maintenance of professional activity. This paradoxical effect could be caused by the attenuation of the resources of vascular bloodcirculation selfregulation during the 1st 12-hour shifts and their under- recovery for the short rest between consecutive shifts. Significant decrease in HR at the 2nd day shifts of low and lower than average strain – near to the level of the night shifts, and increase of HR at the 2nd day shifts of the average strain – to the level of the 1st day shifts, probably, are of compensatory character.

At the night shifts, physiological maintenance of working strain was realising mainly for the account of the increase in the minimum reached scores of HR, in the 1st night shifts – also BPS, and did not effect BPD, and in the 2nd night shift – BPS. Hence, working strain in the night shifts is maintained mainly by vascular bloodcirculation

selfregulation, that is especially pronounced in the 2nd night shifts – against the background of the remained fatigue after the 1st 12-hour shifts. Possibly, known from the literature the increased tiredness of the night works it is worth to link to the biorhythmically caused attenuation of vascular part of bloodcirculation selfregulation at night.

CONCLUSIONS

So, an increase in a working strain leads to the increase in CVS activity, which is realising by different mechanisms at different times of day and working week. Maintenance of the working strain in the day shifts is provided mainly by vascular part of bloodcirculation selfregulation, in the night shifts – by cardiac part, that is probably connected with biorhythmically caused attenuation of vascular part of bloodcirculation selfregulation at night.

Effect of working strain on CVS in operators interacts with an effect of fatigue. Partially, remained fatigue after the 1st 12-hour shift leads to the paradoxical reaction from BP to working strain increase in the 2nd consecutive day shifts and to the smoothed reaction from BPS – in the 2nd night shift. This allows us to link the state of the remained fatigue to the attenuation primary of vascular part of bloodcirculation selfregulation.

To maintain the periods of the increased working strain in the night shifts, the start level of cardiovascular system functioning plays the basic role, while at the day shifts its current functional reserves are in active use as well. The last circumstance is biorhythmically maintained by the resources of body activity during the active phase of 24-hour cycle.

Thus, principal differences in physiological maintenance of the operator’s working strain from CVS that have been revealed at different times of day and working week, probably, evidence quite high physiological price of night working and 2nd consecutive 12-hour shifts. In connection with this, the necessity of complete rest of human-operators on the eve of the night and 2nd consecutive 12-hour shifts has to be of a special importance.

ACKNOWLEDGEMENTS

The assistance and support of staffs at the Controllers’ Services of the Central Regional Controllers’ Centre of Ukraine and the Kiev-Energy Centre are gratefully acknowledged. Thanks are also due to the research technicians Mrs. O.V. Buntovaya and Mrs. O.N. Alekseyeva for their assistance.

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