Does shift work increase the levels
of blood pressure and cholesterol among hospital nurses? A historical cohort
study
Vazirinejad R,
PhD 1*, Esmaeili A, PhD 2, Hassanshahi GH, PhD 3,
Razi L 4
1- Associate Prof., Dept. of Epidemiology, School
of Medicine, Occupational Environmental Research
Center, Rafsanjan University of Medical Sciences, Rafsanjan, Iran. 2. Associate
Prof., Dept. of Social Medicine, Medical
School, Rafsanjan University of Medical Sciences, Rafsanjan, Iran. 3- Associate
Prof., Dept. of
Molecular Medicine Research Centre , Medical School, Rafsanjan University of
Medical Sciences, Rafsanjan, Iran. 4- Medical student, Rafsanjan University of Medical Sciences,
Rafsanjan, Iran.
Abstract:
Received: July 2011, Accepted: October
2011
|
Introduction: Controversies exist in publications about the association
between shift work and hypertension. This study aimed to explore the
relationship between shift work and the level of systolic and diastolic blood
pressure as well as serum cholesterol among nurses in Kerman province,
Southeast, Iran. Materials
and Methods: In this historical cohort study 100
hospital nurses who had been employed for more than five years were
recruited. Respondents were divided into two groups based on their working
schedule (shift work/day work). After one year follow up, both systolic and
diastolic blood pressures were measured. A blood sample was also taken for
recording the level of serum cholesterol. Data were analysed
using SPSS (version 14) and paired t-test and Mann Whitney U test and
relative risk with 95% CI was also calculated. Results:
Demographic variables as well as the mean working years between shift working
(10.98±3.6yrs) and day working (11.18±4.4yrs) nurses were not significantly
different. The results of age-adjusted relative risk calculation showed that
even after adjusting for respondents’ age, there was not any significant risk
of suffering from higher blood pressure among shift workers. The mean levels
of cholesterol in the serum of shift workers (192.18± 43.6) and day workers
(186.7 ±47.7) were not significantly different. Conclusions:
Our findings confirmed the previous studies’ results that reported no
significant association between shift working and the level of blood pressure
among nurses. More investigations are needed to explore factors, which could
increase the risk of hypertension and cardiovascular diseases among shift
working nurses. |
Keywords: Shift Work, Systolic, Diastolic, hypertension,
Cholesterol, Nurses
Introduction
In the last few decades,
the new life style resulting from modern technology has changed the patterns of
morbidity and mortality rates of diseases in the human communities. Disorders
including cardiovascular diseases are known as the main group of such resulting
health problems. Hypertension has been known as the most effective element in
developing cardiovascular disorders. On the other hand, investigations have illustrated
that shift work is a risk factor for generating this group of disease [1-2].
Saksvik and[*]colleagues explained that
“shift work implies any work organization of working hour that differs from the
traditional diurnal work period” [3]. Kawachi says “Shift work
refers to work patterns that extend beyond the conventional 8-hour
work day and that potentially disrupt workers' normal biological
and/or social diurnal rhythms” [1].
Studies have confirmed that shift work may cause several health
problems for working people. Broad aspects of employees’ life can be affected
by shift work including physical and mental health, safety, social life and
work performance/effectiveness. Saksvik and colleagues believe that while some
employees develop serious problems due to shift work exposure, some others
tolerate the exposure to shift work well [3]. This shows that people react to
the potential challenges that come from shift work in different ways. The
theory of “web of causation” shows that a group of effective risk factors come
together to generate suitable situation for creation of a disease. Regarding
this theory, individual ability to adapt to shift work without adverse
consequences depends on many variables including social [4] and cultural
status. For instance, individual feelings and satisfaction about both private and
social lives could affect individual ability to adapt to shift work without
adverse consequences. Andlauer and colleagues explain that shift work tolerance
is associated with behavioural and biological dispositions such as digestive
troubles, persisting fatigue and sleep alterations [5].
Therefore, investigations are needed to explore the level of risk
for health problems in order to decrease the adverse effect of shift work in
different communities. The results would help with determining the most suitable
plan for people who are working in a definite shift-work job. Nurses
working in hospital are a group of employees who most often must work in shift
work hours. Despite the fact that the majority of shift workers such as firefighters, factory workers, taxi drivers,
cooks, and security personnels are men, women most often work in these
jobs, too. women face the risk of job-related health problems, but few studies
have worked on the adverse disadvantages of shift work among this group. De
Gaudemaris and colleagues found a significant difference of blood pressure
between the two groups of employees with and without shift work. Nevertheless,
they concluded that poor relationships within the two groups are related to
high blood pressure among hospital workers [6].
Identifying individual
determinants of shift work tolerance is therefore important not only for
personnel selection purposes, but also to make a base for new research
exploring different shift work systems and work conditions suitable for different
individuals. Although, many investigations reported the association between
shift work and the generation of heart diseases [2, 4, 7-11], Kawachi and
colleagues criticized the results of cross-sectional or retrospective investigations,
which reported a higher prevalence of coronary risk factors among rotating
shift workers [1]. Controversies also exist in publications about the
association between shift work and hypertension [6, 9-12]. Therefore, in this
study, we decided to conduct a historical cohort study exploring the
relationship between shift work and the levels of systolic blood pressure
(SBP), diastolic blood pressure (DBP) and serum cholesterol among nurses
working in an educational hospital in Kerman province, Southeast of Iran.
The
setting in this historical cohort, was Ali-ebn Abitaleb Hospital in Rafsanjan
where there are about 2000 people employed. Out of this number, 320 are nurses
with an age range of 22 to 57 years, who are engaged in nursing work. For this
cohort study, a sample size of 50 was calculated based on the data derived from
a pilot study for each group (a=0.01,
β=0.2). Therefore, a cohort of 100 nurses who were working in the hospital
was recruited for the study. A written consent form was taken from those who
accepted to help with the study after receiving all details about the study
methods and objectives. Respondents were also asked for the permission to
review their medical records. Medical records were reviewed by
physicians who were blinded to exposure status.
Respondents
were ensured that their information would remain anonymous and the data would
be kept in a safe place and would not be used for any purposes other than for
the present study. An approval was also taken from the ethics committee of the
university. Respondents were divided
into two groups based on their working status (day working/shift working). The
two groups were matched based on age, gender, educational status, marital
status, their body mass index (BMI), smoking, and the number of years they had
been working as nurses. All respondents were working in the hospital for at
least five years. There was no history of hypertension and/or other medical
problems in the file of respondents. From the starting point of the study,
respondents were followed up for one year. After one year, both SBP and DBP
were measured in three occasions with one week intervals. A blood sample was
also taken from respondents in each occasion of measuring blood pressure for
recording the level of serum cholesterol (three samples for each respondent). An average was calculated for both
systolic and diastolic blood pressures as well as for the level of serum
cholesterol. Study
checklist was designed for recording some demographic information as well as
the data collected from measuring blood pressure and serum cholesterol. This
study checklist was completed by a trained physician. Nurses who worked at least 3 nights per month in addition to days or evenings
in that month were allocated to the shift working group. Shift work was defined in this study as “working
between 7 pm and 7 am”.
Respondents were asked to have a rest
for at least five minutes just before reading their blood pressure. They were
also sitting down, preferably at a desk or a table, in a quiet place, with
their arms resting on a firm surface. It was important that respondents’ arm
was supported so that the cuff around the arm was at the same level as their
heart. In each occasion respondents’ blood pressure was measured three times
each about two minutes apart and the highest read was recorded. The other
important points in standard blood pressure reading were also regarded [13]. Data
were analysed using SPSS (version 14), parametric tests (such as paired t-test).
Non-parametric tests (such as Mann Whitney U test) were used for comparing the
level of SBP, DBP and serum cholesterol in the two groups. For the purpose of
comparing the level of blood pressure, we used the mean level of SBP and DBP as
well as the mean level of serum cholesterol among the two groups. Although, for
calculation of the relative risk of hypertension incidence among shift working
respondents, those respondents who had an average of SBP more than 140 mmhg,
and/or DBP more than 90 mmhg were considered as hypertensive respondents. The results
of this work are derived from a medical student's thesis.
Results
Out of 350 nurses who were working in
the hospital, a cohort of 100 nurses who were similar based on some effective
variables which were also divided into the two groups of day working (routine)
and shift working was recruited. We compared the characteristics of the 50
nurses who were shift working with the 50 nurses who were day working. There
was not any smokers (or ex-smokers) in the cohort, and variables of age,
gender, marital status, educational status, BMI (body mass index), and the
number of years they were working as nurses, were not significantly different
between the two groups (table 1).
In terms of educational status, there
were three levels of this variable among respondents including MSc, BSc and associate
degree. There were four nurses with MSc degree of which three were working as
routine and remaining one had shift work. Despite this discrepancy, there were
not any significant differences between the two groups in terms of educational
status. About three quarter of the
respondents in both groups (74%) were single and this proportion was
identically equal between the two groups.
Table 1: Demographic
characteristics of respondents in the two groups of shift working and day
working nurses.
|
Characteristics |
Groups |
|||
|
Shift working |
Day working |
|||
|
N % |
N % |
|||
|
Age <30 30-39 +40 |
14 21 15 |
28 42 30 |
11 24 15 |
22 48 30 |
|
Gender female male |
17 33 |
34 66 |
17 33 |
34 66 |
|
Education
status MSc BS Bachelor* |
1 33 16 |
2 66 32 |
3 32 15 |
6 64 30 |
|
Marital
status Single Married |
38 12 |
76 24 |
38 12 |
76 24 |
|
BMI <20 +20 |
21 29 |
42 58 |
25 25 |
50 50 |
|
Working
years** <10 10-19 +20 |
29 9 12 |
58 18 24 |
25 10 15 |
50 20 30 |
* The first university’s degree which is taken after 2-3 years study in the university
** The number of years respondents were working as nurse
BMI was calculated for all
respondents based on their weight and height. Although, the mean of BMI between
the two groups was not identically similar, the two groups were not
significantly different based on this variable. As it is illustrated in table1,
BMI in 21 nurses (42%) in shift working group was less than 20, whereas this
number among day working nurses was 25 (50%). The average of working years for
respondents in the two groups of shift working and day working nurses were
10.98±3.6 and 11.18±4.4 years, respectively.
These two averages were not significantly different. The average levels
calculated from three occasions measuring DBP, were 77.2±8.2 and 79.4±6.7 mmhg among
routine (day) and shift working respondents, respectively.
These levels for SBP among the two
groups of day working and shift working respondents were 119.9±13.4 and
119.1±17.5, respectively. There was no significant differences between the two
groups of shift working and day working nurses based on the mean level of SBP
and DBP. The levels of SBP and DBP of respondents in the two groups are shown
in table 2. The risk levels of hypertension incidence for both SBP and DBP
among those who were shift working were not significantly higher than this risk
among those who were not shift working.
Table 2: Frequency
distribution of respondents in the two groups of shift working and day working
nurses based on the mean level of both systolic and diastolic blood pressures.
|
Blood pressure (mmhg) |
Age groups (years) |
|||
|
20-29 |
30-39 |
40-49 |
Total |
|
|
N % |
N % |
N % |
N % |
|
|
Systolic Shift
working <140 +140 |
12 85.7 2 14.3 |
16 76.2 5 23.8 |
11 73.3 4 26.7 |
39 78.0 11 22.0 |
|
Day
working <140 +140 |
10 90.9 1 9.1 |
23 95.8 1 4.2 |
11 73.3 4 26.7 |
44 88.0 6 12.0 |
|
Total <140 +140 |
22 88.0 3 12.0 |
39 86.7 6 13.3 |
22
73.3 8 26.7 |
83 83.0 17 17.0 |
|
|
|
|
|
|
|
Diastolic Shift working <90 +90 |
12 85.7 2 14.3 |
18 85.7 3 14.3 |
13 86.7 2 3.3 |
43
86.0 7 14.0 |
|
Day
working <90 +90 |
11 100.0 0 0.0 |
23 95.8 1 4.2 |
11 73.3 4 26.7 |
45 90.0 5 10.0 |
|
Total <90 +90 |
23 92.0 2 8.0 |
41
91.1 4 8.9 |
24 80.0 6 20.0 |
88 88.0 12 12.0 |
More important, the results of
age-adjusted relative risk calculation also showed that even after adjusting
for respondents’ age, there was not any significant risk of suffering from
higher blood pressure among those who were shift working. In the other word, compared
with nurses who had never done shift work, nurses who reported doing shift work
in the past did not show a higher age-adjusted incidence of hypertension
(neither for SBP nor for DBP) (table2). The mean levels of cholesterol in the
serum of respondent´s blood samples were 192.18± 43.6 and 186.7 ±47.7 in the two
groups of shift working and day working nurses, respectively. No significant
differences was observed between these two means of cholesterol level in the
two groups.
Discussion
Comparison of the two groups of shift
working and day working respondents showed that the two groups were properly
matched based on resondents’ age, gender, BMI, educational and marital status,
and the number of years they were working as nurses. This confirms that the
results of risk calculation for suffering from hypertension due to shift
working obtained in this study are accurate. In this historical cohort study, we examined
whether alternating shift work increases blood pressure and serum cholesterol
or not? The main finding of this study was that alternating shift work had not
a significant effect on the level of SBP, DBP and serum cholesterol of nurses. This
finding is consistent with the results of Sfreddo and colleagues who reported
that there is not any significant association between shift working and the
level of blood pressure among a group of nurses [9]. However, Sfreddo and
colleagues obtained this result from a cross-sectional study and our result is
derived from a cohort which also helped us with estimating the level of risk.
The risk of hypertention among shift working respondents obtained from our data
was not statistically significant. The causal relationship studied in our
investigation confirmed that the lack of association had been showen in Sfreddo
survey. The results obtained from Hublin and colleagues study also do not support
an association between shift-work and cardiovascular morbidity [14].
In contrast, there are many other
studies which have shown the association between shift work and high blood
pressure [6,15-17]. For instance, Suwazono et al who designed a historical
cohort study in male Japanese workers revealed that alternating shift work is a
significant and independent risk factor for an increase in blood pressure over
time [15]. In controversy with our results they confirmed the effect of shift
work on blood pressure.
McCubbin and colleagues concluded
from their results that “night shift with sleep deprivation may contribute to
blood pressure dysregulation in persons with a positive family history of
hypertension”[18]. This result indicates that a genetic susceptibility is
required for the effect of shift working on the blood pressure. De Gaudemaris
and collegues also insisted that working conditions play important roles and
should be considered further among other risk factors as a pathway to primary
prevention of hypertension [12]. This means that different variables could
intervene and modify the association between shift working and high blood
pressure and it is not easy to declare that shift work by itself can increase
the level of blood pressure. Furthermore, the notion of Individual differences
in tolerance to shift work could be revealed due to this important point[3]. There
have been studies regarding seasonal variations in tolerance against shift work
[19]. Therefore, a wide range of different variables affect the adverse
consequences of shift working and it is very difficult to judge about the
effect of shift working on a definite variable such as blood pressure.
The results of those studies which
are cross-sectional or descriptive and have reported the association between
shift working and hypertension should be interpreted carefully. Not every
association is a causal relationship; however each causal relationship is an
association. Well-designed experimental and analytical studies are needed to
explore the causal relationship between shift work and hypertension; also the
effects of confounding variables should be eliminated as much as possible.
In the other section of our main
results, the effect of shift working on the level of serum cholesterol was
explored. The average level of serum cholesterol in the two groups of shift
working and day working nurses at the end of our cohort study were not
significantly different. This result is in concordance with the results of Chen
and colleagues [20]. Chen and colleagues reported that they did not find any
significant differences in triglyceride and high-density lipoprotein
cholesterol among women working in different schedules [20]. Ghiasvand and colleagues
showed that high serum total cholesterol and LDL-C level were more common in
shift workers than in day workers [21]. This result is in contrast with our
results. The significant difference between the two groups of shift working and
day working obtained in Ghiasvand study might be due to the large number of
respondents in each group. In our study the optimum number of respondents in
each group was calculated statistically, and our results might be more reliable
than the results of Ghiasvand study.
Overall, the
methodology of research on shift work has several limitations [4]. In the
present study we do not have a clear definition of shift work. Also, the
duration of shift working for respondents might be different. The sample size
in our study was calculated based on the proportion of nurses with and without
hypertension and there was not enough respondents in each group to classify
based on the duration of shift working. It is suggested that this variable be
considered in the investigations for calculation of the dose response. The
other limitation in our study was the social class difference between the two
groups of shift working and day working nurses which was not easy to control.
Conclusions
In this study it was concluded that there
is not any significant relation between shift work and neither blood pressure
nor the level of cholesterol concentration among hospital nurses. Although
these results are in contrast with the result of students conducted on this subject,
some other research projects confirm our results. Further investigations are
recommended.
Acknowledgment
We would like to thank all the nurses
who agreed to help with this research. The authors would also like to thank the
management of Occupational Environment Research Centre, Rafsanjan University of
Medical Sceinces for supporting the project financially.
Conflict of interest: Non declared
References
[*]corresponding author: Reza Vazirinejad, Dept. of Epidemiology, School of Medicine, Occupational
Environmental Research Center, Rafsanjan University of Medical Sciences,
Rafsanjan, Iran.
E-mail: rvazirinejad@yahoo.co.uk