Indian Journal of Public Health

ORIGINAL ARTICLE
Year
: 2010  |  Volume : 54  |  Issue : 1  |  Page : 7--10

Seasonal variation in prevalence of hypertension: Implications for interpretation


Pragya Sinha1, DK Taneja2, NP Singh3, Renuka Saha4,  
1 Senior Resident, Department of Community Medicine, Maulana Azad Medical College, New Delhi, India
2 Professor, Department of Community Medicine, Maulana Azad Medical College, New Delhi, India
3 Professor, Department of Medicine, Maulana Azad Medical College, New Delhi, India
4 Assistant Professor, Biostatistics, Department of Community Medicine, Maulana Azad Medical College, New Delhi, India

Correspondence Address:
Pragya Sinha
Senior Resident, Department of Community Medicine, Maulana Azad Medical College, New Delhi
India

Abstract

Objectives: To study seasonal variation in prevalence of hypertension. Materials and Methods: The study was carried out in the year 2006, in Gokulpuri, an urban slum located in eastern part of Delhi. 275 females 18-40 years of age were examined in summer. Blood pressure was measured in two seasons, summer and winter. Nutritional status of each individual was assessed by BMI. Results: The prevalence of hypertension based on SBP was 12.72% in summer which increased to 22.22% in winter. The prevalence of hypertension, using DBP criteria increased to more than double (summer vs. winter, 11.27% vs. 26.59%, P< 0.001). Overall prevalence of hypertension (SBP≥140 or DBP≥90 mm of Hg) was 1.9 times during winter compared to summer (P<0.001). Greater increase in prevalence of hypertension during winter among older females and underweight as well as normal females was observed. Conclusion: Significant increase in prevalence of hypertension during winter compared to summer indicates need for considering this factor while comparing prevalence reported in different studies as well as interpreting the surveillance data based on repeat surveys.



How to cite this article:
Sinha P, Taneja D K, Singh N P, Saha R. Seasonal variation in prevalence of hypertension: Implications for interpretation.Indian J Public Health 2010;54:7-10


How to cite this URL:
Sinha P, Taneja D K, Singh N P, Saha R. Seasonal variation in prevalence of hypertension: Implications for interpretation. Indian J Public Health [serial online] 2010 [cited 2021 Dec 5 ];54:7-10
Available from: https://www.ijph.in/text.asp?2010/54/1/7/70537


Full Text

 Introduction



Seasonal variation in the prevalence of hypertension has not been studied much. Various surveys and studies have documented the association between temperature and blood pressure in various countries among adults, the elderly, and children. [1],[2] Twenty-four-hour blood pressure studies also generally show higher blood pressure during winter. [3] The suggested etiology is that cold increases sympathetic tone, evidenced by elevated blood pressure and plasma and urinary noradrenaline concentrations. The lower blood pressure in warm temperatures is attributed to cutaneous vasodilatation and loss of water and salt from sweating. [4] This variation in blood pressure is likely to affect the prevalence of hypertension in different seasons because of the fact that increase in blood pressure in winter will shift the proportion of the subjects from normotensive to the hypertensive category. The studies on prevalence of hypertension may not be comparable without taking this seasonal variability into account. The issue is also important for surveillance of hypertension as data collected in different seasons may not depict the true trends. Therefore, this study was planned with the objectives of assessing the seasonal variation in prevalence of hypertension and its association with the age and nutritional status.

 Materials and Methods



Study setting: The study was carried out in the year 2006 in Gokulpuri, an urban slum located in eastern part of Delhi, the capital of India. The area was chosen as it is the field practice area of the department of Community Medicine, Maulana Azad Medical College, New Delhi where the study was conceived. The area has four blocks with 19,316 population living in 3672 households.

The city was hottest in the month of May and June (mean maximum and minimum temperature: 39.4±2.15ºC and 26.6±2.27ºC). The coolest months were December and January (mean maximum and minimum temperature: 20.45±2.36ºC and 7.12±2.51ºC). This information was obtained from the meteorological department, Government of India.

Study subjects: Females who were residents of Gokulpuri, Delhi and aged 18-40 years, were eligible to participate in the study. It was decided to include only women in the study due to the feasibility and availability in the follow-up visit as men are mostly away for work during the day. The women who were more than 40 years of age were excluded because perimenopause and postmenopause is associated with a rise in blood pressure. [5] The women who were less than 18 years of age were excluded because the study was planned on adult individual for whom JNC VII criteria for hypertension is applicable. Participation was limited to those who were free from severe/debilitating disease and not pregnant on first or follow up visits. Nearly 90% of study subjects were married and housewives by occupation, mostly doing the household (indoor) activities of sedentary nature.

Sample size and follow up: As there is no study available on seasonal variation in prevalence of hypertension, seasonal variation in blood pressure was taken to calculate sample size. For a mean systolic blood pressure difference of 4.5 mm of Hg and standard deviation of 5.1 between summer and winter (based on study of Kristal-Boneh et al.) [6] with permissible error of 1% and power 95%, minimum sample size worked out to be 46. Allowing for a non-response of 20% on follow up visits and 10% for exclusion criteria a minimum of 61 women were required to be sampled for the study. Women in the age group of 18-40 years residing in the 4 service blocks of Urban Health Center (UHC) formed the universe of the study. All the households in this area constituted the sampling frame. It was assumed that there would be at least 1 woman in the age group of 18-40 year in every household. Systematic random sampling was applied to the households to get the required sample size. If more than one female of the specified age was there in any household, only one was taken by random selection.

A total of 320 females were sampled and contacted to determine their willingness to participate in the study. Participation was limited to individuals who were 1) not taking tobacco in any form e.g smoking, chewing and snuffing 2) not taking oral contraceptives or steroids on the first or follow up visits, 3) not taking alcohol daily/ regularly, 4) free from chronic/debilitating disease (cancer, renal disease, heart failure, TB) 5) not known diabetics, 6) not pregnant on first or follow up visits as these factors may affect blood pressure and confound the assessment of the seasonal variation in prevalence of hypertension. From the study sample, 292 met initial eligibility criteria and consented for the study. Out of 292, 17(5.8%) females were known hypertensive and were on antihypertensive medication. They were excluded from the study since their blood pressure variation without drugs could not be studied due to ethical reasons. Finally 275 females were examined in the summer. At the second follow up examination in winter only 252 women could be studied as 5 were lost to follow up because of change of residence and 18 females became pregnant.

Ethical considerations: The institutional review board approved all subject recruitment and data collection procedures. Each subject signed an approved informed consent form prior to entering the study.

Those who were detected to be hypertensive were informed about the benefits of antihypertensive treatment and were advised to get treatment either from the health center under the department or any source of their choice. Similarly medical advice was given for other conditions if treatment was not being taken.

Data collection: Data was collected from individual study participants at their homes using a pre-designed and pre-tested semi-structured proforma containing the items of socio-demographic data.

Blood pressure was measured in two seasons, summer (May 15 to June 15) and winter (December 26 to January 26). These months were chosen as these were the hottest and the coldest months respectively as per the meteorological department of Government of India. Blood pressure was measured thrice in right arm at one minute interval using adult cuff size in sitting position and arm comfortably placed on an elevated surface/table so that it was at the level of heart. [7] Finally mean of second and third observations was considered and the first measurement was discarded. [8] Same investigator recorded blood pressure in both the seasons. Diurnal variation in blood pressure was controlled by recording blood pressure at fixed timing, i.e. in the morning by 10 am to 1 pm. Hypertension was diagnosed as per JNC VII criteria.

Nutritional Status of each individual was assessed by measurement of Body Mass Index (BMI) which was categorized using standard protocol. [7]

Data Analysis: Data was entered and analyzed in SPSS version 11.01 for windows. For differences in the categorical variable McNemar test was used. A value of P<0.05 was considered statistically significant.

 Results



Information on season wise distribution of environmental variables was obtained from the meteorological department, Government of India.

Mean indoor temperature decreased by 15.35±2.16ºC from 33.18±1.41ºC in summer to17.83±1.85 ºC winter.The mean maximum outdoor temperature decreased by 18.95ºC from summer (39.4±2.15ºC) to winter (20.45±2.36ºC). Minimum outdoor temperature decreased by 19.47ºC from summer (26.6±2.27ºC) to winter (7.12±2.51ºC). The maximum relative humidity was higher in winter (91.48±2.59%) than summer (71.56±11.8%). Minimum relative humidity also was higher in winter (46.28±15.84%) than summer (38.42±7.83%).

The mean day-length was maximum in summer (13.5±0.06 hrs.) and minimum in winter (10.26±0.07 hrs.).

Season - wise Systolic and Diastolic Blood Pressure

Mean Systolic Blood Pressure (SBP) increased by 11.07 mm of Hg from 114.35±11.74 mm Hg in summer to 125.42±11.29 mm Hg in winter. The difference of blood pressure between summer and winter was statistically significant (P<0.001).

Similar trend was observed with Mean Diastolic Blood Pressure (DBP). It increased by 6.79 mm of Hg from 75.78±9.47 mm Hg in summer to 82.57±8.13 mm Hg in winter. The difference of blood pressure between summer and winter was statistically significant (P<0.001).

[Table 1] shows that the prevalence of hypertension based on systolic blood pressure was 12.72% in summer while it increased to 22.22% in winter using the SBP criteria. Similarly when DBP criterion was used to define hypertension the prevalence increased to more than double (summer vs. winter, 11.27% vs. 26.59%) which was significant (P< 0.001). Overall prevalence of hypertension (SBP≥140 or DBP≥90 mm of Hg) was 1.9 times during winter compared to summer. The difference was statistically highly significant (P<0.001){Table 1}

Prevalence of hypertension based on SBP [Table 2] doubled during winter compared to summer in the age group 35-40 years and the difference was highly significant (P<0.001). There was no hypertensive in the summer month in the females aged 18 to 24 years, which increased to 3.38% in winter. The prevalence was almost similar in the 25-34 category of age in summer and winter.{Table 2}

Prevalence of hypertension based on DBP [Table 2]was more than double during winter compared to summer in the age group 25-34 and 35-40 years and the difference were highly significant (P<0.001). There was no hypertensive in the summer month in the females aged 18 to 24 years, which increased to 5.08% in winter.

Prevalence of hypertension (SBP≥140 or DBP≥90 mm of Hg) was 1.75 times more in 35-40 years [Table 2] whereas 1.91 times more in the 25-34 years females in winter as compared to summer. There was no hypertensive in the summer month in the females aged 18 to 24 years, which increased to 8.47% in winter.

The prevalence of hypertension based on SBP almost doubled in winter in underweight, normal and overweight females as compared to summer [Table 3]. However, statistical significance (P<0.001) was found for underweight and normal females only. It was similar in both the seasons in obese females.{Table 3}

The prevalence of hypertension based on DBP was 2.5 times more than in winter [Table 3] in underweight and normal females as compared to summer (P<0.001). The hypertension in the overweight females doubled in winter as compared to summer but it was statistically not significant (P>0.05). It was similar in both the seasons in obese women.

The overall prevalence of hypertension (SBP≥140 or DBP≥90 of Hg) was almost doubled in winter in underweight, normal and overweight females (P<0.001) as compared to summer whereas it was similar in both the seasons in obese females [Table 3].

 Discussion



The proportion of females with hypertension was almost twice during winter as compared to summer. This is line with the study of Narang [9] who reported higher proportion of patients with hypertension with or without coronary artery disease during winter months. Greater increase in prevalence of hypertension during winter among older females may be due to their higher sensitivity to cold. Greater increase in systolic and diastolic blood pressure during winter among older age groups has been reported in a number of studies. [10],[11] This seasonality of hypertension was maintained across all the categories of BMI except in the obese females. No seasonal variation in hypertension among obese females may be due to the small number of females in this category.

 Conclusion



Significant increase in prevalence of hypertension during winter compared to summer indicates need for considering this factor while comparing prevalence reported in different studies as well as interpreting the surveillance data based on repeat surveys. Studies on larger population in different settings are recommended to determine correlation of meteorological factors, particularly temperature with the prevalence of hypertension.

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