Abstract
Background
Isolated diastolic hypertension (IDH) is a largely unrecognized subtype of hypertension, more commonly seen in the younger age group.
Aims
(1) To determine the prevalence of IDH in the adult population of Kanpur district. (2) To study the associated risk factors of IDH.
Methods
A community-based cross-sectional study was conducted in 801 subjects, aged 20 years and above, using multistage stratified random sampling technique.
Results
The prevalence of IDH was 4.5%, which was 6.2% in men and 3.1% in women. A significant proportion of IDH was seen in the 40–49 years age group. Multivariate logistic regression analysis of the associated risk factors showed that gender, physical activity and BMI were significantly associated with IDH.
Conclusion
Isolated diastolic hypertension is an emerging problem in developing countries. IDH is more common among men, sedentary individuals and those with a higher BMI.
Keywords: Isolated diastolic hypertension, Young adults, Risk factors
1. Introduction
Globally 26.4% of the adult population in 2000 had hypertension and 29.2% were projected to have this condition by 2025.1 Worldwide, 7.1 million deaths (approximately 12.8% of the global total) and 64.3 million Disability Adjusted Life Years (DALYs) (4.4% of the global total), were estimated to be due to non-optimal blood pressure and approximately two-thirds of the attributable burden of disease occurred in the developing world.2 The World Health Organization (WHO) has estimated that globally hypertension is directly responsible for about 62% of cerebrovascular disease and 49% of ischemic heart disease.2
The risks of stroke and coronary heart disease (CHD) are directly related to both the levels of systolic blood pressure (SBP) and diastolic blood pressure (DBP).3 The argument concerning the relative importance of SBP and DBP is not new. While isolated systolic hypertension (ISH) has been identified as a specific clinical entity, isolated diastolic hypertension (IDH) has not been reported as a separate group. IDH was defined as diastolic BP more than 90 mmHg and systolic BP less than 140 mmHg.
According to the Framingham Heart Study, in younger adults, IDH is more common than systo-diastolic hypertension (SDH).4 The Third National Health and Nutrition Examination Survey (NHANES III, 1988–1991) showed that IDH was the most frequent form of diastolic hypertension in young adults <40 years old and comparable to SDH in prevalence from age 40–49 years.5 Together, IDH and SDH accounted for more than 75% of younger adult individuals with untreated hypertension.
According to the Framingham report, subjects with IDH were more frequently men and smokers and their mean body mass index (BMI) was higher than their normotensive counterparts. It has recommended that the subjects with IDH, although possibly not at increased risk in the short or middle term should be carefully monitored over time.6–9 The recommendation is that in these patients, the goal of treatment should be the control of all cardiovascular risk factors, not solely the BP normalization.
IDH is a largely under-rated risk factor for cardiovascular mortality, nevertheless its prevalence in the younger age group, its concurrence with other cardiovascular risk factors and above all paucity of data regarding IDH in developing countries necessitate that it should be investigated. Therefore, the present study was planned with the objective to determine the prevalence of IDH in the adult population of Kanpur district and to identify the associated risk factors of IDH.
2. Methods
2.1. Study population
The study population consisted of the total population of Kanpur district aged more than 20 years.
2.2. Study design and sample size
This was a population based cross-sectional study. The sample size required (N = 372) was calculated taking a prevalence of 6.2%, as reported in the Five City study from Moradabad, with a precision of 2.5% and a confidence level of 95%.10,11 The formula used, n = Z2(1−α/2) pq/d2 (where Z(1−α/2) was taken at 95% confidence; p = prevalence of obesity, q = 1 − p; d = absolute precision). For this study, p = 6.2%; q = 93.8%; d = 2.5%. Adding a 10% for incomplete answers, the total number came out to be 409. Since it was a multistage stratified random sampling, a design effect of 2 was included to minimize any error due to inherent variation in the population. The calculated sample size was multiplied by 2 to obtain the sample size of 818. The data was analyzed for 801 subjects only who had provided complete answers.
2.3. Sampling technique
Urban Kanpur has 110 wards and a total population of 2,797,511 according to 2001 census, and rural Kanpur has 10 blocks and a population of 1,370,488 which means that about two-thirds of the total population of Kanpur district is urban population.12 Therefore, applying Probability Proportional to Size (PPS), out of 818 subjects, two-thirds, that is 545 were selected from the urban population and 273 were selected from the rural population. Multistage stratified random sampling technique was used to select representative subjects of Kanpur district. At the first stage, 8 wards were randomly selected to study the urban population.13 Similarly, to study the rural population, 4 blocks were randomly selected.14 At the second stage, 1 urban locality from each ward was randomly selected. Similarly, 1 village from each block was randomly selected. A total of 68 subjects from each urban locality/village were interviewed to complete the required sample size.
2.4. Selection of subjects
The households in every urban area/village were selected for the study by systematic sampling. Depending upon the population of the particular urban locality/village, a random number was chosen and every nth household was selected for the study. This process was continued till the required sample size was completed. In every household, only one member, aged more than 20 years was randomly selected. Data collection was done from December 2006 to February 2007.
2.5. Data collection
A pre-tested structured questionnaire was used to elicit the required information from the study participants. Informed verbal consent was taken from each of the participants.
Social class was calculated using modified Kuppuswamy scale in the urban population and Pareek scale in the rural population.15,16 Physical activity can be expressed as increments of BMR.17 In the present study, subject's BMR factor was calculated by questioning him/her about the type of activity and time spent in performing each activity in last 24 h.18 On the basis of the BMR factor, the level of activity was classified as follows:
Average daily level of activity of adults classified as sedentary, moderate or heavy, expressed as a multiple of BMR:
| Sex | Sedentary | Moderate | Heavy |
|---|---|---|---|
| Men | 1.55 | 1.78 | 2.10 |
| Women | 1.55 | 1.64 | 1.82 |
A standard mercury sphygmomanometer was used for recording blood pressure. Before the measurement was taken, the patient was seated comfortably for at least 5 min. Care was taken that the arm muscles were relaxed and the arm was supported at heart level. The cuff was applied to the right upper arm and was inflated until the manometer reading was 30 mmHg above the level at which the radial pulse disappears, and then slowly deflated at approximately 2 mmHg/s. During this time, the Korotkoff sounds were monitored using a stethoscope placed over the brachial artery. The first (appearance) and the fifth (disappearance) Korotkoff sounds were recorded as the systolic and diastolic blood pressure, respectively. Blood pressures were measured twice and their mean was recorded.
According to JNC-7,19 normal blood pressure was defined as a systolic blood pressure (SBP) < 120 mmHg and a diastolic blood pressure (DBP) < 80 mmHg, pre-hypertension as SBP 120–139 mmHg and/or DBP 80–89 mmHg, Stage I hypertension as SBP 140–159 mmHg and/or DBP 90–99 mmHg and Stage II hypertension as ≥160 mmHg and/or DBP ≥ 100 mmHg. In the present study, subjects in Stage I and Stage II were considered as hypertensive. A subject was classified as having IDH if the diastolic BP was more than or equal to 90 mmHg and systolic BP less than 140 mmHg. ISH was defined as SBP ≥ 140 mmHg and DBP < 90 mmHg.
Respondents were defined as smokers if they were smoking at the time of the survey and had smoked more than 100 cigarettes in their lifetime.20 An alcoholic was defined as a subject who suffers from alcoholism, generally taken to refer to chronic continual drinking or periodic consumption of alcohol which is characterized by impaired control over drinking, frequent episodes of intoxication, and preoccupation with alcohol and the use of alcohol despite adverse consequences.21 A non-vegetarian was defined as any subject consuming non-vegetarian food items including eggs at least once a week or more. JNC-7 has recommended a daily intake of salt of no more than 100 mEq/L, which is equivalent to 6 g/day of sodium chloride or 2.4 g/day of sodium. Therefore, the association of ISH with salt intake was analyzed after dividing the subjects into those consuming <6, 6–8 and >8 g/day. Subjects were classified as having a positive family history if either of the parents or siblings had a history of hypertension. Self-reported diabetes mellitus was noted.
Body weight was measured to the nearest 0.5 kg and height was measured to the nearest 0.5 cm. Body mass index (BMI) was calculated as weight in kilograms divided by the square of the height in meters.
Data was analyzed using the software SPSS 10.0.1 for Windows. Discrete data was analyzed using Pearson's Chi-square test. In case the expected values in some cells were <5, Fischer's Exact test was used in place of Chi-square test. The significance of various associated risk factors was estimated by forward stepwise multiple logistic regression analysis with presence of IDH as the dependent variable and risk factors as the independent variables. Two-tailed p-value less than 0.05 was considered significant.
3. Results
In the present study, 801 subjects were examined, 533 in urban and 268 in rural area. Percentage of women was 55.6% as compared to 44.4% men. The overall prevalence of hypertension was observed to be 22.5% (95% CI 19.6%–25.5%). The prevalence of ISH was 4.2% (95% CI 2.8%–5.7%) and IDH was 4.5% (95% CI 3.0%–5.9%) (Table 1). The prevalence of IDH was highest in the 40–49 years age group (8.3%), but IDH was absent in the >60 years age group.
Table 1.
Age-wise distribution of subjects according to type of hypertension.
| Age (years) | SDH |
ISH |
IDH |
Normotensive |
Total | ||||
|---|---|---|---|---|---|---|---|---|---|
| No. | % | No. | % | No. | % | No. | % | ||
| 20–29 | 21 | 7.1 | 1 | 0.3 | 13 | 4.4 | 259 | 88.1 | 294 |
| 30–39 | 25 | 10.9 | 4 | 1.7 | 8 | 3.5 | 192 | 83.8 | 229 |
| 40–49 | 29 | 18.6 | 5 | 3.2 | 13 | 8.3 | 109 | 69.9 | 156 |
| 50–59 | 23 | 31.9 | 11 | 15.3 | 2 | 2.8 | 36 | 50.0 | 72 |
| >60 | 13 | 13.9 | 13 | 26.0 | 0 | 0 | 24 | 48.0 | 50 |
| Total | 111 | 13.9 | 34 | 4.2 | 36 | 4.5 | 620 | 77.4 | 801 |
SDH = Systolic Diastolic Hypertension, ISH = Isolated Systolic Hypertension, IDH = Isolated Diastolic Hypertension.
Isolated diastolic hypertension was significantly more common in men than in women(Table 2). Prevalence of IDH was higher in the upper class (9.1%). IDH was significantly associated with level of physical activity, being prevalent 7.5% in sedentary workers, 3.5% in moderate workers and absent in heavy workers. Isolated Diastolic Hypertension was most common in subjects with BMI greater than 30 kg/m2 (15.8%) and was minimal in subjects with BMI less than 18.5 kg/m2 (1.0%). Isolated Diastolic Hypertension was more prevalent in smokers and alcoholics although the differences were not statistically significant. Salt intake and eating habit had no effect on IDH. Subjects with a family history of hypertension had a significantly higher prevalence of IDH (13.3%). A greater proportion of subjects had coexisting diabetes mellitus.
Table 2.
Baseline characteristics of subjects with isolated diastolic hypertension.
| Determinants | Total (n = 656) | IDH (n = 36) | IDH % | Normotensive (n = 620) | Normotensive % | P-value |
|---|---|---|---|---|---|---|
| Gender | ||||||
| Male | 274 | 22 | 8.0 | 252 | 92.0 | 0.015* |
| Female | 382 | 14 | 3.7 | 368 | 96.3 | |
| Place of residence | ||||||
| Urban | 411 | 34 | 8.3 | 377 | 91.7 | 0.001* |
| Rural | 245 | 2 | 0.8 | 243 | 99.2 | |
| SES | ||||||
| Upper | 220 | 20 | 9.1 | 200 | 90.9 | 0.004* |
| Lower | 436 | 16 | 3.7 | 420 | 96.3 | |
| Religion | ||||||
| Hindu | 473 | 27 | 5.7 | 446 | 94.3 | 0.849 |
| Others | 183 | 9 | 4.9 | 174 | 95.1 | |
| Marital status | ||||||
| Married | 538 | 30 | 5.6 | 508 | 94.4 | 0.832 |
| Others | 118 | 6 | 5.1 | 112 | 94.9 | |
| Family type | ||||||
| Nuclear | 481 | 28 | 5.8 | 453 | 94.2 | 0.534 |
| Joint | 175 | 8 | 4.6 | 167 | 95.4 | |
| Physical activity | ||||||
| Sedentary | 385 | 29 | 7.5 | 356 | 92.5 | 0.012* |
| Moderate | 199 | 7 | 3.5 | 192 | 96.5 | |
| Heavy | 72 | 0 | 0.0 | 72 | 100.0 | |
| BMI | ||||||
| <18.5 | 191 | 2 | 1.0 | 189 | 99.0 | <0.001* |
| 18.5–24.9 | 409 | 9 | 2.2 | 400 | 97.8 | |
| 25–29.9 | 163 | 19 | 11.7 | 144 | 88.3 | |
| >30 | 38 | 6 | 15.8 | 32 | 84.2 | |
| Smoking | ||||||
| Smoker | 58 | 4 | 6.9 | 54 | 93.1 | 0.549 |
| Non-smoker | 598 | 32 | 5.4 | 566 | 94.6 | |
| Alcoholism | ||||||
| Alcoholic | 14 | 2 | 14.3 | 12 | 85.7 | 0.176 |
| Non-alcoholic | 642 | 34 | 5.3 | 608 | 94.7 | |
| Salt intake (g/day) | ||||||
| <6 | 136 | 9 | 6.6 | 127 | 93.4 | 0.319 |
| 6–8 | 166 | 5 | 3.0 | 161 | 97.0 | |
| >8 | 499 | 22 | 4.4 | 477 | 95.6 | |
| Eating habit | ||||||
| Vegetarian | 555 | 27 | 4.9 | 528 | 95.1 | 0.149 |
| Mixed | 101 | 9 | 9.1 | 92 | 91.1 | |
| Family history (n = 558)a | ||||||
| Present | 83 | 11 | 13.3 | 72 | 86.7 | 0.011* |
| Absent | 475 | 24 | 5.1 | 451 | 94.9 | |
| Diabetes | ||||||
| Diabetic | 5 | 1 | 20.0 | 4 | 80.0 | 0.247 |
| Non-diabetic | 651 | 35 | 5.4 | 616 | 94.6 | |
*p-value <0.05 is significant.
n = 558 for those subjects who responded for family history.
Forward stepwise multiple logistic regression analysis showed that regardless of age, place of residence and socio-economic status, gender (OR = 0.337), physical activity (OR = 0.394) and BMI (OR = 3.717) were significantly associated with IDH (Table 3). Males were more prone to developing IDH. Increasing level of physical activity was inversely associated with IDH whereas increasing BMI was positively associated.
Table 3.
Multivariate logistic regression analysis for association of various risk factors with isolated diastolic hypertension.
| Determinants | β Coeff | SE | Odds ratio | Lower limit | Upper limit | P-value |
|---|---|---|---|---|---|---|
| Constant | −0.690 | 1.356 | 0.611 | |||
| Age (per 1 year) | −0.031 | 0.017 | 0.969 | 0.937 | 1.003 | 0.070 |
| Place (Ref = Urban) | −0.054 | 0.418 | 0.947 | 0.417 | 2.150 | 0.897 |
| Sex (Ref = Male) | −1.086 | 0.417 | 0.337 | 0.149 | 0.763 | 0.009* |
| SES (Ref = Upper class) | −0.238 | 0.406 | 0.788 | 0.356 | 1.748 | 0.558 |
| Family history (Ref = Family history absent) | −0.601 | 0.407 | 0.549 | 0.247 | 1.218 | 0.140 |
| Physical activity (Ref = Sedentary) | −0.931 | 0.446 | 0.394 | 0.165 | 0.944 | 0.037* |
| BMI (per 1 kg/m2) | 1.313 | 0.253 | 3.717 | 2.265 | 6.101 | <0.001* |
The reference category is normotensive, *p-value <0.05 is significant.
Ref = Reference category, SES = Socio-economic Status, BMI = Body Mass Index.
4. Discussion
In the present study, 13.9% subjects were observed to have SDH, 4.2% had ISH and 4.5% had IDH. In a similar study from China, it was reported that 7.4% of the adult population had SDH, 7.6% had ISH, and 4.4% had IDH.22
Our study shows that the prevalence of IDH was higher in men (6.2%) than in women (3.1%). The prevalence of IDH was 6.4% in the urban population and 0.7% in the rural population. Prevalence seen in the urban population was comparable to that reported in a study from New York wherein IDH was observed in 15 out of 175 subjects (8.6%).23 In another study from Belgium, diastolic hypertension was found in 5% females and 1% males among adolescents.24 None of the Indian studies on hypertension have reported the prevalence of the IDH subtype.
The question of the significance of IDH is largely unresolved. Very few studies are available on the subject. Patients with IDH are usually young and are therefore at low cardiovascular risk.22 As seen in our study, the prevalence of ISH and IDH is nearly equivalent, but ISH is considered as important and treated as a separate clinical entity. Given the fact that IDH is more common in the younger age group and is associated with modifiable risk factors, tracking and controlling IDH might be of greater public health importance.
Pickering concluded that if the systolic blood pressure is <140 mmHg, a high diastolic pressure is not associated with an adverse prognosis.7 However, it cannot be said that IDH is harmless, since it is possible that a younger adult with IDH may convert to ISH or develop SDH with increasing age.25
A recent study from Finland reported that patients with SDH and ISH both had approximately double the relative risk whereas patients with IDH had a relative risk of 1.12 for all-cause mortality.9 However, according to the Framingham Heart Study, given the propensity for increased BMI and weight gain in the development of new-onset of IDH and the high probability of transition of IDH to SDH, it is likely that IDH is not a benign condition.4
According to a Framingham report, the view that IDH is a low-risk condition should be reconsidered or reformulated. Unfortunately, no information on cardiovascular events associated with evolution of hypertension subtypes was available from the report. However, the subjects with IDH at entry had a cluster of features of increased risk. They tended to be more frequently men (65.3%) and smokers (57%). The mean BMI of subjects with IDH was 28.0 ± 4 g/m2, and, although not reported, metabolic syndrome might have been frequent in this group. Similarly in our study, IDH was more frequent among men and those with a higher BMI. However, there was no association with smoking. It was recommended that the subjects with IDH should be monitored over time and their management should focus on the control of all cardiovascular risk factors, and not solely the optimization of BP.23
In our study, salt intake was not found to be associated with IDH. This is in concordance with the findings of a systematic review of 11 long-term controlled randomized trials wherein a small decrease (1.1 mmHg) in median systolic but not diastolic blood pressure was reported with a reduced dietary sodium intake.26
There are some limitations of this study. Firstly, ‘non-smoker’ had been defined taking a cut off of the past one year, irrespective of the total duration of smoking. This could have led to a misclassification bias and dilution of the observed effect on IDH. Smokeless tobacco had not been considered in this study, but an association with IDH needs to be explored. Secondly, being a cross-sectional study, this study does not elucidate any causative association between IDH and the associated risk factors.
In the present study, multivariate logistic regression analysis showed that male sex, lack of physical activity and BMI were the associated risk factors of IDH. Our findings correspond with those reported in the Framingham Heart Study wherein it was observed that the predictors of IDH were younger age, male sex, and BMI at baseline.4 In the NHANES III survey, IDH was observed to be the most frequent form of diastolic hypertension in young adults <40 years old.5 IDH and SDH together were responsible for more than 75% of untreated hypertension among younger adults. However, age was not observed to be a risk factor in our study.
Lack of physical activity and increased BMI are independent predictors of IDH as revealed in our study. This emphasizes the need for further research to document the impact of modifiable risk factors on IDH and the effect of primary prevention in bringing down the burden of IDH. Given the risk of evolution of SDH in patients with IDH and subsequent cardiovascular disease, screening of hypertension and health education programs regarding weight reduction and increase in physical activity may be considered as a cost-effective approach in dealing with the morbidity attributed to IDH and cardiovascular diseases.
5. Conclusion
Our findings provide an insight into the magnitude of the problem of IDH in developing countries. The most important factors associated with IDH are sedentary lifestyle and increased BMI. Screening of IDH in the younger age group is recommended and a public health approach to curb the development of risk factors will be a viable way forward.
Conflicts of interest
All authors have none to declare.
References
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