Background
Stroke is becoming the major cause of death and behavioral disorders because of the rapid increase of the elderly population with the elongation of life expectancy[
1].
Hypertension and aging are known to be the most common risk factors for stroke worldwide[
2,
3]. Aging is an irreversible factor, but hypertension is controllable. It has been reported that appropriately treated hypertension reduces the risk of stroke by 40% and every 20/10 mmHg incremental increase in systolic blood pressure (SBP) and diastolic blood pressure (DBP) above115/75 mmHg doubles the risk of stroke[
4]. The Framingham study in the USA, a well-regarded prospective epidemiologic study on chronic diseases initiated in 1948, elucidated that the risk factors for stroke include age, SBP, use of antihypertensive agents, diabetes, smoking, history of cardiovascular diseases (CVD), atrial fibrillation, and left ventricular hypertrophy (LVH)[
5‐
7]. In Korea, it was reported that the average 10-year probability of stroke in the Korean Cancer Prevention Study (KCPS) cohort was 3.5% for males and 3.7% for females[
8]. In addition, the average 10-year probability of stroke in the 55-84 year old group within the KCPS cohort was 10% for males and 9% for females[
8]. In Western countries, the 10-year risk of stroke in hypertensive patients is approximately 3-4 times higher than the risk of stroke in the general population[
9]. We reported that the 10-year probability of stroke by the Framingham risk score in hypertensive Korean patients was approximately 1.7 times higher than of the risk of stroke in of the KCPS cohort[
10]. However, this study population was derived from a highly selected group of patients who were attending private clinics and the results did not reflect the real 10-year probability of stroke in Korean hypertensive patients. Therefore, we set out to determine the 10-year probability of stroke in hypertensive patients attending tertiary hospitals in addition to private clinics using the Framingham risk score and we planned to compare the 10-year probability of stroke by the Framingham risk score between the two groups.
Discussion
This study is a report designed to evaluate the 10-year probability of stroke in hypertensive Korean patients and to compare the probability in hypertensive patients attending tertiary hospitals and private clinics using the Framingham risk score.
The main finding of the present study was that the 10-year probability of stroke by the Framingham risk score in hypertensive Korean patients was approximately 2.2 times higher than that of stroke in the KCPS cohort and the 10-year probability of stroke by the Framingham risk score in hypertensive patients attending tertiary hospitals was higher than hypertensive patients attending private clinics.
Table
1 showed the different profiles between the two groups. These different profiles could be caused by the fact that patients attending tertiary hospitals usually more severe and had more comorbidity than patients attending private clinics. Patients attending tertiary hospitals were older and had more atrial fibrillation, LVH, history of CVD, and smoking than patients attending private clinics (Table
1).
In the present study, the 10-year probability of stroke by the Framingham risk score in hypertensive patients attending the tertiary hospitals was higher than the risk in patients attending the private clinics. This result was attributed to the increased values and/or prevalence of the risk factors (age, smoking, history of CVD, atrial fibrillation, and LVH) in the tertiary hospital group than the private clinic group; there were more high risk patients in the tertiary hospital group. In addition, the correlation coefficient between various factors and 10-year probability of stroke by the Framingham risk score were higher in atrial fibrillation, LVH (Table
4). Therefore, we thought that these differences could also cause the difference in 10-year probability of stroke by the Framingham risk score between patients attending tertiary hospitals and patients attending private clinics. In other words, the higher probability of stroke in hypertensive patients attending tertiary hospitals is not entirely due to their hypertension but is due to the other factors used in the Framingham score. In Korea, most of patients (especially patients with more comorbidity) prefer the tertiary hospitals to private clinics and they can any time attend tertiary hospitals without referral of private clinics. The disposition to go tertiary hospitals could cause the different comorbidity between tertiary hospitals and private clinics and then cause a greater stroke risk in patients attending tertiary hospitals than patients attending private clinics.
In our study, the proportion of treated hypertensive patients among hypertensive Korean patients was 79% (Table
1) and the proportion of patients who had uncontrolled hypertension among patients treated with antihypertensive agents was 49% (Table
1). These results were high compared to those of the Korean National Health and Nutrition Examination Survey (2005)[
15]. Since the study population was derived from a select group of patients who were suspected to have, or had established cardiac disease, the proportion of antihypertensive agents and the proportion of patients who had uncontrolled hypertension was thought to be high. In addition, age and diabetes were associated with uncontrolled hypertension (Table
3). Age cannot be controlled but, diabetes can be controllable. Therefore, these results suggest that appropriate management of diabetes could be an important factor in the control of hypertension.
In Western countries, the 10-year risk of stroke in hypertensive patients was approximately from 3-4 times higher than stroke in the general population[
9]. We reported that probability of stroke in Korean hypertensive patients attending community based-hospitals[
10]. This study is the first study to compare the 10-year probability of stroke in hypertensive Korean patients attending tertiary hospitals and private clinics based on the Framingham risk score. As mentioned (
vide supra), hypertension and aging are known to be most common risk factors of stroke and hypertension is controllable. In Korea, since the proportion of patients who have uncontrolled hypertension despite use of antihypertensive agents was 49% (Table
1), we suggest that the appropriate control of BP is an important approach to prevent the risk of stroke. In addition, the control of risk factors, such as SBP, antihypertensive therapy, diabetes mellitus, smoking, history of CVD, atrial fibrillation, and LVH, was thought to also be important for the preventing stroke since these risk factors were common in hypertensive patients.
This study has some limitations. First, the study population is not representative of a real hypertension population in Korea. Because it was restricted to patients in the population between 55 and 84 years of age, and did not include patients attending the other public health centers. Although it would be appropriate to include private clinics, tertiary hospitals, and other public health centers in the study, our study did not include hypertensive patients from other public health centers. Therefore, the cardiovascular risk factors were overrepresented and it cannot be denied that the enrolled subjects were the hypertensive patients with a higher risk than the actual hypertension population.
Second, since patients may see the tertiary center before the primary and secondary levels, it could be possible that there was an overlap in services offered by primary and tertiary centers. Since we had the information about a resident registration number of all patients, we confirmed that there was not overlap in services offered by primary and tertiary centers during progression of the study. In addition, we enrolled subjects who had attended constantly the same private clinics or tertiary hospitals for at least 2 years, therefore there was not overlapped in services offered by primary and tertiary centers for at least 2 years
Third, we predicted the 10-year probability of stroke in hypertensive patients using the Framingham risk score. However, Framingham risk score by hypertensive status when these people must have higher scores because of their hypertension. In addition, these people have higher Framingham risk score because hypertensive patients had more comorbidity than general population. Therefore, 10-year probability of stroke in our subjects could be overestimated.
Fourth, the variables used in the Framingham risk score are associated with the duration of hypertension. Therefore, the duration of hypertension could be an important factor and have an effect on Framingham risk score. However, we had no data about the duration of hypertension.
Fifth, this study was a simple cross-section study and we had no information about the practical stroke event. Therefore, we did not compare the 10-year probability of stroke by the Framingham risk score with the actual stroke incidence. In addition, risk factors were uneven distributed between the two groups. Some risk factors (old age, smoking, afib, LVH, history of CVD etc.) were more in tertiary hospitals, and other risk factors (high BP, frequency of diabetes and dyslipidemia) were more in private clinics. Therefore, whether real strokes will be more or less frequent over 10 years in those treated at larger institutions remains unknown. If a well-controlled, large prospective study is conducted for hypertensive patients, we could analyze the relationship between the 10-year probability of stroke by the Framingham risk score and the actual stroke incidence.
Sixth, the Framingham stroke score in Korean population have not been validated yet. The Framingham Heart Study has contributed to the identification of risk factors for stroke and has developed multivariate functions to predict absolute stroke risk. However, there have been no large cohorts that compare the practical stroke event and probability of stroke. In addition, other ethnic populations may differ from Caucasians or people living in suburban as Framingham in terms of diet, life style, social environment, or genetic predisposition. Therefore, in Korean population, there are limitations in applications of the risk functions obtained from the Framingham study.
Seventh, we measured the smoking history and the family history of stroke with a single, self-reported questionnaire. Therefore, a non-differential misclassification was possible.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
CUC participated in the design of the study, analyzed and interpreted data and drafted the manuscript. CGP conceived of the study and participated in its design and critically revised the manuscript. All authors read and approved the final manuscript.