WHO MONICA Project e-publications, No. 22

Do trends in population levels of blood pressure and other cardiovascular risk factors explain trends in stroke event rates? - methodological appendix to a paper published in Stroke

August 2002

Hanna Tolonen1, Kari Kuulasmaa1, Kjell Asplund2, Markku Mähönen1 for the WHO MONICA Project3

1 Department of Epidemiology and Health Promotion, National Public Health Institute (KTL), Helsinki, Finland
2 Department of Medicine, University Hospital, Umeå, Sweden
3 Annex: Sites and key personnel of the WHO MONICA Project

Correspondence to Hanna Tolonen (hanna.tolonen@ktl.fi)


© Copyright World Health Organization (WHO) and the WHO MONICA Project investigators 2002. All rights reserved. 

Contents

1. Introduction

This document is the methodological appendix to the paper titled "Do trends in population levels of blood pressure and other cardiovascular risk factors explain trends in stroke rates? Comparisons of 15 populations in 9 countries within the WHO MONICA Stroke Project." published in Stroke in 2002 [1]. It covers:

2. Quality scores

2.1 Overall quality score

The overall quality score, which was used for weighting the data in the regression analyses in reference [1], has values between zero and two. If the score is two, no problems were identified in the quality of the data for a population, whereas a score of zero indicates major concern about the data quality. The overall quality score was derived from the quality scores of the individual data components. The values of the overall quality score and its components are shown in Table A1. The definition of the overall quality score is included in the following description of the columns of Table A1

2.2 Quality score for individual risk factors

The quality scores for individual risk factors (systolic blood pressure, daily cigarette smoking, total cholesterol and BMI) also have values between zero and two, with value two indicating a good quality and value zero indicating major concern about the data quality. The values of the quality score for each risk factor and its components are shown in Table A2. The definition of the quality scores for each risk factor are otherwise the same as for the overall quality score, except that the "Risk factors mean" is replaced with the quality score of the individual risk factor in question.

3. Stroke risk score

Stroke risk score was derived from the follow-up of the Finnish MONICA risk factor surveys conducted in 1982 and 1987. Subjects were 25-64 years old during the baseline examination. The total number of subjects was 14902 (7195 men and 7707 women). They were followed-up until the end of 1995. By that time there were a total of 553 fatal and non-fatal stroke cases (ICD = 430-439). The follow-up procedure has been described elsewhere [9].

Risk factors used for the stroke risk score were systolic blood pressure measurement (mmHg), daily smoking (0/1), total cholesterol (mmol/L) and BMI (kg/m2).

The risk score was defined as a linear combination of the levels of the risk factors, where the coefficients were obtained using the Cox-proportional hazards model [10]. The  coefficients are given in Table 1.

Table 1. Coefficients for stroke risk score
Variable Men Women
Coeff. S.E. P-value Risk ratio Coeff. S.E. P-value Risk ratio
Systolic blood pressure (mmHg) 0.011 0.003 0.0001 1.011 0.010 0.003 0.0005 1.010
Daily smoking (%) 0.607 0.117 0.0001 1.836 0.409 0.193 0.0339 1.506
Total cholesterol (mmol/l) 0.055 0.048 0.2557 1.056 -0.004 0.053 0.9439 0.996
BMI (kg/m2) 0.054 0.014 0.0002 1.055 0.043 0.013 0.0007 1.044

In the analysis, the coefficients for systolic blood pressure and total cholesterol of Table 1 were multiplied by 1.5 to compensate the regression dilution [11-13].

The coefficients of Table 1 are similar to coefficients obtained from other studies. For example, in the Kaunas-Rotterdam Intervention Study [14] the risk ratios for men were: 1.02 for systolic blood pressure, 2.01 for smoking, 0.97 for total cholesterol and 1.03 for BMI. 

4. Sensitivity analyses

The parent publication [1] refers to sensitivity analyses which were performed to assess the robustness of the results. The results of the sensitivity analyses are shown in Table A3a for the simple regression analysis using systolic blood pressure as the explanatory variable. Table A3b shows the results by other individual risk factors (daily smoking, total cholesterol and BMI) as explanatory variables. Table A3c shows the results when systolic blood pressure and smoking are used as the explanatory variables in a multiple regression analysis. The results for the simple regression analysis using the risk score are given in Table A3d. In each table, the analysis was repeated:

References

  1. Tolonen H, Mähönen M, Asplund K, Rastenyte D, Kuulasmaa K, Vanuzzo D, Tuomilehto J, for the WHO MONICA Project. Do trends in population levels of blood pressure and other cardiovascular risk factors explain trends in stroke event rates? Comparisons of 15 populations in 9 countries within the WHO MONICA Stroke Project. Stroke 2002;33:2367-2375.
  2. Moltchanov V, Kuulasmaa K, Torppa J, for the WHO MONICA Project. Quality assessment of demographic data in the WHO MONICA Project. (April 1999). Available from: URL:http://www.thl.fi/publications/monica/demoqa/demoqa.htm, URN:NBN:fi-fe19991073.
  3. Mähönen M, Asplund K, Tolonen H and Kuulasmaa K, for the WHO MONICA Project. Stroke event trend quality score for the WHO MONICA Project. (2001). Available from: URL:http://www.thl.fi/publications/monica/strokescore/score.htm, URN:NBN:fi-fe20011554.
  4. Wolf H, Kuulasmaa K, Tolonen H, Ruokokoski E, for the WHO MONICA Project. Participation rates, quality of sampling frames and sampling fractions in the MONICA surveys. (September 1998). Available from: URL:http://www.thl.fi/publications/monica/nonres/nonres.htm, URN:NBN:fi-fe19991076.
  5. Molarius A, Kuulasmaa K, Evans A, McCrum E, Tolonen H, for the WHO MONICA Project. Quality assessment of data on smoking behaviour in the WHO MONICA Project. (February 1999). Available from: URL:http://www.thl.fi/publications/monica/smoking/qa30.htm, URN:NBN:fi-fe19991077.
  6. Kuulasmaa K, Hense HW, Tolonen H, for theWHO MONICA Project. Quality assessment of data on blood pressure in the WHO MONICA Project. (May 1998). Available from: URL:http://www.thl.fi/publications/monica/bp/bpqa.htm, URN:NBN:fi-fe19991082.
  7. Ferrario M, Kuulasmaa K, Grafnetter D, Moltchanov V, for the WHO MONICA Project. Quality assessment of total cholesterol measurements in the WHO MONICA Project. (April 1999). Available from: URL:http://www.thl.fi/publications/monica/tchol/tcholqa.htm, URN:NBN:fi-fe19991083.
  8. Molarius A, Kuulasmaa K, Sans S, for the WHO MONICA Project. Quality assessment of weight and height measurements in the WHO MONICA Project. (May 1998). Available from: URL:http://www.thl.fi/publications/monica/bmi/bmiqa20.htm, URN:NBN:fi-fe19991079.
  9. Jousilahti P, Vartiainen E, Tuomilehto J, Puska P. Sex, age, cardiovascular risk factors, and coronary heart disease. A prospective follow-up study of 14 786 middle-aged men and women in Finland. Circulation 1999;99:1165-1172.
  10. Clayton D & Hill M. Statistical Models in Epidemiology. New York: Oxford University Press; 1993.
  11. Clarke R, Shipley M, Lewington S, et al. Underestimation of risk associations due to regression dilution in long-term follow-up of prospective studies. Am J Epidemiol 1999;150:341-353.
  12. MacMahon S, Peto R, Cutler J, et al. Blood pressure, stroke and coronary heart disease, part 1: prolonged differences in blood pressure - prospective observational studies corrected for the regression dilution bias. Lancet 1990;355:765-774.
  13. Law MR, Wald NJ, Wu T, Hackshaw B, Bailey A. Systematic underestimation of association between serum cholesterol concentration and ischaemic heart disease in observational studies: data from the BAPU study. BMJ 1994;308:363-366.
  14. Rastenyte D, Tuomilehto J, Domarkiene S, Cepaitis Z & Reklaitiene R. Risk factors for death from stroke in middle-aged Lithuanian men. Stroke 1996;27:627-676.

Acknowledgements

We thank Pekka Jousilahti for his help in deriving score risk score from Finnish data. 

The MONICA Centres are funded predominantly by regional and national governments, research councils, and research charities. Coordination is the responsibility of the World Health Organization (WHO), assisted by local fund raising for congresses and workshops. WHO also supports the MONICA Data Centre (MDC) in Helsinki. Not covered by this general description is the ongoing generous support of the MDC by the National Public Health Institute of Finland, and a contribution to WHO from the National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland, USA for support of the MDC. The completion of the MONICA Project is generously assisted through a Concerted Action Grant from the European Community. Likewise appreciated are grants from ASTRA Hässle AB, Sweden, Hoechst AG, Germany, Hoffmann-La Roche AG, Switzerland, the Institut de Recherches Internationales Servier (IRIS), France, and Merck & Co. Inc., New Jersey, USA, to support data analysis and preparation of publications.