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Comments on cardiovascular mortality – Comparing risk factor associations within couples and in the total population – The HUNT study
International Journal of Cardiology 242 (2017) 7
Contents lists available at ScienceDirect
International Journal of Cardiology journal homepage: www.elsevier.com/locate/ijcard
Correspondence
Comments on cardiovascular mortality – Comparing risk factor associations within couples and in the total population – The HUNT study Saeid Safiri a, Erfan Ayubi b,c,⁎ a b c
Managerial Epidemiology Research Center, Department of Public Health, School of Nursing and Midwifery, Maragheh University of Medical Sciences, Maragheh, Iran Department of Epidemiology, School of Public Health, Shahid Beheshti University of Medical Sciences, Tehran, Iran Department of Epidemiology & Biostatistics, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 26 February 2017 Accepted 27 February 2017
Dear Editor, We read with great interest the article published by Bjørngaard and colleagues in 2017 [1]. The authors aimed to compare the effect of established risk factors at baseline on cardiovascular mortality within couples and the population as a whole. The design of their study was a large population based cohort study with long-term follow-up. The authors used standard Cox proportional hazard model to evaluate the effect of systolic blood pressure (SBP) at baseline on cardiovascular death. Despite the previous studies in which larger effect of SBP on cardiovascular mortality have been reported [2,3], the results of the study conducted by Bjørngaard and colleagues found that for each 20 mm Hg increase in SBP, hazard ratios (95% confidence interval) of cardiovascular death within couples and the population as a whole were 1.28 (1.17, 1.40) and 1.16 (1.12, 1.20), respectively [1]. We think the main limitation of the study conducted by Bjørngaard and colleagues is that the concept of time varying exposures and confounders is missed in the analyses. Since SBP is subject to fluctuations within individuals, using the baseline measurement of risk factors may dilute the true association and lead to biased estimate. It is worth mentioning that, the degree of bias will be large within long term follow-up periods [4]. On the other hand, there is a time-varying association between smoking and mortality because smoking is a time-
⁎ Corresponding author at: Department of Epidemiology, School of Public Health, Shahid Beheshti University of Medical Sciences, Tehran, Iran. E-mail address: [email protected] (E. Ayubi).
http://dx.doi.org/10.1016/j.ijcard.2017.02.145 0167-5273/© 2017 Elsevier B.V. All rights reserved.
varying confounder [3]. In such situations, we have to use suitable methods to correct the source of bias; otherwise, the results should be interpreted with caution. As take home message for readers is that regression dilution bias is common in biomedical research due to using single measurements at baseline [5]. Conflict of interest The authors report no relationships that could be construed as a conflict of interest. Acknowledgment The authors would like to thank statistics consultants of Research Development Center of Sina Hospital for their technical assistance. This work was not supported by any organization. References [1] J.H. Bjorngaard, G.A. Vie, S. Krokstad, I. Janszky, P.R. Romundstad, L.J. Vatten, Cardiovascular mortality - comparing risk factor associations within couples and in the total population - the HUNT study, Int. J. Cardiol. 232 (2017) 127–133. [2] A.S. Koh, M. Talaei, A. Pan, R. Wang, J.M. Yuan, W.P. Koh, Systolic blood pressure and cardiovascular mortality in middle-aged and elderly adults - the Singapore Chinese Health Study, Int. J. Cardiol. 219 (2016) 404–409. [3] K. Tilling, J.A. Sterne, M. Szklo, Estimating the effect of cardiovascular risk factors on all-cause mortality and incidence of coronary heart disease using G-estimation: the atherosclerosis risk in communities study, Am. J. Epidemiol. 155 (2002) 710–718. [4] D. Wormser, I.R. White, S.G. Thompson, A.M. Wood, Within-person variability in calculated risk factors: comparing the aetiological association of adiposity ratios with risk of coronary heart disease, Int. J. Epidemiol. 42 (2013) 849–859. [5] S. Safiri, S. Khazaei, K. Mansori, M. Sani, E. Ayubi, Differing predictive relationships between baseline LDL-C, systolic blood pressure, and cardiovascular outcomes: methodological issues, Int. J. Cardiol. 229 (2017) 141.
Contents lists available at ScienceDirect
International Journal of Cardiology journal homepage: www.elsevier.com/locate/ijcard
Correspondence
Comments on cardiovascular mortality – Comparing risk factor associations within couples and in the total population – The HUNT study Saeid Safiri a, Erfan Ayubi b,c,⁎ a b c
Managerial Epidemiology Research Center, Department of Public Health, School of Nursing and Midwifery, Maragheh University of Medical Sciences, Maragheh, Iran Department of Epidemiology, School of Public Health, Shahid Beheshti University of Medical Sciences, Tehran, Iran Department of Epidemiology & Biostatistics, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 26 February 2017 Accepted 27 February 2017
Dear Editor, We read with great interest the article published by Bjørngaard and colleagues in 2017 [1]. The authors aimed to compare the effect of established risk factors at baseline on cardiovascular mortality within couples and the population as a whole. The design of their study was a large population based cohort study with long-term follow-up. The authors used standard Cox proportional hazard model to evaluate the effect of systolic blood pressure (SBP) at baseline on cardiovascular death. Despite the previous studies in which larger effect of SBP on cardiovascular mortality have been reported [2,3], the results of the study conducted by Bjørngaard and colleagues found that for each 20 mm Hg increase in SBP, hazard ratios (95% confidence interval) of cardiovascular death within couples and the population as a whole were 1.28 (1.17, 1.40) and 1.16 (1.12, 1.20), respectively [1]. We think the main limitation of the study conducted by Bjørngaard and colleagues is that the concept of time varying exposures and confounders is missed in the analyses. Since SBP is subject to fluctuations within individuals, using the baseline measurement of risk factors may dilute the true association and lead to biased estimate. It is worth mentioning that, the degree of bias will be large within long term follow-up periods [4]. On the other hand, there is a time-varying association between smoking and mortality because smoking is a time-
⁎ Corresponding author at: Department of Epidemiology, School of Public Health, Shahid Beheshti University of Medical Sciences, Tehran, Iran. E-mail address: [email protected] (E. Ayubi).
http://dx.doi.org/10.1016/j.ijcard.2017.02.145 0167-5273/© 2017 Elsevier B.V. All rights reserved.
varying confounder [3]. In such situations, we have to use suitable methods to correct the source of bias; otherwise, the results should be interpreted with caution. As take home message for readers is that regression dilution bias is common in biomedical research due to using single measurements at baseline [5]. Conflict of interest The authors report no relationships that could be construed as a conflict of interest. Acknowledgment The authors would like to thank statistics consultants of Research Development Center of Sina Hospital for their technical assistance. This work was not supported by any organization. References [1] J.H. Bjorngaard, G.A. Vie, S. Krokstad, I. Janszky, P.R. Romundstad, L.J. Vatten, Cardiovascular mortality - comparing risk factor associations within couples and in the total population - the HUNT study, Int. J. Cardiol. 232 (2017) 127–133. [2] A.S. Koh, M. Talaei, A. Pan, R. Wang, J.M. Yuan, W.P. Koh, Systolic blood pressure and cardiovascular mortality in middle-aged and elderly adults - the Singapore Chinese Health Study, Int. J. Cardiol. 219 (2016) 404–409. [3] K. Tilling, J.A. Sterne, M. Szklo, Estimating the effect of cardiovascular risk factors on all-cause mortality and incidence of coronary heart disease using G-estimation: the atherosclerosis risk in communities study, Am. J. Epidemiol. 155 (2002) 710–718. [4] D. Wormser, I.R. White, S.G. Thompson, A.M. Wood, Within-person variability in calculated risk factors: comparing the aetiological association of adiposity ratios with risk of coronary heart disease, Int. J. Epidemiol. 42 (2013) 849–859. [5] S. Safiri, S. Khazaei, K. Mansori, M. Sani, E. Ayubi, Differing predictive relationships between baseline LDL-C, systolic blood pressure, and cardiovascular outcomes: methodological issues, Int. J. Cardiol. 229 (2017) 141.