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EDITORIAL COMMENTARIES

Birth cohort or measurement protocol differences?

Wiley, Joshua F.

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doi: 10.1097/HJH.0000000000001363
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Studies have reported age-related differences in blood pressure (BP) utilizing cross-sectional designs [1] as well as trajectories of BP in longitudinal designs [2]. However, little research examines whether and in what way the age-related trajectories of BP change over time. In this issue of Journal of Hypertension, Joas et al.[3] compared the trajectories of BP over 10 years in 70–80-year-olds from two Swedish birth cohorts approximately 30 years apart.

Joas et al.[3] reported lower levels of SBP and DBP across all ages from the 1930 versus 1901–1902 birth cohort. Stratified by sex, in the 1901–1902 birth cohort, women had higher SBP and DBP than men, but in the 1930 birth cohort women had comparable or lower BP. Interpreting these findings is complicated by limited information on mechanisms to explain why such differences might exist. The authors note potential ‘white-coat’ differences as there was a measurement protocol difference in BP assessments between the two birth cohorts, with the 1901–1902 cohort assessments conducted by physicians but the 1930 cohort assessments conducted by nurses. Indeed, not only are there potential differences, early research on the white-coat effect showed a mean SBP difference in the same individuals when assessed by physicians versus technicians of 6 mmHg in normal volunteers, rising to 14 mmHg in adults with hypertension [4]. These effects are large enough to explain much of the observed differences between birth cohorts, and call for caution in conclusions regarding any differences.

If the observed cohort differences represent true changes in population BP over time and not differences due to the sample or measurement, these findings have practical implications. First, the findings suggest that the management of BP has improved substantially, on average. This finding is surprising insofar as research demonstrates a growing obesity epidemic [5,6], and indeed at age 70 years, Joas et al.[3] reported that adults from the 1930 cohort had an average BMI 1.25 higher than those in the 1901–1902 cohort. Considering the links between BMI and hypertension [7], based on BMI, participants in the 1930 birth cohort should have had higher, not lower, BP. A natural assumption may be that higher rates of antihypertensive treatment in the 1930 birth cohort explain their lower BP. However, sensitivity analyses adjusting for greater rates of antihypertensive treatment in the 1930 versus 1901–1902 birth cohort failed to fully account for the observed birth cohort differences.

Considering that Joas et al.[3] reported an average difference in SBP of over 20 mmHg in women between birth cohorts, if representative of true population differences, a second implication is that caution is warranted when interpreting and comparing age-related and sex-related differences in longitudinal studies conducted at different points in time. Likewise studies that pool multiple datasets (e.g. [2]) and meta-analyses on summary or individual patient data should consider accounting for birth cohort differences in addition to adjusting for age-related differences.

To move forward, research is needed that does more than quantify the differences between birth cohorts. Population databases or national health registries [8] are needed that over time systematically track risk factors related to BP beyond antihypertensive treatment, such as diet [9], exercise [10], sleep [11], and other established risk factors. Only with this additional information will we be able to understand not only that birth cohort differences exist, but why they exist and what can be done to continue improving population health.

ACKNOWLEDGEMENTS

Conflicts of interest

There are no conflicts of interest.

REFERENCES

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