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Findings

No nationally representative blood pressure (BP) data for Canadian children and adolescents have been collected since the 1978 Canada Health Survey.¹ With the results of the 2007-2009 Canadian Health Measures Survey (CHMS), launched by Statistics Canada in partnership with Health Canada and the Public Health Agency of Canada, it is possible to address this data gap.^2-5 The CHMS is the most comprehensive direct health measures survey ever conducted in Canada. In addition to a detailed health interview, the survey involves direct measurement of indicators and of risk factors for chronic diseases, infectious diseases, environmental exposures, nutritional status, physical activity and physical fitness.^2-5

Elevated BP is one of the most important causes of death and disability worldwide,⁶ accounting for 7.6 million premature deaths and 92 million disability-adjusted life years annually. In adolescence, hypertension is associated with increased left ventricular mass, diastolic dysfunction,⁷ fatty streaks and fibrous plaques in the coronary arteries and the aorta,⁸ and arterial wall thickening.⁹ BP levels track from childhood to adulthood,^10,11 indicating that elevated BP at young ages is a risk factor for the development of hypertension in adulthood. The strength of BP tracking increases with body mass index (BMI), such that tracking is strongest in overweight and obese youth.^12,13

Population information about BP levels in children and adolescents can be useful from a public health and clinical perspective to guide prevention planning, help establish norms, and monitor trends over time. However, Quebec is the only Canadian province to have relatively recent measures for youth: in 1999, 12% to 23% of youth aged 9, 13 and 16 years had high-normal or elevated BP.¹⁴ A 2004 study of BP levels among American youth found that from 1988-1994 to 1999-2000, mean systolic blood pressure (SBP) increased 1.4 mmHg, and mean diastolic blood pressure (DBP) increased 3.3 mmHg.¹⁵ A longer-term review of trends in American youth from 1963 to 2002 also demonstrated a slight upturn in the prevalence of elevated BP in the last decade.¹⁶ But such findings have not been consistent. For example, a study of 15-year-old Russian adolescents between 1995 and 2004 found a significant decrease in DBP among boys, and a significant decrease in SBP among both sexes.¹⁷ As well, comparisons of results from past studies are complicated by different survey methods, including different measurement devices.

Based on data from the 2007-2009 CHMS, this study presents BP distributions and estimates of elevated BP for a representative sample of Canadian children and adolescents aged 6 to 19 years.

Methods

Data source

Data are from cycle 1 of the Canadian Health Measures Survey (CHMS), which collected information at 15 sites from March 2007 through February 2009. The CHMS covered the population aged 6 to 79 years living in private households. Residents of Indian Reserves or Crown lands, institutions and certain remote regions and full-time members of the regular Canadian Forces were excluded. Approximately 96.3% of Canadians were represented.¹⁸

Health Canada’s Research Ethics Board gave ethics approval to conduct the survey. Informed written consent was obtained from respondents aged 14 years or older. For younger children, a parent or legal guardian provided written consent, in addition to written assent from the child. Participation was voluntary; respondents could opt out of any part of the survey at any time.

The response rate for households selected for inclusion in the CHMS was 69.6%—meaning that in 69.6% of selected households, the sex and date of birth of all household members were provided by a household resident. In each responding household, one or two members were chosen to participate; 88.5% of selected 6- to 19-year-olds completed the household questionnaire, and 86.9% of those who completed the questionnaire participated in the subsequent examination centre component. The final response rate for 6- to 19-year-olds, after adjusting for the sampling strategy, was 53.5%. This article is based on 2,079 examination centre respondents aged 6 to 19 years (after removing 8 with missing BP data) (Appendix Table A).

Appendix Table A
Sample sizes for selected characteristics, by sex and age group, household population aged 6 to 19 years, Canada, March 2007 to February 2009

Measures

At the respondent’s home, an interviewer administered a questionnaire covering socio-demographic characteristics, medical history, current health status and lifestyle behaviours (Table 1). In the chronic conditions component of the questionnaire, respondents aged 12 years or older were asked if they had high BP (diagnosed by a health professional and expected to last or had already lasted six months or more) and if they had taken “medicine for high blood pressure” in the past month.

Table 1
Selected characteristics of sample (weighted), by age group and sex, household population aged 6 to 19 years, Canada, March 2007 to February 2009

One day to six weeks after the home interview, the respondent visited a mobile examination centre for a battery of physical measurements, including anthropometry, BP, heart rate, spirometry, physical fitness, oral health and biospecimen collection.⁴ BMI was calculated as weight in kilograms divided by height in meters squared (kg/m²), and respondents were classified as overweight, obese, or neither.^19,20 BP was measured after urine collection, but before blood collection and fitness testing.⁴

BP and heart rate were measured with the BpTRU™ BP-300 (BpTRU Medical Devices Ltd., Coquitlam, British Columbia). The BpTRU™, an automated electronic monitor, automatically inflates and deflates the upper-arm cuff and uses an oscillometric technique to calculate SBP and DBP. It has passed international validation protocols for accuracy.^21,22

An advantage of an automated device is that it enables BP to be measured in the absence of another person, thereby eliminating observer errors such as digit bias, zero preference and incorrect deflation rates, and reducing “white-coat hypertension” (a rise in BP associated with the presence of the health care professional and the measurement procedures).²³ For more detailed information on the procedures and protocol, including staff training, equipment calibration, and quality assurance and control, see Resting blood pressure and heart rate measurement in the Canadian Health Measures Survey cycle 1.²⁴

Definitions

Measures of SBP and DBP were calculatedas the average of the first set (last five of six measures taken one minuteapart) of valid BP measurements.²⁴ For those aged 6 to 17 years, based on age and sex, each respondent’s height and average SBP and DBP were converted to z-scores, which were used to calculate individual BP percentiles as per the equations in Appendix B of the fourth report of the National High Blood Pressure Education Program Working Group on High Blood Pressure in Children and Adolescents (NHBPEP4).²⁵ With these calculated percentiles, children and youth in this age group were classified into BP categories. As well, respondents who reported taking medicine for high BP in the past month were classified as having “elevated” BP, regardless of their BP percentile value (fewer than 10 respondents). The seventh report of the Joint National Committee on Prevention, Detection, Evaluation and Treatment of High Blood Pressure (JNC7) was used to classify youth aged 18 or 19 years.²⁶ The NHBPEP4 classification parallels that of the JNC7.

Normal BP for respondents aged 6 to 17 years was defined as a calculated SBP percentile and DBP percentile less than the 90th percentile. For respondents aged 18 or 19 years, it was defined as a measured mean SBP less than 120 mmHg and a measured mean DBP less than 80 mmHg. This corresponds to the “normal” category proposed by the NHBPEP4 and JNC7.

Borderline BP for respondents aged 6 to 17 years was defined as a calculated SBP percentile or DBP percentile greater than or equal to the 90th percentile, but less than the 95th percentile, or a measured SBP/DBP greater than 120/80 mmHg, even if less than the 90th percentile. For respondents aged 18 or 19 years, it was defined as a measured mean SBP of 120 to 139 mmHg and a measured mean DBP of 80 to 89 mmHg; or SBP of 120 to 139 mmHg and DBP lower than 80 mmHg; or SBP lower than 120 mmHg and DBP 80 to 89 mmHg. This corresponds to the “prehypertension” category proposed by the NHBPEP4 and JNC7.

Elevated BP for respondents aged 6 to 17 years was defined as a calculated SBP percentile or DBP percentile greater than or equal to the 95th percentile, or the respondent’s report of using BP medication in the past month. For respondents aged 18 or 19 years, elevated BP was defined as a measured mean SBP/DBP of 140/90 mm Hg or higher, or the respondent’s report of BP medication use in the past month. This corresponds to the “Stage 1 or Stage 2 hypertension” category proposed by the NHBPEP4 and JNC7.

Analytical techniques

Weighted data were analyzed separately by sex and age. Estimates of proportions, means, standard errors, and percentiles were produced. Standard errors, coefficients of variation and 95% confidence intervals (CI) were estimated using bootstrap weights to account for the complex survey design of the CHMS.^27,28 Gender differences in SBP and DBP were tested using t-tests. Analyses were conducted with SUDAAN.

Results

Mean SBP (standard error) rose with age from 91(1) mmHg among boys aged 6 to 7 years to 104(1) mmHg at 18 to 19 years; for girls, the increase was from 92(1) to 99(1) mmHg (Table 2). Mean SBP was similar in boys and girls from ages 6 to 7 through 10 to 11 years, and also at 14 to 15 years. However, at 12 to 13 years and 16 through 19 years, mean SBP was higher in boys (p<0.01). Median SBP was very close to the mean in all age/sex categories.

Table 2
Percentile distribution of measured systolic blood pressure (SBP) (mmHg) values, by sex and two-year age group, household population aged 6 to 19 years, Canada, March 2007 to February 2009

The sample size was too small to obtain percentile values by single-year-of-age or 95th percentile values for most two-year age groups. At ages 6 to 11 years, the 95th percentile (95% CI) for SBP was 105 (102 to 107) mmHg among boys and 106 (104 to 108) mmHg among girls; at ages 12 to 19 years, the 95th percentile for SBP was 116 (113 to 119) mmHg among boys and 111 (108 to 114) mmHg among girls.

Mean DBP also rose with age, but not as much as SBP (Table 3). From ages 6 to 7 to 18 to 19 years, mean DBP increased from 59(1) to 65(1) mmHg among boys and from 60(1) to 64(1) among girls. Mean DBP was similar in both sexes. Median DBP was very close to the mean in all age/sex groups.

Table 3
Percentile distribution of measured diastolic blood pressure (DBP) (mmHg) values, by sex and two-year age group, household population aged 6 to 19 years, Canada, March 2007 to February 2009

In 2007-2009, few Canadian children and adolescents had borderline or elevated BP: 3.7% (2.3% to 6.0%) at ages 6 to 11 years and 2.2% (1.2% to 4.0%) at 12 to 19 years (Table 4).

Table 4
Percentage distribution of measured blood pressure status, by sex and age group, household population aged 6 to 19 years, Canada, March 2007 to February 2009

Mean SBP was higher among children and adolescents who were overweight or obese (Table 5). The SBP differences between BMI categories reached statistical significance among boys aged 12 to 19 years, girls aged 6 to 11 years, and in both age groups when the genders were combined. Differences in DBP by BMI category were less apparent, reaching statistical significance only among obese boys aged 12 to 19 years.

Table 5
Mean measured value of systolic (SBP) (mm/Hg) and diastolic blood pressure (DBP) (mm/Hg), by age group, sex and body mass index (BMI) category, household population aged 6 to 19 years, Canada, March 2007 to February 2009

Discussion

The main finding of this analysis is the remarkably low overall prevalence of borderline or elevated BP among Canadian children and adolescents.

However, echoing the results of other studies,^14,15 mean SBP was significantly higher among boys aged 12 to 19 years and girls aged 6 to 11 years who were overweight or obese. Excess weight is believed to influence BP through increased sympathetic nervous system activation, which is associated with SBP. The association of weight with DBP was much less pronounced.

The generally low levels of BP obtained from the CHMS appear inconsistent with the rise of childhood and adolescent obesity in Canada.^29,30 And despite a trend toward excess weight among youth in other countries, BP levels have not shown consistent increases.³¹Hence, population-level increases in BP may not necessarily be a consequence of rising weight. More research is required to explain this apparent paradox.

For each age and sex category, mean child and adolescent SBP in Canada was about 10 mmHg lower than the most recent United States National Health and Nutrition Examination Survey (NHANES) data.³² The only other recent BP data from a large, representative sample of youth in Canada were collected in 1999 by the Quebec Child and Adolescent Health and Social Survey (QCAHS) from respondents aged 9, 13 and 16 years.¹⁴ Compared with the results of the QCAHS, mean SBP at these ages in the CHMS was 9, 16 and 20 mmHg lower in boys, and 8, 17 and 16 mmHg lower in girls.

CHMS values for DBP generally exceeded the NHANES results, with a mean difference of 5 mmHg higher in boys and 2 mmHg higher in girls. And compared with the QCAHS, the CHMS values were 8, 7 and 7 mmHg higher in boys aged 9, 13 and 16 years, respectively, and 9, 5 and 7 mmHg higher among girls of the same ages.¹⁴

Differences in measurement instruments and procedures may, in part, explain the disparities in BP levels in the three surveys. The CHMS used the BpTRU™ device; QCAHS used the DINAMAP (Critikon Co, FL) device; and NHANES used mercury sphygmomanometers. The last has been the gold standard for BP assessment for many years, but its use in children is decreasing because mercury-containing instruments are being removed from pediatric environments, and because auscultatory methods are subject to various biases (digit preference, rounding, white coat hypertension, etc.). The substantial differences between the CHMS and QCAHS may be due to opposing systematic differences between BP measured by mercury manometers and by the DINAMAP and the BpTRU. The DINAMAP has been reported to overestimate SBP by about 10 mmHg and slightly underestimate DBP, whereas the BpTRU may slightly underestimate DBP (by 2.1 mmHg), compared with the mercury manometer.³³ Most cases of borderline or elevated BP among CHMS participants had diastolic rather than systolic elevation, whereas clinically, most reported cases of pediatric hypertension are the result of an increase in SBP, thought to reflect, at least in part, hyperactivity of the sympathetic nervous system. Counterintuitively, children aged 6 to 11 years were somewhat more likely to have borderline or elevated BP than were adolescents aged 12 to 19 years.

The CHMS procedures may also have contributed to lower mean SBP. Measurement in a quiet room and in the absence of staff may have been conducive to maximal subject relaxation, which could decrease sympathetic activation and lower SBP. By contrast, the QCAHS measurements took place in school settings, usually a room where other survey-related measures were going on and in the presence of a staff member recording BP readings.^14,34

Limitations

The overall CHMS response rate was slightly above 50%. Although survey weights were adjusted to the socio-demographic characteristics of the Canadian population, it was not possible to adjust for many factors that could be associated with BP levels. Selection bias would be present if the BP levels of non-participants differed systematically from those of participants. In addition, the logistical and cost constraints associated with the use of mobile examination centres restricted the number of collection sites to 15.¹⁸ Whether this sampling strategy affected the results is unknown.

Conclusion

A small percentage of Canadians aged 6 to 19 years have borderline or elevated BP. More research is required to improve our understanding of BP levels and their determinants in order to help maintain healthy levels over the life-course.

Funding

Gilles Paradis holds a Canadian Institutes of Health Research (CIHR) Applied Research Chair in Public Health. Arnaud Chiolero holds a Ph.D. fellowship from CIHR. Ian Janssen holds New Investigator Awards from the CIHR and Ontario Ministry of Research and Innovation.

Acknowledgements

We thank Liane Fransblow and Nathalie Theoret for their contributions.