Survey population. Data analysed were taken from the Core Content and National Health Survey components of the Australian Health Survey (AHS),95 conducted between 2011 and 2013 by the Australian Bureau of Statistics (ABS). The survey was a nationally representative sample of individuals from 20,500 private dwellings across Australia (n=31,837).96 Data on 20,716 individuals (≥18 years old) who had two BP readings in the AHS were included in the primary analysis and secondary analysis was performed among participants who had three BP readings (n=5,189). An age threshold of 50 years was used to delineate younger from older individuals based on the well documented age-related changes in haemodynamic patterns after age 50.97 Additional information on survey design, ethics approval, anthropometry, statistical details, BP exclusionary criteria, classification of BP control, anti-hypertensive medications,
57 biomedical measures and health conditions are provided in supplementary material (Appendix 2).
Blood pressure. Duplicate BP readings were taken by non-clinicians using a validated,98 automated BP monitor (A&D:UA-851) on the left arm using an appropriate cuff size at the end of a survey interview with participants seated and relaxed. If there was a difference between the first and second BP reading of >10 mmHg in either SBP or DBP, then a third reading was taken.99 BP measurements were consecutive; the precise time of each measurement was not recorded. BP readings were excluded if they were outside the extremes of physiological range and/or measurement error was suspected. These exclusion thresholds were: SBP >260 or <70 mm Hg, DBP >150 or <40 mm Hg, and pulse pressure (PP) >150 or <20 mm Hg. Participants with only one BP measure were excluded from this analysis. The above measurement protocol enabled assessment of BP classification using all the international guidelines previously mentioned. Primary outcomes and measures included the reclassification of BP category, the direction of change from first (SBP1) to second SBP (SBP2) readings and the absolute difference between the SBP1 and SBP2 readings (|ΔSBP|). Reclassification of BP category was defined as the change of a participants’ BP status either from hypertension at SBP1 (≥140 mmHg) to normal average SBP (average SBP<140 mmHg) based on the above protocols, or from normal SBP1 (<140 mmHg) to hypertension (average SBP≥140 mmHg) based on the above protocols. Therefore, for the purpose of examining reclassification; when BP is classified based on one reading we use the term “normal SBP1” or “high SBP1”. When BP is classified based on the average of more than one readings we use the term “normal average SBP” or “high average SBP”. The cut-off of 140 mmHg was chosen to show normality of SBP based on the ESH/ESC guidelines; however, we acknowledge that other definitions may apply according to other guidelines (i.e. 130-139 mmHg might refer to prehypertension). Table 4. 1 summarizes the CHEP, NICE, ESH/ESC and JNC-7 hypertension guideline recommendations.
58 Hypertension definition and classification of blood pressure category. For the primary analysis, hypertension was defined as SBP ≥140 mmHg based on five protocols relevant to international guidelines and the goals of this study. The five protocols presented in Table 4.1 required either one, two or three BP readings for BP classification and were compared based on reclassification as described previously: 1) the average of SBP1 and SBP2 (not taking into account the magnitude of ΔSBP), 2) the average of SBP1 and SBP2 if ΔSBP≥10 mmHg, 3) the average of SBP1, SBP2 and third SBP (SBP3) readings if ΔSBP≥10 mmHg, 4) the average of SBP2 and SBP3, discarding SBP1 and; 5) the lower of the last two SBP readings if ΔSBP≥10 mmHg. Following hypertension classification based on SBP, classification based on DBP ≥90 mmHg was also undertaken. SBP was classified as; low (SBP <90 mmHg), normal (90-129 mmHg), high normal (130-139 mmHg), grade I (140-159 mmHg), grade II (160-179 mmHg) and grade III (≥180 mmHg) hypertension. Similarly, DBP was classified as; low (<60 mmHg), normal (60-84 mmHg), high normal (85-89 mmHg), grade I (90-99 mmHg), grade II (100-109 mmHg) and grade III (≥110 mmHg) hypertension.
Health conditions.Participants were asked whether they had been diagnosed with a medical or health condition and whether this condition was: 1) still-current and long-term, 2) still current but not long-term or 3) not current.100 The specific classification of long-term health conditions reported by the participants was based on the International Classification of Diseases.101
Anti-hypertensive medications. 20,500 of the individuals participating in the Australian Health Survey had also provided detailed information regarding the use of medications (including antihypertensives) which was retrieved from the National Health Survey, 2011-2012 component of the survey. Information regarding the use of medications as collected by the National Health Survey has been previously published.102
59 Statistical analysis. Means (continuous data) and percentages (categorical data) are reported as summary measures, together with 95% confidence intervals because analyses were weighted using weights supplied by the ABS. The analysis utilised person-weights96 provided by the ABS, which ensured that any disproportionate sampling of certain groups was taken into account. Replicate weights provided by the ABS were used to calculate standard errors and 95% confidence intervals using the Jackknife delete-1 method. Linear regression was used to estimate the relationship of |ΔSBP| with SBP1 and age as predictor variables, with the square of SBP1 included to capture non-linearities and product terms (age x SBP1, age x squared of SBP1) used to capture interactions between age and SBP1. Final models included covariates to adjust for sex and body mass index. Further analysis was performed to evaluate the effect of cardiovascular disease or antihypertensive medications on the main findings. Analysis was also performed on secondary outcomes (supplementary material, [Appendix 2]; the difference between SBP2 and SBP3; the overall variability in 3 SBP measures [coefficient of variation; SBP CV]; the difference between DBP readings; the difference between pulse pressure (PP) readings). Stata 10 was used for all analyses (StataCorp, College Station, Tx). A two-sided p value <0.05 was considered significant.