Wednesday December 21 2011
Many people do not realise they have high blood pressure
“Blood pressure control is key to a healthy heart,” the Daily Express has reported. The newspaper said that keeping your blood pressure levels low before you turn 55 dramatically cuts your risk of cardiovascular disease.
The news is based on US research that looked for a potential link between blood pressure in middle-aged people (defined as 41-55 years old) and the risk of cardiovascular disease over the rest of their life. The study combined the results of seven previous studies featuring 61,585 men and women and looked at their risk of both fatal and non-fatal health issues after the age of 55, including heart attacks and strokes.
As would be expected, the researchers found that the overall cardiovascular risk was slightly higher for men than for women, and that ethnicity also had an effect (cardiovascular risk has been observed to be greater in people of African or Asian backgrounds). They also found that people who reduced or maintained their blood pressure at normal levels between the ages of 41 and 55 had lower cardiovascular risk than those who maintained or developed high blood pressure (hypertension) over the same period.
High blood pressure has long been a well-established risk factor for cardiovascular disease, and this study lends further support to the importance of controlling blood pressure. While some risk factors, such as ethnicity, cannot be changed, the research suggests that controlling so-called “modifiable” factors, even at a younger age, can have notable benefits.
Where did the story come from?
This US study was performed by researchers from Northwestern University’s Feinberg School of Medicine, and the Southwestern Medical Center, Dallas. The individual authors and the overall research programme received various sources of financial support, including from the US National Heart, Lung, and Blood Institute. The study was published in the peer-reviewed medical journal Circulation.
The media reflected the findings of this study accurately.
What kind of research was this?
This research, called the Cardiovascular Lifetime Risk Pooling Project, investigated whether changes in blood pressure during middle age affect the risk of cardiovascular disease, such as coronary heart disease (CHD) and strokes, later in life. The researchers say that earlier research has examined the effect of blood pressure on cardiovascular disease, but only by assessing blood pressure at a particular age. It did not take into account how blood pressure changes over time. To investigate this issue, the researchers pooled and analysed data from seven US cohort studies that had examined this topic.
What did the research involve?
The Cardiovascular Lifetime Risk Pooling Project was made up of 17 US cohort studies, which all met certain criteria. They had to:
- feature a community or representative population sample
- assess participants at least once at the start of the study, recording demographic, personal and medical history and taking blood pressure and body measures
- follow participants for at least 10 years
- assess the outcomes of cause-specific or cardiovascular deaths, and non-fatal cardiovascular disease events
The authors of this latest review were particularly interested in cohorts that included mixed ethnic groups so that they could compare the cardiovascular risk between groups. Full and complete data were obtained from seven of these cohorts and were used in this analysis.
Blood pressure in all the studies was recorded as the average of two or three measurements. People were categorised as having:
- normal blood pressure: blood pressure (BP) lower than 120/80mmHg, while on no BP medication
- pre-hypertension: systolic BP (when the heart beats) of 120-139mmHg or diastolic BP (when the heart rests) of 80-89mmHg, while on no BP medication
- stage 1 hypertension: systolic BP of 140-159mmHg or diastolic BP of 90-99mmHg, while on no BP medication
- stage 2 hypertension: either systolic BP higher than 160mmHg or diastolic BP higher than 100mmHg, or if the person was already being treated for hypertension
The age at which the researchers started following participants for cardiovascular outcomes (known as the index date) was 55 years old. However, most people in the cohorts had received their first blood pressure measurement an average of 14 years before this, at the age of 41. The researchers could therefore look at how blood pressure had changed before the index date of 55, allowing them to see whether it had remained the same, increased or decreased. Blood pressure was also reassessed during follow-up into older middle age.
Each person in the study was followed from the age of 55 until a first cardiovascular disease event, death, or the age of 95, whichever came first. For most cohorts, all available medical records were used to assess fatal or non-fatal cardiovascular disease events, with the National Death Index used for gathering death certificate data on the underlying cause of death. Cardiovascular disease risk was assessed in relation to the participants’ blood pressure at ages 45, 55, 65 and 75. Separate analyses were made for white and black individuals due to the link between race and heart risk. Starting at age 55, the researchers followed 61,585 men and women across the seven cohorts. This provided an average of 700,000 person-years of follow-up data.
What were the basic results?
Over half of all men and women had consistent blood pressure from the age of 41 through to 55. Almost 20% of men and 10% of women experienced decreases in their blood pressure over this time, and 30% of men and 40% of women had an increase in blood pressure. At 55 years:
- 25.7% of men and 40.8% of women had normal blood pressure
- 49.4% of men and 47.5% of women had pre-hypertension
- 18.1% of men and 9.6% of women had hypertension
- 6.8% of men and 2.2% of women had stage 2 or treated hypertension.
Starting at age 55, the risk of having a cardiovascular disease outcome in a person’s remaining lifetime was 52.5% for men (95% confidence interval [CI] 51.3 to 53.7) and 39.9% (95% CI 38.7 to 41.0) for women. Risk of cardiovascular disease was also higher for black people than for white people, and was higher among individuals who had a higher blood pressure at age 55.
People who had maintained or decreased their blood pressure to normal levels before the age of 55 had the lowest risk of cardiovascular disease in their remaining lifetime (22-41%). Those who had or developed hypertension between the ages of 41 and 55 had the highest risk (42-69%).
How did the researchers interpret the results?
The researchers concluded that people who have an increase in blood pressure in middle age have a higher risk of developing cardiovascular disease over their lifetime. Conversely, those who have a decrease in blood pressure in middle age have a lower risk. They advise that cardiovascular disease prevention strategies should focus on the importance of avoiding hypertension.
This research is said to be the first study to examine how changes in blood pressure throughout middle age (taken as the average change from the age of 41 to 55) can affect subsequent lifetime risk of cardiovascular disease, including events such as coronary heart disease and strokes. It looked at a large, multi-ethnic US population and found that individuals who maintained or reduced their blood pressure to normal levels by the age of 55 had the lowest risk of cardiovascular disease in their remaining lifetime, at around 22-41%. Those who had or developed high blood pressure between these ages had the higher cardiovascular risk, at 42-69%.
In some ways, the findings are not surprising: cardiovascular diseases have various risk factors, some that can’t be changed (such as age, gender, ethnicity and family history) and some that can be influenced by healthy lifestyle changes or appropriate medical management. These “modifiable” risk factors include smoking, diabetes, being overweight or obese, high cholesterol and high blood pressure. However, the new and perhaps most interesting finding is that maintaining lower blood pressure earlier in life may have notable benefits in later life.
There are, however, some limitations to the research, particularly as so many factors can influence both blood pressure and cardiovascular disease risk. For example, although the researchers adjusted their results for several factors that could have affected the outcome, including age, gender and ethnicity, many other potential factors that could affect both high blood pressure and cardiovascular risk were not assessed, including lifestyle factors such as smoking and alcohol, excess weight and socioeconomic status. In addition, combining data from cohort studies with slightly different methods of population sampling and follow-up may affect the accuracy of the final study’s results. Also, it is not possible to pick apart the effect of individual factors that may have influenced a participant’s blood pressure, for example decreasing cholesterol, starting blood pressure medication and making lifestyle changes.
Nevertheless, high blood pressure has long been a well-established risk factor for cardiovascular disease, and this study lends further support to the importance of controlling blood pressure in middle age, not just in later life.