Monday February 25 2008
Rates of coronary artery disease have been linked to lifestyle
“Heart risk rises for live-now generation” warns the headline in The Times today. The newspaper goes on to say that heart disease, which has been declining for 30 years, appears to be “back on the rise mainly among the young”. Evidence from the UK, US and Australia suggests that while death rates from heart disease continue to fall in the old, they are levelling off or rising in men and women aged 35 to 54 years old, The Times says.
The story is based on more than one piece of research; however, the newspaper mainly refers to a small study of autopsies in Minnesota, which looked at the severity of coronary artery disease in adults who had died for “non-natural” reasons (i.e. suicide, accident or homicide) over a 24-year period. The results supported those of other studies, finding a decrease in coronary artery disease overall since the 1980s. However, when broken down by year, the decline in coronary artery disease seemed to reverse after about 2000.
Another study mentioned in The Times, but not discussed in detail here, showed similar results. It found an increase in mortality associated with coronary artery disease in young adults after 2000. The changes in the rates of heart disease suggested by these recent studies must be considered by health professionals; however, further studies are needed to confirm these results and any possible reasons for them.
Where did the story come from?
Dr Peter Nemetz and colleagues from the University of British Columbia and the Mayo Clinic College of Medicine in Minnesota carried out this research. The study was funded by grants from the National Institutes of Health and the AJ and Sigismunda Palumbo Foundation. It was published in peer-reviewed medical journal Archives of Internal Medicine.
What kind of scientific study was this?
The research was a time-trend study, which repeats cross-sectional analysis over time, of autopsies of people who died from non-natural causes in Minnesota between January 1 1981 and December 31 2004. Only people aged 16 to 64 years were included.
The researchers used health records to identify the deaths in Minnesota between January 1 1981 and December 31 2004, where the cause of death was not natural and an autopsy was carried out. The researchers reviewed autopsy records and pathology reports, and looked at the severity of coronary artery disease in each patient. The researchers then assigned a “grade” of coronary artery disease to each patient, a measure based on the degree of blocking of each main coronary artery. After this process, 425 cases were available for analysis. The researchers assessed whether the year of death was associated with the severity of coronary artery disease and how the prevalence of coronary artery disease changed over time.
What were the results of the study?
The researchers found that, over time, significantly more non-natural deaths had evidence of coronary artery disease, though there was no change over time in the average age at which death occurred.
Over the 24 years of the study, only 35 of the 425 deceased people had high-grade coronary artery disease (i.e. severe coronary artery disease). The proportion of people with high-grade coronary artery disease declined over the whole time of the study, particularly in males and in younger people compared with older people.
When researchers looked at changes over time more closely, they found that reductions in the severity of coronary artery disease ended after 1995 and the grade of coronary artery disease “may have increased since 2000”.
A study on a similar theme by Earl Ford and Simon Capewell that compared coronary heart disease mortality among young adults in the USA between 1980 and 2002 found that the rate of decline in coronary artery disease mortality had slowed.
What interpretations did the researchers draw from these results?
The study found that, over the full study period, the prevalence of coronary artery disease decreased. This result supports other studies that show reductions in coronary artery disease over time. However, the researchers found that the decline in the grade of coronary artery disease seen at autopsy had ended and possibly reversed. They state that this provides the “first data to support increasing concerns that declines in heart disease mortality may not continue”. The researchers add that further investigation is needed to confirm whether recent trends can be attributed to obesity and diabetes mellitus.
What does the NHS Knowledge Service make of this study?
Importantly, none of the subjects included in this study had died from heart disease. Though the researchers say that “information drawn from autopsies has been considered the gold standard for case detection and enumeration”, they go on to discuss at some length the potential biases of this type of study. Firstly, rates of autopsy are not high and have been declining, with the rate of decline the steepest in older age groups. Secondly, the decision to conduct an autopsy is significantly associated with the diagnosis of coronary artery disease. This means that those with coronary artery disease may be under- or over-represented in an autopsy sample. However, the researchers suggest that an autopsy study in people who died of non-natural causes largely avoids this bias because rates of autopsy are not influenced by whether the person had coronary artery disease and that all ages are as likely to be autopsied. There are some other limitations to this study that the researchers discuss:
- The study was carried out in a single county in the USA. The findings may not apply to other counties and populations. Similarly, the trends observed cannot be generalised to people who die of other causes (i.e. natural causes). Deaths from coronary artery disease will largely be classified as deaths from natural causes.
- Importantly, the study was in non-elderly people. As there was no comparison with trends in the elderly, for whom the authors say autopsy rates are lower, implying that the apparent reversal in decline of coronary artery disease after 1995 was only evident in the young is not strictly accurate. Also, the study did not analyse trends by age of individual because the sample sizes were too small. Instead, it looked at the change in trends over different calendar years.
- The study relied on the “grade” of coronary artery disease as determined by the pathologist conducting the autopsy. The researchers note that there were changes in staff over time. This means that the way coronary artery disease was recorded was likely to have changed too.
These limitations mean that on their own the results of this study do not provide very strong evidence of a change in trends of heart disease. However, another recent study that looked at changes in rates of death due to coronary heart disease over a similar time period found similar results, i.e. that, overall, rates have declined since the 1980s, but that trend has levelled out since about 2000 in young adults. Neither study can conclusively give reasons for this change.