Statistics in the News: Chapter 12 Estimation
Assessing the Risk of Cardiovascular Disease
Pharmaceutical companies are forever searching for new and
better drugs that may dramatically impact their profits. For
that reason, they seek to learn all they can about the causes
of diseases and how these causes might respond to various
drugs. The Framingham Heart Study recently provided interesting
data on the relationship between blood pressure and the incidence
of cardiovascular disease (including such "events" as congestive
heart failure, myocardial infarction, and strokes).
The 10-year study involved 6,859 subjects-all of whom were
initially free of hypertension (high blood pressure) and cardiovascular
disease. The subjects were divided into three blood pressure
categories, as defined in Table A.
Table A. Blood Pressure Categories
|
Pressure
|
Optimal
|
Normal
|
High-Normal
|
|
Systolic
|
<120 mm Hg
|
120-129 mm Hg
|
130-139 mm Hg
|
|
Diastolic
|
<80 mm Hg
|
80-84 mm Hg
|
85-89 mm Hg
|
The study showed a clear positive relationship between blood
pressure and the incidence of cardiovascular disease: The
higher the blood pressure, the greater was the risk of such
disease. Some of the relevant data for men and women appear
in Table B.
Table B. The 10-Year Incidence of Cardiovascular Disease
|
Blood Pressure Category
|
Number of Subjects in Study
|
Number of Cardiovascular Events
|
Incidence in Percent
|
95% Confidence Interval
|
|
Men
|
|
Optimal
|
1,005
|
55
|
5.8
|
4.2 - 7.4
|
|
Normal
|
1,059
|
96
|
7.6
|
6.0 - 9.1
|
|
High-Normal
|
903
|
108
|
10.1
|
8.1 - 12.1
|
|
Women
|
|
Optimal
|
1,875
|
26
|
1.9
|
1.1 - 2.7
|
|
Normal
|
1,126
|
40
|
2.8
|
1.9 - 3.8
|
|
High-Normal
|
891
|
72
|
4.4
|
3.2 - 5.5
|
Note the rising incidence numbers (along with their associated
confidence intervals in the last column of Table B),
as blood pressure rises. Compared to optimal blood pressure,
the authors noted, higher blood pressure categories for either
sex were invariably associated with greater risk of cardiovascular
disease.
Table C provides an alternative view. It shows how much more
hazardous higher blood pressure is after adjustments are made
for the subjects' age, body mass index, total cholesterol
levels, presence or absence of diabetes mellitus, and smoking
status. The authors fitted a complex regression model
for the purpose. Statisticians will be interested to learn
that the p values associated with the Table
C data (indicating the likelihood of being able to attribute
the results to mere sampling error) were 0.01 for men and
< 0.001 for women.
Table C. HAZARD RATIOS
|
Blood Pressure Category
|
Hazard Ratio
|
95% Confidence Interval
|
|
Men
|
|
Optimal
|
1.0
|
|
|
Normal
|
1.3
|
1.0 - 1.9
|
|
High-Normal
|
1.6
|
1.1 - 2.2
|
|
Women
|
|
Optimal
|
1.0
|
|
|
Normal
|
1.5
|
0.9 - 2.5
|
|
High-Normal
|
2.5
|
1.6 - 4.1
|
Sources: Adapted from Ramachandran S. Vasan et al,
"Impact of High-Normal Blood Pressure on the Risk of Cardiovascular
Disease," The New England Journal of Medicine, November
1, 2001, pp. 1291-1297, and Julio A. Panza, "High-Normal Blood
Pressure-More 'High' Than 'Normal'" ibid., pp. 1337-1340.
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