Platelet Aggregation and Coronary Heart Disease Risk Factor Variation in Australian Populations with Different Coronary Heart Disease Mortality Andrew Wilson, MBBS, FRACP, Stephen Leeder, MBBS, PhD, FRAU’, Jerry Koutts, MBBS, MD, FRACP, Richard Heller, MBBS, MD, FFCM, Tom Exner, PhD, and Andrew Dinale, B App Sci In a cross-sectional analytic study, we examined the diff erences in coronary heart disease (CHD) risk factors, including coagulation factors and platekr aggregation, among males from southern European countries and those of Anglo-Celtic descent who had widely different CHD standardized mortality ratios. The participants included I69 men aged 40 to 49 years, 27% of whom were born in southern European countries. The subjects had no history of heart disease and no orher clinical conditions, or were not taking medications known to affect hemostasis. Data obtained included their medical history and CHD-related risk behaviors, blood pressure, height, weight, abdominal and pelvic circumference, and coagulation, jibrinolysis, platelet activity, lipids, and lipoproteins profiles. There were signifcant differences between the two groups in the prevalence of a positive family history, mean apolipoprotein Al levels, and platelet aggregation responses to ADP. Other established risk factors, including coagulation factor levels, were not significantly different. Ann Epidemiol 1992;2:495-508. Coronary

KEY WORDS:

heart disease, risk factors, ethnicity, coagulation,

platelets, lipids.

INTRODUCTION Thrombosis plays a major role in the clinical presentation of coronary heart disease (CHD) and probably in the formation of atherosclerosis (1, 2). Factors associated with the formation

of thrombus

also predict

the risk of CHD.

In six cohort

studies,

fibrino-

gen level was a strong, independent risk factor for CHD (3-8). In one study (5), factor WC (FVIIc) level was an independent risk factor for CHD, especially within the first 5 years of enrollment,

and two other

studies

showed

similar

trends

(4, 7).

Plasminogen activator inhibitor and platelet hyperreactivity predict re-infarction lo), and factor VIII, factor X, and antithrombin III may be clinically important

(9, but

await

confirmation in epidemiologic studies (11). Studies of cross-cultural differences in risk factor prevalence have contributed substantially to our knowledge of the determinants of CHD, which differences in risk factor prevalence often running parallel with differences in CHD mortality among countries

(12).

However,

those

that compare

different

populations

living

in the same

country have not always found as consistent a relationship between the differences in risk factors and mortality. Moreover, differences in CHD risk within a country predicted from differences in risk factors from place to place are often smaller than From the Department of Social and Preventive Medicine, University of Queensland, Brisbane, Queensland (A.W.); the Departments of Community Medicine (S.L.) and Haematology (J.K., T.E., A.D.), Westmead Hospital, Sydney and the Centre for Clinical Epidemiology and Biostatistics, University of Newcastle, Newcastle, Australia. Address reprint requests to Andrew Wilson, MBBS, FRACP, Department of Social and Preventive Medicine, University of Queensland Medical School, Herston, Queensland, Australia 4006. Accepted September 4, 1992. 0 1992 Elsevier Science Publishing

Co., Inc.

1047-2797/92/$05.00

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Wilson et al. CHD RISK FACTORS

observed

differences

in CHD.

For example,

Australia

has a rich mixture

of immigrant

groups of non-Anglo-Celtic origin, and among this population there is considerable variation in CHD mortality (13). Among the white population, Australian-born males of British descent exhibit two- to threefold higher age standardized mortality ratios

(SMRs)

compared

with those

in men from southern

European

countries.

How-

ever, Armstrong and colleagues found that differences between samples of Italian migrants and Australian-born subjects in smoking rates, blood pressure, and serum cholesterol level predicted little of the large difference in mortality (14). The principal objective of this study was to examine whether differences

in

coagulation factor levels explain further the risk differences between these two groups of Australian men with major differences in CHD mortality. The secondary objective was to test for differences in a range of hemostatic variables, including platelet aggregability, that have been reported in various clinical and epidemiologic research as being different in groups with different risks of CHD. The third objective was to examine the more

the relation

between

diet and variation

This article presents a comparison major CHD risk factors-smoking, recently

described

between blood

in hemostatic

variables

in these men.

the two groups of men in relation to pressure, and blood lipids-and the

risk factors-fibrinogen

and FVIIc.

Additionally,

sons of the two groups are presented for other clotting indices, hemostatic platelet aggregation. Dietary data are currently being analyzed.

comparifactors,

and

METHODS

Sampling and Recruitment The

population

weighted maximum

high proportion Yugoslavia. minimize

sample

was selected

using

a two-stage

sampling

process.

First,

a

random sample was drawn comprising census collector’s districts, each with a size of 200 households. The sample was weighted for areas with a relatively of men aged 40 to 49 years who were born in Italy, Greece,

The areas had to be within the time between

20 minutes’

the collection

driving

and processing

districts, starting from a random start point, all households men aged 40 to 49 years and born in Australia, Britain,

Malta,

time of the laboratory, of blood.

or to

In ten collector’s

were approached and all Ireland, or the southern

European countries listed above were invited to participant. Those with a history of heart disease or any condition known to affect clotting factors, or currently regularly using medications known to affect platelet function or clotting time, were excluded. Three potential subjects, to treat vascular disease.

all Australian-born,

were excluded

because

they used aspirin

Initially, 256 men aged 40 to 49 years agreed to participate; 232 were born in the countries of interest. All potential participants completed a questionnaire concerning their medical history, medications, demographic details, and CHD-related risk factors. They were also given a questionnaire assessing frequency of food intake. One investigator then contacted each participant to make an appointment for blood collection, and participants were requested not to use any aspirin-containing medications for 1 week prior to the appointment. In the event of minor aches and pains, it was recommended that they use paracetamol. All participants were examined at home in the morning prior to work, between 6:00 and 9:00 AM. Body measurements consisted of blood pressure, height, weight, and mid-abdominal and pelvic circumferences. Height and weight were measured while the subject was without shoes and in light clothing and the body mass index (BMI) calculated (weight [kg]/height [m2]). Midab-

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497

dominal circumference was measured at the point midway between the xiphistemum and the symphysis pubis, and the pelvic circumference at the level of the greater trochanter. These measurements are reported as the ponderosity index (the ratio of these two measurements). Blood pressure was measured twice with the individual seated using a standard mercury sphygmomanometer with an appropriately sized cuff. A positive family history was restricted to reports of a mother or father having a probable coronary event before the age of 70 years.

Laboratory

Procedures

A venous blood sample (50 mL) (f o 11owing an overnight fast) was carefully collected by syringe and 21-gauge butterfly needle, with minimal use of tourniquet, and immediately transferred to one lo-mL plain glass tube, one 2-mL fluoride oxalate tube, one 5-mL ethylenediaminetetraacetic acid (EDTA) tube, and three lo-mL tubes containing 1 mL of 0.12 M sodium citrate. Blood for platelet studies was drawn last without stasis. During transport and prior to analysis, the blood samples, including those for platelet studies were kept at room temperature. The platelet studies were undertaken immediately on receipt of the sample in the laboratory and completed within 4 hours of collection; a full blood cell count, kaolin clotting time (KCT), prothrombin time (PT), activated partial prothrombin time (aPTT), and recalcification time of platelet-rich plasma (PRP) were performed on the day of collection. The remaining samples were centrifuged and the: plasma separated and stored at -80°C for batch processing. VIIc was measured by a one-stage coagulation method using factor-deficient plasma. Von Willebrand factor antigen was measured by enzyme-linked immunosorbent assay (ELISA). Platelet aggregation was studied using turbidometric methods. The sample was spun at 1000 rpm at room temperature for 10 minutes to obtain PRP. The platelet count was then adjusted to 250 x 109/L (+I25 x 109/L) using platelet-poor plasma (PPP) obtained from the same sample by spinning at 3000 ‘pm. From a standard stock, dilutions of the agonists (ADP, adrenalin, and collagen) were prepared daily using normal saline solution. The Aggregometer (Payton Scientific Lumiaggregation Module Model 1000) was set to zero aggregation with the blank cuvette containing PRP only. Then, the theoretical 100% aggregation tracing was determined using PPP. Each test cuvette containing 450 PL of PRP was then incubated in the Aggregometer for 2 minutes before 50 PL of the appropriate dilution of the agonist was added. If no aggregation response or no obvious release occurred to ADP or adrenalin, then arachidonic acid was used to test platelet response. If there was still doubt about release, then Chromo-lume reagent (Chronolog Corp, Cl-Haem, Melbourne) was used to detect ATP release. For each chart recording, the slope of primary aggregation, the lowest agonist dilution at which release occurred (threshold response), and the total percentage aggregation (maximum optical density change observed with the agonist divided by the theoretical maximum measured on PPP) were recorded. For collagen, the lag phase to secondary response was also recorded. All lipid and apolipoprotein measurements except lipoprotein(a) (Lp(a)) were performed by the Hunter Biochemisry Service, a participating laboratory in the MONICA project with standards meeting the requirements of that project (15). Total cholesterol was measured by an automated enzymatic calorimetric method (Boehringer Mannheim Monotest). High-density lipoprotein (HDL) cholesterol was measured by the same method following precipitation with 20% polyethylene glycol. Apolipoproteins Ai and Bree were measured by turbidometric method (Raichem SPIA ApoA-1 and ApoB Reagents, Reagents Applications, San Diego CA). The Lp(a)

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Wilson et al. CHD RISK FACTORS

estimation Sweden).

was assayed

by an ELISA

technique

using TintElize

Lp(a)

(Biopool

AB,

For the purpose of this analysis, two groups were defined: those born in a southern European country (group S-E) and the Anglo-Celtic group (group A-C) consisting of those who were born or whose parents were born in England, Scotland, Ireland, or Wales. Means of continuous variables and prevalence of categorical variables were compared using Student’s t tests and chi-square test, respectively. Where appropriate, one-way

and two-way

analysis

of variance

or covariance

was used when

more than two

means were being compared. Because of the non-normality of the Lp(a) results, log transformation was used in comparisons. Because of the same concern, the KruskalWallis h statistics was used to test for trend in the platelet aggregation responses. Confidence limits (95% CI) around the differences in group means are given as an indication of the power of the study to detect important differences.

RESULTS Of 232 eligible

subjects

the questionnaire, most often missed

initially

agreeing

to participate,

169 participants

completed

and the examination and blood collection procedures. The step (n = 53) was the examination and blood collection, usually due to

the impossibility of finding an appropriate time for the individual or a refusal to have blood taken. One subject was excluded because of probable angina. Overall, southern European men made up 29.7% of the subjects but only 26.6% of those with complete data, as shown in Table 1. Those not completing the examination and blood collection phase were similar residence in Australia, grading.

This

analysis

to the participating sample with regard smoking status, history of hypertension, is confined

to those

who completed

to age, duration of and occupational

all phases.

The mean ages of the groups were very similar (43.5 and 44.2 years for group A-C and group S-E, respectively). Prevalence rates for self-reported risk factors and family history

of CHD

are shown

although the difference A-C reported a positive

in Table

2. There

were more current

smokers

in group S-E

was not statistically significant. Over half (50.8%) of group family history, compared with just over one-fourth (27.5%) of

the group S-E (I’ = 0.017). paternal history only of CHD, percent of group A-C reported

About one-third (32.5%) of group A-C reported a compared to one-fifth (20.0%) of group S-E. Thirteen a maternal history only of CHD, compared to only 5%

among group S-E. There was no significant treatment for hypertension. Table 3 compares the anthropomorphic

difference and blood

in the proportions pressure

measurements

receiving of the

two groups. There were no significant differences in the mean BMI or the ponderosity index (midabdominal-pelvic circumference ratio). There was no significant difference in the mean systolic and diastolic blood pressures overall, with or without inclusion of

TABLE

1

Proportion

of different groups completing Anglo-Celtic % n

Completmg all procedures Questionnam only Total enrolled

124 39 163

76 24

all examination

stages

Southern European n % 45 24 69

65 35

Total 169 63 232

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TABLE

2

Prevalence

of reported

499

CHD risk factors in Anglo-Celtic and southern

European men, aged 40 to 49 years Southern European

Anglo-Celtic n %

Total Smoking status Never Exsmoker Current Hypertension” Family history of CHDb

124

b Father

or mother squared

with

had

32.2% 34.2% 31.5% 10.5%

18 10 17 5

40.0 22.2 37.7 12.2

64 50.8%

11

27.5L

pressure.

a probable

Yates

%

45

40 45 39 13

* Ever treated for high blood ’ Chi

n

coronary

correction

= 5.69,

event. P = 0.017.

those subjects taking antihypertension medications. Because of the known association between BMI and blood pressure, mean blood pressure was also compared after adjusting for BMI. This marginally increased the differences in mean systolic and diastolic blood pressure but the difference was still not significant. The confidence intervals around the difference in means indicate that the power of the study was adequate to detect potentially important differences for all variables except possibly systolic blood pressure. There was a significant difference in the levels of apolipoprotein Al (P = 0.034) and consequently in the apolipoprotein Al/Bloc ratio (P = 0.033), as shown in Table 4. Mean plasma total cholesterol and HDL cholesterol levels and the HDL cholesterol-total cholesterol ratio were not significantly different. The distribution of Lp(a) was highly skewed but after natural log transformation there was no significant difference in the mean values of the two groups. No differences were found between the two groups in a range of screening hemostatic and clotting indices, as shown in Table 5. Specifically, mean values for the hematocrit and mean corpuscular volume (MCV), other automated red blood cell indices, platelet, and white blood cell counts were not significantly different. Activated and nonactivated PTT, KCT, and re-calcified clotting time of PRP were not significantly different, although the variances of these measures were large.

TABLE

3

Mean CHD risk factor levels in Anglo-Celtic

and southern Southern European (n = 45)

Anglo-Celtic (n = 124) Mean

Systolic BP Diastolic BP0 BMIb Ponderosiq+

’ 127.3 84.9 26.1 0.91

n Mean blood pressure(BP) b BMI

=

r Ponderosity

weight

(mm

Hg)

excluding

SD

Mean

SD

11.9 7.9 3.5 0.08

125.7 85.0 27.3 0.91

13.3 6.1 3.4 0.90

those

on antihypertension

circumference-pelvic

circumference.

(kg)/height(m’)

index

= midabdominal

European men, aged 40 to 49 years

medication.

Difference

1.6 -0.1 -1.2 0

(95%

(-2.6-5.8) (-1.39-1.19) (-1.8--0.6) (-0.03-0.03)

CI)

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Wilson et al. CHD RISK FACTORS

AEP Vol. 2, No. 4 July 1992: 495-508

TABLE 4 Mean plasma lipoprotein men, aged 40 to 49 years

and apolipoprotein

levels in Anglo-Celtic

HDL cholesterol (mm&L) CholesteroliHDL ratio Apolipoprotein Bloa (g/L) Apolipoprotein A, (g/L) Bloo/Alm ratio Log lipoprotein(a) (g/L) ” Student

t=

European

Southern European (n = 45)

Anglo-Celtic (n = 123)

Total plasma cholesterol

and southern

Mean

SD

Mean

SD

5.83 1.10 5.67 0.75 1.22 0.63

1.01 0.39 1.71 0.16 0.24 0.16

5.84 1.01 6.22 0.77 1.13 0.69

1.15 0.27 2.35 0.19 0.15d O.l@

1.59

0.55

1.50

Difference (95% #CI) -0.01 (-0.17-0.19) 0.09 (0.03-O. 15) -0.55(-0.88-0.22) -0.02 (-0.04-0.00) 0.09 (0.06-o. 12) -0.06 (-0.09--0.03) 0.09 (0.00-0.18)

0.41

2.13,df = 161,P = 0.034.

As shown in Table 6, there were no significant von Willebrand factor antigen levels. There status and fibrinogen level, with significantly (mean,

3.19

g/L;

95%

confidence

(mean, 2.81 g/L; 95% confidence 2.83 g/L; 95% confidence limits, prevalence mean

of current

smokers

differences

in FVIIc,

fibrinogen,

or

was a strong relation between smoking higher levels being present in smokers

limits,

3.02-3.36)

compared

to never-smokers

limits, 2.66-2.96; P = 0.001) or exsmokers (mean, 2.63-3.03; I’ = 0.007). Controlling for the higher

in group S-E did not alter significantly

fibrinogen levels between the two groups. Consistent differences were found in the response

of platelets

the differences to different

in

aggre-

gating agents (Table 7). For the two groups, the threshold aggregating levels of ADP were significantly different (I’ = 0.002) but not to collagen (P = 0.074) or adrenalin (P = 0.181).

Table

7 shows that a higher

proportion

of group A-C had lower aggrega-

tion thresholds (the concentration at which they commenced aggregation) than group S-E. The same trend was evident in the responses to collagen and adrenalin, as the platelet lin, the statistical

samples

were allocated

first to ADP,

TABLE 5 Mean hematologic to 49 years

and clotting

indices in Anglo-Celtic

collagen,

and finally adrena-

and the results did not reach this further, showing the mean

and southern

European

men, aged 40

Southern European (n = 40)

Anglo-Celtic (n = 120)

Total white blood cell count Hematocrit (%) Mean corpuscular volume Activated partial prothrombin time (PPT) Nonactivated PPT Kaolm clotting time(s) Recalcification time of platelet-rich plasma (s)

then

total numbers studied were slightly smaller significance. Figures 1, 2, and 3 demonstrate

did but

Mean

SD

Mean

SD

Difference (95% #CI)

6.67 45.6 89.4

1.75 3.5 4.2

7.06 46.0 88.2

2.07 2.9 3.5

-0.3 (-0.95-0.35) -0.4 (-1.16-0.80) 1.2 (-0.25-2.65)

29.2 243.8 73.7

3.5 50.2 13.7

29.2 241.1 71.0

3.2 64.9 10.7

0.0 (-1.23-1.23) 2.7 (-13.0-18.4) 2.7 (-1.98-7.30)

274.1

80.7

264.9

73.9

9.2 (-19.2-37.6)

Wilson et al. CHD RISK FACTORS

AEP Vol. 2, No. 4 July 1992: 495-508

TABLE 6 49 years

501

Mean plasma coagulation factor levels in Anglo-Celtic and southern European men, aged 40 to Southern European

Anglo-Celtic (n = 117)

Fibrinogen (g/L) Factor VIIc (%) van Willebrand factor antigen (%)

Mean

SD

Mean

SD

2.92 111.4

0.66 30.6

2.99 110.9

0.56 31.9

53.7

120.3

86.2

121.2

Difference (95% #CI) -0.07 (-0.19-0.05) 0.5 (-5.12-6.12)

0.9 (10.55-12.35)

maximum aggregation at different concentrations of the aggregating agent for the two groups. There was consistently lower maximum aggregation in response to ADP and collagen

in group S-E except at the highest concentration.

For adrenalin,

the differ-

ence was present at all concentrations. The maximum slope and the rate of aggregation, when plotted against concentration, showed the same pattern of difference between the two groups.

DISCUSSION Substantial differences in CHD mortality have been observed among migrant groups in Australia (13, 16-20). Generally, these studies showed that CHD age standardized

TABLE 7 Platelet aggregation responses in Anglo-Celtic and southern European men, aged 40 to 49 years’ Anglo-Celtic n

ADP concentrations 0.0625 0.125 0.25 0.5 1.0

concentration

37

0.9 9.5 25.9 49.1 14.7

0.0 0.0 18.9 43.2 37.8

113

36

28.3 50.4 20.4 0.9

11.1 61.1 27.8 0.0

98

29

22.4 19.4 11.2 46.9

10.3 13.8 20.7 55.2

(PM/L)’

n

Adrenalin 0.125 0.25 0.5 1.0

116 (PM/L)~

n

Collagen 0.25 0.5 1.0 2.0

Southern European

concentration

(PM/L)“

a Percentage reaching aggregation threshold at different concentratmns of aggregating agent. b Kmskal-Wallis h = 9.552. df = 1, P = 0.002; chi square (trend) = 9.991, df = 1. I’ = 0.002. ’ Kruskal-Wallis d Kmskall-Wallis

h = 3.189, df = 1, P = 0.074; c h’I square (trend) = 2.913, df = 1, P = 0.88. h = 1.790, df = 1, P = 0.181; c h’Lsq uare(trend) = 2.234, df = 1, P = 0.135.

502

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I

-

ANGLocELTic

+

STH

EUROPEAN __-

0’ 0.0625

0.25

0.125 ADP

0.5

CONCENTRATIONS

(log

FIGURE 1 Anglo-Celtic

Mean maximum platelet aggregation to increasing and southern European men aged 40 to 49 years.

FIGURE 2 Anglo-Celtic

Mean maximum platelet aggregation to increasing and southern European men aged 40 to 49 years.

,O

MEAN ._ ~~

MAXIMUM

AGGREGATION

1.0

scale)

concentrations

concentrations

of ADP

in

of collagen

in

(%I

I’ ’ ,/”

40

30 # / 20 /

10 --

0 0.25

ANGLOCELTIC

1.0

0.5 COLLAGEN

CONCENTRATION

+

2.0 (log

scale)

STH

EUROPEAN

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503

Wilson et al. CHD RISK FACTORS

MEAN

MAXIMUM

AGGREGATION

(%)

35,

5/ -+-

I 0’ 0.125

FIGURE 3

mortality one-half,

-46

STH

EUROPEAN

J

0.25

0.5

ADRENALIN

Anglo-Celtic

ANGLOCELTIC

CONCENTRATION

(log

1.0

scale)

Mean maximum platelet aggregation to increasing concentrations and southern European men aged 40 to 49 years.

of most migrant groups from non-Anglo-Celtic than that of the Australian-born population.

of adrenalin

in

countries is lower, by up to Migrants from Anglo-Celtic

countries have CHD age SMRs that are usually higher than those of the Australianborn population. Migrants from the Indian subcontinent and from some eastern European countries also have higher SMRs. Similar differences have been found among different ethnic groups in Britain and the United States (21, 22). Usually, but not always (the major exception

being the Indian subcontinent),

CHD SMRs are in the same direction and their adopted country.

these differences

in

as the difference between their home country

The differences

in SMRs decrease with duration of resi-

dency in Australia but SMRs do not reach those of the Australian population even after 25 years (18). It is unlikely that any one factor explains these differences in CHD mortality. Possible contributing factors include the healthy migrant effect, differences in risk factors, genetic differences, and differences in access to medical care. The differences in risk factor prevalence

evident in studies comparing samples from different countries

would suggest that this should be a major contributor

to the difference

between

migrants and their adopted population. However, it would also be expected that with increasing duration of residency and increasing adoption of the life-style of the host countries, the risk factor profile of migrants would be increasingly similar to that of the host population. In this study we compared the prevalence of established risk factors between two ethnic groups with substantially different CHD SMRs: migrants from southern European countries (Italy, Greece, and Malta) and migrants or their descendants from Anglo-Celtic countries (England, Ireland, Scotland, and Wales). The study was restricted to men between the ages of 40 and 49 years without a history of CHD and

504

Wilson et al. CHD RISK FACTORS

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subjects were randomly selected from the community to minimize selection bias. All anthropomorphic measurements were performed by the same observer using the same instruments. Venous blood samples were collected at similar times, and the subjects were fasted overnight. Laboratory assays were all performed in the same laboratories by the same technicians who had no knowledge of the ethnic origin of the specimens. The

only statistically

(smoking,

significant

differences

found in established

risk factors

BMI, systolic and diastolic blood pressures, and blood lipids) were a higher

frequency of maternal and paternal history of CHD in group A-C and a lower mean apolipoprotein Ai level in group S-E. As a result of the latter, there was also a difference in the apolipoprotein Al/B ioe ratio. The 95% confidence limits around the differences in means for the risk factors indicate that it is unlikely the study missed epidemiologically

important

differences

for established

risk factors,

except

systolic blood pressure for which the upper bound of the possible difference mm Hg. Apolipoprotein

Al is the principal protein component

possibly is 5.8

of the HDL fraction.

A

similar Australian study compared a random sample of Italian migrants with a random sample of Australian-born persons of Anglo-Celtic descent (14). The study included men and women aged 20 to 79 years. The Italian men had lower mean systolic and diastolic blood pressures but higher mean BMI than did Australian-born

men. There

were no statstically significant differences in serum total cholesterol, HDL cholesterol, or apolipoprotein A among men, but HDL cholesterol was significantly lower among Italian women. However, in both this and the earlier Australian study, the trend was toward lower HDL cholesterol levels in Italian men. The direction of the difference in the mean diastolic blood pressure and the BMI was the same in this study as that in the earlier study and the magnitudes of the differences study were they statistically significantly different.

were very similar but in neither

We found significant differences in the aggregation responses of platelets between group S-E and group A-C. significantly

The threshold concentration

for aggregation to ADP was

higher in group S-E and for collagen and adrenalin the trend was in the

same direction. The mean maximum platelet aggregation and the rate of aggregation were higher in group A-C. These results demonstrate that the southern Europeans have platelets that are less likely to aggregate in response to a standard stimulus than those of subjects from Anglo-Celtic backgrounds. Platelet aggregation has been suggested as a possible marker of CHD risk in several clinical studies but there are few epidemiologic studies on the direct relationship between platelet reactivity and CHD (23, 24). The Caerphilly study demonstrated that cross-sectionally, platelet aggregation responses to ADP were associated with a twofold increased risk of prevalent CHD (25). R es ponses to thrombin showed a less marked association and there was no association with collagen. In that study, the methods used were essentially the same as in this study. The only study reporting prospective results found that platelet hyperreactivity, as measured by spontaneous platelet aggregation, was associated with an increased risk of reinfarction in patients surviving a recent infarct (10). The relative risks, compared to nonrespondents, were 1.6 for the intermediate group and 5.4 for the strongly positive group. The relationship existed throughout the 5-year follow-up. In our study a measure similar to hyperreactivity would be the proportion with lower aggregation threshold to ADP. For example, the proportion of group A-C with an ADP threshold of less than 0.5 mmol/L (36.3%) was almost twice that of group S-E (18.9%). There were no significant differences between the two groups in the mean levels of fibrinogen and FVIIc, the two clotting factors with the strongest evidence linking

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Wilson et al. CHD RISK FACTORS

505

them to a risk of CHD. The slightly higher mean fibrinogen level in group S-E is consistent with the slightly higher prevalence of current smoking in that group. No other Australian epidemiologic studies have described variations in hemostatic, clotting, and platelet function indices in the population and only a few wellcontrolled studies from elsewhere examined the variation in hemostatic and platelet function among different ethnic groups with different CHD risk. In India, CHD mortality varies markedly between the northern and the southern regions. No significant differences were found in the mean atherosclerotic indices between the two areas. This led Malhotra to look for differences in thrombotic tendency (26). He compared clotting indices between nonsmoking, age-matched, railway employees in the two parts of the country. Whole blood clotting time was significantly longer in the low CHD risk group from southern India. Clot retraction, platelet count, and platelet adhesiveness were not significantly different. Among Bangladeshi migrants to Britain, however, who have higher CHD mortality than the British-born population, levels of FVIIc but not fibrinogen were lower among men (27). Among immigrants from India in Britain, there was no difference in fibrinogen and factor VII, although platelet counts and mean platelet volume were slightly higher compared to those in AngloCeltics (27). In Britain, blacks have a CHD mortality rate that is many times less than that of the white population. Meade and coworkers (1978) found higher cigarette and alcohol consumption, blood pressures, cholesterol, triglyceride, FVIIc, and red blood, white blood, and platelet cell counts in the whites compared to the blacks in one workforce (28). Blacks had lower factor VIII levels and fibrinolytic activity. Fibrinogen levels and platelet adhesion were not significantly different. In a later article, Meade and colleagues (1985) reported that platelet aggregation responses to ADP were greater in whites than black but that platelet aggregation to adrenalin was greater in blacks (29). In North America, most, but not all studies have found a lower CHD incidence among blacks, compared to whites. Szczeklik and colleagues reported that mean blood fibrinolytic activity and euglobulin plasminogen concentrations were higher among a sample of blacks than a matched sample of whites in southern Georgia (30). Iso and associates compared hemostatic variables among four different samples consisting of Japanese farmers, Japanese urban workers, Japanese-Americans, and white Americans (31, 32). Comparatively, CHD mortality was lowest in the Japanese population, intermediate among Japanese-Americans, and highest among white Americans. Mean fibrinogen concentration and factor VII coagulation activity were higher among the white American and Japanese-American sample after controlling for other risk factors. Factor VIII activity and antithrombin III activity were not independently significantly different. Mean tissue plasminogen activator antigen levels were also higher in the high CHD risk groups. Iso and associates concluded that these differences probably contribute to the differences in CHD risk. The differences in platelet aggregation responses in our study are especially interesting because experimentally, platelet responsiveness is sensitive to relatively small changes in diet, particularly the polyunsaturated-saturated fatty acid ratio and omega3 fatty acid intake (33, 34). Moreover, the effects of diet can occur relatively quickly because of the short life span of platelets. Platelet aggregation response is affected by smoking (29), although this effect could not explain the differences observed here as the smoking rate was higher in group S-E. Platelet aggregation may also be influenced by the plasma lipoprotein concentrations, with sensitivity to adrenalin influenced by low-density lipoprotein (LDL) and total cholesterol concentrations but not HDL concentration (35). In this study, the only difference in lipoproteins was in apolipo-

506

AEP Vol. 2, No. 4 july 1992: 495-508

Wilson et al. CHD RISK FACTORS

protein

At,

which

is therefore

not consistent

ences explain the aggregation error and biologic variability

with the theory

differences. are reported

that

lipoprotein

differ-

Variability due to technical measurement to be substantial for platelet aggregation

assays (36). In this study, every effort was made to ensure that variability was not different between the groups but variability would tend to mask the differences observed. In conclusion, between

the

migrants

origin. appear

study

found

from southern

no differences

European

in fibrinogen

countries

and

and Australians

FVIIc

levels

of Anglo-Celtic

Therefore, differences in the mean levels of these coagulation factors to contribute significantly to the difference in risk of CHD, evidenced

do not by the

markedly different standardized mortality rates from CHD between these two groups. The only significant differences in established CHD risk factors were the higher frequency of a family history of CHD in the Anglo-Celtic lower apolipoprotein Ai level in the southern European showed ples.

significant

This

tributor

differences

study suggests

that

to the differences

this hypothesis primary CHD.

in platelet

aggregation

differences

in platelet

in CHD

requires

further

risk between

confirmation

responses

between

reactivity

these

that

group and a paradoxically group. However, the study could

two ethnic

platelet

the two sam-

be a major groups.

reactivity

con-

However,

is a predictor

This project was funded by the National Health and Medical Research Council Australia and undertaken while Dr. Wilson was a NHMRC Public Health Development Committee Fellow.

of

(NHMRC) of Research and

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