Results

A total of 2157 patients were admitted with AMI over 1994–1998 (1395 men, 762 women; M:F 1.8:1). Mean age for AMI admission for men and for women were 62.4 and 71.6 years respectively (t=18, p<0.0001). Patients were subdivided into four groups: <60, 60–69, 70–79 and >79 years. The male to female ratio decreased significantly along these groups, at 6.6, 2.1, 1.1 and 0.6 respectively (χ² for trend=288, p<0.0001). There were 212 deaths from 2157 AMI admissions, a hospital mortality of 9.8%, with 108 men and 104 women (mortalities of 13.6 and 7.7% respectively). Mortality was significantly higher in women (OR=1.53, 95% CI 1.15 to 2.04; χ=8.6, p=0.003). Mean ages for deaths of AMI for men and for women were 70.1 and 76.8 years respectively (t=5.2 p<0.0001).

Crude admission rates (per 100 000 population) for men, women and overall were 606, 432 and 618 respectively. The equivalent age adjusted rates were 642 (95% CI: 607 to 676), 247 (95% CI: 229 to 266) and 434 (95% CI: 415 to 452). For AMI admission deaths, crude rates for men, women and overall were 62, 59 and 61 respectively. The equivalent age adjusted rates were 47 (95% CI: 38 to 56), 29 (95% CI: 24 to 35) and 37 (95% CI: 32 to 43).

A highly significant seasonal variation was found for AMI admissions (θ=37°, Feb table 1, p<0.0001, fig 1), with a peak in January and a trough in June to July. The main contribution was female despite the smaller number of female admissions (θ=37°, p<0.0001), with little male contribution (θ=38°, Feb, p=0.076). The first quarter peak in admissions was consistent for all ages except for the under 60 years age group (θ=37–47°, Feb, p⩽0.006). Seasonal variation was produced by women at almost all age groups, with no significant male seasonal variation at any age group. Mortality overall showed a peak in January (θ=1, p=0.001), entirely caused by female variation (θ=25, p<0.0001).

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Figure 1

Percentage variation in admission with myocardial infarction for all patients, stratified by gender.

The mean monthly minimum and maximum temperatures were 9.3°C and 33.9°C respectively. Ranked mean monthly ambient temperatures correlated highly with monthly admissions (r=−0.44, p<0.0001). Female admissions were more highly correlated (r=−0.54, p<0.0001) than were male admissions (r= −0.19, p=0.16). Monthly mortality of AMI admissions also correlated with mean monthly ambient temperatures (r=−0.33, p=0.009). Again, this correlation was far stronger for women (r=0.42, p=0.001) than for men (r=0.10, p=0.45).

Discussion

An increase in winter AMI has been reported1 as well as increased winter mortality ranging between 22% to 53% in Hawaii, New Zealand, Scotland, China and the United States.2-4 A direct effect between the extent of temperature decrease and mortality has been noted.5 Conversely, seasonal variation in Taiwan (with less temperature fluctuations) was not found.6 To the best of our knowledge, ours is the first study that demonstrates gender specificity in seasonal variability in AMI admissions and mortality. This was not found by Spencer et al on specific subgroup analysis of a large AMI cohort, by age and gender.3

We speculate that low winter temperatures may be more likely to trigger AMI in older patients,7 and as women tend to present with AMI at a later age than men, women will therefore tend to exhibit a more marked seasonal variation. A later age at presentation with AMI will also lead to increased mortality of AMI, with a resulting seasonal variation. However, men did not show any significant seasonal variation at any age group, and we therefore cannot exclude the involvement of other gender specific factors.