Results

Figure 2 shows the incidence rate of “total” myocardial infarction cases per 100 000 woman years, by one year age bands. The rate increased steeply from a level of 3.0 at age 33, to a maximum of 20.2 cases per 100 000 woman years at age 44. The youngest cases were aged 19, of which there were three. The age standardised rate of “total” cases with myocardial infarction in the age group 16–44 was 4.8 per 100 000 woman years.

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Figure 2

Rate of “total” myocardial infarction, by age.

Figure 3 is a map showing the distribution of the unadjusted rates for “total” myocardial infarction in postal areas. It shows clearly that the highest rates were in Scotland and north Wales, with relatively high rates in a belt stretching east-west from Liverpool to Hull, south Wales and south west England. The lowest rates were in the south east of England. The range of values was from 0 to 33, (mean 7.01, SD 4.59), cases per 100 000 woman years, with most of the highest rates in Scottish areas.

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Figure 3

Myocardial infarction per 100 000 women age 25–44 in quartiles.

Table 1 shows the rates in the four regions, both unadjusted and standardised by age and smoking habit, using those myocardial infarction cases who were interviewed (and thus smoking history was known) as the numerator (n=448). It shows increasing risk from south to north, both in the unadjusted and adjusted rate ratios: Scottish women had a risk of myocardial infarction 3.7 times that of their southern English counterparts, even when adjusting for differences in age distribution and prevalence of smoking. Some of the difference between Scotland and England in this analysis may have been accounted for by a difference in interview rates (in Scotland 77.5% of confirmed cases, and in the south of England, for example, 56.9%). When the expected number of cases in the south of England was increased to allow for this, the adjusted rate ratio for Scotland became 3.07 (95% CI 2.69, 3.5).

The overall, unadjusted, mortality rate in women aged 16–44 for England and Wales was 1.17 per 100 000 woman years, and for Scotland, 1.51 per 100 000 woman years. Case fatality for the different regions and selected urban areas are shown in table 2. There was a gradient of risk of fatality that was the inverse of the incidence rate of myocardial infarction: in the south of England, the proportion was 31% (95% CI 25.9, 36.0), whereas in Scotland, it was 18.1% (95% CI 12.7, 24.6). The decreasing trend from south to north was statistically significant (χ² for trend=9.81, p=0.002). There was wide variation between the urban areas, the proportion in London being twice that in Liverpool, although the difference was not statistically significant (χ²=3.63, p=0.06).

This same trend of case fatality was observed: (a) if we made the assumption that all the potential cases with no hospital notes (n=145) were not true myocardial infarction cases, and (b) if we only included those cases that were confirmed as valid by the diagnostic committee (n=743, see fig 1). Data from these analyses are not shown.

Discussion

The most important findings from this study are firstly that the unadjusted regional variation in incidence of myocardial infarction, as reflected by a rate ratio between Scotland and the south of England of 4.5 (95% CI 3.4, 5.8), persisted even after adjustment for smoking and age (adjusted rate ratio 3.7, 95% CI 3.2, 4.2). Age was a very important factor in the aetiology of myocardial infarction, as shown in figure 2, and the population attributable fraction of smoking as a risk factor for myocardial infarction in this age group of women was 73%.5 It is unlikely that the other known cardiovascular risk factors can fully account for this marked regional variation: other unknown factor(s) (environmental or genetic) probably play a part.

Secondly, there was a surprising difference in the fatality rate between Scotland (18.6%) and southern England (31%), which was the reverse of that expected from the incidence rates of myocardial infarction. This could have been because of under ascertainment of the number of deaths from myocardial infarction in Scotland, relative to England. WHO MONICA data on myocardial infarction in north Glasgow between 1985 and 1991 showed a case fatality of 36% among 35–44 year old women.9 However, north Glasgow is not typical of Scotland, as it is an area of socioeconomic deprivation.10Furthermore, the period of data collection was five to 10 years earlier than MICA. There could also have been under ascertainment of the number of surviving myocardial infarction cases in England, relative to Scotland. It seems unlikely that such a systematic bias should exist in these government data, and the fatality rates for this study in England were compatible with case fatality of 33% for women aged <45 years quoted in another study from three health districts in England and Wales.11 However, this does not exclude the possibility that there is under diagnosis of myocardial infarction in young women in England and Wales, compared with Scotland.

Assuming that the data were not biased, it is possible that the difference in case fatality is attributable to random error. The total number of fatal cases in Scotland was 33 over two years. However, the Register General's Office mortality data shows the mean (SD) annual number of deaths for women aged 15–44 for the years 1993–1996 inclusive to have been 17.75 (3.83), giving a mean case fatality of 20.1% (95% CI 14.7, 27.0). Although the confidence intervals for this fatality rate overlap those for the south of England, there is still a large difference in the point estimate.

If there is a true difference between the case fatality in Scotland and the south of England, the causes are most likely to relate predominantly to ambulance and patient response times from the onset of symptoms. Norris and colleagues showed that 91% of fatalities in patients under 55 occur outside hospital.11 Another study from Nottingham confirms that most deaths from myocardial infarction occur outside hospital.12

A “chain of survival”, proposed by the American Heart Association, has shown that the length of time between onset of symptoms or collapse, and the arrival of a trained ambulance crew is vital.13 Ambulance delays of longer than eight minutes to victims of cardiac arrest leads to a high probability of an unfavourable outcome.14 Prompt attempts at cardiopulmonary resuscitation and defibrillation are particularly important in younger people, who are more likely to respond to resuscitation attempts.14 There is evidence of considerable variation in ambulance response times between England, Scotland and Wales during the period of case incidence of the MICA study. The average proportion of cardiac arrest calls in England with response time of eight minutes or less was 56% in 1994–1995, but the range was from 13% (London) to 75% (Mersey). In Scotland, as a whole, the equivalent figure was 61% for the period 1991–1995. In Scotland, the Heartstart Programme had resulted in all emergency ambulances being equipped with defibrillators and a trained crew by mid-1990.15 This programme was completed before similar initiatives in England and Wales. It is thus possible that the lower case fatality proportion in Scotland reflects better response times and actions by ambulance crews, relative to their colleagues in the south of England. The especially poor ambulance response rates in London appear unique and may contribute to the high case fatality in this city. When the London cases were excluded from those in the south of England, the case fatality (30.5%), was still significantly higher than that of Scotland (18.1%, χ²=7.36, p=0.007). A study by Leslie and colleagues has suggested that the impact of the Heartstart Scotland programme on coronary mortality in north Glasgow was minimal.15 In contrast, another study in Scotland suggested that the Heartstart programme has had a considerable impact on survival of patients after collapse in the community with ventricular fibrillation.16

Another contributory factor to the regional difference in case fatality could be variability in efficiency of treatment for myocardial infarction within hospitals, for example, use of low dose aspirin, β blockers or thrombolytic therapy. This does not seem very plausible, but merits further investigation.

The lower case fatality in Scotland is in keeping with WHO MONICA project results that have shown that case fatality for women, aged 35–64, was significantly lower in countries with a higher event rate for myocardial infarction, for example, in Finland (Kuopio).17

These data from the MICA study have shown that the regional variations of myocardial infarction incidence in Scotland and England for men applied to young women as well. These differences persisted after adjustment for age and smoking. The case fatality in Scotland was significantly lower than in the south of England, which may be explained, by different diagnostic rates, or by better ambulance response. This finding merits further research, and exploration of other factors such as the extent of use of cardiopulmonary resuscitation by witnesses, delay between onset of symptoms and the call to an ambulance, and the consistency of therapeutic interventions in hospital.