PREVALENCE OF ATRIAL FIBRILLATION

Analysis of ECG data for the original cohort of men and women aged 45–64 years (n = 15 406) found 100 (0.65%; 95% CI 0.53 to 0.79%) documented cases of AF.

Prevalence of AF increased with age and there were more cases detected in men (53 of 7052) than women (47 of 8354). In the subset of subjects who participated in the second survey of this cohort (n = 8532) there were 46 (0.54%, 95% CI 0.40 to 0.72%) documented cases of AF. Figure1 summarises the overall prevalence of AF according to age and sex using the combined data from the two screening periods.

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Figure 1

Age and sex specific prevalence of atrial fibrillation (AF) in a population cohort from the west of Scotland.

DEMOGRAPHIC AND CLINICAL ASSOCIATES OF ATRIAL FIBRILLATION

Table 1 shows the characteristics of this population cohort at baseline according to the presence or absence of AF on an age adjusted basis. Both men and women in AF were older and more likely to have a history of stroke, symptoms indicative of possible transient cerebral ischaemia, ECG evidence of myocardial ischaemia, and left bundle branch block and cardiomegaly as determined by chest radiography. They were also more likely to have a lower adjusted FEV₁. Women but not men in AF were found to have higher diastolic blood pressure and blood sugar concentrations, lower total cholesterol concentrations, and a history of angina, and were more likely to have ECG evidence of left ventricular hypertrophy. Men but not women in AF were more likely to have symptoms indicative of chronic bronchitis and ECG evidence of past MI (pathological Q waves) and to come from a manual social class. Of the 100 subjects with AF, 13 reported exertional dyspnoea with all 13 fulfilling the Medical Research Council criteria for chronic bronchitis.11

Table 2 summarises the independent correlates of AF according to baseline screening data. Male sex, a history of stroke, and ECG evidence of myocardial ischaemia were independently associated with AF. Overall, however, cardiomegaly was the strongest correlate of AF. When this variable was excluded from the model, older age also appeared to be independently associated with AF (OR 2.1, 95% CI 1.1 to 10.2 per decade, p < 0.05).

INCIDENCE OF ATRIAL FIBRILLATION DETECTED AT FOLLOW UP SCREENING

Of the 8532 subjects who participated in the follow up screening of the Renfrew/Paisley cohort, 14 men and five women had newly documented AF. Figure 2 shows the age (based on age at first screening) and sex specific rate of incident cases of AF per 1000 person years of follow up. The rate of incident cases increased notably with age and was greater in men than in women. Paired analysis of data collected during the first and follow up examinations found no significant changes in systolic and diastolic blood pressure, cardiothoracic ratio, blood sugar concentration, plasma cholesterol concentration, and adjusted FEV₁ for those who developed new AF. For example, of the 19 subjects with new AF, eight had radiographic cardiomegaly detected on both occasions. However, four subjects reported new angina pectoris. Thus, 12 of 19 subjects with new AF had a history of angina. Five (including the four with new onset angina) subjects had newly documented myocardial ischaemia on their repeat ECG. Although two subjects no longer had ECG changes indicative of myocardial ischaemia during the second survey, 13 of the 19 subjects with newly documented AF had some manifestation of myocardial ischaemia (angina or ECG changes) during the second examination. Moreover, five subjects reported new symptoms indicative of transient cerebral ischaemia (making a total of six subjects with this problem), though there were no new cases of stroke.

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Figure 2

Rate of new incident cases of AF detected during rescreening of 8532 men and women (34 891 person years of follow up). (Age groups are based on age at baseline.)

INDEPENDENT PREDICTORS OF INCIDENT ATRIAL FIBRILLATION

According to the multivariate analysis, male sex (OR 3.4, 95% CI 1.1 to 10.2), greater age (OR 1.29, 95% CI 1.2 to 1.4/decade), and cardiomegaly (OR 14.0, 95% CI 5.1 to 10.2) were the only independent predictors (p < 0.05) of new AF. In the absence of cardiomegaly, all of the above variables were retained in the logistic regression model, with the addition of ECG evidence of myocardial ischaemia (OR 4.1, 95% CI 1.2 to 14.2) detected at baseline.

INCIDENT HOSPITALISATIONS ASSOCIATED WITH A DIAGNOSIS OF ATRIAL FIBRILLATION

As the age of the Renfrew/Paisley cohort ranged from 45–64 years at baseline, we were able to document incident hospitalisations for AF in this cohort up to the age of 84 years. During the first four years of follow up of the original Renfrew/Paisley cohort we found that five of the 19 subjects with new AF were hospitalised. All were coded at discharge with a diagnosis of AF. There were no hospitalisations associated with a diagnosis of AF in subjects without AF who had a follow up examination.

During 20 years follow up, 255 of the 6999 men (3.6%, 95% CI 3.2% to 4.1%) and 282 of the 8307 women (3.4%, 95% CI 3.0% to 3.8%) in the Renfrew/Paisley cohort without AF at baseline were discharged from hospital with a diagnosis of AF. The proportion of men for whom such a hospitalisation was recorded increased from 3.3% (95% CI 2.5% to 3.2%) in those initially aged 45–49 years to 4.2% (95% CI 3.3% to 5.4%) in those aged 60–64 years. The equivalent figures for women were 2.3% (95% CI 1.7% to 3.0%) and 4.4% (95% CI 3.5% to 5.4%), respectively. Figure 3 shows the age and sex specific rates of incident hospitalisations with AF per 1000 person years of follow up.

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Figure 3

Rate of incident AF hospitalisation in 15 306 men and women during 20 year follow up. (Age groups are based on age at baseline.)

Overall, this cohort recorded 808 hospitalisations where AF was coded in any position during 20 years follow up (38% of these as the principal diagnosis). The most common concurrent diagnoses (coded in any position at discharge) were heart failure (38%), coronary artery disease other than MI (34%), stroke (23%), and MI (14%). Valvar heart disease was documented in approximately 7% of cases, while thyroid disease was documented in < 1.5% of cases.

INDEPENDENT PREDICTORS OF INCIDENT HOSPITALISATION WITH ATRIAL FIBRILLATION

Table 3 shows the independent predictors of incident hospitalisation with AF based on original screening data in patients not found to be in AF at baseline (n = 15 306). Once again, age and cardiomegaly were independent predictors for new AF detected in this manner, although there was little difference between men and women. Other predictors were a history of symptoms indicative of angina and a higher systolic blood pressure recorded at baseline. In the absence of cardiomegaly, a higher body mass index was also retained in the final model. Not surprisingly, subjects who were in AF at baseline were also about three times more likely to be hospitalised with a diagnosis of AF during follow up (15%; adjusted OR 2.9).

PREVALENCE OF ATRIAL FIBRILLATION: COMPARISON WITH PREVIOUS STUDIES

Our findings are in keeping with those of the Framingham heart study, which found an initial screening prevalence of 5/1000 in those aged 50–59 years and rising to 18/1000 in those aged 60–69 years.1 ,15-18 The prevalence rates in subjects aged 50–59 years in the Renfrew/Paisley cohort were 7.2/1000 in men and 2.9/1000 in women. The male and female rates in those aged 60–69 years were 10.2 and 7.4/1000, respectively, somewhat less than in Framingham. Our findings are also agreement with reports from Denmark,19 Iceland,20 and the Netherlands21 that have described smaller populations.

INCIDENCE OF ATRIAL FIBRILLATION: COMPARISON WITH PREVIOUS STUDIES

Of more interest are our data on the incidence of AF because of the paucity of data in this regard. The Framingham heart study reported two yearly incidence rates of approximately 1.9 and 0.9/1000 person years in men and women, respectively, aged 50–59 years.1 ,15-18 Our equivalent rates over four years in those aged 55–64 years were 1.31 and 0.44/1000 person years. Over 38 years of follow up, the Framingham study found an overall incidence of approximately 3/1000 person years in men and 2/1000 person years in women aged 55–64 years.1 We observed a 20 year rate of incident hospitalisation with AF of 2.1 and 1.7/1000 person years in men and women, respectively, in this age range.

The only other incidence study encompassing subjects in the same age range and with long term follow up is the Manitoba follow up study in Canada, which examined 3983 male aircrew recruits with an average age of 31 years in 1948.22 These subjects were followed for 44 years. The incidence rate was 2/1000 patient years overall, being as low as < 0.5/1000 before the age of 50 and rising to 2.3/1000 person years by the age of 60 years.

INCIDENT HOSPITALISATION OVER 20 YEARS

Our rate of incident hospitalisation for all men over 20 years' follow up was 2.0/1000 person years. This almost certainly is an underestimation of its true incidence because we had no means to detect AF in those who avoided hospitalisation. Consequently, our population incidence over 20 years is likely to have been higher than the hospitalisation rates and, therefore, higher than in either the Framingham.1 ,15-18 or Manitoba cohorts.22This is, perhaps, not surprising with regard to the Canadian study, which recruited fit young men, whereas our cohort was unselected and from a population with a high rate of coronary and other cardiovascular disease.4 Interestingly, however, it seems that although we found a lower initial prevalence than Framingham, our subjects went on to have a higher long term incidence, again presumably reflecting the impact of other cardiovascular diseases in later years.1 ,15-18 Indeed, one of the most important findings of our study is the high burden of incident hospitalisation related to AF, with 3% of men and over 2% of women aged 45–49 years at baseline going on to experience such an event in the subsequent two decades of follow up.

CORRELATES OF ATRIAL FIBRILLATION

We were also able to examine the baseline demographic and clinical features of subjects with and without AF and the independent correlates of AF in this population cohort. As in other studies, we found male sex and existing cardiovascular disease to be independently correlated with AF. We also found chronic lung disease and, in particular, cardiomegaly to be important. We found hyperglycaemia to be a much stronger correlate than in the cardiovascular health study.2 ,3

PREDICTORS OF NEW ATRIAL FIBRILLATION

Of more interest are the predictors of new AF. Here we found age and sex to be important independent predictors, as in other reports. We observed a doubling of risk for each decade of age, in close agreement with the Framingham investigators' findings.1 However, we have also been able to show that cardiomegaly, on the chest radiograph, is by far the most powerful predictor in the multivariate analysis, an observation not made previously. In the Framingham study, heart failure and ECG determined left ventricular hypertrophy were the most powerful predictors of incident AF with a four- to sixfold increase risk due to the former and a three- to fourfold increased risk from the latter.1 A later cohort of 1924 subjects aged 50–94 years did undergo echocardiography in the Framingham study, and left atrial size, reduced fractional shortening, and wall thickness were found to increase modestly the risk of developing AF.15-18 In contrast, we found that radiological cardiomegaly led to a 14-fold increase in the likelihood of new AF. These findings support the view that cardiothoracic ratio is a simple and powerful predictor of adverse cardiovascular outcomes.23 ,24 Therefore, patients with radiological cardiomegaly should, if not already under regular review, be kept under close surveillance from the point of view of their heart rhythm (and, in any case, merit further cardiological investigation).

Though this population cohort was rescreened only once, after four years, importantly, we have also been able to obtain information on incident hospitalisations with AF for up to 20 years. Clearly, these data underestimate the true incidence of new AF in our population, as mentioned earlier. Nevertheless, this manifestation of AF is clearly of interest. Once again, age and cardiomegaly were important predictors, though the latter was much less powerful a predictor for this outcome in the longer term. Of considerable interest was the finding that systolic hypertension was an independent predictor of hospitalisation with AF, approximately doubling the chances of this event. These data are difficult to interpret. Though each hospitalisation had a discharge coding of AF, it was usually a secondary diagnosis. In the approximately 800 such hospitalisations accumulated over 20 years, AF was the principal diagnosis in 38% of cases, the most frequent other principal diagnoses being heart failure (11%), MI (9%), and stroke (7%). The predictive relation we have described may, therefore, merely reflect an indirect association between systolic blood pressure and AF—for example, the association between systolic blood pressure and MI, and MI and AF. If, however, there is a direct relation, this may be of considerable importance and raises a number of interesting questions. Can AF account for some of the relation between systolic blood pressure and stroke or heart failure? This certainly seems to be an area for further investigation.

STUDY LIMITATIONS

As with any large epidemiological study, there are limitations to our analysis. Baseline information on valvar heart disease and chronic heart failure, both important causes of AF, was not collected. It should be noted, however, that only 13% of subjects with AF reported exertional dyspnoea at the time of screening, all of them fulfilling the criteria for chronic bronchitis. Moreover, only three subjects were hospitalised with heart failure within three years of screening and only a small proportion of hospitalisations for AF (< 5%) were associated with a concurrent diagnosis of valvar heart disease. In the majority of cases, a single ECG recording was made, meaning that it was not possible to differentiate between paroxysmal and sustained AF, though this limitation is true of other studies also. These data also do not provide information concerning treatment. Lastly, the potential influence of selection bias (in favour of healthier people) as part of the rescreening process is an important caveat when considering incidence data.

CONCLUSIONS

In summary, the prevalence of AF in Scotland is similar to that in comparable age groups in other countries. The incidence of AF is also similar, although in the short term (four years) only about a quarter of these cases are associated with a hospital discharge related to AF. In the longer term (20 years) almost 4% of middle aged men and women will have an incident hospitalisation associated with AF. The most powerful predictor of the future development of this arrhythmia is radiological cardiomegaly, which increases the adjusted risk 14-fold in the short term and 2.5-fold in the longer term.