The Electrocardiogram in Epidemiological Studies

The means available for identifying coronary disease in the community are regrettably limited. A history of chest pain may be elicited and judicious questions may enable one to decide if this was cardiac or not. If cardiac pain has been experienced, clinical examination may reveal conditions other than coronary disease (congenital, valvular, or hypertensive disease, for example) which could be responsible. Finally an electrocardiogram (ECG) may be taken. If the resting ECG is normal, evidence suggestive of coronary disease may be obtained in a proportion of cases by repeating it under some form of stress, usually exercise. Unfortu nately abnormal electrical patterns shown by the ECG, even when they are highly suggestive, are not specific for coronary disease. Furthermore, there may be considerable difficulty in differentiating normal from abnormal tracings (Simonson, 1958). It is not surprising, therefore, that a number of studies have revealed variation between different observers in their classification of ECGs as normal or abnormal and especially of whether the abnormality is due to coronary disease or not (Davies, 1958; Thomas, Cochrane, and Higgins, 1958; Segall, 1960; Acheson, 1960; Epstein, Doyle, Pollack, Pollack, Robb, and Simonson, 1961; Higgins, Cochrane, and Thomas, 1963). Despite these reservations, however, the ECG remains a valuable screening tool for coronary disease. It is objective, simple, rapidly obtained, and painless, and largely independent of co-operation or language differences. Variation in interpretation of an ECG for coronary disease may arise either because certain items in the tracing are recorded by some observers but not by others, or because of differences of opinion on the meaning of items which have been observed by all. Resolution of observer variation into its two com ponent parts, namely that of observation and that of interpretation, is the logical approach. To do this, Blackburn, Keys, Simonson, Rautaharju, and Punsar (1960) suggested a detailed classification of electrocardiograms based on precisely defined criteria (Appendix). Wherever possible these criteria are determined by objective measurements. This standardized coding system has become known as the Minnesota Code and it has been widely recommended for initial classification of ECGs for epidemiological purposes. A World Health Organi zation scientific study group on comparable methodo logy for the epidemiological study of hypertension and ischaemic heart disease (WHO, April 6, 1963) endorsed this approval and advocated studies of the Code's reproducibility and validity. Its validity, the report suggested, might be best established by correlating the ECG code items with the presence or development of disease detected by other means. The main purpose of the present study was to determine the implication of the various code items as measures of risk of developing or dying of coro nary disease. In the course of the work some obser vations were also made on the reproducibility of the classification and these have been included in the paper. Finally, the use of the code for comparing the frequency of ECG abnormalities in different popu lations is briefly considered. The implication of ECG items can be assessed by following a sufficiently large group of people, whose ECGs have been recorded, for 5 to 10 years. Studies of this kind, in which the Minnesota Code has been used, are in fact in progress in various countries and 53