Besides the presence of T wave abnormalities, we identified high BMI, high Lp (a) and smoking at age 50 as independent risk factors for the development of major abnormal Q/QS patterns at age 70. T wave abnormalities, together with high fasting blood glucose were also important predictors for the development of ST segment depression while participants with higher BMI had an increased risk of developing T wave abnormalities.
This is a longitudinal community based study, with long follow-up period, where all subjects from one birth cohort in the community were invited to participate. The same two nurses registered the ECGs at both surveys, contributing to consistency in electrode placement. Three physicians carried out the coding of all ECGs, whereby the coding consistency can be expected to be high. However, coding errors may confound analysis of ECG data. An obvious limitation of our study is that women were not included. There was no validation of the outcome detection system, however, the myocardial infarction diagnosis in the Swedish hospital register has been found to be correct in 93–97% of cases with less than one percent being missed [8
Our study is unique in that it uses ECG variables as outcomes instead of clinical outcomes. By choosing major abnormal Q/QS patterns as end-point, silent myocardial infarctions have also been included in the analyses. According to most studies a clinically unrecognized MI is believed to be associated with the same risk as recognized symptomatic MI [1
] although a lower risk for unrecognized MI has also been described [10
]. The prevalence of silent MI increases with age [1
] and varies from 21% to 68% [2
], with an average around 30% [11
]. In our study, 63% of the subjects with a major abnormal Q/QS pattern on ECG at the 70-year survey had not been previously hospitalized due to MI. While the majority of major Q waves are believed to be due to myocardial infarction, some may be due to other causes, including myocarditis, cardiac amyloidosis, and cardiomyopathy [12
]. Also displacement of the diaphragm and the heart, as in chronic obstructive lung disease may cause the appearance of a non-infarction Q wave and non-infarction Q waves may be seen in subjects with pre-excitation syndrome and left ventricular hypertrophy (LVH) [12
]. The low specificity of Q/QS pattern to recognize silent myocardial necrosis is an additional limitation to our study.
Since some of the conventional risk factors [13
] were not strong predictors of the development of major abnormal Q/QS patterns, our study suggests that predictors of major abnormal Q/QS patterns may be different from those predicting symptomatic MI. This is further strengthened by our additional analysis where the major Q/QS pattern end-point was substituted by death due to acute MI before the 70-year examination and where conventional risk factors such as diabetes mellitus and DBP became significant predictors. Other authors have debated whether the predisposing factors for silent MI are simply the traditional risk factors for coronary atherosclerosis, or whether silent MI patients have unique characteristics [4
]. However, no general conclusions can be drawn from our study since our major abnormal Q/QS finding will include silent MI, clinically recognized MI and some non-infarction Q waves.
Loss of Q wave could explain some of the cases where diagnosis of MI was not accompanied by a visible major Q/QS pattern at age 70. Even though development of Q waves after acute MI has been considered an indicator of myocardial necrosis, studies have shown that Q wave loss may occur in 11–20% of patients after transmural MI [14
]. The clinical significance of such an ECG phenomenon has not been fully clarified [16
]. Functional recovery of stunned and/or hibernating myocardium [16
], as well as regeneration of myocytes [17
] has been proposed as possible mechanisms.
The pathophysiologic basis of ST segment depression on resting ECG remains speculative, and ST segment depression is seen both in left ventricular hypertrophy and myocardial ischemia [1
]. One of the possible etiologies for development of left ventricular hypertrophy is altered glucose metabolism with insulin resistance and hyperinsulinemia [18
]. On the other hand, patients with diabetes mellitus are known to have an increased risk of developing arteriosclerosis [20
], so an impaired glucose metabolism could well be a predictor for ST segment depression due to either left ventricular hypertrophy or coronary heart disease.
While permanent negative T waves in the chronic stage of MI have indicated the presence of transmural infarction [21
]. T waves in a post-myocardial phase have been shown to be associated with the presence of viable myocardium at jeopardy [22
]. T waves should therefore be regarded as a dynamic substrate. Even though subjects not surviving the MI were not included in our main analyses, we found that the prevalence of major T wave abnormalities in those who died during the period due to acute MI was statistically significantly higher for this group compared to those who survived, suggesting that the predictive power of T wave abnormalities is not restricted to survivors of a Q wave MI.
High BMI predicted both major abnormal Q/QS pattern and abnormal T wave. One possible mechanism is horizontal displacement of the heart [23
], but left ventricular hypertrophy may also play a role.
Lp (a) is believed to have potential pathogenic effects on atherosclerosis and thrombosis [24
]. Some previous studies have identified elevated Lp (a) as a risk factor leading to premature MI [27
], while others have shown no association between Lp (a) and risk for CHD [24
]. One study has, in diabetic patients, specifically found Lp(a) to be associated with silent CHD [29
]. Our study supports the finding that Lp (a) may be important an important predictor of CHD and silent MI.
The present approach using ECG-based diagnosis should be regarded as a complement to the more traditional approach used in most previous studies. One should therefore be careful not to generalize the conclusion of this study to other population groups, and further studies for confirmation of our findings are motivated.