In large scale clinical trials of fibrinolytic treatment in STEMI, the amount of resolution of ST segment elevation has been shown to correlate with recovery of left ventricular function and to predict long term prognosis.2–10
These observations have led to the hypothesis that ST segment changes during the course of STEMI are better at predicting clinical outcome than patency rates of the infarct related epicardial coronary vessel because they reflect myocardial tissue perfusion. A less consolidated belief of many investigators is that the so called “complete” resolution of ST segment deviation corresponds to a small myocardial infarct with limited myocardial necrosis. In this study, we have explored the correlation between ST segment changes and myocardial tissue perfusion.
A first important finding was that categorisation of resolution of ST segment deviation showed substantial differences depending on which method of ECG analysis was used, the percentage of patients categorised as having complete resolution varying between 22–40%. It can therefore be expected that the different ECG analysis methods differ in their ability to predict infarct size and prognosis. These differences can at least in part explain the discrepancies between the results obtained from different studies and seem to preclude pooling of these data.
We found a significant relation between the ST segment changes on the ECG and the extent of myocardial hypoperfusion as assessed by PET, thus confirming the hypothesis that ST segment changes in the course of STEMI reflect myocardial tissue perfusion.
Importantly, no significant correlation was found between the relative resolution of ST segment deviation and any of the PET indices of myocardial damage. This implies that relative resolution does not allow any firm conclusions to be drawn regarding infarct size in the individual patient. This contrasts with the findings in large patient populations in which the average relative resolution of ST segment deviation shows significant prognostic power.2–10
To explain this paradox, one needs to know the exact relation between relative resolution of ST segment elevation and the percentage of jeopardised myocardium saved by reperfusion therapy. Presuming that this relation exists, we hypothesise that the explanation for the paradox is as follows: if in a patient with a large myocardial infarct half the jeopardised myocardium is saved by reperfusion, this patient is still left with a rather large infarct. When, in contrast, half of a small area of jeopardised myocardium is saved, that patient will end up with a very small infarct. In both patients, the relative resolution of ST segment deviation may have been the same, but final infarct sizes differ greatly. When large patient populations are considered, however, pooling many patients with different amounts of jeopardised myocardium, the patients in whom a greater percentage of the jeopardised myocardium is saved will, on average, have smaller infarcts and consequently a better prognosis. This is illustrated by the figures obtained by Dong and colleagues,19
who studied the correlation between ST segment resolution and myocardial salvage assessed by Tc99m sestamibi scintigraphy in a large number of patients (n
243) after reperfusion therapy for acute myocardial infarction. They found that ST segment resolution correlated with myocardial salvage, albeit with a very wide distribution of the scintigraphy results—for example, the final infarct size was 12.5 (12.0)% of the left ventricle in the group with complete ST segment resolution, 20.0 (13.9)% in the group with partial ST segment resolution, and 22.7 (19.4)% in the group with no resolution (p < 0.001; values are mean (SD)). As with the results we have obtained, such a wide distribution would preclude any prediction of final infarct size in the individual patient.
When patients were subdivided into tertiles according to their myocardial damage severity index, summed ST segment elevation at baseline emerged as the most powerful predictor of final infarct size. After reperfusion therapy, residual ST segment deviation in the single lead with maximum ST segment deviation was found to predict infarct size at least as well as summed ST segment deviation. This finding is in agreement with a recently published study in which residual ST segment elevation in the lead with maximum ST segment deviation after reperfusion therapy emerged as the best predictor of 180 day cardiac mortality.14
It provides the clinician with a simple tool that is readily and universally accessible to estimate the final result of reperfusion therapy.
The major limitation of our study was the absence of PET examinations preceding reperfusion therapy. However, in view of the well known time dependency of the benefit of coronary reperfusion therapy, postponing this treatment by performing PET scintigraphy was deemed unethical. Likewise, no 13NH3 PET re-studies after 4–6 weeks were done, which could have improved the diagnostic value for viability.
The size of the index infarct was not homogeneous in the study population. This may have reduced the possibility of finding a correlation between resolution of ST segment deviation and infarct size. Furthermore, it should be acknowledged that reperfusion is a dynamic phenomenon, and a single ECG recording may have missed possible ischaemic events in the hours following reperfusion.
Methods of ECG analysis differ in their ability to predict the absence of myocardial tissue reflow. Final infarct size as assessed by PET is significantly, but only moderately, correlated with the ST segment deviation score obtained before and after reperfusion therapy, but not with relative ST segment deviation resolution.
Residual ST segment deviation in the single lead with maximum ST segment deviation after reperfusion therapy seems to be equally as good as summed ST segment deviation as a predictor of the extent and severity of myocardial damage.