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Abstract & Commentary
Synopsis: The Q-wave/non-Q-wave distinction is useful clinically, and the primary determinant of the presence of Q-waves is the total size of the underlying infarction, rather than its transmural extent.
Source: Moon JCC, et al. The Pathologic Basis of Q-Wave and Non-Q-Wave Myocardial Infarction. J Am Coll Cardiol. 2004;44:554-561.
There has been considerable controversy regarding the dichotomous labeling of an acute myocardial infarction (AMI) as transmural vs non transmural; Q-wave vs non-Q-wave; ST elevation (STEMI) vs non-ST elevation (NSTEMI), and more recently, the microinfarcts, defined by isolated troponin elevation without CK or CKMB abnormalities. Prior pathology studies have demonstrated that many Q-wave infarctions are often non transmural, and conversely, transmurality may occur in the absence of Q-waves. The present study using a new imaging technique, late gadolinium enhancement cardiovascular magnetic residence (CMR), was used to evaluate 91 patients with previous myocardial infarction to determine the transmural extent and anatomic features of the MI. The CMR imaging technique employed a 1.5-T CMR scanner, sectioning the left ventricle (LV) into 2 long axis slices, and up to 10 short-axis slices, encompassing the entire LV myocardium. Intravenous gadolinium was injected, with imaging acquired after 10 minutes for late enhancement image analysis. LV function, volume, and mass were calculated, as well as the degree of enhancement within each of the 17 LV sections analyzed, utilizing a scoring system rating ranging from 0-25% to 100% infarction. Each segment was assessed for the presence, at any point, of transmural involvement. Anatomic regions analyzed were anterior, inferior, and lateral; the latter equivalent to posterior MI. Total size of MI was calculated for each territory. ECG analysis was made using both TIMI criteria, as well as those of the ESC/ACC. Of interest, the former were more accurate in correlating with CMR images.
The results demonstrated a better correlation of Q-waves, with total size of the prior MI vs the degree of transmurality. Furthermore, LV function, as assessed by ejection fraction (EF), also correlated with the size of the MI; receiver operating curve 0.85 for all MI (0.90 for anterior and 0.77 for inferior territory). There was no relationship for lateral MI with any parameter. Classification into Q-wave/non-Q-wave MI was a good diagnostic test for the size of MI. In addition, there was an increased likelihood of prior Q-wave MI being present, as the extent of transmural MI territory increased. This was true for both anterior and inferior MI, but not lateral. The total size of MI was a better predictor of Q-wave vs non-Q-wave classification than the transmural extent; the latter did not significantly increase the area under the ROC curve with multivariant analysis, and was not an independent predictor of Q vs non-Q MI. Systolic function was worse in patients with any Q-wave MI in the anterior region vs non-Q MI, (47% EF vs 55%; P = 0.02). Anterior Q-wave MI had a lower EF than anterior non-Q MI, (45% vs 55%; P =0.003), but there was no difference between inferior Q-wave vs non-Q MI. Although ESC/ACC definition of Q-wave MI resulted in fewer patients classified as Q-wave MI, both TIMI and ESC/ACC ECG criteria correlated with infarct size, most prominently for anterior MI. Six of 21 anterior Q-wave MI patients were defined as non-transmural, and 34 of 48 transmural MI were classified as Q-wave MI. Thus, non transmural Q-wave infarctions were observed, as well as transmural non-Q-wave infarction. Moon et al conclude that: 1) the Q-wave/non-Q-wave distinction is useful clinically, and 2) the primary determinant of the presence of Q-waves is the total size of the underlying infarction, rather than its transmural extent. Thus, the larger the MI, the more likely Q-waves will be present; conversely Q-waves on an ECG suggest a large infarction and lower EF. Subendocardial infarction was not predicted by the presence, or absence, of Q-Waves. Moon et al opine that MIs have a complex structure, with varying transmural extent, making the transmural/non-transmural division overly simplistic. Previous silent infarctions were not uncommon, even when the index infarction was believed to be the first event. Late gadolinium enhancement CMR is suggested as an excellent technique to evaluate MI. Moon et al suggest that there should be a reinterpretation of anatomic and pathologic bases of the ECG, with new criteria resulting from further research. Although the size of an infarct correlates with outcome, there is no clear evidence that transmural extent is an independent predictor of clinical events compared to the size of the MI. As Moon et say, "MIs are rarely simply one or the other."
Comment by Jonathan Abrams, MD
The terminology of MI has been a subject of many reports over the years, and remains somewhat arcane and confusing. Years ago, it was believed that "non-transmural"or "non-Q-infarction" were more benign than Q-wave MI, until it was realized that long term follow-up demonstrated a merging of survival and event curves, often with worse prognosis for non-Q MI. In the recent era, the terms STEMI and NSTEMI have become popularized; it is well known that morbidity and mortality is higher immediately with STEMI, but during long-term follow-up, NSTEMI subjects have substantial morbidity and mortality. Lytic therapy and/or direct angioplasty are accepted as the appropriate emergent therapy for STEMI, but individuals with NSTEMI or non-Q-wave MI are more complicated to treat, and are candidates for variety of anti-platelet/thrombotic medications, as well as early catherization and possibly early coronary intervention. It is well accepted that the presence of Q-waves usually indicates substantial LV dysfunction, which may or may not be true with NSTEMI without Q-waves.
The present study by Moon et al confirms that the presence of Q-waves are important with respect to predicting LV function. Transmurality of an infarct is not necessarily concordant with Q-waves on the electrocardiogram. Physicians should keep in mind the difference in pathophysiology of ST segment elevation MI compared to non-ST MI. While a large infarction is likely to be present when extensive Q-waves are found, the converse may not be true, ie, LV systolic function can be unexpectantly decreased in the absence of Q-waves. STEMI are generally associated with a total occlusion of a large coronary artery, with significant loss of myocardium. This study, as have others, indicates that the territory subserved by the left anterior descending artery is significantly greater than that supplied by the right coronary artery, helping to explain the better prognosis with interior MI, usually a smaller infarction. NSTEMIs are usually the result of non-occlusive thrombus, often with multiple complex lesions throughout the coronary tree, and leave the patient with "more to come" unless vigorous ACS therapy is instituted. On the other hand, STEMI or Q-waves MI are likely to be larger, have a higher early mortality, and less likely to be accompanied by recurrent infarction or unstable angina after the event. Finally, in spite of attempts to estimate the degree of involvement of LV with troponins, CK, and ECG findings, prompt assessment, of LV function with ultrasound, is critical for all infarctions to provide an accurate baseline ejection fraction and extent of regional wall motion abnormalities. Transmural vs non-transmural infarction is a taxonomy that should be discarded.
Dr. Abrams, Professor of Medicine, Division of Cardiology, University of New Mexico, Albuquerque, is on the Editorial Board of Clinical Cardiology Alert.