The effect of the localization of Q wave myocardial infarction on ventricular electromechanics.

Background: The exact location of a Q wave myocardial infarction has an important effect on overall left ventricular function.

Objectives: To assess the effect of localization of Q wave infarction on left ventricular minor and long axis function, with particular reference to electromechanical disturbances.

Methods: We studied 72 patients with Q wave myocardial infarction; 35 anterior, age 61+/-15 years and 37 inferior, age 62+/-12 years. ECG intervals were automatically measured by Hewlett-Packard Pagewriter and LV dimension and filling velocities studied by transthoracic echocardiography and simultaneous phonocardiogram. Findings were compared with 21 controls of similar age.

Results: Heart rate and all ECG intervals were similar in the two patient groups and controls. QRS axis was more to the left in patients with inferior MI. Normal septal q wave was absent in lead V5 and V6 in 33/35 (94%) patients with anterior MI and in only 3/37 (8%) with inferior MI, p<0.001. LV minor axis dimensions were enlarged vs. normal (p<0.001) in the two patient groups and to a greater extent in anterior MI compared with inferior MI, p<0.05. Isovolumic relaxation time was prolonged only in-patients with an inferior MI, p<0.01. Long axis amplitude was globally reduced (p<0.001) in the two patient groups as were shortening and lengthening velocities (p<0.001). The onset of septal long axis shortening with respect to the q wave was delayed by 30 and 40 ms in inferior MI and anterior MI and that of lengthening with respect to A2 by 20 and 30 ms, respectively, compared to normal (p<0.001 for both). Post ejection shortening was localized to the septal long axis in 32/35 patients with anterior MI but was generalized involving all three LV long axes in inferior MI, p<0.001. Transmitral Doppler flow velocities and the frequency of mild mitral regurgitation were similar in the two groups.

Conclusion: These results confirm a close association between anterior Q wave infarction, septal incoordination and absent septal q waves. The global incoordinate long axis behaviour in inferior Q wave MI may be due to significant papillary muscle dysfunction, and results in significant shape change in early diastole. This disturbance in electromechanical behaviour might play an important role in the differing outcomes between the two different sites of myocardial infarction.