Myocardial Infarction

• Introduction

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• ST Elevation Infarction
• Thrombolytic Therapy- Reducing Mortality
• Thrombolytic Therapy- Additional Benefits

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• Assessing Extent of Infarction
• ECG and Q Waves

Assessing Extent of Myocardial Infarction with ECG

It is common practise to look for Q waves on ECG and generally, the presence of Q waves tend to be regarded as evidence for transmural infarction. However, the presence of Q waves tends to reflect the total size of infarction rather than the transmural extent of infarction.

One study published in 1995 examined the the ECGs of patients that were enrolled in a trial of thrombolytic therapy for ST elevation infarction.

This study found that over 50 percent of patients had Q waves on the ECG within one hour of onset of symptoms. The presence of Q waves was correlated with the extent of infarction as measured by quantitative thallium scanning.

A more recent study using cardiovascular magnetic resonance imaging found:

• the probability of Q waves increased with size of infarction and transmural extent for anterior and inferior infarction but not for lateral infarcts
• 28% of subendocardial MI had Q waves
• 29% of transmural MI did not have Q waves
• 48% of all MIs had infarction areas of full transmural infarction
• 99% of MIs had areas of non-transmural infarction

References:

• The Pathologic Basis of Q-Wave and Non–Q-Wave Myocardial Infarction. A Cardiovascular Magnetic Resonance Study. J Am Coll Cardiol 2004;44:554–60
• Appearance of abnormal Q waves early in the course of acute myocardial infarction: implications for efficacy of thrombolytic therapy, J Am Coll Cardiol 1995;25:1084-8