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The ST Compass: A vectorial approach to ST deviation analysis for the electrocardiographic diagnosis of acute myocardial infarction

 

Mads Peter Andersen

Summary

Acute Myocardial Infarction (AMI) – more commonly known as ‘a heart attack’ – is one ofthe leading causes of death in the developed countries. AMI is caused by occlusion of one ormore coronary arteries causing limited blood supply to the cardiac muscle (myocardialischemia).Because time-to-treatment is a major factor for the prognosis of AMI, there is profoundinterest in early detection of myocardial ischemia. The electrocardiogram (ECG) is wellsuited for the early diagnosis, because the ECG waveform is instantly affected by theischemic condition.In clinical practice the presence or absence of ST segment elevation on the 12-lead ECGdetermines the treatment of the AMI patient. However, more detailed information on thelocation and extent of myocardial ischemia may be obtained from ST segment measurementsby analyzing the spatial aspects of the electrocardiographic recording. To a certain degree,the cardiac electrical activity can be described as a fixed dipole source and the ST segmentlevels in each lead of the twelve leads can be perceived as linear projections of a singlecurrent, the ST injury current. Theoretically, the ST injury current flows towards areas oftransmural myocardial ischemia.A graphical tool for visualization of ST deviations, the ST Compass, was developed to clarifythe spatial aspect of ST deviations to the clinical end-user. Additionally, a method to estimatea vector describing the ST injury current from the ST Compass was developed. Modelperformance studies showed that this ST injury vector covers approximately 90% of the STdeviation information contained in the original 12-lead ECG.The characteristics of the ST injury vector were investigated in transmural myocardialischemia and we found strong agreement between the direction of the ST injury vector andthe location of myocardial ischemia as confirmed by Myocardial Perfusion Imaging. Therewas a moderate correlation between the magnitude of the ST injury vector and the extent ofmyocardial ischemia (r = 0.29). ST injury vectors of significant magnitude in the posteriordirection were seen in patients who did not fulfill the classical ST-elevation criteria for AMI,indicating that the ST injury vector may be useful to improve the diagnostic sensitivity toposterior transmural ischemia. In a study of exercise-induced subendocardial ischemia, we found ST injury vectors in the superoseptal direction indicating that the direction of the ST injury vector may be used to iscriminate between posterior transmural ischemia and subendocardial ischemia in the acute phase of AMI.