Coronary artery disease, as all physicians know, presents in different ways in different people. We have anecdotes of people who die suddenly without ever being aware that their coronary arteries were blocked. On the other end of this coronary artery disease spectrum we have the condition called chronic stable angina where patients experience chest pain only on exertion. And in between these two extremes lie the various categories of acute coronary syndrome. Intuitively we might believe that the milder manifestation of chronic stable angina represents the benign end of the spectrum where the coronary arteries are only minimally involved. This is not always true because we have learnt that even in patients with chronic stable angina there can be extensive coronary involvement.
What matters in coronary artery disease are the answers to three questions: Which artery is blocked? Is that artery blocked proximally or distally? What is the degree to which it is blocked? And of the two coronary arteries and their branches, high grade blocks in the left main coronary artery and in the proximal portion of the left anterior descending artery are the most dangerous. So when faced with patients who have chronic stable angina, one of the important things I want to know is whether they have involvement of the left main coronary artery or the proximal portion of the left anterior descending artery. Determining the ST segment deviation vector from the ECG is an attempt to answer this question without a coronary angiogram.
The ST segment deviation vector is determined in much the same way as the cardiac axis is determined from the QRS complexes. We use the limb leads and look at the ST segments in them. Wherever ST segments are depressed, the vector is moving away from those leads and wherever ST segments are elevated, the vector is moving towards those leads. And the vector is perpendicular to any lead whose ST segment is neither elevated nor depressed. With these basic electrophysiological principles, we can proceed to determine the ST segment deviation vector, if any, in patients with chest pain. A general rule is that when the ST segment deviation vector is in the quadrant between minus 90 degrees and minus 180 degrees (the "aVR quadrant"), there is a high probability of significant stenosis in either the left main or the proximal left anterior descending coronary arteries.
I use this information as a guide in deciding which patients with chronic stable angina should be referred for a coronary angiogram after an exercise stress test.
For an understanding of how ST segment deviations will look like in patient with significant blocks in the left main coronary and proximal left anterior descending coronary arteries, please see:
Proximal left anterior descending artery block
Left main coronary artery block
