| MadSci Network: Medicine |
Cristina, Constructing a QRS vector loop from an EKG is basically calculating the direction of electrical activity or electrical axis of the various cardiac chambers during depolarization and repolarization. The axis can be calculated exactly by plotting vectors on a triaxial reference grid, which is nothing more than a rearranged Einthoven triangle. Since you said that you understand how to plot the vector, I will be brief in my simplified explanation. First you have to determine the algebraic sum of the QRS in millimeters. You can determine any axis by using any two frontal leads. I usually use leads one and three. To do this in lead one (assuming on an electrocardiogram that 1 small box is 1mm and therefore 1 lg box is 5mm) add the #mm of the Q wave to the #mm of the R wave. then draw a perpendicular line down from that lead on the triaxial diagram. The same can then be done for lead 3. With that complete a line is then drawn from the center of the triaxial diagram to the intersection of the two perpendicular lines.This represents the electrical axis of that specific cardiac cycle. A normal electrical axis is +105 degree's to -30 degree's. Further right than + 105 degree's represents a right axis deviation and further left than -30 degree's constitutes a left axis deviation. Basically, determining the electrical axis of a patients EKG is like determining the "tilt" of the patients heart within the chest cavity. This is important because we know that certain disease processes can cause cause these characteristics (right and left axis deviation). Some example's of these are the following: For right axis deviation- chronic obstructive pulmonary disease,severe pulmonary hypertension, pulmonary emboli and cor pulmonale. For left axis deviation- hypertension, aortic valvular disease and ischemic heart disease. I hope this helps. For more information a basic electrocardiography manual is a good place to start. Stephanie Loslo Department of Cardiology School of Medicine
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