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| Vectors are used to describe electrical current flow
in the heart. They represent direction, polarity, and
magnitude of currents. They may be added or subtracted to
create mean vectors representing several seperate
vectors. These vectors are used in cardiology to measure
the direction of travel of action potentials through the
heart. By examining vectors, you may detect abnormalities
in conduction, ischemia, or myocardial infarction. Each
lead measures current along a different vector. By
examining the magnitude of electrical current along
specific lead/vectors the heart can be mapped with
significant accuracy. By analyzing lead vectors in a
stepwise fashion you can determine the vectors of ECG
components. |
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This diagram shows the vector relationship of the limb and augmented leads. These leads measure vectors in the coronal plane. The limb leads are
shown in RED. Please take some time to learn the vector directions and relationship between these vectors. This is important when trying to determine the axis of the QRS complex and other compenents. |
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This diagram presents the vectors associated with the 6 precordial leads. The diagram is in the axial (transverse) plane. It is similar to a CT image. (except down is anterior on this diagram) These leads add to the picture obtained from the limb leads and help to locate areas of ischemia and infarction. Myocardial infarction is associated
with these specific findings: Imagine these two diagrams superimposed
over the heart. The specificed leads line up with the
areas of infarction. Examination of ECGs often allows
clinicians to pinpoint the area of infarction to a
specific area of the heart, and its corresponding blood
supply. Note: ECGs can be non-specific in myocardial
infarction and must not be the sole method of diagnosis. |
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The limb leads are used to determine the QRS axis. This is a relevant measurement and helps to diagnose conditions such as ventricular enlargement or hypertrophy, or bundle branch blocks. We will present one method for determining the QRS axis from an ECG tracing. Remember 1) Look at lead I. If the QRS complex
has a positive deflection we can conclude that the QRS
axis is between -90 and +90 degrees. This is shown in
purple on the diagram. Proceed to the next lead. |
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2) Examine lead II. If lead II is positive, than we know the current is running toward the positive electrode. We combine the information from lead I and II to predict a new axis range. The QRS axis lies between -30 and +90 degrees. |
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3) Examine lead aVF. If avF is positive, the axis range is further limited. The QRS axis must now lie between 0 and +90 degrees. |
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4) Contine to examine the limb leads in a systematic order. Lead III is positive. This further limits the QRS axis range: +30 to +90. This is a fairly small range. We can draw conclusions from this information. Look again at lead II. If it has the highest magnitude, and no negative inflection, and we can estimate the axis to lie around +60. Another way of confirming axis is to look at the lead perpendicular to the suspected axis vector. Lead avL is perpendicular to lead II. If the QRS axis is +60 degrees, than the vector perpendicular to that (-30 degrees) should have no deflection, or a biphasic deflection on the ECG tracing. The above method is a decent method of
estimating the QRS axis by direct tracing observation. |
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An alternative involves looking at fewer leads. Look for a lead that is equiphasic or has no deflection. This indicates that the current is running perpendicular to the lead vector. In this example lead III is equiphasic. Therefore the QRS axis must point toward -150 degrees or +30 degrees. To determine which direction, examine additional leads. Lead I and II are both positive. This limits the axis range to: -30 to +90 degrees. We can conclude that the vector points toward +30 degrees. |
VECTORS AND AXIS
