Patient: Young woman 23 years of age, asymptomatic, with no prior history and a normal cardiac ultrasound;
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QRS axis
ECG: Normal QRS-complex with normal axis at 75°;
Comments: The electrical axis of the QRS in the frontal plane is the direction of a single vector which represents the sum of all instantaneous vectors generated by ventricular depolarization. The left ventricle dictates the overall electrical activity of the heart and determines the position of this mean vector which is physiologically directed to the left and downward. The electrical axis is influenced by the anatomical position of the heart in the chest (patient anatomy, body shape, etc.), the anatomy of the heart itself (dilatation, hypertrophy, infarction sequelae, etc.) and the ventricular activation sequence (narrow QRS, bundle branch blocks, hemiblocs, etc.). The electrical axis is an index of normality or of abnormality even if a significant variability in normal patterns in limb leads explains the wide range of normality of the axis in the frontal plane.
The measurement of the axis in the frontal plane is determined from the limb leads and obeys simple rules:
- A vector traveling in the direction of the explorer electrode registers a positive deflection
- A vector traveling away from the explorer electrode registers a negative deflection
- A vector located perpendicularly to the explorer electrode yields either two deflections, the first positive, the second negative (or vice versa) of similar algebraic magnitude, or no visible deflection, i.e. no modification of the isoelectric line
Several methods have been proposed for measuring this axis, two of which are the most frequently used in daily practice:
- Axis evaluation on the basis of leads 1 and aVF: the thorax is divided into 4 quadrants of identical dimensions; this method determines the quadrant in which the axis of the QRS-complex is found, obtained from the absolute value of the amplitude of the QRS measured in these two leads; to achieve this, the amplitude of the QRS-complex exceeding the isoelectric line is measured and the portion below the isoelectric line is subtracted (positive deflection – negative deflection); the amplitude in these two leads is plotted on a graph whose horizontal axis is lead I and vertical axis is aVF in order to obtain the resultant vector; If the QRS-complexes are positive in leads I and aVF, the axis is normal; if the QRS-complexes are positive in lead I and negative in aVF, the axis can be considered normal up to -30°; the axis is left beyond -30°; if the QRS-complexes are positive in aVF and negative in lead I, the axis is considered as right; if the QRS-complexes are negative in aVF and lead I, the axis is considered as extreme-right (various names are used: undeterminate axis, “no man’s land”, extreme-left axis or probably the most appropriate denomination, extreme-right axis);
- determination based on the “flattest” (smallest) QRS-complex and the tallest QRS: to achieve this, one must determine the QRS-complex among the frontal leads whose sum vector is close to zero (amplitude of the positive deflections is equal to the amplitude of the negative deflections); the axis is perpendicular to the isoelectric lead (there are two perpendicular directions) and is oriented in the direction of the lead recording the greatest positive deflection;
The mean axis of the heart in the horizontal plane is estimated by evaluating the transition zone from the precordial leads. The amplitude of the R waves gradually increases with a concomitant decrease in the amplitude of the S wave, beginning with a rS complex in V1 and ending with a qR complex in V5, V6. The R wave can be taller in V5 than in V6 in conjunction with the attenuation generated by the lungs. The precordial QRS electrical transition zone corresponds to the precordial lead where the change in polarity of the QRS-complex occurs (R waves and S waves of equal voltage, R/S amplitude=1).
- The transition is considered normal if it occurs in V3 or V4;
- The transition is considered early (rapid progression of the R wave) if it occurs in V1 or V2; this is called a counterclockwise rotation; this abnormality is observed in the presence of poor electrode placement, right ventricular hypertrophy, posterior infarction, right bundle branch block or ventricular pre-excitation;
- The transition is considered late (slow progression of the R wave) if it occurs in V5 or V6; this is called a clockwise rotation; this abnormality is observed in the presence of poor electrode placement, previous infarction, dilated cardiomyopathy, left ventricular hypertrophy, chronic lung disease, left anterior fascicular block;
- The transition is reversed if, paradoxically, the amplitude of the R wave decreases; this abnormality is observed in the presence of a prior myocardial infarction or dextrocardia;
Take-home message: The normal axis of the QRS-complex in the frontal plane is between -30 and 90°; the normal electrical transition zone on the horizontal plane is located between V3 and V4.
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On this ECG, the axis of the QRS-complex is:
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