Axis of ECG

The average direction of spread of the depolarization wave through the ventricles as seen from the front is called the ‘cardiac axis’. The mean electrical axis of the QRS complex is the average of the total depolarization of the ventricles in the frontal plane. There is a correlation between the mean electrical axis of the QRS complex and the electrical activity of the heart. It is useful to decide whether this axis is in a normal direction or not. Therefore, in pathologic hypertrophy of either ventricle (i.e. when there is more muscle being depolarized), the axis tends to shift in the direction of the hypertrophied ventricle.

Direction of Depolarization (Vector) of the QRS Complex

1) The left ventricle is thicker so the mean QRS vector is down and to the left. (The origin of the vector is the AV node with the left ventricle being down and to the left of this).

2) The vector will point toward hypertrophy (thickened wall) and away from the infarct(electrically dead area).

The QRS complex morphologies in frontal plane leads are variable. The mean QRS axes more positive than + 90 degrees represent right axis deviation. The axis more negative than – 30 degrees represents left axis deviation. Mean axes lying between – 90 and 180 degrees (or equivalently between + 180 and + 270 degrees) are referred to as extreme axis deviations. The designation indeterminate axis is applied when all six extremity leads show biphasic (QR or RS) patterns; this finding can occur as a normal variant or may be seen in a variety of pathological conditions. Usually if the axis is abnormal aVF is predominantly negative.

Axis of ECG

 

 

 

 

 

Negative axis of ECG

Leads VR and II look at the heart from opposite directions. Seen from the front, the depolarization wave normally spreads through the ventricles from 11 o’clock to 5 o’clock, so the deflections in lead VR are normally mainly downward (negative) and in lead II mainly upward.

How to Determine Frontal Plane QRS Axis?

Lead I and aVF

Since lead I and aVF are perpendicular to each other, one can use those two leads to quickly determine the quadrant in which the QRS axis falls. Lead I runs from right to left across a patient’s body, positive at the left hand. If the QRS in lead I is positive (mainly above the baseline), the direction of depolarization will be in the positive half (right half) of the circle above. Lead aVF runs from top to bottom across a patient’s body, positive at the feet. If the QRS in lead aVF is positive (mainly above the baseline), the direction of depolarization will be in the positive half (lower half) of the circle above. If one realizes that there are two leads to consider and a positive (+) or (-) orientation for each lead, there would be four possible combinations.

Memorize the following axis guidelines.

Axis deviation of ECG

Subtract the number of little squares below the baseline in lead I from the number above the line (+ 5 mm in the example) and plot horizontally towards the lead. Repeat for lead AVF (+11 in the example) and plot vertically towards the lead. If the sum is negative then plot the number of squares away from the lead. The bottom line is, if the axis is shifted out of the normal quadrant,evaluate the reasons for this.Lead I, II and III for Axis Determination

The direction of the axis can be derived easily from the QRS complex in leads I, II and III. A normal 11 o’clock–5 o’clock axis means that the depolarizing wave is spreading towards leads I,II and III and is therefore associated with a predominantly upward deflection in all these leads;the deflection will be greater in lead II than in I or III If the right ventricle becomes hypertrophied, the axis will swing towards the right the deflection in lead I becomes negative(predominantly downward) and the deflection in lead III will become more positive(predominantly upward). This is called ‘right axis deviation’. It is associated mainly with pulmonary conditions that put a strain on the right side of the heart, and with congenital heart disorders.

Axis in Degrees-using Six Limb Leads

The cardiac axis is at right angles (90°) to the lead in which the R and S-waves are of equal size.The axis points towards any lead where the R-wave is larger than the S-wave. It points away from any lead where the S-wave is larger than the R-wave.The cardiac axis is sometimes measured in degrees though this is not clinically particularly useful. Lead I looks at the heart 0°; lead II from +60°; lead aVF from +90°; and lead III from +120°. Leads aVL and aVR are said to look from –30° and –50°, respectively. The normal cardiac axis is in the range –30° to +90°. For example, if in lead II the size of the R-wave equals that of the S-wave, i.e, QRS is equiphasic, the axis is at right angles to lead II. In theory, the axis could be at either –30° or +150°. If lead I shows an R-wave greater than the S-wave, the axis must point towards lead I rather than lead III. Therefore, the true axis is at –30° – this is the limit of normality towards what is called the ‘left’. If in lead II the S-wave is greater than the R-wave, the axis is at an angle of greater than –30°, and left axis deviation is present. Similarly, if the size of the R-wave equals that of the S-wave in lead I, the axis is at right angles to lead I or at +90°.This is the limit of normality towards the ‘right’. If the S-wave is greater than the R-wave in lead I, the axis is at an angle of greater than +90°, and right axis deviation is present.

Summary

Using leads I and aVF the axis can be calculated to within one of the four quadrants at a glance.If the axis is in the “left” quadrant take your second glance at lead II.

• both I and aVF +ve = normal axis

• both I and aVF -ve = axis in the northwest territory

• lead I - e and aVF +ve = right axis deviation

• lead I +ve and aVF -ve

• lead II +ve = normal axis
•
 lead II -ve = left axis deviation

Causes of Northwest Axis (No Man’s Land)

• emphysema

• hyperkalaemia

• lead transposition

• artificial cardiac pacing

• ventricular tachycardia

Causes of Right Axis Deviation

• normal finding in children and tall thin adults

• right ventricular hypertrophy

Chronic Lung Disease Even Without Pulmonary Hypertension

• anterolateral myocardial infarction

• left posterior hemiblock

• pulmonary embolus

• Wolff-Parkinson-White syndrome-left sided accessory pathway

• atrial septal defect

• ventricular septal defect

Causes of Left Axis Deviation

• left anterior hemiblock

• Q-waves of inferior myocardial infarction

• artificial cardiac pacing

• emphysema

• hyperkalaemia

— Wolff-Parkinson-White syndrome-right sided accessory pathway

— tricuspid atresia

— ostium primum ASD

— injection of contrast into left coronary artery