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The 12-lead system

Created: 28/8/2006
Updated: 9/8/2006

Standar limb ECG leads

The standard limb leads/bipolar limb leads (leads I, II, III)

With the bipolar system, one limb is connected to the positive terminal of the recording galvanometer and another limb to its negative terminal. Three limbs (right arm-RA, left arm-LA and left leg/foot-LL) are used. The right leg was used as "earth", to minimise interference.

We have the following bipolar leads:
Lead I: RA (-) to LA (+)
Lead II: RA (-) to LL (+)
Lead III: LA (-) to LL (+)

Augmented/unipolar limb leads (frontal plane)

Initially, unipolar limb leads were used, in which recordings were made from the RA (lead R), LA (lead L) and LL (lead F for "foot").

Any voltage measurement requires two electrodes: a "reference electrode", whose potential does not change during the cardiac cycle, and an "exploring electrode" attached to the limb in question. A voltage or "V" lead was used as an indifferent electrode by joining R, L and F together.

Nowadays, the limb connections are slightly modified, to "augment" the size of the deflections obtained from leads L, R and F. These are named as follows:

aVR: RA (+) to [LA & LL] (-)
aVL: LA (+) to [RA & LL] (-)
aVF: LL (+) to [RA & LA] (-)

The precordial/chest leads (horizontal plane)

With each precordial lead, the positive (recording) terminal of the galvanometer is connected to an electrode at an agreed site on the chest wall, and the negative terminal is connected to an indifferent electrode, the"V" electrode (see above).

Hence, the chest leads are designated as V1, V2, V3, V4, V5 and V6.

The sites of the above electrodes are as follows:

V1: Right sternal margin at 4th intercostal space (ICS)
V2: Left sternal margin at 4th ICS
V4: Intersection of 5th ICS and left mid-clavicular line
V3: midway between V2 and V4
V5: Intersection of left anterior axillary line with a horizontal line through V4
V6: Intersection of left mid-axillary line with a horizontal line through V4 and V5

Einthoven's triangle hypothesis

Published in 1913, this hypothesis attempts to explain the principles of electrocardiography on a scientific basis. It is based on four assumptions which are not completely true, but do provide some basis. The four assumptions are as follows:

1. The trunk is a homogeneous volume conductor.
2. The mean of all the electrical forces generated during the cardiac cycle can be considered as originating from a dipole situated at the heart’s centre.
3. The limb leads pick up voltage changes in the frontal plane only.
4. The attachments of the three extremities used in making the limb leads (R, L and F) form the apices of an equilateral triangle with the dipole at its centre.

Determination of the electrical axis

The electrical axis of the heart can be derived from the six frontal plane, and leads to an accuracy of +/- 15°. The axis is measured by reference to the hexaxial reference system (see figure above).

Calculation of the axis requires determining the algebraic sum of the QRS deflections in each limb lead. This is done by adding the positive deflections and subtracting the negative deflections of the QRS complex in any given lead.

Follow these steps for calculating the QRS axis:

1. By inspection, find the frontal-plane lead in which the algebraic sum of the QRS complex deflections most closely approximates to zero (not necessarily the smallest QRS complex!).

The axis will be approximately at right angles to this lead and must therefore lie in one of two approximate directions.

For example, if the algebraic sum of the QRS complex deflections most closely approximates zero in lead I, then the axis must lie approximately at either -90° or +90°, which are the two directions perpendicular to lead I.

2. Now examine the QRS complex in that limb lead which occupies a position at right angles to the original lead (where the algebraic sum of the QRS deflections was close to zero) - i.e. in lead aVF.

If the QRS complex deflection in this lead is dominantly positive, then the QRS axis should be +90° (direction of aVF). But if predominantly negative, then the QRS axis will be -90°.

The above calculation gives accuracy to the nearest 30°. However, resolution to the nearest 15° can be obtained by further adjustments (not discussed here).

Examples are as follows:

(1) Normal axis:

Normal ECG axis

  • Lead aVF is the isoelectric lead.
  • The two perpendiculars to aVF are 0° and 180°.
  • Lead I is positive (i.e. orientated to the left).
  • Therefore, the axis has to be 0°.

(2) Axis in the left axis deviation (LAD) range:

Left axis deviation

  • Lead aVR is the smallest and isoelectric lead.
  • The two perpendiculars are -60° and +120°.
  • Leads II and III are mostly negative (i.e. moving away from the + left leg)
  • The axis, therefore, is -60°.

(3) Axis in the right axis deviation (RAD) range:

Right axis deviation

  • Lead aVR is closest to being isoelectric (slightly more positive than negative)
  • The two perpendiculars are -60° and +120°.
  • Lead I is mostly negative; lead III is mostly positive.
  • Therefore, the axis is close to +120°. Because aVR is slightly more positive, the axis is slightly beyond +120° (i.e. closer to the positive right arm for aVR).

Normal appearances in precordial leads

Click for larger image

P waves: Upright in V4-V6.
Upright or biphasic in V1-V2 (negative component should be smaller if biphasic)

QRS complexes:
(1) Morphology: V1 shows an rS pattern

V6 shows a qR pattern
The size of the r wave increases progressively from V1 to V6

Transition zone: the initial part of the QRS deflection is positive in the right precordial leads. The transition zone is the point between V1 and V6, where the initial deflection ceases to be positive and becomes negative.

To the left of this point of transition, the dominant deflection is positive, and to the right of this, it is negative. When this zone is between V3 and V4, the heart is intermediate in position (neither clockwise nor counter-clockwise rotated). However, when the transition zone is between V2 and V3/V1 and V2, there is counter-clockwise cardiac rotation and when the transition zone is between V4 and V5/V5 and V6, there is clockwise cardiac rotation.

(2) Dimensions:

  • QRS duration < 0.12 s 
  • At least one R wave in the precordial leads must exceed 8 mm 
  • The tallest R wave in the precordial leads must not exceed 27 mm 
  • The deepest S wave in the precordial leads must not exceed 30 mm 
  • The sum of the tallest R wave in the left precordial leads and the deepest S wave in the right precordial leads must not exceed 40 mm 
  • Precordial q waves must not equal/exceed 0.04 s in duration 
  • Precordial q waves must never have a depth greater than one quarter of the height of the R wave which follows them.

ST segments: Must not deviate above or below the isoelectric line by more than 1 mm. Normal ST segment elevation occurs in leads with large S waves (e.g. V1-3), and the normal configuration is concave upward.

ST segments

T waves: Upright in V4-V6.

Often inverted in V1, may be inverted in V2 (provided already inverted in V1).
The T wave height should not be more than two-thirds and not less than one-eighth of the height of the preceding R wave in any of the leads V3-V6.

Normal appearances in limb leads

Click for larger image

P waves: Best seen in lead II (Small rounded waves)

  • P wave height = 2.5 mm 
  • P wave duration = 0.12 s

QRS complexes: Mean frontal plane QRS axis range is between +90°and -30 °; this implies that the QRS is mostly positive (upright) in leads I and II.

R wave in aVL must not exceed 13 mm and R wave in aVF must not exceed 20 mm.

Normal q waves reflect normal septal activation (beginning on the LV septum); they are narrow (<0.04 s duration) and small (<25% the amplitude of the following R wave). They are often seen in leads I and aVL when the QRS axis is to the left of +60°, and in leads II, III, aVF when the QRS axis is to the right of +60°. Septal q waves should not be confused with the pathological Q waves of myocardial infarction.

ST segments: Must not deviate above or below the isoelectric line by more than  1mm.

T waves: In the normal ECG, the T wave is always upright in leads I, II, V3-6, and always inverted in lead aVR.

The U wave: 

  • U wave amplitude is usually < one-third T wave amplitude in same lead. 
  • U wave direction is the same as T wave direction in that lead.

U waves are more prominent at slow heart rates and usually best seen in the right precordial leads.

The normal rhythm of the heart

The heart is said to be in sinus rhythm based on the following criteria: 

  • P waves must be present and be regular. 
  • P wave frequency should be within the range of 60-100 per min. 
  • There must be one P wave for each QRS complex. 
  • The P wave must precede each QRS complex. 
  • The PR interval must be normal and constant. 
  • The morphology of the P waves and QRS complexes must be the usual form for that patient.

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