ECG Quiz

This is the tutorial / quiz used for a study of ECG versus ECG-VCG for ECG interpretation by medical students.

Electrical activation of the heart

Electrical activity of the heart can be recorded from the body surface
This is typically represented as an electrocardiogram (ECG)
In the ECG, multiple leads are shown, each of which is a "view" of the heart from an angle
Vectorcardiogram (VCG) is an alternative representation
In VCG, electrical activation is represented as a series of vector
Two dimensional representations of this 3D vector are shown in frontal, tranverse and sagittal planes

Typical ECG

Frontal plane VCG encapsulates information in the frontal leads (I, II, III, aVR, aVL and aVF)
Transverse plane VCG encapsulates information in the transverse leads (V1 - V6)
These two planes are generally sufficient to understand the activation
We will be using VCGs in these two planes to illustrate electrical activation in normal and abnormal situations

The leads I, II, III, aVR, aVL, aVF lie in the frontal plane.
Axes drawn using information from these leads at various instants of one cardiac cycle would give a VCG in the frontal plane
The green area denotes the limits of normal vector amplitude

The precordial leads V1 - V6 lie in the horizontal plane.
Axes drawn using information from these leads at various instants of one cardiac cycle would give a VCG in the horizontal plane
The green area denotes the limits of normal vector amplitude

Electrical axis is the dominant direction of electrical activity
When we say "axis", we typically mean the dominant direction in the ‘frontal’ plane
In the ECG, this can be identified from two frontal leads.
For example, lead I (left) and lead aVF (down)
Axis that is left and down is normal, left and up is left axis, right and down is right axis and right and up is north-west axis
In the VCG, axis can be identified from the dominant direction of the vector in the ‘frontal’ plane

Lead I is positive (left) and aVF is negative (up)
This gives axis in left upper quadrant which is called left axis deviation (LAD)

Looking at the frontal vector loop, dominant axis is left and up
This gives axis in left upper quadrant which is called left axis deviation (LAD)

Lead I is negative (right) and aVF is positive (down)
This gives axis in right lower quadrant which is called right axis deviation (RAD)

Looking at the frontal vector loop, dominant axis is right and down
This gives axis in right lower quadrant which is called right axis deviation (RAD)

Hypertrophy is the thickening of heart muscle
This is reflected in increased amplitude of the ECG and axis deviation
With left ventricular hypertrophy, there is increased amplitude of R wave in V6 and S wave in V1. Frontal vector may also be deviated leftwards
In VCG, this increased amplitude is directed to the left in frontal plane and posteriorly in horizontal plane
With right ventricular hypertrophy, there is increased amplitude of the R wave in V1 and V2. There may also be right axis deviation
In VCG, this increased amplitude is directed anteriorly and to the right in the horizontal plane

S wave depth in V1 plus the R wave height in V5/V6 more than 35 mm indicates LVH
Frontal axis is also deviated to the left

The vector loop goes beyond the green areas. This is to the left in the frontal plane and posterior in the horizontal plane
Frontal axis is deviated to the left

The vector loop goes beyond the green areas. This is to the right in the frontal plane and anterior/right in the horizontal plane
Frontal axis is deviated to the right

The left and right bundle branches are important for rapid activation of all regions of the heart
With block of one of the bundles QRS becomes broad
When there is right bundle branch block, delayed activation of right ventricle occurs
With left bundle branch block, there is delayed activation of the left ventricle
This manifests as a "M" shape (rSr`) in V1 for RBBB and V6 for LBBB