The ECG record
Where is the ECG recorded?
It's recorded on graph paper.
The height is related to the stimulus voltage.
The width is related to the time.
What's the difference between positive, negative and isodiphasic waves?
A positive wave is any wave that goes predominantly upwards.
A negative wave is any wave that goes predominantly downwards.
An isodiphasic wave is one in which the wave area upwards is equal to the wave area downwards.
Electrodes: the eyes of the ECG
As we've already seen, the electrical stimuli of the heart are captured by electrodes on the patient's skin, and thus recorded on the ECG.
What types of electrodes are there?
Exploring electrode (positive)
When it's hit by a positive charge, a positive wave will be recorded on the ECG.
When the depolarization vector is in the opposite direction to the exploring electrode, a negative wave will be recorded on the ECG.
The other electrode, if present, is the negative electrode.
I'll reveal the biggest secret of electrocardiography.
The exploring electrode is basically a camera.
A camera which, when it sees an electrical signal coming towards it, creates a positive wave.
What is the cardiac electrical axis?
The cardiac electrical axis is the vector resulting from the sum of the depolarization wave vectors.
What is the normal axis?
Between 0 and +90.
Vectors and electrodes
If a positive charge (e.g. cardiac depolarization) has the same orientation and direction as the exploring electrode, it will have a maximum positive amplitude
Depolarization will not always have an exact alignment with the exploring electrode.
So what do we do?
Simple! Decompose vectors!
Note the decrease in the wave when the depolarization vector does not coincide towards the exploring electrode.
What is an isodiphasic wave?
It's a wave whose elevation is equal to its slope, with the sum of its rise subtracted from its slope = zero.
When does it occur?
When the vector has an orientation perpendicular to that of the exploring electrode.
When will negative waves occur?
Based on the above, we can conclude that negative waves will occur when the depolarization vector has the opposite orientation to the exploring electrode!
They obey the same logic when it comes to size: when they're parallel, they'll be bigger (more negative).
Depolarization with the same direction, but a different orientation from the exploring electrode: we'll have the greatest possible negativity.