The value of time and voltage just discussed is very important to understand, but it must be combined with an understanding of the grid or graph paper the tracing is printing upon that you are analyzing.
The grid is broken down in small boxes (1 X 1 millimeter) and heavier darkened lined boxes.
There are five small boxes going from left to right and bottom to top of each large box (5 X 5 mm).
Each small box has a time value of 0.04 second.
If there are five small boxes going across each large box, then the value of time for each large box equals 0.20 second
(5 small boxes X 0.04 second = 0.20 second)
(5 LARGE BOXES X 0.20 SECOND = 1 SECOND)
Voltage is important when measuring the height or depth of a waveform.
The height or depth provides an indication of the amount of electrical activity occurring within the heart.
Voltage can be affected by a number of different factors to include the amount of viable muscle mass in the heart.
When analyzing cardiac tracings, the question always comes up “where do I measure from”?
To keep it as clear as possible, it is important to establish the isoelectric line or baseline.
There are times when this can prove to be impossible, but in most cases if you can see the PR segment, ST segment and TP segment, you should have a strong sense that this is where the isoelectric line is located.
However, all three segments are not always on the isoelectric line
Where two out of the three of them are located when analyzing horizontally, this is typically where the isoelectric line is located.