First-Degree AV Block - ECG
First-degree AV block is an extension of PQ interval (the beginning of the P wave to beginning of the QRS complex) over 200ms. In a classic ECG record, one little square equals 40ms. Therefore, first-degree AV block is the extension of the interval over 5 small squares.
PQ interval is prolonged to about 7 little squares, which gives us the diagnosis of first degree AV block. This is best seen in lead II.
Conclusion: From a clinical point of view, the first-degree AV block is mostly only an incidental finding. It can be related to usage of certain drugs such as beta-blockers, digoxin, some calcium channel blockers, etc. It can also be related to blood levels of minerals, especially calcium.