How to read an Electrocardiogram (ECG) - Part 4, The PR Interval

Following the P wave is the PR segment. (NOTE: the PR segment and the PR interval are NOT the same thing.) The PR segment is not routinely measured, but may be commented on if it is depressed or elevated. During the PR segment, the electrical wave moves slowly through the atrioventricular (AV) node. This activity is not seen on the ECG.

The PR interval is the time from the beginning of the P wave until the beginning of the QRS complex. It is normally between 0.12 and 0.2 seconds (three to five small boxes) in length.

The PR interval may be prolonged when conduction of the electrical wave through the AV node is slow. This may be seen with degenerative disease of the node, or with digoxin, hyperkalemia, hypercalcemia, or hypothermia.

The PR interval may be unusually short when conduction is rapid. A mildly short PR interval may be seen with hypokalemia or hypocalcemia. An artificially-short PR interval occurs when the QRS complex begins early, as happens with an extra conducting bundle Wolff-Parkinson White Syndrome (WPW).

Part 1 PQRST Waves
Part 2 ECG Graph Interpretation
Part 3 The P Wave
Part 4 The PR Interval
Part 5, The QRS Complex
Part 6, The ST Segment