Lesson 7 of 21 · Waves & Intervals
Key Intervals: PR, QRS, QT
Three intervals carry most of the diagnostic weight. Memorize the normal ranges and you'll catch blocks, pre-excitation, and arrhythmia risk.
01PR interval
Start of the P wave to start of the QRS. Normal 0.12–0.20 s (3–5 small boxes).
- Long (> 0.20 s): 1st degree AV block
- Short (< 0.12 s): pre-excitation (WPW) or junctional rhythm
- Variable: Wenckebach or AV dissociation
II
II
02QT interval
Start of the QRS to the end of the T wave. It shortens as heart rate rises, so correct it (QTc).
Key takeaways
- PR 0.12–0.20 s
- QRS < 0.12 s
- QTc < 450 (M) / 460 (F); > 500 ms = high torsades risk