ECG Basics · Lesson 5 · Reading the paper
Waves, segments and intervals
A segment is a stretch of baseline between waves; an interval includes at least one wave. Getting the start and end points right matters more than memorising numbers.
Where to measure
- PR interval: start of the P wave to the start of the QRS (whether that is a Q or an R wave).
- QRS duration: first deflection of the QRS to the J point, where the QRS meets the ST segment.
- ST segment: J point to the start of the T wave. Judge it against the baseline (TP or PR segment).
- QT interval: start of the QRS to the end of the T wave. Use the lead with the longest clear T, often II or V5.
Adult reference values
| Measure | Typical adult range | Outside the range suggests… |
|---|---|---|
| P wave | < 120 ms wide; < 2.5 mm tall in II | Atrial enlargement or delay |
| PR interval | 120–200 ms | > 200 ms: first-degree AV block. < 120 ms: pre-excitation or low atrial/junctional origin |
| QRS | < 120 ms (usually 80–100) | ≥ 120 ms: bundle branch block, ventricular origin, pacing, pre-excitation, drugs or electrolytes |
| ST segment | At baseline | Elevation or depression: see lesson 12 |
| QTc | Upper limit about 450 ms (men), 460 ms (women) | Long QT; > 500 ms carries a clearly higher torsades risk |
QT shortens as heart rate rises, so it is corrected for rate (QTc). Bazett's formula divides QT by the square root of the RR interval in seconds; Fridericia's uses the cube root and over-corrects less at fast rates.
The figure's numbers are the engine's model measurements for that single beat. The live simulator's Intervals button shows the same kind of values for the running rhythm. They are educational model values, not clinical intervals.
Key points
- Intervals contain waves; segments are baseline between waves.
- PR 120–200 ms, QRS under 120 ms, QTc under about 450–460 ms.
- Always correct QT for heart rate before calling it long.
See it move in the live simulatorFirst-degree AV blockSinus + Long QT
References
- Kligfield P, et al. Recommendations for the standardization and interpretation of the electrocardiogram, Part I: the electrocardiogram and its technology. Circulation 2007;115:1306–1324.
- Rautaharju PM, et al. Recommendations for the standardization and interpretation of the electrocardiogram, Part IV: the ST segment, T and U waves, and the QT interval. J Am Coll Cardiol 2009;53:982–991.
- Surawicz B, et al. Recommendations for the standardization and interpretation of the electrocardiogram, Part III: intraventricular conduction disturbances. J Am Coll Cardiol 2009;53:976–981.