ECG Basics Part 1: Start here Lesson 4 of 22
P, QRS and T: the parts of one beat
One heartbeat draws three main waves. Name them, know what each one shows, and measure them with the grid.
One beat, labelled
| Part | Electrical event | What the heart does next | Normal look in lead II | Normal adult size |
|---|---|---|---|---|
| P wave | Atria depolarise | Atria contract | Small, rounded, upright | < 0.12 s wide (< 3 small boxes); < 2.5 mm tall |
| PR interval (start of P to start of QRS) | Delay in the AV node, then His–Purkinje conduction | Ventricles finish filling | Flat line after the P | 0.12–0.20 s (3–5 small boxes) |
| QRS complex | Ventricles depolarise | Ventricles contract; the first heart sound follows just after the QRS starts | Sharp, mostly upright | < 0.12 s (< 3 small boxes), usually 0.08–0.10 s |
| ST segment | Ventricles fully depolarised | Blood is ejected | Flat, at the baseline | — |
| T wave | Ventricles repolarise (recover) | Relaxation begins near the end of the T; the second heart sound falls about here | Rounded, upright, same direction as the QRS | — |
| QT interval (start of QRS to end of T) | The whole ventricular electrical cycle | — | — | Depends on heart rate; corrected as QTc |
Atrial repolarisation happens during the QRS and is hidden by it. A small U wave sometimes follows the T wave. Rate-corrected QT values are in Lesson 8: Waves, segments and intervals.
Naming the QRS
- Q: a downward deflection before the first upward one.
- R: any upward deflection.
- S: a downward deflection after an R.
Large waves get capital letters and small ones lower case. A complex that is entirely negative is called QS. Not every QRS has all three parts, but the whole complex is still called the QRS. In the beat above (lead II) there is a small q, a tall R and a small s: a qRs complex. In lead V1 the same normal heartbeat gives a small r and a deep S, an rS complex:
Later you will meet other shapes, such as rSR′ in V1 with right bundle branch block (Lesson 17: Narrow vs wide QRS: bundle branch block).
Measuring in small boxes
Each small box is 0.04 s (40 ms). The engine's measurements for the beat above are:
- P wave 92 ms: just over 2 small boxes.
- PR interval 158 ms: about 4 small boxes.
- QRS 84 ms: about 2 small boxes.
- QT 351 ms at 75 /min: about 9 small boxes.
A useful memory hook: the PR interval is 3–5 small boxes and the QRS is under 3.
Which way the waves point
In lead II the normal P, QRS and T are all upright. In aVR all three normally point down. A T wave can be inverted in V1, and sometimes in lead III, in healthy adults. A T wave pointing the opposite way to its QRS in a lead where it is usually upright deserves a closer look (Lesson 18: ST segment and QT).
Key points
- P = atria depolarise; QRS = ventricles depolarise; T = ventricles repolarise.
- PR 0.12–0.20 s (3–5 small boxes); QRS under 0.12 s (under 3 small boxes).
- Q is down before R, R is up, S is down after R.
- In lead II the normal P, QRS and T are upright.
Check yourself
A PR interval measures 6 small boxes. Is it normal?
Answer: no. 6 × 0.04 s = 0.24 s, longer than 0.20 s: this is first-degree AV block.
Which wave shows the ventricles recovering?
Answer: the T wave.
A QRS is 2.5 small boxes wide. Narrow or wide?
Answer: narrow. 2.5 × 0.04 s = 0.10 s, under 0.12 s.
In the live simulator, the Intervals button shows these measurements for the running rhythm.
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.
- Mason JW, Hancock EW, Gettes LS, et al. Recommendations for the standardization and interpretation of the electrocardiogram, Part II: electrocardiography diagnostic statement list. Circulation 2007;115:1325–1332.
- 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.
- Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. 14th ed. Elsevier; 2021.
SIMULATED · NOT FOR DIAGNOSIS · TracingsMD ECG Basics