ECG Basics · Lesson 3 · From cell to trace
From cells to the surface ECG
An electrode on the skin cannot see single cells. It records the sum of activity across the heart as a wavefront moves toward it or away from it.
One rule explains most deflections
- A depolarisation wave moving toward the positive pole of a lead gives an upward deflection.
- A wave moving away gives a downward deflection.
- A wave moving at right angles to the lead gives a small or biphasic (equiphasic) deflection.
Repolarisation is a wave of the opposite charge, so the rule flips. Repolarisation moving away from an electrode also gives an upward deflection.
Naming the parts of one beat
| On the ECG | Electrical event |
|---|---|
| P wave | Atrial depolarisation, spreading from the SA node |
| PR segment | Slow conduction through the AV node, then His–Purkinje; too few cells to see |
| QRS complex | Ventricular depolarisation (phase 0 across the ventricles). Atrial repolarisation is usually hidden here |
| ST segment | Plateau (phase 2): ventricular cells sit at similar voltages, so the line is near baseline |
| T wave | Ventricular repolarisation (phase 3) |
Why the T wave points the same way as the QRS
Depolarisation travels from endocardium to epicardium. If repolarisation followed the same path, T would point opposite to QRS. In fact epicardial cells have shorter action potentials than the cells deeper in the wall, so although they are activated last they recover first. Repolarisation therefore runs broadly from epicardium back toward endocardium, which by the rule above gives an upright T in leads with an upright QRS.
When depolarisation itself is abnormal, as in bundle branch block or a ventricular beat, repolarisation follows the abnormal path and the T wave points opposite to the main QRS deflection. These are secondary ST–T changes.
Key points
- Toward the positive pole = up; away = down; perpendicular = biphasic.
- P = atria depolarise, QRS = ventricles depolarise, T = ventricles repolarise.
- The normal T is upright because the outer wall (epicardium) repolarises before the deeper layers.
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.
- Bueno-Orovio A, Cherry EM, Fenton FH. Minimal model for human ventricular action potentials in tissue. J Theor Biol 2008;253:544–560 (model used for the cell figures).