ECG Basics Part 1: Start here Lesson 3 of 22
What an ECG records
An ECG is a graph of voltage over time, measured between electrodes on the skin. Learn what an electrode, a lead and the baseline are before reading any wave.
Electricity, not pumping
An ECG (electrocardiogram) plots voltage on the vertical axis against time on the horizontal axis. Each contraction follows its electrical signal a moment later, but the ECG only records the electricity. It cannot show whether blood is actually moving: it gives no pulse and no blood pressure (see Lesson 22: Arrest rhythms, pacing and artifact).
Electrodes and leads
- An electrode is the sticky pad on the skin.
- A lead is the voltage measured between electrodes. Each lead is one viewpoint on the same heartbeat.
A bedside monitor usually uses three electrodes (right arm, left arm, left leg) or five (adding a right-leg neutral and one chest electrode). A diagnostic 12-lead ECG uses ten electrodes to make twelve leads (Lesson 14: The 12 leads and what they see).
| Electrode | AHA colour | IEC colour |
|---|---|---|
| Right arm (RA / R) | White | Red |
| Left arm (LA / L) | Black | Yellow |
| Left leg (LL / F) | Red | Green |
| Right leg (RL / N) | Green | Black |
Colour codes differ between regions, so always read the letters printed on the clip.
One rule for up and down
- A wave of activation travelling toward a lead's positive electrode draws an upward deflection.
- Travelling away draws a downward deflection.
- Travelling across the lead draws a small or biphasic (up-and-down) deflection.
Lead II measures from the right arm (negative) to the left leg (positive). The normal impulse runs from the SA node at the top right of the heart toward the ventricles at the bottom left, roughly toward lead II's positive electrode. So in lead II the P wave, QRS and T wave are all upright, which is one reason monitors and rhythm strips so often show lead II.
Lead aVR looks at the heart from the right shoulder. The same beats, recorded at the same moment, travel away from it, so they point downward:
The grid in one minute
At the standard speed of 25 mm/s, one small box is 0.04 s and one large box is 0.20 s, so five large boxes make one second. Vertically, 10 mm is 1 mV. The full details are in Lesson 7: The grid and calibration.
The baseline
The flat line between the end of one T wave and the next P wave (the TP segment) is the baseline. Other parts of the trace are judged against it.
Signals that are not from the heart
Movement, shivering, loose electrodes and mains interference all add artifact, which can hide the rhythm or imitate a dangerous one (Lesson 22: Arrest rhythms, pacing and artifact). Look at the patient, not only at the screen.
Key points
- The ECG records electrical activity only; it cannot show a pulse.
- Electrodes are pads; leads are viewpoints made from them.
- Toward the positive electrode = up; away = down.
- In lead II the normal P, QRS and T are upright.
Check yourself
A wave of activation travels toward a lead's positive electrode. Does the trace go up or down?
Answer: up.
Which electrode is positive in lead II?
Answer: the left leg electrode.
Can a normal-looking ECG prove that the patient has a pulse?
Answer: no. The ECG shows electrical activity only.
To see all twelve viewpoints at once, open normal sinus rhythm in the 12-lead view.
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.
- Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. 14th ed. Elsevier; 2021.
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