The ECG
Every heartbeat starts with a tiny electrical signal, and a surprising amount of that signal leaks all the way out to the surface of the skin. An electrocardiogram turns those faint, fleeting voltages into a trace doctors can read like a story — one with a beginning, a climax and a resolution, repeated roughly once a second for an entire lifetime.
What an ECG actually measures
An electrocardiogram (ECG) is a recording of the electrical activity generated by the heart as it beats, plotted as a voltage against time. It's easy to think of the heart purely as a mechanical pump, but every contraction is triggered by an electrical signal that sweeps through the heart muscle in a very specific pattern and timing. That electrical signal doesn't stay neatly confined inside the heart — the body's tissues and fluids are reasonably good conductors, so tiny, time-varying potential differences produced by the heart's activity spread out and can be detected at the surface of the skin, at points some distance from the heart itself.
This is the core principle behind obtaining an ECG trace: rather than measuring anything inside the body directly, electrodes are attached to the skin at several standard positions — typically a combination of points on the limbs and across the chest — and the small potential differences between pairs of these electrodes are recorded continuously over time. As the heart's electrical signal changes from moment to moment during each beat, so does the pattern of potential differences measured at the skin, producing the characteristic rising and falling trace seen on an ECG recording.
- Electrodes: conductive pads placed at fixed positions on the body surface, making electrical contact with the skin so that potential differences can be picked up.
- Multiple electrode positions: because the heart's electrical activity spreads through the chest in a particular direction at each stage of the beat, comparing potential differences between several different pairs of points gives a much fuller picture than a single pair of electrodes could — this is why a full clinical ECG uses more than just two contacts.
- Signal size: the potential differences involved are extremely small, typically no more than a few millivolts, because only a fraction of the heart's internal electrical activity survives being conducted out through the surrounding tissue to the skin.
Amplifying a very small signal
A raw signal of a few millivolts, changing over a fraction of a second, is far too small to drive a pen, a screen trace or a digital recording directly, and it's easily swamped by electrical noise from elsewhere in the body (such as nearby muscle activity) or from the recording equipment itself. Before the signal from the electrodes can be usefully displayed, it must therefore be passed through an amplifier — a circuit that increases the size of the voltage signal while preserving its shape, so that the pattern of the original heart activity is reproduced faithfully, just at a much larger and more usable scale.
This need for amplification is not unique to the ECG. Any situation where a physical process produces a genuinely tiny electrical signal — far below what display or recording equipment can work with directly — requires the same basic principle: boost the signal's amplitude electronically before doing anything else with it, without distorting its shape or introducing significant extra noise. The details of how amplifier circuits achieve this are a topic in their own right, but the key idea for the ECG is simple: amplification is what turns an undetectably faint skin-surface voltage into a clear, readable trace.
Reading the trace: P, QRS and T
Once amplified, a normal ECG trace from a single heartbeat has an unmistakable and highly repeatable shape: a small bump, followed shortly after by a sharp spike, followed by a broader bump — then a flat stretch before the whole pattern repeats for the next beat. Each of these three named features corresponds to a distinct, well-defined stage of the heart's electrical cycle, and being able to identify and describe each one is the single most testable part of this topic.
| Feature | Shape | What it corresponds to |
|---|---|---|
| P wave | Small, rounded deflection | Electrical activity as the atria (the heart's upper chambers) contract |
| QRS complex | Sharp, tall spike — the most prominent feature | Electrical activity as the ventricles (the heart's lower, larger chambers) contract |
| T wave | Broader, lower deflection | Electrical "reset" of the ventricles — recovery to their resting state, ready for the next beat |
The order in which these appear within one heartbeat is always the same: P, then QRS, then T. This ordering directly mirrors the sequence of electrical and mechanical events in the heart — the atria are triggered to contract first (P wave), then a short delay later the much larger mass of ventricular muscle is triggered to contract (QRS complex), and finally the ventricles' electrical state recovers in preparation for the next cycle (T wave). The QRS complex is normally by far the largest feature on the trace, simply because the ventricles contain far more muscle tissue than the atria, so their electrical activity produces a proportionally larger detectable signal at the skin.
The size of each deflection on an ECG trace broadly reflects how much heart muscle is electrically active at that moment, not just whether an event is "important." The ventricles have to pump blood around the entire body (or to the lungs), so they contain substantially more muscle than the thin-walled atria — and that extra muscle mass is exactly why the QRS complex towers over the P and T waves on a real trace.
Worked example: finding heart rate from the trace
Because the QRS complex is the sharpest, most easily identified feature on the trace, it's the standard reference point for timing successive heartbeats. Specifically, the tallest point of the QRS complex — the R peak — gives a precise, repeatable marker of when each beat occurs, so measuring the time between consecutive R peaks gives the time period of one full heartbeat, from which the heart rate follows directly.
The same method works for any measured R–R interval: a shorter interval between R peaks means a shorter beat period and therefore a higher heart rate, while a longer interval means a lower heart rate. Because $f = 1/T$, doubling the time between beats halves the calculated heart rate, and vice versa — a useful check when sanity-testing an answer.
The real diagnostic power of the ECG doesn't come from simply confirming that the heart is beating — it comes from the fact that the P wave, QRS complex and T wave each correspond to a specific, well-understood stage of the heart's electrical cycle. Because each feature has a known cause, any deviation in its timing, shape, size or spacing relative to the others can point toward a specific problem with how the heart's electrical signal is being generated or conducted, rather than just a generic sign that "something is wrong." A trace is read feature-by-feature, which is what makes it such a precise diagnostic tool rather than a simple pulse counter.
Key takeaways
- An ECG records the small, time-varying potential differences produced at the body's surface by the heart's own internal electrical activity, which triggers each muscle contraction.
- Electrodes are placed at several standard positions on the body (typically across the limbs and chest) to detect these potential differences between different pairs of points.
- The raw signal detected at the skin is only a few millivolts, so it must be significantly amplified before it can be usefully displayed or recorded.
- A single heartbeat produces three named features in a fixed order: the P wave, the QRS complex, then the T wave.
- The P wave corresponds to atrial contraction, the QRS complex to ventricular contraction, and the T wave to the ventricles' electrical recovery before the next beat.
- The QRS complex is normally the largest feature because the ventricles contain far more muscle mass than the atria.
- Heart rate can be calculated from the time interval between successive R peaks using $f = 1/T$, then converting to beats per minute.
- The ECG's clinical value comes from interpreting each feature individually — deviations in a specific wave's timing, size or shape point to specific abnormalities, not just to the fact that a heartbeat occurred.