Electrocardiogram: Reading the Electrical Activity of the Heart

Why does a beating heart produce electrical signals at the skin?

The heart's working muscle cells are excited by changing electrical potentials across their cell membranes. As depolarisation and repolarisation spread through a large mass of heart muscle, the body conducts small potential differences that can be detected at the surface. An electrocardiograph senses these differences through skin electrodes and displays or records them; the tracing is called an electrocardiogram, abbreviated ECG.

An ECG is not a direct photograph of the heart, nor does it directly show how much blood is ejected. It shows an electrical pattern that usually precedes and coordinates muscular contraction. Its shape depends on which electrodes or lead arrangement is used. The simplified standard trace described here illustrates the timing and interpretation of the major deflections.

What do the P, QRS and T components mean?

The first small rise, the P wave, reflects the depolarisation of the atria as the SA-node impulse spreads. Atrial contraction follows electrical excitation. The tall, comparatively rapid QRS complex represents ventricular depolarisation, the electrical event that initiates ventricular contraction shortly afterwards. The T wave represents ventricular repolarisation, as ventricular muscle cells recover toward the resting electrical state.

The electrical meaning should not be confused with muscular motion. The QRS wave is an electrical event, not the volume of blood leaving the aorta. The T wave marks ventricular repolarisation rather than ventricular blood refilling. Atrial repolarisation also occurs, but it is usually hidden by the larger QRS complex in a typical standard recording.

How a standard recording is obtained

In the introductory arrangement presented in the chapter, leads are connected near both wrists and the left ankle to register potential differences associated with cardiac activity. Clinical ECG systems commonly use multiple limb and chest leads because different viewpoints provide more diagnostic information. An individual lead records a projection of electrical activity, so the polarity and shape of deflections may differ between lead placements.

To read a simplified tracing, first identify the P wave before QRS, then locate the narrow ventricular complex, and finally find the T wave following it. A repeating, ordered series usually indicates organised electrical activity. The shape and timing require careful interpretation; a 'normal-looking' small diagram cannot establish whether every cardiac condition is absent.

Heart rate from repeating electrical events

Each ordinary ventricular activation produces one QRS complex, so the number of complexes counted during a known interval can estimate heart rate. For example, if 12 QRS complexes occur over 10 seconds, the estimated rate is 12 × 6 = 72 beats per minute. An alternative is to use the interval between consecutive R peaks when the tracing provides a calibrated time axis; rate in beats/min is 60 divided by the R–R interval in seconds.

This calculation assumes that QRS complexes are identifiable and the recorded pattern represents actual ventricular electrical activations. In some abnormal rhythms, electrical rate and effective pumping may not correspond one-to-one; a trace alone does not demonstrate an effective pulse.

Why ECGs matter

Changes in heart rate, rhythm, conduction timing or waveform morphology may indicate a problem with electrical initiation or propagation, and some patterns can reflect injury or altered function. Traces from different people have broadly recognisable components when compared using similar lead positions. Clinicians interpret deviations alongside symptoms, physical findings and other tests. The school-level P–QRS–T model is a foundation for this reasoning, not a complete diagnostic manual.

A useful causal chain is SA-node firing → atrial depolarisation and P wave → AV conduction → ventricular depolarisation and QRS → ventricular mechanical contraction → ventricular repolarisation and T wave. Always keep the electrical record distinct from blood pressure and valve sounds, even though all three are linked in time.

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