CorpusHuman Anatomy

Cardiac Conduction System & ECG

Impulse spread through the heart, synchronised with a lead-II ECG

Left: the specialised conduction tissue (yellow) seen through an anterior view of the heart. The patient's right is on the viewer's left. Right: a lead-II ECG on standard paper. Small squares are 0.04 s by 0.1 mV and large squares are 0.2 s by 0.5 mV.

One heartbeat at 60 beats/min is played at half speed (2 s per loop). Sparks show the impulse travelling along each pathway. The yellow wash shows myocardium that is depolarised. The marker on the ECG moves in step with events in the heart. The SA node fires just before the P wave, and the atria then depolarise during the P wave. The impulse is held up in the AV node during the PR segment. It then races through the His–Purkinje system, and the ventricles depolarise from the septum outward (QRS). The plateau phase gives the flat ST segment, and the ventricles repolarise during the T wave.

Atrial repolarisation is hidden inside the QRS complex.

Step-by-step

  1. 1 · SA node fires

    Pacemaker cells in the sinoatrial node depolarise spontaneously from about −60 mV. The funny current (If) and T-type Ca²⁺ current drive this slow rise until threshold is reached at about −40 mV, when an L-type Ca²⁺ action potential fires. The node is too small to register on the surface ECG, so this moment falls in the flat line just before the P wave.

  2. 2 · Atrial depolarisation → P wave

    The impulse spreads through atrial muscle and along the internodal pathways at about 1 m/s. Bachmann bundle carries it across to the left atrium. Depolarisation of both atria produces the P wave, which lasts less than 0.12 s.

  3. 3 · AV nodal delay → PR interval

    In the AV node conduction slows to about 0.05 m/s, delaying the impulse by about 0.1 s. The reasons are that the node's cells are small, have few gap junctions and fire Ca²⁺-dependent action potentials. The ECG is flat during this delay (the PR segment), so the atria can finish emptying into the ventricles. The PR interval is normally 0.12–0.20 s.

  4. 4 · Bundle of His

    The bundle of His pierces the central fibrous body, the only electrical route through the insulating fibrous skeleton. It runs along the membranous septum and divides at the crest of the muscular septum.

  5. 5 · Bundle branches and septal activation

    The left bundle branch, with its anterior and posterior fascicles, and the right bundle branch, which travels partly through the moderator band, carry the impulse down the septum. The septum depolarises first, from left to right, producing the small q wave at the start of the QRS.

  6. 6 · Purkinje fibres → QRS complex

    The Purkinje fibres conduct at 1.5–4 m/s, the fastest in the heart. They activate the ventricular muscle from apex to base and from endocardium to epicardium. Both ventricles depolarise within about 0.08 s, producing the QRS complex. The ventricles then contract from the apex upward.

  7. 7 · Plateau → ST segment

    During the plateau (phase 2) all ventricular cells are depolarised to a similar voltage, held there by L-type Ca²⁺ current. No current flows between them, so the ST segment is flat. This is when the ventricles eject blood. Injured myocardium shifts the ST segment.

  8. 8 · Repolarisation → T wave; QT interval

    Ventricular repolarisation (phase 3, carried by K⁺ efflux) produces the T wave. The QT interval, from the start of the QRS to the end of the T wave, measures total ventricular electrical activity. Corrected for heart rate (QTc), it is normally up to about 440 ms in men and 460 ms in women. The heart then waits for the next SA node discharge.

Labelled parts