The Cardiac Conduction System

There are four main components of the conducting system of the heart, these are the SA node, AV node, the bundle branches and the Purkinje fibres. Also, we have subcomponents called the tracts and branches, each of these components have specialised cells that are able to conduct electrical impulses.



Sinoatrial Node ( SA node)

This is the first primary pacemaker of the heart and is able to beat 60-100bpm. The SA node contains specialised cells which are located in the right atrium adjacent to the orifice of the superior vena cover.

Bachmann’s Bundle

This is the interatrial pathway which facilitates the transmission of electrical impulses from the right to the left atrium.

Internodal Tracts

There are three internodal tracts, namely, inner middle and outer which transmits impulses from the SA node to the AV junction

Atrioventricular Node

The AV node is the second pacemaker of the heart and is able to initiate 40-60 bpm. It contains specialised pacemaker cells which are located above the intraventricular septum in the right atrium near the orifice of the coronary sinus ostium. The AV nodes regulate the number of electrical impulses which are conducted in the ventricles which promote ventricular filling during atrial contraction.

Bundle of His

The bundle fibres extend down the right side of the intraventricular septum and receive electrical impulses from the AV junction then transmits them to the bundle branches.

Right Bundle Branch

Descends from the bundle of his to the right septal wall and transmits electrical impulses to the ventricular Purkinje fibres.

left Bundle Branch

Descends to the right septal wall and divides into the anterior and posterior branches which send impulses to the left ventricular Purkinje fibre network.

The Purkinje System

They are lowest pacemakers of the heart and are capable of initiating less than 40 bpm. This network is found in the all the inner surfaces of the ventricles and stimulates myocardial cells.

Ectopic Pacemakers

These are cells that are located on the outside of the regular conductive system which is able to initiate electrical impulses. They are essentially contractible cells that are located in the atria, ventricles at the AV node.

The Cardiac Electrophysiology

Before the heart can mechanically contract and pump blood, depolarisation of the cardiac muscle cells must happen. A difference in the electrical muscle cells must occur for the electrical current to be generated.

The electrolytes that affect the heart are Sodium, potassium and calcium. The exchange of electrolytes in the cardiac cells creates an electrical activity which then shows up on the ECG waveform. In a healthy heart, depolarisation is then followed by cardiac muscle contraction. Repolarisation happens which the electrical charge returns to what it was before depolarisation.

We also have a refractory period which is a period in which the cardiac muscle cell either cannot respond to the incoming stimuli or will only react to the stimuli if it is stronger than usual.

EKG lead placement


Normal Sinus Rhythm



Components of the normal Cardiac Electrical Impulses 

P Waves 

  • Represents arterial depolarisation
  • Smooth and rounded
  • Usually no more than 2.5 cm in height and 0.11 second in duration
  • Should be upright in all lead except aVR
  • Maybe notched, peaked or inverted

PR Interval 

Represents the atrial depolarisation and repolarisation of the atria

  • The P wave plus the PR segment equal the PR interval
  • Begins with the onset of the P wave and ends with the beginning of the QRS complex
  • A typical PR interval indicates the electrical impulses were normally conducted through the atria, AV node, Bundle of His, bundle branches and Purkinje fibres
  • Normally measures 0.12 to 0.20 seconds in adults may be shorter in children and longer in older adults.
  • PR interval is normally shorter as the heart rate increases

QRS Complex 

  • Ventricular depolarization
  • Measured by counting boxes the small squares multiplied by 0.04

ST segment

  • Ventricular depolarization
  • The period between the completion of ventricular depolarization and the beginning of the final repolarisation

T wave 

  • Represents normal recovery










ventricular FLUTTER STRIP-1

Ventricular Tachycardia



van Weerd, J. H., & Christoffels, V. M. (2016). The formation and function of the cardiac conduction system. Development143(2), 197-210.

van Weerd, J. H., & Christoffels, V. M. (2016). The formation and function of the cardiac conduction system. Development143(2), 197-210.


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