Sinus Node Rhythms and Arrhythmias

The conduction system handles the spreading of an electrical signal through the heart. The normal sinus rhythm begins in the sinus node and goes via the AV node to the His bundle where it splits via the right and left bundle branch.

During normal sinus rhythm, every atrial contraction (P-wave) is followed by a ventricular contraction (QRS complex).

Normal sinus rhythm with a positive P-wave in I, II and AVF, and a biphasic P-wave in V1.

The normal heart rhythm is sinus rhythm. That means that the rhythm has its origin in the sinus node, the heart's fastest physiological impulse generator.

The sinus node (SA) is located in the upper part of the wall of the right atrium. When the sinus node generates an electrical impulse, first the cells of the right atrium depolarise, then the cells of the left atrium, the AV (atrioventricular) node follows and at last the ventricles are stimulated via the His bundle.

With this knowledge it is quite simple to recognise normal sinus rhythm on the ECG.

Criteria for normal sinus rhythm (see also Basics)

A P wave (atrial contraction) precedes the QRS complex
Every P is followed by a QRS complex
The rhythm is regular, but varies slightly while breathing
The frequency ranges between 60 and 100 beats per minute
The P waves maximum height is 2.5 mm in II and/or III
The P wave is positive in I and II, and biphasic in V1

These last two definitions will be discussed in the topic P wave morphology. Heart rhythms which are not sinus rhythm are arrhythmias.

An example of a normal ECG.