Sinus Node Rhythms and Arrhythmias

«Basics

Step 2: Heart Rate»

Author(s)	J.S.S.G. de Jong
Moderator	I.A.C. van der Bilt
Supervisor
some notes about authorship

Please eread the introduction to the 7+2 step plan first.

The sinus node (SA) is located in the roof of the right atrium. It is the fastest physiological pacemaker. When the sinus node generates an electrical impulse, the surrounding 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.

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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 morphology P wave (atrial contraction) precedes every QRS complex The rhythm is regular, but varies slightly while breathing The rate 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

As you see, at least knowledge of heart rate and P wave morphology are necessary to determine the rhythm. Still, we have put Rhythm as step 1 as it is of such importance in the interpretation of the ECG, as arrhythmias include the most lifethreatening ECG abnormalities. In most settings however, the rhythm will be sinus rhythm in the vast majority of ECGs.

If the rhythm is not sinus rhythm, the arrhytmias algorithm should be followed.

Sinus arrhythmias

Some variants of sinusrhythm exist:

• Asystole
• Sinustachycardia (>10 beats per minute)
• Sinusbradycardia (<60 beats per minute)
• Sinus arrest or pause
• Sino-atrial exit block
• Sick Sinus Syndrome

Arrhythmias are discussed in the Arrhythmias chapter.

If the heart rate exceeds 100 bpm, the tachcyardia flow chart should be followed.

Examples

• 

  An example of normal sinus rhythm.

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  Another example of normal sinus rhythm.