Cardiovascular System
Sinus rhythm on an ECG
Sinus rhythm is the rhythm started by the sinoatrial node, the heart's natural pacemaker, seen as a P wave (atrial activation) before each QRS complex (ventricular activation).
Remember
Sinus rhythm means a P wave of the same shape, upright in lead II (one of the views of the heart that the ECG records), comes before each QRS complex at a constant PR interval (the time from the start of the P wave to the start of the QRS complex). Rate, regularity and a narrow QRS complex (one under 0.12 second) do not prove it.
Sinus rhythm is the normal rhythm that the sinoatrial (SA) node, the heart's natural pacemaker, starts. The impulse spreads through the atria to give the P wave, is held briefly at the atrioventricular (AV) node (which passes the impulse on), and then reaches the ventricles to give the QRS complex.
As the same pathway is used every time, the P waves have the same shape and are upright in most leads, including lead II. The PR interval, the time from the start of the P wave to the start of the QRS complex, stays constant. It is normally 120 to 200 ms.
Every QRS complex should have a P wave ahead of it, and every P wave should have a QRS complex after it, which makes a one-to-one sequence. The rhythm is regular, apart from small normal variations with breathing, and in adults the rate of a normal sinus rhythm is usually 60 to 100 beats per minute. But rate, regularity and a narrow QRS complex do not prove where the beat began, so look at the P wave.
An irregular rhythm (beyond small normal variations with breathing), missing or abnormal P waves, or failed one-to-one pairing points to a different rhythm or a lost link:
- Atrial fibrillation (the atria quiver chaotically): no clear P waves and completely random spacing between QRS complexes, which is called irregularly irregular.
- Atrial flutter (a rapid regular circuit in the atria): flutter waves in a saw-tooth pattern in place of ordinary P waves.
- Second-degree AV block: some atrial impulses fail to reach the ventricles, so some P waves have no QRS complex, while the SA node may fire normally.
- Junctional rhythm (the beats start in the AV node): P waves are absent or abnormal and may appear just after the QRS complex.
Memory aid
like carriages always following their engine
Think of the P wave as the engine and the QRS complex as the carriage that follows it.
Key facts
- Pacemaker
- the sinoatrial node starts every beat in sinus rhythm.
- P wave
- the same shape from beat to beat, and upright in most leads, including lead II.
- PR interval
- constant from beat to beat, normally 120 to 200 ms.
- Rate
- in adults at rest the rate of a normal sinus rhythm is usually 60 to 100 beats per minute.
- Three checks
- is it regular, are there P waves, and does each P wave have a QRS complex after it?
- Less specific features
- rate, regularity and QRS width also occur in rhythms that do not start in the sinoatrial node.
Exam traps
- A regular rhythm does not confirm sinus rhythm, because several other rhythms are regular too, for example a junctional rhythm.
- A narrow QRS complex does not show that the beat began in the sinoatrial node, because an impulse starting in the atria or the AV node rather than the sinoatrial node also gives a narrow QRS complex.
- A normal rate does not prove sinus rhythm, because some junctional rhythms can fall within the same range.
- P waves after each QRS complex do not fit sinus rhythm, because they mean the atria are being activated backwards from the AV node or ventricles.
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