12-lead ECG engine

How to identify Normal Sinus Rhythm

The reference rhythm. The impulse starts in the SA node and takes the normal path — atria, AV node, His-Purkinje, ventricles — so every other rhythm in this library is described as a deviation from this one.

The systematic read

Rate

60–100/min. The same rhythm below 60 is sinus bradycardia and above 100 is sinus tachycardia: the mechanism is unchanged, only the rate.

Regularity

Regular. Mild variation with the respiratory cycle (sinus arrhythmia) is normal and is most obvious in young, fit patients.

P waves

One P before every QRS, one QRS after every P, and every P the same shape. Upright in II, III and aVF, inverted in aVR — that pattern is what SA-node origin looks like, because the impulse spreads down and to the left, toward lead II and away from aVR.

PR

120–200 ms (3–5 small squares), and the same on every beat.

QRS

Under 120 ms (under 3 small squares). Narrow means the ventricles were depolarised down the His-Purkinje system rather than muscle-to-muscle.

ST / T

ST on the isoelectric line. T upright in most leads and asymmetric — slow up, faster down. Inverted in aVR, and may normally be inverted in V1 and III.

Also typical

• R-wave progression across the chest leads: rS in V1, transition around V3–V4, qR in V6.
• Small, narrow septal q waves in I, aVL, V5 and V6 — normal, and not the same thing as a pathological Q.
• QRS axis between −30° and +90°.

Watch out for

• Normal sinus rhythm is not the same as a normal ECG. STEMI, ischaemia, hyperkalaemia and bundle branch block all sit on top of sinus rhythm. A normal rate, regularity, P, PR and QRS tells you the conduction sequence is normal — nothing more.
• An upright P in aVR is not a sinus beat. It usually means the limb leads are reversed, or the pacemaker is an ectopic atrial focus, not the SA node.
• Regularity is checked against the grid, not judged by eye on a moving trace.
• The trace shows electrical activity only. Sinus rhythm on the monitor with no palpable pulse is PEA, and it is treated as a cardiac arrest.

Why it matters

It is the baseline you measure everything else against, not a statement that the patient is well. Read the ST segments and the morphology on every sinus strip.