Normal Sinus Rhythm
normalControls
Normal Sinus Rhythm
The reference trace: an upright P before every QRS, a fixed PR, a narrow QRS, and a rate between 60 and 100.
Normal sinus rhythm is four findings that happen to coincide, and it is worth holding them separately, because every other rhythm in this simulator is one of them going wrong. The sinus node is in charge, which shows as an upright P wave of constant shape before every QRS. Conduction through the AV node is intact and unhurried, which shows as a PR interval between 120 and 200 ms that does not move. The ventricles are activated through the His-Purkinje system, which shows as a QRS under 120 ms. And the rate sits between 60 and 100. Take any one away and the trace has a different name. The rhythm being normal is also not the same as the ECG being normal, which is a distinction worth keeping.
On the trace
Read the strip in this order. This is the order worth using on every rhythm, and it is easiest to learn where the answer is already known.
- Find a P wave before every QRS. One, upright, 0.10 mV, and the same shape every time. Upright in lead II means the atria were depolarised from the top down, which is where the sinus node sits.
- Check that every P has a QRS after it, and every QRS a P before it. Thirty-six of each in thirty seconds. Nothing dropped, nothing extra.
- Measure the PR interval. 175 ms, from the start of the P to the start of the QRS, identical on every beat. The normal range is 120 to 200 ms.
- Measure the QRS. 72 ms. Narrow means the impulse used the His-Purkinje system, which is the only way to activate both ventricles that fast.
- Take the rate. 72 bpm, R-R 833 ms. Between 60 and 100 is sinus rhythm; outside it, the same trace becomes bradycardia or tachycardia and nothing else changes.
- Look at how regular it really is. The R-R intervals run 805 to 861 ms, a spread of 56 ms. Not metronomic, and not supposed to be. Past 120 ms of variation this becomes sinus arrhythmia, which is a different article and still a normal heart.
How to recognise it
| Feature | Value | Notes |
|---|---|---|
| Rate | 60 to 100 bpm | 72 here. Rate alone separates this from bradycardia and tachycardia. |
| P wave | Upright in lead II, one per QRS | 0.10 mV here, constant shape. Shape is what says sinus. |
| PR interval | 120 to 200 ms | 175 here, fixed. |
| QRS | Under 120 ms | 72 here. |
| Regularity | Regular, but not exactly | 56 ms of R-R spread. Over 120 ms is sinus arrhythmia. |
| Conduction | 1:1 | Every impulse arrives, and none arrives from anywhere else. |
The strip
Mechanism
The sinus node depolarises spontaneously, faster than anything else in the heart, so it sets the rate and suppresses every other pacemaker before it can reach threshold. The impulse spreads across both atria, which inscribes the P wave, and reaches the AV node, which holds it for about a tenth of a second. That delay is the flat part of the PR interval, and it exists so the atria can finish emptying before the ventricles contract.
From the node the impulse enters the His bundle and the bundle branches, which carry it far faster than muscle conducts. Both ventricles depolarise almost at once, which is why the QRS is narrow.
Go deeperShow less
Why "normal sinus rhythm" is four findings, not one
Rate, rhythm, P wave and PR each rule something out. The rate excludes the two rate variants. The one-to-one relationship excludes the blocks, in which P waves outnumber QRS complexes. The upright P in lead II excludes every rhythm driven from lower down, all of which either invert the P or lose it. The narrow QRS excludes a ventricular origin. Drop any one and the rhythm is something else with a name.
The heart is never metronomic
A perfectly regular R-R is not a sign of health. Beat-to-beat variation comes from vagal traffic tracking the breath, and losing it is associated with poorer outcomes in heart failure and after infarction. What separates normal from sinus arrhythmia is only how much: over 120 ms of variation earns the second name, and it too is a normal finding in a young person.
Where sinus rhythm stops being reassuring
The label describes the origin of the beat and nothing else. A patient can be in sinus rhythm with a large infarct, severe hyperkalaemia or a life-threatening electrolyte disturbance. It says the sinus node is in charge, not that the heart is well.
The P wave is a shape, not a bump
Any rhythm can put a positive deflection before the QRS. What makes this P a sinus P is that it is upright in lead II, negative in aVR, and the same on every beat. An ectopic atrial focus produces a P of a different shape, and often the only way to see that is to compare it with an earlier ECG.
Clinical impact
There is nothing to act on. The value of knowing this trace well is that every abnormality is a departure from it, and the departure is easier to name when the baseline is familiar.
Management
Nothing, for the rhythm
Sinus rhythm at a normal rate needs no treatment by definition.
Read the rest of the ECG
The rhythm being normal says nothing about the ST segments, the QT interval, the axis or the voltages. Those are separate questions and the answer to one does not cover the others.
Compare with a previous tracing where you can
A P wave that has changed shape, or a PR that has lengthened, is only visible against an earlier baseline.
Treat the patient when the rhythm and the patient disagree
A well patient with an odd trace and an unwell patient with a normal one are both common, and the second is the more dangerous.
Differential
The same beat with more variation. Over 120 ms of R-R spread earns the name, and it is still a normal heart.
Identical in every respect except the rate, which is under 60.
Identical again, over 100. At the top of its range the P starts hiding in the preceding T.
Also a P before every QRS, but the P is the wrong shape because the focus is not the sinus node. Compare the P morphology, not the rate.
References
- Normal Sinus Rhythm — Life in the Fast Lane, ECG Library, 2024
- Sinus Arrhythmia — StatPearls, NCBI Bookshelf NBK537011, 2023