Sinus rhythm, sinus tachycardia, bradycardia and arrhythmia

What it is

The sinoatrial node, high in the right atrium near the entry of the superior vena cava, depolarises spontaneously and sets the rate; the autonomic nervous system speeds it (sympathetic) or slows it (vagal). Because atrial depolarisation spreads from the top right of the heart downwards and to the left, a sinus P wave is upright in leads I and II and inverted in aVR. Sinus rhythm means every QRS is preceded by an identical sinus P wave with a constant PR interval, at 60–100 bpm. Sinus tachycardia is the same pattern above 100 bpm: almost always a normal response to something else (fever, pain, fear, hypovolaemia, bleeding, sepsis, hypoxia, pulmonary embolism, anaemia, thyrotoxicosis, stimulants, or withdrawal), so the treatment is the cause. Sinus bradycardia is the pattern below 60 bpm: normal in athletes and during sleep, but also caused by drugs (beta-blockers, non-dihydropyridine calcium channel blockers, digoxin, amiodarone), hypothyroidism, hypothermia, raised intracranial pressure, inferior myocardial infarction, obstructive sleep apnoea and sinus node disease. Sinus arrhythmia is sinus rhythm whose P–P interval varies by more than 120 ms; the respiratory form (faster on inspiration, slower on expiration) is normal and marked in children and young adults.

How to recognise it

• Sinus rhythm: a P before every QRS and a QRS after every P; P upright in I and II, inverted in aVR; rate 60–100 bpm.
• Sinus tachycardia: same P morphology at above 100 bpm; at fast rates the P may sit on the preceding T wave.
• The maximum sinus rate falls with age (roughly 220 minus age); a resting rate far above that is unlikely to be sinus.
• Sinus bradycardia: same P morphology below 60 bpm; the PR stays normal and every P is conducted.
• Sinus arrhythmia: P–P variation above 120 ms with identical P waves; respiratory form speeds on inspiration and slows on expiration.
• Rhythm strip: use lead II (and V1) to see P waves best; measure several cycles before calling the rhythm regular.

Why it happens

• The sinoatrial node has the fastest intrinsic rate of the conducting system, so it normally sets the beat.
• Sympathetic stimulation and circulating catecholamines speed phase 4 depolarisation of the nodal cells and raise the rate.
• Vagal tone slows the node: high in athletes, during sleep and with pain, vomiting or raised intracranial pressure (with hypertension, the Cushing reflex).
• On inspiration, vagal tone falls and venous return rises, so the rate speeds; on expiration it slows: respiratory sinus arrhythmia.
• Sinus node disease (fibrosis with age, ischaemia, infiltration) gives inappropriate bradycardia, pauses or alternating brady- and tachycardia.

Step by step

1. Look at lead II for P waves — a sinus P is upright in I and II and inverted in aVR.
2. Check that every P is followed by a QRS with a constant PR — otherwise think of AV block.
3. Calculate the rate and check regularity over several cycles — sinus rhythm varies little except with breathing.
4. If fast, ask whether onset was gradual and whether there is a cause — fever, pain, bleeding, sepsis, hypoxia or drugs.
5. If slow, review drugs and look for adverse features — shock, syncope, chest pain or heart failure need urgent action.
6. If irregular with identical P waves, check its link with breathing — respiratory sinus arrhythmia needs no treatment.

The sinus rhythms side by side

Differential and pitfalls

• Atrial flutter with 2:1 block at about 150 bpm looks like sinus tachycardia: look for sawtooth waves in II, III, aVF and a second P in V1.
• Atrial tachycardia has an abnormal P axis (for example, inverted in II); re-entrant SVT starts and stops abruptly and has no visible P or a retrograde one.
• Inappropriate sinus tachycardia and postural tachycardia syndrome are diagnoses of exclusion after secondary causes are ruled out.
• Rate-controlling a compensatory sinus tachycardia (bleeding, sepsis, pulmonary embolism) can be fatal: the tachycardia keeps cardiac output up.
• A slow pulse can hide a 2:1 AV block or blocked premature atrial beats with P waves buried in the T wave; a junctional escape rhythm has no preceding P.
• Sinus arrhythmia is not atrial fibrillation: every QRS still has an identical P wave; three or more P shapes suggest wandering atrial pacemaker or multifocal atrial tachycardia.

Next step

• Sinus tachycardia: treat the cause (fluids, analgesia, oxygen, antibiotics, blood) and reassess the rate as the cause improves.
• Bradycardia with shock, syncope, ischaemia or heart failure: call the senior, give atropine by the local protocol and prepare transcutaneous pacing.
• Stable bradycardia: review and stop slowing drugs where possible; check potassium, thyroid function, temperature and glucose.
• Unexplained or symptomatic sinus bradycardia or pauses: refer to cardiology for ambulatory monitoring and assessment of sinus node disease.
• Respiratory sinus arrhythmia in a well patient: no treatment; explain that it is normal.

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Elsewhere in DurgiPedia

• ECG basics: waves, intervals and leads
• The SA node pacemaker potential
• The cardiac conduction system
• Arterial pulse: rate, rhythm and amplitude

Connections

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Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Sinus Tachycardia (StatPearls (NCBI Bookshelf), 2023); Sinus Bradycardia (StatPearls (NCBI Bookshelf), 2026); Sinus Arrhythmia (StatPearls (NCBI Bookshelf), 2022).

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