Sinus node dysfunction

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Intrinsic degeneration or disease of the sinus node and surrounding atrium can impair impulse formation or exit. Extrinsic autonomic, metabolic and pharmacological influences can mimic it, so a reversible-cause review precedes a permanent-device decision whenever the patient is stable.

Symptoms range from fatigue and exercise intolerance to presyncope and syncope. Because asymptomatic nocturnal bradycardia and pauses can be physiological, the key evidence is temporal agreement between a symptom and clinically plausible bradycardia or failure to raise heart rate with activity.

Acute resuscitation and chronic therapy are separate. Atropine and temporary pacing restore perfusion during deterioration; a dual-chamber permanent pacemaker is an established option for symptomatic SND without AV block, selected after specialist assessment.

Key points

Sinus node dysfunction (SND) includes inappropriate sinus bradycardia, sinus arrest, sinoatrial exit block, chronotropic incompetence and tachy-brady syndrome.
There is no single heart-rate or pause threshold that diagnoses clinically important SND; demonstrate a symptom-rhythm correlation.
Exclude reversible extrinsic causes such as rate-limiting medicines, hypothyroidism, electrolyte disturbance, ischaemia, hypothermia and sleep apnoea.
Tachy-brady syndrome combines atrial tachyarrhythmia with long pauses or marked bradycardia, often after the atrial rhythm terminates.
In adverse bradycardia, RCUK atropine is 500 micrograms IV every 3-5 minutes to a total of 3 mg.
If atropine fails, use transcutaneous pacing as a bridge and arrange early transvenous pacing; isoprenaline or adrenaline infusion are drug bridges.
Permanent pacing treats symptomatic bradycardia; it does not treat AF stroke risk or remove the need to manage atrial tachyarrhythmias.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Age-related atrial degeneration

Progressive fibrosis around the sinus node and atrial conduction tissue reduces pacemaker cell function and impulse propagation, making intrinsic dysfunction more common with age.

Cardiac injury or structural disease

Ischaemia, cardiomyopathy, infiltrative disease and prior cardiac surgery can injure the sinus node, its arterial supply or surrounding atrial tissue.

Rate-limiting medicines

Beta-blockers, rate-limiting calcium-channel blockers, digoxin and antiarrhythmics suppress automaticity or conduction and may reveal latent dysfunction, particularly when several agents are combined.

Metabolic, autonomic and sleep-related causes

Hypothyroidism, electrolyte disturbance, hypothermia, high vagal tone and sleep apnoea can produce reversible sinus slowing or pauses that mimic intrinsic disease.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Pacemaker tissue is impaired
Degeneration, ischaemia or extrinsic suppression can reduce spontaneous depolarisation within the sinus node or slow conduction from the node into surrounding atrial myocardium.
2
Bradycardia or pauses develop
Impaired impulse formation produces inappropriate sinus bradycardia or arrest, while impaired impulse exit can create sinoatrial block despite continuing nodal activity.
3
Atrial disease promotes tachy-brady behaviour
The same atrial remodelling predisposes to atrial fibrillation or flutter. When the tachyarrhythmia terminates, delayed sinus-node recovery can expose a prolonged pause.
4
Cardiac output becomes inadequate
A slow resting rate, prolonged pause or failure to increase rate with exertion lowers cardiac output, causing fatigue, exercise intolerance, presyncope, syncope or haemodynamic compromise.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Sinus bradycardia

A sinus P wave precedes each QRS at an inappropriately slow rate for the clinical setting; athletic conditioning and sleep may be physiological alternatives.

Sinus arrest or exit block

A pause lacks a sinus P wave. A pause that is an exact multiple of the basic P-P interval suggests sinoatrial exit block; non-multiple timing favours sinus arrest.

Chronotropic incompetence

Heart rate fails to rise appropriately with exertion, producing fatigue, breathlessness or exercise limitation despite an acceptable resting rate.

Tachy-brady syndrome

AF, flutter or atrial tachycardia alternates with bradycardia, and termination of the tachyarrhythmia may expose a prolonged pause.

Adverse bradycardiaRed flag

Shock, syncope, myocardial ischaemia or severe heart failure attributable to bradycardia requires immediate treatment.

Risk of asystoleRed flag

Recent asystole, Mobitz II block, complete heart block with broad QRS or a ventricular pause longer than 3 seconds increases concern and prompts pacing readiness, although these may represent AV rather than sinus-node disease.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
12-lead ECGFirst step
Why
Confirm sinus origin and distinguish SND from AV block, blocked atrial ectopy or AF with a slow ventricular response.
Interpretation and limitations
P-wave timing and its relation to QRS complexes are essential; do not label every pause sinus arrest.
02
Ambulatory ECG or implantable loop recorder
Why
Correlate intermittent symptoms with bradycardia, pauses or tachy-brady transitions.
Interpretation and limitations
A symptom marker during a plausible pause is more useful than the longest asymptomatic pause found incidentally.
03
Exercise ECG
Why
Assess chronotropic response when exertional fatigue is the main complaint.
Interpretation and limitations
Failure to augment rate despite adequate effort supports chronotropic incompetence after medicine and fitness effects are considered.
04
U&Es, magnesium and thyroid function
Why
Identify reversible metabolic or endocrine causes.
Interpretation and limitations
Correct abnormalities and reassess rhythm before permanent pacing when it is safe to wait.
05
Echocardiogram
Why
Assess chamber size, ventricular function and structural disease before device selection.
Interpretation and limitations
LV dysfunction and anticipated pacing burden may alter the optimal pacing strategy.
06
Medication and sleep assessment
Why
Identify beta-blockers, verapamil/diltiazem, digoxin, antiarrhythmics, sedatives and sleep apnoea.
Interpretation and limitations
Withdrawal or dose reduction may resolve extrinsic SND; nocturnal bradycardia alone is not an automatic pacing indication.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Physiological sinus bradycardia

Athletic conditioning, sleep and high vagal tone can cause marked slowing without symptoms; appropriate rate increase with activity and absence of daytime events are reassuring clues.

Atrioventricular block

Continuing P waves that intermittently or completely fail to conduct, with diagnostic PR relationships, favour AV block rather than failure of sinus impulse formation. A broad-QRS escape suggests distal conduction disease but is not specific by itself.

Blocked premature atrial beat

An early non-conducted P wave hidden within the preceding T wave can reset the sinus node and mimic a pause from sinus arrest.

Atrial fibrillation with slow ventricular response

Absent organised P waves and an irregular ventricular rhythm identify atrial fibrillation; rate-limiting medicines or AV-node disease may explain the slow response.

Extrinsic reversible bradycardia

A temporal medicine change, thyroid or electrolyte abnormality, hypothermia or sleep apnoea supports secondary suppression and should be addressed before permanent-device decisions when safe.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01acuteBradycardia with adverse featuresFirst stepBradycardia is causing shock, syncope, ischaemia or severe heart failure.

1Perform ABCDE assessment, monitor ECG/BP/SpO2, record a 12-lead ECG if this does not delay treatment, obtain IV access and correct immediate causes.
2Give atropine 500 micrograms IV; repeat every 3-5 minutes if needed to a maximum total of 3 mg.
3If ineffective, start transcutaneous pacing as a bridge and consider isoprenaline 5 micrograms/min IV or adrenaline 2-10 micrograms/min IV while arranging early transvenous pacing.
4Do not give atropine after cardiac transplantation, and do not use it for high-grade AV block with a wide QRS; move directly to effective pacing/appropriate alternatives with expert help.

02stableStable suspected SNDBradycardia or pauses are present without life-threatening features.

1Review rate-limiting medicines, thyroid and electrolytes, ischaemia, hypothermia, sleep apnoea and physiological conditioning.
2Select ambulatory-monitor duration based on symptom frequency and ask for time-stamped symptom recording.
3Use exercise testing for exertional limitation or suspected chronotropic incompetence.
4Refer to cardiology when symptoms correlate with SND or unexplained syncope persists; observe asymptomatic physiological bradycardia without unnecessary pacing.

03tachy-bradyTachy-brady syndromeAtrial tachyarrhythmia alternates with symptomatic pauses or marked bradycardia.

1Document both components and assess stroke risk independently when AF or flutter is present.
2EscalationAvoid simply escalating rate-limiting medication when post-conversion pauses are causing symptoms.
3Obtain electrophysiology/device assessment; pacing may permit safer rate or rhythm treatment when symptomatic bradycardia is established.
4Continue anticoagulation according to stroke risk regardless of pacemaker implantation.

04permanentPermanent pacingSymptoms are attributable to irreversible SND after reversible causes are addressed.

1Confirm symptom-rhythm correlation and discuss goals, alternatives and procedural risk.
2For symptomatic SND without AV block, offer dual-chamber pacing as a NICE-recommended option unless patient-specific factors favour another system.
3Programme rate response when chronotropic incompetence is important and minimise unnecessary ventricular pacing when appropriate.
4Arrange wound review, device interrogation and ongoing monitoring for AF, lead performance and battery life.

Key medicines and prescribing safety3 treatments · regimens, roles and cautions

First-line RCUK drug for adverse symptomatic bradycardia when not contraindicated by context.

Atropine

500 micrograms IV, repeat every 3-5 minutes if needed to a maximum total of 3 mg.

Do not use after cardiac transplantation; avoid high-grade AV block with wide QRS because it is ineffective and may worsen block. Do not delay pacing when response is poor.

Second-line bridge when atropine is ineffective and pacing is being established.

Isoprenaline

Start 5 micrograms/min by IV infusion and titrate with continuous monitoring.

Can provoke tachyarrhythmia and myocardial ischaemia; specialist monitored use.

Alternative second-line bridge for atropine-refractory adverse bradycardia.

Adrenaline

2-10 micrograms/min by IV infusion, titrated to perfusion and rate.

Use a controlled infusion with continuous monitoring; avoid confusing this peri-arrest infusion with the 1 mg cardiac-arrest bolus.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Syncope, falls and injury

Long pauses or an inadequate rate can transiently reduce cerebral perfusion, causing unpredictable syncope, traumatic injury and loss of driving or occupational independence.

Heart failure or myocardial ischaemia

Persistent severe bradycardia lowers cardiac output and may reduce systemic and coronary perfusion pressure, potentially worsening congestion or angina, especially in people with limited ventricular reserve.

Atrial tachyarrhythmia and embolic stroke

Tachy-brady syndrome commonly includes atrial fibrillation or flutter. The resulting thromboembolic risk requires separate assessment because pacing does not prevent atrial clot formation.

Asystole and circulatory collapse

Prolonged failure of impulse formation without a reliable escape rhythm can cause profound hypotension, seizure-like syncope, cardiac arrest or death.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Correlate symptoms with ECG rate, pauses and tachy-brady transitions.
After changing a rate-limiting drug, reassess pulse, symptoms and the rhythm rather than assuming causation.
During temporary pacing, confirm both electrical capture and a mechanical pulse at an adequate output.
After permanent pacing, monitor wound, sensing, capture threshold, lead impedance, battery and arrhythmia log.
Reassess AF/flutter burden and anticoagulation need independently of the paced heart rate.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

No magic pause duration

A long asymptomatic pause during sleep does not automatically equal clinically important SND; symptoms and context determine significance.

Blocked PAC is a mimic

An early non-conducted P wave hidden in the T wave can reset the sinus node and imitate sinus arrest.

Atropine has exceptions

RCUK specifically advises against atropine after heart transplantation and in high-grade AV block with wide QRS.

Pacing capture has two parts

A pacing spike followed by QRS proves electrical capture; an associated pulse or arterial waveform proves mechanical capture.

A pacemaker is not an anticoagulant

Device support can make tachyarrhythmia treatment safer, but it does not prevent embolic stroke from AF/flutter.

11Common pitfallsFrequent interpretation and management errors.

01
Implanting a pacemaker for asymptomatic sleep-related bradycardia without addressing sleep apnoea.
02
Diagnosing SND from a rate number without symptom correlation.
03
Giving repeated atropine while an unstable patient waits for pacing.
04
Using the 1 mg cardiac-arrest adrenaline bolus for bradycardia with a pulse instead of a titrated infusion.
05
Treating tachy-brady symptoms with more AV-nodal blocker before documenting the pauses.