11/14/24, 10\:43 AM Sick Sinus Syndrome

Sick Sinus Syndrome

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Sick sinus syndrome\: SAN dysfunction causing bradyarrhythmias or tachyarrhythmias; common in older adults.
Aetiology\: intrinsic (idiopathic
diseases, congenital abnormalities, iatrogenic) and extrinsic (drugs,
'hypos'
,
'hypers'
, autonomic dysfunction).
Pathophysiology\: various arrhythmias due to SAN dysfunction (tachy-brady syndrome, sinus bradycardia and arrest,
sinoatrial exit block, slow atrial
Risk factors\: advancing age, cardiac disease, electrolyte derangement, thyroid disease, medications (negative
chronotropes/antiarrhythmics).
Clinical features\: often asymptomatic; symptoms include fatigue, dizziness, palpitations, pre-syncope/syncope, angina.
Investigations\: resting 12-lead ECG, U&Es, thyroid function tests, drug levels, ambulatory ECG monitoring, implantable loop
recorder.
Management\: remove extrinsic causes (correct electrolytes, stop medications, treat metabolic disturbances), implant
pacemaker (dual-chamber preferred), beta-blocker therapy for tachy-brady syndrome.
Complications\: syncope/pre-syncope, rate-related myocardial ischaemia, acute heart failure, heart block,
thromboembolic events, sudden cardiac death.
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A comprehensive topic overview

Introduction

Sick sinus syndrome, also known as sinus node dysfunction, occurs when sinoatrial node (SAN) dysfunction causes
bradyarrhythmias or tachyarrhythmias.
1,2
The condition predominantly a
syndrome accounts for more than half of pacemaker implantations. Due to an ageing population, the incidence continues
to rise.
3

Aetiology

The SAN contains two types of specialised cells. Pacemaker cells have intrinsic pacemaker activity and initiate action
potentials at regular intervals. Transitional cells facilitate the propagation of the impulse across the atria (Figure 1).
3
For more information on the cardiac conduction system, see the Geeky Medics article here.
In sick sinus syndrome, dysfunction of the SAN leads to an atrial rate that is inappropriate for normal requirements.
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Figure 1. The electrical conduction system of the heart.
The causes of sick sinus syndrome can be divided into intrinsic and extrinsic causes.
1,3
Intrinsic causes include\:
Idiopathic
Ischaemic heart disease (e.g. myocardial infarction, ischaemia)
Myocarditis
Pericarditis
Rheumatic heart disease
In
Congenital abnormalities
Iatrogenic\: for example, damage to the SA node during open-heart surgery
Extrinsic causes include\:
Drugs\: digoxin, beta-blockers, calcium channel blockers, anti-arrhythmics
‘Hypos’
\: hypothermia, hypothyroidism, hypoxia
‘Hypers’
\: hyperkalaemia, hyperthyroidism
Autonomic dysfunction
The exact cause is often di
degree of age-related
infarct, or by the addition of medications such as beta-blockers.

Pathophysiology

Several di
cells vs. transitional cells).
Arrhythmias seen may change over time and are often intermittent. Some important examples of arrhythmias include
tachycardia-bradycardia syndrome, sinus bradycardia and sinus arrest, sinoatrial exit block and slow atrial
3
Tachycardia-bradycardia ("tachy-brady") syndrome
Tachy-brady syndrome is identi
commonly atrial
This is caused by abnormal conduction within the atrial tissue and is the most common manifestation of sick sinus
syndrome, a
Figure 2a and 2b are two ECGs taken from a patient with tachy-brady syndrome.
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Figure 2a. Bradycardia at a rate of 46 bpm
Figure 2b. Tachycardia (paroxysmal atrial
Sinus bradycardia and sinus arrest
Severe, inappropriate sinus bradycardia is often a feature of sick sinus syndrome (Figure 3)
A pause of three seconds or more without any atrial activity (p waves) is known as sinus arrest. This re
pacemaker cells to generate an action potential (Figure 4).
Figure 3. Sinus bradycardia at a rate of approximately 50 bpm
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Figure 4. A period of sinus arrest (no p-waves seen for >3 seconds
To prevent asystole and syncopal episodes, the heart may ‘rescue’ a severe sinus bradycardia or sinus arrest by utilising
pacemaker tissue that is outside of the SAN to generate a new action potential and allow systole to occur. This is called an
escape rhythm and it acts to preserve cardiac output.
Escape rhythms may arise from atrial tissue (atrial escape rhythm) the AVN (junctional escape rhythm) or the ventricular
myocytes (ventricular escape rhythm) and give characteristic appearances on the ECG.
4,5
Sinoatrial exit block
Sinoatrial exit block is similar to atrioventricular heart blocks but instead of a
cause is a failure of the sinoatrial node transitional cells to propagate the impulse across the atria. There are multiple
di
Atrial
Atrial with a slow ventricular response may be seen in the absence of beta-blocker therapy. The ECG will show a
chaotic but bradycardic ventricular rhythm (irregularly irregular) with no evidence of any organised atrial activity in the
form of p waves.

Risk factors

Risk factors for the development of sick sinus syndrome include\:
1,3
Advancing age\: the most important risk factor
Cardiac disease\: ischaemic, in
Electrolyte derangement\: especially hyperkalaemia (e.g. in patients with renal impairment or in those on medications
which cause hyperkalaemia)
Thyroid disease
Medication\: particularly negative chronotropes/antiarrhythmics which may unmask subclinical sinus node dysfunction in
those with additional risk factors
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Clinical features

Sick sinus syndrome often develops over a long time course and many patients are asymptomatic.
Symptoms and signs arise due to arrhythmias which reduce cardiac output and lead to end-organ hypoperfusion.
Therefore, the clinical presentation is not speci
1

History

Typical symptoms of sick sinus syndrome may include\:
Fatigue
Dizziness
Palpitations during periods of arrhythmia
Pre-syncope or syncope due to cerebral hypoperfusion
Angina\: often caused by rate-related myocardial ischaemia
Other important areas to cover in the history include\:
Past medical history\: underlying cardiac disease, multi-system in
amyloidosis) and thyroid disease
Drug history\: negative chronotropes including beta-blockers, rate-limiting calcium channel blockers and other
antiarrhythmics may all contribute to the pathogenesis of sick sinus syndrome and precipitate its onset in those with
other predisposing conditions

Clinical examination

There are no speci

Di

The di
Other tachyarrhythmias to consider include the supraventricular tachycardias\:
Atrial
AV-nodal re-entrant tachycardia (AVNRT)
Atrioventricular re-entrant tachycardia (e.g. Wol
Atrial tachycardia
Other bradyarrhythmias to consider include\:
Atrioventricular node blockade (heart block)
Sinus bradycardia\: due to beta-blockers or other drugs
Physiological bradycardia\: for example amongst endurance athletes

Investigations

Sick sinus syndrome can be challenging to diagnose because the arrhythmias seen are often intermittent and can change
over time.
For a diagnosis of sick sinus syndrome, there must be correlation between ECG evidence of bradyarrhythmias alongside
typical clinical symptoms.

Bedside investigations

Relevant bedside investigations include\:
1,3
Resting 12-lead ECG\: diagnosis based on a resting ECG is unusual because the arrhythmias are often episodic

Laboratory investigations

Relevant laboratory investigations include\:
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U&Es\: to assess plasma electrolyte levels. Hyperkalaemia may be a cause of sinus node dysfunction.
Thyroid function tests\: hyperthyroidism and hypothyroidism can both cause sinus node dysfunction
Drug levels\: for example, digoxin levels (a recognised cause of sinus node dysfunction)

Other investigations

Ambulatory ECG (‘Holter’ monitor)
Ambulatory ECG monitoring for 24 - 48 hours can demonstrate episodes of bradycardia and correlate these with the
patient’s symptoms.
The patient wears a device that captures an ECG rhythm strip over one or two days. The data is then captured and
analysed.
Sometimes a wearable device is required for longer periods (an event recorder). The patient can activate the device
following symptoms such that the previous 5-minute recording is then saved for analysis.
Implantable loop recorder
If ambulatory ECG fails to reveal the diagnosis, then an implantable loop recorder may be placed to monitor the heart
rhythm over a longer period of time.
These small battery-operated devices (approximately 4-5cm long and less than 1cm wide) which are inserted under the
skin in the left parasternal region under local anaesthetic. This is a minor procedure, and they can record for up to three
years.

Management

The management of sick sinus syndrome
electrolyte abnormalities, stopping unnecessary medications and treating any metabolic disturbance.
3
De
A dual-chamber pacemaker with both an atrial lead and a ventricular lead (rather than a ventricular lead alone) is
preferred. This provides a more physiological form of pacing and protects against heart block due to AVN
chamber pacing has also been shown to reduce the risk of AF.
6
Patients with tachy-brady syndrome will require beta-blocker therapy and pacemaker implantation.
Whilst this may seem counter-intuitive, the pacemaker allows the use of su
episodes of tachycardia, whilst protecting from symptomatic bradycardia.
3
Pacemakers are not appropriate for patients with sick sinus syndrome who are asymptomatic, or for those in whom a
reversible cause can be identi
2
For more information on cardiac pacemakers see the Geeky Medics article here.
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Figure 5. Example of a pacemaker device
Figure 6. PA chest X-ray showing a left-sided dual-chamber pacemaker.

Complications

Complications of sick sinus syndrome include\:
Syncope and pre-syncope (sometimes with injury)
During periods of profound tachycardia (e.g. fast AF) patients may experience rate-related myocardial ischaemia, which
can cause a troponin rise.
Tachyarrhythmias may also precipitate acute heart failure
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Heart block due to AVN
Patients with tachy-brady syndrome (paroxysmal atrial
Sudden cardiac death (rare, more common in AVN disease/heart block)

References

Tidy C (Patient.info Professional Reference). S i c k S i n u s S y n d r o m e . Published in 2014. Available from\: [LINK]
Burns E (LITFL.com). S i n u s N o d e D y s f u n c t i o n ( S i c k S i n u s S y n d r o m e ) . Published in 2019. Available from\: [LINK]

Jabbour F (StatPearls). S i n u s N o d e D y s f u n c t i o n . Published in 2020. Available from\: [LINK]
Burns E (LITFL.com). V e n t r i c u l a r E s c a p e R h y t h m . Published in 2019. Available from\: [LINK]
Acute Coronary Syndrome (ACS)
Burns E (LITFL.com). J u n c t i o n a l E s c a p e R h y t h m . Published in 2019. Available from\: [LINK]
Acute Heart Failure
Lamas GA et al., Atrial Fibrillation (AF)
V e n t r i c u l a r P a c i n g o r D u a l -C h a m b e r P a c i n g f o r S i n u s-N o d e D y s f u n c t i o n . New England Journal of Medicine.
Published in 2002. Available from\: [LINK]
Atrioventricular Block
Figures
Brugada Syndrome
Figure 1. Madhero. T h e E l e c t r i c a l C o n d u c t i o n S y s t e m o f t h e H e a r t . License\: [CC BY-SA]. Available from\: [LINK]
Figure 2a. Fruitsmaak S. T w o E C G s t a k e n f r o m a P a t i e n t w i t h T a c h y-B r a d y S y n d r o m e . License\: [CC BY-SA]. Available from\:
[LINK]

Test yourself

Figure 2b. Fruitsmaak S. T w o E C G s t a k e n f r o m a P a t i e n t w i t h T a c h y-B r a d y S y n d r o m e . License\: [CC BY-SA]. Available from\:
[LINK]

Figure 3. Meyerson A. E C G S h o w i n g S i n u s B r a d y c a r d i a . License\: [CC BY-SA]. Available from\: [LINK]
Figure 4. CardioNetworks. E C G d e m o n s t r a t i n g a p e r i o d o f s i n u s a r r e s t . License\: [CC BY-SA]. Available from\: [LINK]
Introduction
Figure 5. Fruitsmaak S. S t J u d e M e d i c a l P a c e m a k e r D e v i c e . License\: [CC BY]. Available from\: [LINK]
Figure 6. CardioNetworks. P A C h e s t R a d i o g r a p h D e m o n s t r a t i n g L e f t-S i d e d D u a l -C h a m b e r P e r m a n e n t P a c e m a k e r i n s i t u .
Aetiology
License\: [CC BY-SA]. Available from\: [LINK]
Risk factors

Reviewer

Clinical features
Dr Hazel White
Di
Consultant Cardiologist with a special interest in complex device implantation
Investigations
Mid Yorkshire Hospitals NHS Trust
Management
Source\: geekymedics.com
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