Syncope toolkit

Syncope is very common and will affect 42% of people during their lifetime

Cardiac syncope

Approximately 9% of patients presenting with syncope were found to have an identifiable cardiac cause.

In this section:

Cardiac syncope and mortality

The mortality associated with syncope is highest when the episode has a cardiac cause and therefore it is important not to assume that a faint is vasovagal in nature without appropriate assessment.

In general, when heart conditions cause syncope, the cause is usually:

  • Structural heart disease in older people
  • Rarer genetic cardiac conditions in the young.

Sudden death in young athletes is rare but has obvious devastating consequences and is discussed in more detail in our section about syncope in athletes. In comparison, there is no increased risk of death amongst people with vasovagal syncope.

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Cardiac Arrhythmias

Arrhythmias cause syncope due to reduced cardiac output.

Inherited disorders causing arrhythmia and syncope

Long QT syndrome (LQTS):

  • Long QT syndrome is diagnosed if the corrected QT interval (QTc) is more than or equal to 480 ms (or QTc is more than or equal to 460 ms in the presence of syncope)
  • Computerised ECG machine reporting of QT interval is generally accurate although this can over or under report
  • 13 genetic mutations have been identified (LQTS types 1-13). Prevalence of the autosomal dominant form is 1 in 2500
  • Jervell and Lange-Nielsen is a rare syndrome which causes profound congenital sensorineural hearing loss, syncope and sudden cardiac deaths due to LQTS.
  • The mean age at presentation of LQTS is 14 years
  • Triggers for arrhythmias can include exercise, fright, the post-partum period, and QT prolonging drugs
  • The risk of sudden cardiac death is 0.9%, but this increases to 5% in people who experience syncope
  • Beta blockers can reduce this risk, but an implantable cardioverter defibrillator (ICD) will be considered for those who continue to experience unexplained syncope despite beta blockers.

See an example of a long QT ECG on ECG Library

Short QT syndrome:

  • Short QT syndrome is very rare, but highly lethal
  • It can present in the first month of life (for examples as sudden infant death syndrome) and over 40% of patients experience cardiac arrest by the age of 40 years
  • A short QT can be diagnosed if the QTc less than or equal to 340 ms or less than or equal to 360 ms if other risk factors are present.

Brugada syndrome:

  • Patients with Brugada syndrome and syncope are more likely to have arrhythmic events such as VT and VF
  • However non-arrhythmogenic syncope is at least as common in this group as in the general population and investigation with an implantable loop recorder may be indicated to determine whether an ICD would be recommended
  • Some drugs can trigger arrhythmias in affected patients and more information including lists of offending medications can be found at

Congenital Accessory Pathways - Wolff-Parkinson-White syndrome (WPW):

  • WPW can be asymptomatic, or present with syncope, presyncope or palpitations.
  • Sudden death is very rare, and tends to occur during exercise
  • Treatment is usually with catheter ablation
  • Digoxin and calcium channel blockers diltiazem and verapamil should never be used in patients with WPW as this combination can provoke lethal ventricular arrhythmias See the British National Formulary (BNF)
  • Atrial fibrillation in WPW is a worrying finding as this arrhythmia can degenerate into ventricular fibrillation.

Visit the British Heart Foundation to see more on Wolff-Parkinson-White Syndrome.

Visit ECG Library for examples of a Wolff-Parkinson-White syndrome ECG and a WPW with atrial fibrillation ECG.

Catecholaminergic polymorphic ventricular tachycardia (CPVT)

Visit the British Heart Foundation to see more on Catecholaminergic polymorphic ventricular tachycardia.


Sinoatrial (SA) node disease

A variety of conditions where sinus arrest or SA block are seen on ECG to cause syncope. A pacemaker may be indicated.

Bradycardia-tachycardia syndrome

An example of this is a prolonged sinus node pause following self-termination of a tachycardia, resulting in syncope.

Atrioventricular (AV) node disease:

Bifascicular bundle branch block (right BBB plus left anterior or left posterior hemiblock or left BBB):

  • Patients with bifascicular block and syncope are at risk of intermittent complete heart block and may benefit from a permanent pacemaker - a routine echocardiogram is worthwhile to see if these patients also have structural heart disease
  • Patients with syncope, bifascicular block and a left ventricular ejection fraction less than 35% benefit from an implantable defibrillator as they are at risk of ventricular tachycardia and ventricular fibrillation.


Paroxysmal SVT:

  • AV nodal re-entrant tachycardia
  • AV re-entrant tachycardia
  • Atrial flutter - see example of atrial flutter ECG
  • Ectopic atrial tachycardia.

All can be treated with ablation or anti-arrhythmic drugs.

Paroxysmal VT

In ventricular tachycardia with syncope, ablation, drug therapy or an implantable cardioverter defibrillator (ICD) may be indicated. Visit ECG Library for an example of a ventricular tachycardia ECG.

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Arrhythmia caused by medication (Long QT interval)

Medication can provoke a variety of brady- and tachyarrhythmias, for example beta blockers can cause sinus bradycardia. However, one that commonly causes concern for GPs is drug-induced long QT syndrome.

Torsades de pointes is a common form of ventricular tachycardia in this situation and can be due to medication that prolongs the QT interval. If it self-terminates it can cause syncope, but torsade de pointes can also progress to ventricular fibrillation and sudden death. This arrhythmia is more likely to occur in the presence of low potassium or magnesium levels.

Drugs that carry the greatest risk of prolonging the QT interval are selected antiarrhythmic drugs; however, other drugs including antipsychotics, antidepressants, antibiotics, and antihistamines are also culprits. Polypharmacy increases this risk.

Consulting room computer software or the British National Formulary app can assist in identifying drug interactions. A useful website is CredibleMeds.

Visit ECG Library for examples of a Toursade de Pointes ECG, ventricular tachycardia ECG and ventricular fibrillation ECG.


Patient on QT prolonging drugs should be advised to report lightheadedness, palpitations or syncope.

If a QT prolonging medication results in a QTc interval of 470-500ms in men, or 480-500ms in women, the dose should be reduced, or the drug stopped. If the QTc is more than 500ms the medication should be reviewed urgently in association with their secondary care specialist, if appropriate and stopped if possible.

Repeat the ECG after reducing or stopping the offending medication. Details of drugs that prolong the QT interval can be found at CredibleMeds.

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Structural Heart Disease

Myocardial ischaemia and its structural cardiac complications

Syncope is almost twice as common in people with cardiovascular disease than those without.

In acute ischaemia, the mechanism can be vasovagal (especially in inferior MI), high grade AV block or tachyarrhythmias. During the first weeks after an acute ischaemic event, there is a higher risk of ventricular arrhythmia, and syncope following MI is particularly concerning.

Hypertrophic cardiomyopathy

Unexplained syncope predicts sudden death and ICD may be indicated. Visit the British Heart Foundation for more on hypertrophic cardiomyopathy.

Arrhythmogenic right ventricular cardiomyopathy (ARVC)

Unexplained syncope suggests risk of arrhythmia and sudden death and ICD may be recommended. Visit the British Heart Foundation for more on arrhythmogenic right ventricular cardiomyopathy.

Aortic Stenosis

Syncope usually occurs during exercise in aortic stenosis due to inability to increase cardiac output to match demand and indicates a poor prognosis.

‘SAD’ symptoms are red flags indicating severe aortic stenosis and urgent referral to cardiology or admission to hospital should be considered.

Myotonic and Muscular dystrophies

Myocardial fibrosis and conduction abnormalities can occur. In type 1 myotonic dystrophy, sudden death was documented in 3.6% of affected patients.

Other structural heart disease:

  • Cardiac mass (atrial myxoma, ball thrombus)
  • Pericardial disease
  • Pericardial tamponade
  • Congenital abnormalities of the coronary arteries
  • Prosthetic valve dysfunction.

Other cardiovascular syncope

Pulmonary embolism and pulmonary hypertension

These conditions effectively obstruct the circulation leading to underfilling of the left ventricle. In acute pulmonary embolism the right ventricle may dilate and have markedly reduced function.

Aortic dissection

This is a life-threatening condition which may cause syncope by several different mechanisms:

  • Pain can lead to vasovagal syncope, but with incomplete recovery
  • By obstructing the circulation
  • By rupturing into the pericardium causing cardiac compression.

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Arrhythmia further reading

Cardiac syncope case histories