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Long QT Syndrome

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Long QT syndrome (LQTS) is condition which runs in families and affects 1 in 2000 of the general population. Carriers of the condition are outwardly normal. It affects mostly children and young adults and can cause frequent faints or even sudden death, though most carriers of the condition are unaware of it. The most common forms of LQTS are inherited in an autosomal dominant pattern, which means that each child of an affected parent has a 50% chance of inheriting the disorder. Once a diagnosis is made, treatment is available. However, an accurate diagnosis can be difficult, because not all people affected by LQTS display symptoms.

Usually, LQTS can be seen on an electrocardiogram (ECG). Every heart beat is triggered by an electrical signal that tells the heart's muscle cells to contract. After contracting, these cells must recover - or recharge - before the next heart beat is initiated. The amount of time needed by these cells to recover can be measured on an ECG and this is called the QT interval. People with LQTS have an abnormally long QT interval. If the next electrical signal arrives before the muscle cells have completed their recovery period a dangerously fast, chaotic, heart rhythm can occur leading to a fall in blood pressure and loss of consciousness. It's a bit like the heart having a seizure.

Schematic diagram of the electrical activity of one heart beat recorded by an ECG. The electrical discharge (depolarization) of the ventricle is represented by the QRS complex, with the T wave reflecting electrical recovery (repolarization). The QT interval, corrected for heart rate at the time, is the time interval from the beginning of the QRS complex to the end of the T wave, and is generally less than 0.46 seconds in males and 0.47 in females.

Long QT

In people with LQTS, the QT is usually longer than these values. There is considerable overlap between those with and those without LQTS, and they change with time, so repeated ECGs may be needed. A long QT interval is present on the ECG below:

ECG trace

People with LQTS are sometimes identified after an unexplained sudden fainting episode. In children, boys are more commonly affected than girls, but after puberty, females are more commonly affected than males. Episodes are commonly triggered by surges of adrenaline, such as with intense physical activity, especially swimming. Other triggers, more common in women, include sudden loud noises, intense emotional reactions, especially something which suddenly wakes from sleep. An example of an ECG during a collapse is shown below, the heart rhythm is very fast and chaotic:

 ECG collapse

Events may result in seizures which are commonly misdiagnosed as epilepsy, especially when they occur at night. Currently, the primary treatment for LQTS is a beta blocker medication, which inhibits the effects of adrenaline on the heart.  Some patients (not the majority) may benefit from pacemakers or implantable defibrillators or surgical resection of a group of nerves connecting the brain and the heart.  Everyone with LQTS should avoid medications which are known to prolong the QT interval.  A constantly updated list is available online  

Types of Long QT Syndrome (LQTS)

In 2018 we know of 17 genes which have been linked to LQTS, but the vast majority are of types 1 2 and 3. The genes encode for proteins in the heart muscle cells which build pumps enabling the movement of potassium and sodium across the cell walls. If these pumps don't work well, it takes the heart longer to pump the potassium or sodium across the cell walls to recharge the cell ready for the next beat. This is seen as the long QT interval on the ECG.

Type 1 is the commonest, linked to a gene called KCNQ1. Triggers for a sudden collapse are usually exercise and swimming related. Long QT type 2 is the next commonest and is linked to a gene called KCNH2. Triggers for a collapse are typically loud noise and startle, or sleep, and it seems to be most severe in adult women. Long QT type 3 is less common (about 10%) and causes events most commonly at night. Type 1 LQTS responds particularly well to beta blocker medication.

Aside from the ECG, other factors may be needed to make the diagnosis, such as a family history of sudden death in young people, a history of sudden collapse in the patient, and ECG changes after exercise. Exercise tests and 24 hour ECG monitoring are useful to help establish who has the condition, and genetic testing is also very helpful. About 75% of affected families will have a gene defect which has thus far been identified.

Screening family members

Since the gene runs in families, screening of family members is very important, yet this can be difficult for doctors. Specialist cardiologists are usually required to confirm the diagnosis. Not only are the tests difficult to interpret, but the family members may be spread across New Zealand and the rest of the word. In New Zealand there is a national group of experts and scientists interested in helping people with conditions like LQTS called CIDG (the Cardiac Inherited Diseases Group). They run a national voluntary registry to help with family screening, give expert advice, and offer research into their condition. There is a website and facebook page at and they can be contacted through the administrator ( There are clinical coordinators in Auckland, Hamilton and Wellington.


  1. Everyone with this condition must avoid medications which prolong the QT interval. There are many, including some antibiotics such as erythromycin and some antihistamines. 
  2. Most will need to take a regular beta-blocker every day. In young children we currently usually use Atenolol liquid, which has to be taken twice a day. Once able to take a tablet, Nadolol is preferred.
  3. Severely affected people, or those who cannot take beta blockers (such as those with bad asthma) can have a procedure called a left cardiac sympathectomy. A nerve is cut, using a telescopic procedure under the left armpit.
  4. A few will need an implanted cardioverter defribrillator. This is a sophisticated pacemaker which can detect and shock an abnormal rhythm back to normal. In general, this is for people who have suffered a cardiac arrest or in whom control is not achieved with other therapies.

Other sources of information

There are many web sites to visit, of varying quality.  A good start is SADS Foundation: Sudden Arrhythmia Death Syndromes Foundation and

National Long QT Registry in New Zealand

A registry is based in Auckland co-ordinated by Jackie Crawford (Senior Cardiac Technician) and Dr Jon Skinner as part of the inherited cardiac disease service.  A molecular diagnostic service is available via labplus Auckland, with all tests being referred via CIDG. A research programme is based at Auckland University (Dr Annika Winbo).

Genetic diagnosis

Your supervising physician, after obtaining formal informed consent from yourself, often with the assistance of a genetic counsellor, will send your blood for genetic testing to the place that seems most appropriate for your case at that time.  

The importance of the genetic test is two-fold.  A genetic diagnosis can be found in about 70%

  1. It defines the sub-type of Long QT syndrome, so treatment and advice can be adjusted appropriately
  2. It enables screen of family members to see who may be at risk, and if they could pass it on to their children.

There are three common types. Type 1 and 2 respond well to betablocker treatment, because the triggers for type 1 (60%) is exercise; swimming is especially dangerous.  For type 2 (30%) the commonest triggers are sudden and loud noise (especially at night), and emotional excitement; women tend to be at higher risk.  In type 3 (10%) events usually occur at night, men tend to be at higher risk, and defibrillator pacemakers are most commonly needed in this group in whom beta blocker treatment is of doubtful benefit.

Medications to be avoided or requiring special caution in people with LQTS

This list includes medications which prolong the QT interval and is meant as a guide for people with LQTS, or acquired long QT interval from heart muscle disease, and their parents or guardians. It should not be seen as all inclusive. Those prescribing any medication to someone with LQTS should always check the drug specifications and contra-indications. The list has been compiled by review of publications and/or drug advice sheets provided with medications. Check also and


Erythromycin, Clarithromycin, Gatifloxacin, levofloxacin, Moxifloxacin, Sulfamethoxazole-trimethoprim (Septrin/Bactrim), Spiramycin, Pentamidine


Terfenadine, Astemizole, Diphenhydramine.

(These are particularly to be avoided (even in normal subjects) in combination with Erythromycin or grapefruit juice or the antifungals ketoconazole, miconazole, fluconazole or itraconazole)

Antihistamines that may be used safely are loratidine, cetirizine and fexofenadine, and phenergan

Appetite suppressants

Fenfluramine, phentermine, Sibutramine

Asthma treatments

The Beta-2 agonists (eg. Terbutaline, Salbutamol, Salmeterol) both work against the B-Blockers given to subjects with long QT and lower blood potassium levels and therefore should be given only in hospital with careful monitoring. Inhaled steroids (eg. Becotide/Flixotide) and Ipatropium (Atrovent) are safer.


Ephedrine, phenylephrine, phenylpropanolamine, pseudoephedrine

Psychotropics/ Antidepressants/ Anticonvulsants

  • Antipsychotics (including Thioridazine, Haloperidol Mesoridazine, chlorpromazine)
  • Antidepressants (including Maptiline, Amitriptyline, imipramine, fluoxetine, desipramine, paroxetine)
  • Anticonvulsants (Felbamate and Fosphenytoin are to be avoided).


(used to dilate blood vessels in treatment of heart failure angina or high blood pressure)
Prenylamine Lidoflazine, Fenoxedil, Bepridil


(Pure Beta Blockers are safe! eg Nadolol, Propranolol, Atenolol)
The others are not safe, for example the following:

  • Type IA Quinidine (particularly dangerous), Procainamide, Disopyramide
  • Type IC Encainide, Flecainide
  • Type III Ibutilide, Amiodarone, Sotalol, Sematilide


This lowers potassium level even in small doses, and is dangerous in big doses (more than one liquorice twist) or over long periods. Is present as Glycyrrhizinic acid (GZA), found in many sweets including; Stimorol and Ben Bits Coolmint chewing gum, Fisherman's Friend, turkish Pepper and herbal medicines for gastric ulcer treatments. Drinks include Belgian beers, Pastis, Raki, Ouzo and Pernod.


  • Adrenaline (epinephrine)
  • Amantadine (antiviral and Parkinson's disease agent)
  • Chloral hydrate (sedative)
  • Corticosteroids
  • Diuretics (via electrolyte disturbance-low potassium, magnesium, calcium: Frusemide and other loop diuretics particularly dangerous)
  • Chloroquine (antimalarial)
  • Cocaine
  • Dopamine
  • Dobutamine (used in intensive care)
  • Dolasetron (anti-emetic)
  • Droperidol (sedative)
  • Foscarnet (antiviral)
  • Halofantrine (antimalarial)
  • Isoproterenol (adrenaline like substance)
  • Levomethadyl (narcotic dependance)
  • Midodrine (to treat low blood pressure)
  • Suxamethonium and Atropine (anaesthetic agents)
  • Ritodrine
  • Vasopressin
  • Cisapride (for oesophageal reflux- indigestion)
  • Tacrolimus (to suppress immune responses)
  • Liquid protein diets.

Your dentist should use local anaesthetic WITHOUT ADRENALIN.

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Document Control

  • Date last published: 10 August 2018
  • Document type: Clinical Guideline
  • Services responsible: Paediatric Cardiology
  • Author(s): Jon Skinner
  • Editor: Greg Williams
  • Review frequency: 2 years

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