Find information on thousands of medical conditions and prescription drugs.

Long QT syndrome type 3

The long QT syndrome (LQTS) is a heart disease in which there is an abnormally long delay between the electrical excitation (or depolarization) and relaxation (repolarization) of the ventricles of the heart. It is associated with syncope (loss of consciousness) and with sudden death due to ventricular arrhythmias. Arrhythmias in individuals with LQTS are often associated with exercise or excitement. The cause of sudden cardiac death in individuals with LQTS is ventricular fibrillation. more...

Amyotrophic lateral...
Bardet-Biedl syndrome
Lafora disease
Landau-Kleffner syndrome
Langer-Giedion syndrome
Laryngeal papillomatosis
Lassa fever
LCHAD deficiency
Leber optic atrophy
Ledderhose disease
Legg-Calvé-Perthes syndrome
Legionnaire's disease
Lemierre's syndrome
Lennox-Gastaut syndrome
Lesch-Nyhan syndrome
Leukocyte adhesion...
Li-Fraumeni syndrome
Lichen planus
Limb-girdle muscular...
Lipoid congenital adrenal...
Lissencephaly syndrome...
Liver cirrhosis
Lobster hand
Locked-In syndrome
Long QT Syndrome
Long QT syndrome type 1
Long QT syndrome type 2
Long QT syndrome type 3
Lung cancer
Lupus erythematosus
Lyell's syndrome
Lyme disease
Lysinuric protein...

Individuals with LQTS have a prolongation of the QT interval on the ECG. The Q point on the ECG corresponds to the beginning of ventricular depolarization while the T point corresponds to the beginning of ventricular repolarization. The QT interval is measured from the Q point to the end of the T wave. While many individuals with LQTS have persistent prolongation of the QT interval, some individuals do not always show the QT prolongation; in these individuals, the QT interval may prolong with the administration of certain medications.


The two most common types of LQTS are genetic and drug-induced. Genetic LQTS can arise from mutation to one of several genes. These mutations tend to prolong the duration of the ventricular action potential (APD), thus lengthening the QT interval. LQTS can be inherited in an autosomal dominant or an autosomal recessive fashion. The autosomal recessive forms of LQTS tend to have a more severe phenotype, with some variants having associated syndactyly or congenital neural deafness. A number of specific genes loci have been identified that are associated with LQTS. Following is a list of the most common mutations:

  • LQT1 - mutations to the alpha subunit of the slow delayed rectifier potassium channel (KvLQT1 or KCNQ1). The current through the heteromeric channel (KvLQT1+minK) is known as IKs. This mutation is thought to cause LQT by reducing the amount of repolarizing action potential current that prolongs action potential duration (APD). These mutations tend to be the most common yet least severe.
  • LQT2 - mutations to the alpha subunit of the fast delayed rectifier potassium channel (HERG + miRP). Current through this channel is known as IKr. This phenotype is also probably caused by a reduction in repolarizing current.
  • LQT3 - mutations to the alpha subunit of the sodium channel (SCN5A). Current through is channel is commonly referred to as INa. Depolarizing current through the channel late in the action potential is thought to prolong APD. The late current is due to failure of the channel to remain inactivated and hence enter a bursting mode in which significant current can enter when it should not. These mutations are more lethal but less common.
  • LQT4 - mutations in an anchor protein Ankyrin B which anchors the ion channels in the cell. Very rare.
  • LQT5 - mutations in the beta subunit MinK which coassembles with KvLQT1.
  • LQT6 - mutations in the beta subunit MiRP1 which coassembles with HERG.
  • LQT7 - mutations in the potassium channel KCNJ2 which leads to Andersen-Tawil syndrome.
  • LQT8 - mutations in the calcium channel Cav1.2 encoded by the gene CACNA1c leading to Timothy's syndrome

Other mutations affect the beta subunits ion channels. For example LQT6 affects minK (aka KCNE1) which is the beta subunit that coassembles with KCNQ1 to form IKs channels.


[List your site here Free!]

Recognizing when long QT intervals mean trouble
From Nursing, 4/1/97 by Julia E Bernstein

Here's how to identify patients at risk for ventricular arrhythmias and sudden cardiac death.

John Adams, a 30year-old boxer, was admitted to your emergency department (ED) today complaining of syncope and palpitations. Two weeks ago, he was diagnosed with episodes of nonsustained ventricular tachycardia (VT) and an anterior wall myocardial infarction (MI). He underwent an emergency angioplasty for an occlusion in his left anterior descending coronary artery but continued to experience episodes of nonsustained VT and syncope.

Mr. Adams was started on 480 mg of sotalol (Betapace) P.O. and 20 mg of furosemide (Lasix) daily. When electrophysiology studies confirmed that his VT was no longer inducible, he was discharged home. Today, however, he returned to the ED complaining of dizziness and palpitations.

The ED nurse starts cardiac monitoring and identifies Mr. Adams' rhythm as sinus bradycardia at 40 beats/minute, with premature ventricular contractions and a QT interval of 0.52 second. His blood pressure is stable at 108/60, his isoenzymes are normal, and his potassium is 3.2 mEq/liter (normal is 3.5 to 5.1 mEq/liter). He's receiving oxygen at 2 liters/minute via nasal cannula and an intravenous (I.V.) infusion of 0.9% sodium chloride solution at a keep-vein-open rate. He's also receiving 20 mEq of potassium chloride in 100 ml of D5W to correct his hypokalemia and reduce the risk of torsades de pointes-a type of VT characterized by a ventricular rate of 150 to 250 beats/minute and wide QRS complexes with rotating baselines. (The name "torsades de pointes" means "twisting around the points.") A patient with low potassium, magnesium, or calcium levels is at increased risk for this arrhythmia.

Later that evening, 16-year-old Maggie Webb arrives at the ED unresponsive and intubated. The nurses note tremors and rigidity in all of Maggie's extremities. They place her on a cardiac monitor and start an I.V. infusion of 0.9% sodium chloride solution at 100 ml/hour. Her electrocardiogram (ECG) displays severe sinus bradycardia (30 beats/minute) with bursts of VT at 170 beats/minute and a lengthened QT interval. The nurses prepare atropine and bring the defibrillator to the bedside. Maggie's rhythm changes to sustained VT.

Because of their prolonged QT intervals, Mr. Adams and Maggie are at high risk for ventricular arrhythmias, such as torsades de points, and sudden cardiac death. In this article, I'll explain what's behind long QT intervals, how to recognize abnormal values and patients at risk, how to calculate the ratecorrected QT interval (QTc) (See Determining the QTc Interval), and what medications to give.


As you know, the QT interval is the time needed by the ventricles to depolarize (the Q, R, and S waves) and repolarize (the T wave). You measure the QT interval by counting the number of boxes from the onset of the R wave (or the Q wave, if present) to the end of the T wave (as it intersects the isoelectric baseline) on standard ECG paper (25 mm/second) and multiplying that number by 0.04 second.

A normal QT interval measures 0.44 second or less, although it can extend to 0.50 second in sinus bradycardia. (See The Long and Short of QT Intervals.) QT intervals greater than 0.44 second are abnormal, and those greater than 0.50 second are associated with serious events, such as VT, ventricular fibrillation, and sudden death. You may find QT intervals difficult to evaluate when a wave is absent, the ST segment is elevated, or the T wave is flat.

To evaluate the QT interval, use a 12-lead ECG and select the lead with the longest QT interval as the baseline. This is usually lead II, although V5, V2, or VI may have the longest QT interval in some cases.

WHO'S AT Risk?

When he arrived in the ED, Mr. Adams appeared hemodynamically stable. His assessment revealed hypokalemia (probably caused by furosemide). His bradycardia and prolonged QTc interval were the result of sotalol, a Class III antiarrhythmic agent with beta-blocker activity (see Common Antiarrhythmic Medications that Prolong QT Intervals).

Analyzing Mr. Adams's ECG with the medical resident, you note prolonged QTc intervals ranging from 0.51 second to 0.59 second in all 12 leads. When you review his previous records, you find that Mr. Adams' QTc intervals have increased 0.14 second since his last admission.

The sotalol he's been receiving prolongs the ventricle's action potential duration (shortened by ischemia related to his previous myocardial infarction) and creates a uniform, almost simultaneous, period of ventricular recovery.

This reduces the risk of reentry activity and ventricular tachyarrhythmias. Occasionally, however, areas of the myocardium don't respond to sotalol and repolarization is dispersed across the myocardium. You'll see this as varied QT intervals throughout the 12-lead ECG.

The QT dispersion is the longest measured QT interval minus the shortest measured QT interval. Mr. Adams's sotalol treatment seems inadequate to establish a uniform QT dispersion: His maximum QT interval is 0.59 second and his minimum is 0.51 second, for a dispersion of 0.08 second (a zero dispersion is normal).


In contrast to Mr. Adams, whose long QT intervals are transient and caused by medication, Maggie appears to have long QT syndrome, a congenital condition. Her young age (she's under 21), her presentation with seizures, and a long QTc interval are key findings. Her family history suggests that other family members may have had long QT syndrome: Her younger brother died of sudden cardiac death while playing baseball and a sister died of sudden infant death syndrome. Maggie herself is deaf with frequent episodes of syncope, other signs of long QT syndrome, although she has no symptomatic cardiac history.


Suddenly, Mr. Adams' monitor sounds. His arrhythmia has changed to torsades de pointes. He's immediately defibrillated and his rhythm converts to sinus bradycardia at 56 beats/minute. The resident discontinues Mr. Adams' sotalol because of beta-blockade and transfers him to the intensive care unit (ICU).

The next day, Mr. Adams is given a loading dose of amiodarone (Cordarone), 800 mg P.O. twice a day, to control his arrhythmia. Because amiodarone is absorbed slowly by the gastrointestinal tract, its full onset of action may take 10 days. Several weeks or months are needed to reach a steady plasma concentration of 1 to 2.5 mcg/ml.

Recent studies indicate that this potent antiarrhythmic reduces mortality from ventricular arrhythmias by decreasing the QT dispersion more significantly than sotalol, particularly in patients with myocardial infarction. With Mr. Adams on amiodarone, you can expect his QTc interval to increase above baseline or normal limits without increased risk of torsades de pointes because the QTc dispersion will become more uniform.

You'll draw plasma concentration levels every 3 days to determine the onset of action, therapeutic level, or toxicity of the amiodarone. A plasma level greater than 2.5 mcg/ml is toxic.

If Mr. Adams has an adverse reaction to the amiodarone, such as hypotension, increased ventricular ectopy, or increased cough and dyspnea, call the physician, withhold the amiodarone, and intervene to treat the symptoms. Mr. Adams may need an implantable defibrillator during his hospitalization.

After a week, Mr. Adams' plasma levels are therapeutic at 1.5 mcg/ml and his ECG records normal sinus rhythm with no ectopy. Electrophysiology testing confirms that he's noninducible for torsades de pointes. His amiodarone dose is reduced to a maintenance level-800 mg a day for the next 3 weeks. Before his discharge, you tell him to report any adverse reactions to his physician, such as a slow heart rate (less than 60 beats/minute), blood pressure below 100/60, bluish skin color, bruising, wheezing, fever, or cough. He's scheduled for a repeat amiodarone level and Holter monitor testing in I week. If he remains asymptomatic, his amiodarone will be reduced to 400 mg a day to reduce the risk of long-term adverse reactions, such as thyroid dysfunction and photophobia.

Maggie's arrhythmia also changes to sustained VT and deteriorates to torsades de points. Like Mr. Adams, she's successfully defibrillated in the ED. Her rhythm converts to sinus bradycardia with a prolonged QTc interval at 0.60 second, placing her at high risk for recurrent VT. Transferred to the ICU, she receives 30 mg of oral propranolol twice daily for antiarrhythmic therapy.

Propranolol is standard therapy for congenital long QT syndrome because it blocks increased sympathetic beta-adrenergic activity, reducing the extra systoles that can lead to torsades de pointes. The drug also shortens the QT interval, reducing the vulnerable period when extra systoles might occur.

Maggie should be monitored carefully for premature atrial and ventricular contractions, sinus bradycardia, atrioventricular blocks, and hypotension. She may also have adverse reactions to propranolol, including drowsiness, blurred vision, gastric irritation, cough, and dyspnea. Keep a defibrillator-pacemaker unit and 1 to 2 mg of I.V. atropine nearby.

Her physician suggests inserting a pacemaker and internal defibrillator to prevent sudden cardiac death, but Maggie and her parents refuse, preferring to try medical therapy first. Because she continues to have frequent premature ventricular contractions, her propranolol dosage is increased to a maintenance level of 60 mg twice daily (2 mg/kg/day, based on her weight of 132 pounds [60 kg]).

Before her discharge, teach Maggie about the seriousness of her condition, the risk of sudden death, and her medication therapy. She's arrhythmia-free, with a heart rate of 50 and a QTc interval of 0.50 second on discharge.


Emotional support for Mr. Adams, Maggie, and their families is essential to their recovery. Because these patients are still at risk for sudden cardiac death, they should be followed closely after discharge. To reduce their anxiety, teach them how to monitor their pulse, how to recognize adverse medication reactions and threatening symptoms, and when to notify a physician or call 911. Their families should learn to perform cardiopulmonary resuscitation so they can respond in an emergency. Encourage Mr. Adams and Maggie to join a cardiac support group so they can learn more about their condition and resume a normal lifestyle.


Buckingham. T., et al.: "Differences in Corrected QT Intervals at Minimal and Maximal Heart Rate May Identify Patients at Risk for Torsades de Pointes during Treatment with Antiarrhythmic Drugs;' Journal of Cardiovascular Electrophysiology. 5(5):408-411, May 1994.

Cui, G., et al.: "Effects of Amiodarone, Semahlide, and Sotalol on QT Dispersion." American Journal of Cardiology. 74(9):896-900, November I, 1994.

Drew, B.: "Bedside Electrocardiogram Monitoring." AACN Clinical Issues. 4(1):25-33, February 1993. Garson, A., Jr., et al.: "The Long QT Syndrome in Children: An International Study of 287 Patients," Circulation. 87(6):18661872, June 1993.

Hebra, J.: "The Nurse's Role in Continuous Dysrhythmia Monitoring," AACN Clinical Issues. 5(2):178-185, May 1994. Hohnloser, S., and Woosley, R.: "Sotalol:' The New, England Journal of Medicine. 331(l):31-38, July 7, 1994.


Intermediate Cardiac Surgery Unit Valley Hospital Ridgewood, N.J.

Copyright Springhouse Corporation Apr 1997
Provided by ProQuest Information and Learning Company. All rights Reserved

Return to Long QT syndrome type 3
Home Contact Resources Exchange Links ebay