Long QT syndrome type 3

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.

TIME TO REPOLARIZE

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).

ANOTHER PATIENT, ANOTHER CAUSE

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.

TREATMENT PROTOCOLS

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.

TEACHING YOUR PATIENTS

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.

SELECTED REFERENCES

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.

BY JULIA E. BERNSTEIN, RN, CCRN, CNS-C, MS Staff Nurse

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

Copyright Springhouse Corporation Apr 1997
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