ECG Quiz Answer
Sudden Death in a Young Athlete
G. Michael Vincent, MD
THE PHYSICIAN AND SPORTSMEDICINE - VOL 26 - NO. 7 - JULY 2021
Clinical findings. The rhythm strip (figure 3) reveals QT interval prolongation with a QT interval corrected for heart rate (QTc) of 0.48 seconds and large-area T waves, suggestive of long QT syndrome (LQTS). (See "Calculating the Corrected QT Interval," at right.) These findings could also have been neurogenic due to hypoxic brain injury. Electrolyte disturbance was excluded by laboratory values.
The ECG taken 2 years earlier (figure 4) is also consistent with congenital LQTS, which could have been the underlying cause of the patient's cardiac arrest. The ECG reveals sinus arrhythmia with an average cycle length of 750 milliseconds, a QT interval of 0.41 seconds, a QTc of 0.47 seconds, a normal T-wave form, and somewhat prominent U waves, which are normal for the patient's age.
A QTc of 0.47 seconds in an apparently normal female is very suggestive of LQTS, but not completely diagnostic, since a small percentage of normal women have QTc values of 0.46 to 0.47 seconds. A QTc at or above 0.48 seconds in females or 0.47 seconds in males, in the absence of drugs, electrolyte disturbance, or other conditions that might independently lengthen the QT interval, is probably diagnostic of LQTS (1).
Family evaluation. To further evaluate the possibility of inherited LQTS in this patient, ECGs of her parents and siblings were obtained. QT intervals were normal in the parents. One brother had a QTc of 0.46 seconds and one sister had a QTc of 0.47 seconds, both suggestive of LQTS. Both the brother and the sister were asymptomatic. Exercise ECG testing revealed QT prolongation and bifid T-wave abnormalities during and after exercise in her mother and both of these siblings, confirming the diagnosis of LQTS in these family members. Subsequently, the family was found to have a mutation of the HERG gene on chromosome 7.
LQTS and sudden death. LQTS is an important cause of unexpected syncope and sudden death in children and young adults, causing an estimated 3,000 to 4,000 deaths per year (1,2).
The estimated prevalence of LQTS in the general population is 1 in 7,000. About two-thirds of those who have the condition have one or more syncopal episodes. In the past, LQTS was discovered only when syncope or sudden death occurred; increasingly, it is being discovered by family screening studies.
Two clinical forms of LQTS are known: the predominant and autosomal-dominant Romano-Ward variant and the rare Jervell and Lange-Nielsen variant associated with autosomal recessive inheritance of profound congenital deafness. Inherited LQTS should be considered in the active patient with unexplained syncope or sudden death, and the Jervell and Lange-Nielsen form should be specifically considered in the active patient with severe congenital deafness. Rarely, LQTS occurs as a spontaneous new mutation (a "sporadic" case).
With LQTS, syncope and death usually occur suddenly and without warning during exercise or emotional upset, but sudden death during sleep is not uncommon. The average QTc interval for someone who has LQTS is 0.49 seconds, an abnormal value that is generally not obvious on casual observation of an ECG; for this reason QT interval prolongation is often overlooked. The ECG of a patient who had more obvious QT prolongation (figure 5) shows a mean RR interval of 1.04 seconds, a QT interval of 0.54 seconds, and a QTc of 0.53 seconds.
Pathophysiology. Inherited and sporadic LQTS is caused by mutations of the genes encoding for cardiac ion channels (3,4). To date, mutations of the potassium-channel genes KVLQT1 (chromosome 11), HERG (chromosome 7), and KCNE (chromosome 21), and the sodium-channel gene SCN5A (chromosome 3) have been identified. Approximately 90% of genotyped patients have the potassium-channel genotypes, and 10% have the sodium-channel genotype. Other, as yet unrecognized genes also exist, evidenced by some families in which LQTS is present without these genotypes.
The mutations alter ion channel function, lengthen action potential and QT duration, and predispose patients to the polymorphous ventricular tachycardia known as torsades de pointes, which can lead to syncope and sudden death. The events in the KVLQT1 (LQT1) genotype typically occur during exercise, when a person is startled, or during emotional stress such as fright or anger. In patients with the HERG (LQT2) genotype, events occur with about equal frequency during exercise or emotional stress and during sleep, whereas most events in patients who have the SCN5A variation (LQT3) occur during sleep.
The absence of prior symptoms in the patient or family members and/or a normal to only modestly prolonged QTc interval (as in this patient's mother and affected siblings) does not exclude this syndrome. Approximately 16% of HERG genotype patients (the genotype of this patient's family) and 10% of KVLQT1 genotype patients have a normal QTc (at or below 0.44 seconds) at initial ECG evaluation. Another 20% have borderline QTc intervals (0.45 to 0.47 seconds). Thus, the ECG diagnosis of LQTS may be difficult.
Treatment. Treatment of patients who have LQTS can be quite effective. For a large majority of those who have the KVLQT1 and HERG genotypes, beta-blockers are effective in preventing subsequent symptoms. Potassium supplementation has been suggested but not proven to be helpful, particularly in LQT2 patients. Patients who have the SCN5A genotype may respond less well to beta-blockers, and current studies are evaluating the use of sodium-channel blockers such as mexiletine hydrochloride for treatment of patients with this genotype.
When pharmacologic therapy fails, implantable defibrillator or pacemaker therapy may be used. Some patients appear to benefit from left cervicothoracic sympathectomy.
Sports participation. Because syncope and sudden death often occur in the heat of competition and are a result of the combined effect of exercise and emotion on adrenergic stimulation, it has been considered prudent to restrict LQTS patients from competitive sports. In some instances LQTS patients with absent or chronologically remote symptoms whose exercise tests show normalization of the QTc and absence of T-wave abnormalities during exercise have been allowed to continue competitive activities. But the risks associated with this strategy are not known, and therefore this approach cannot be routinely recommended at this time.
For patients who have had significant cardiac events due to LQTS, it seems best at present to restrict activities even after beta-blocker or other therapy is instituted.
Dr Vincent is chairman of the department of internal medicine, director of medical education, and director of the internal medicine residency program at LDS Hospital in Salt Lake City, and professor and associate chairman of the department of internal medicine at the University of Utah School of Medicine in Salt Lake City. Address correspondence to G. Michael Vincent, MD, LDS Hospital, 8th Ave and C St, Salt Lake City, UT 84143; e-mail to [email protected].
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