Continuing Education Activity

The QT interval on an electrocardiogram (ECG) represents the duration of the ventricular action potential, and this physiologically correlates with the duration of the ventricular depolarization and repolarization. Cardiac events and fatal arrhythmias may occur when the QT interval is prolonged either congenitally or through acquired causes. This activity reviews the causes of QT prolongation and highlights the role of the interprofessional team in its management.

Objectives:

• Identify the etiology of prolonged QT syndrome.
• Evaluate the ECG features of prolonged QT syndrome.
• Determine the treatment and management options available for prolonged QT syndrome.
• Communicate interprofessional team strategies for improving care coordination and communication to advance the management of QT prolongation and improve patient outcomes.

Introduction

The QT interval on an electrocardiogram (ECG) represents the duration of the ventricular action potential, and this physiologically correlates with the duration of the ventricular depolarization and repolarization. Cardiac events and fatal arrhythmias may occur when the QT interval is prolonged either congenitally or through acquired causes.[1][2][3]

Etiology

The causes of QT interval prolongation can be divided into congenital or acquired. Congenital causes are usually a result of mutations in ion channels (potassium, calcium, or sodium) with more than 15 identified mutations. In contrast, acquired QT interval prolongation may result from electrolyte abnormalities or drugs that affect those ion channels (see Image. Prolonged QT interval).[4][5][6]

Epidemiology

The prevalence of congenital causes, also known as Long QT syndrome (LQTS), is difficult to estimate but may be expected in 1 in 2,500 to 1 in 10,000 individuals. It is more common in females and usually presents with cardiac events in childhood, adolescence, or early adulthood. There are, however, case reports of it manifesting in the fifth decade of life. Family history is positive for Long QT syndrome in 40% and for sudden cardiac death in 30% of patients. Acquired causes are relatively more common than congenital causes. Some studies report the prevalence of QT prolongation in as many as 30% of patients in the intensive care unit.[7][8]

Pathophysiology

A QT interval duration largely depends on the duration of the ventricular action potential. This duration largely depends on the heart's closure or opening of ion channels, with the influx of positive ions (sodium, calcium) causing depolarization and the efflux of positive ions (potassium) causing repolarization. Any disturbance in these ion channels that leads to an excess of intracellular positive ions prolongs the action potential, leading to QT prolongation. The pathophysiology of congenital and acquired causes is explained below.

• Congenital: Mutation in genes coding for ion channel proteins results in their malfunction, leading to excess intracellular positivity. Though rare, this entity results in a high risk of sudden death. So far, mutations of any of 15 genes have been linked to Long QT syndrome, with KCNQ1 being the most common gene mutated and is the cause of Long QT syndrome type 1.
• Acquired: More commonly, prolongation of QT interval is acquired. As one can expect, disturbances of electrolytes (hypokalemia, hypocalcemia, hypomagnesemia) prolong QT. Also, certain medications affect those ion channels and lead to QT prolongation. Virtually all drugs that produce Long QT syndrome act by blocking the outward IKr current, which is mediated by the potassium channel encoded by the KCNH2 gene. Medications known to cause this include antiarrhythmic drugs such as sotalol and amiodarone, certain antibiotics such as macrolides and fluoroquinolones, antipsychotics such as haloperidol and olanzapine, and certain gastric motility agents (such as cisapride). A great resource for knowing which drugs prolong QT interval is www.crediblemeds.org.

History and Physical

Syncope is the most common symptom, usually experienced during exercise and high emotions (50% of the genetic variant). Syncope during swimming, including immediately after diving into the water, appears relatively specific for LQT1. Other presentations include near-syncope, cardiac arrest, or seizures. In 10% to 15% of individuals, death is the first sign. Also, certain types of Long QT syndrome have an additional non-cardiac phenotype. For example, hearing loss is present in Jervell and Lange-Nielsen syndrome. Skeletal abnormalities, such as short stature and scoliosis, are present in the LQT7 type (Andersen syndrome). Also, cognitive and behavioral problems and immune dysfunction may be seen in those with LQT8 type (Timothy syndrome).

Evaluation

Diagnosing prolonged QT starts by measuring the QT interval on ECG. This is often done on lead II or V5-6, whichever is longer. This should be done on several successive beats, of which the longest interval is chosen. If a U wave exists and is large (greater than 1 mm), and fused with T-wave, then this should be included in the QT measurement. On the contrary, if the U-wave is small or separate from the T-wave, then it should be excluded. The maximum slope-intercept method defines the end of the T wave. A helpful tip that helps identify prolonged QT intervals during the initial examination of the ECG is that a normal QT interval should be less than half the preceding RR interval.[9][4][10]

Due to the variation of QT interval with heart rate (higher heart rate has shorter QT interval, lower heart rate has longer QT interval), it is important to correct the QT interval for the heart rate. This is known as QTc. QTc is prolonged if it is greater than 440 ms in men or greater than 460 ms in women. A QTc greater than 500 is associated with an increased risk of torsade de pointes. While several equations exist to help correct heart rate variation, the Bazett formula (QTC = QT / √ RR) is the most commonly used. Though the Bazett formula seems relatively accurate in heart rates between 60 to 100 beats/min, it tends to overcorrect with higher heart rates and undercorrect with lower heart rates.

Once QTc is identified as prolonged, the next step in a workup is to look for acquired causes. The most common cause of QT prolongation in an ICU setting is usually drug-related. Serum potassium, calcium, and magnesium levels should be checked, as low serum of each can cause QT prolongation. Also, stimulating thyroid hormone (TSH) levels may be checked in patients with suspected hypothyroidism.

In the absence of reversible or acquired causes of QT prolongation, Long QT syndrome is diagnosed. In those patients, obtaining an electrocardiography of the patient and family members may be very helpful. Noncardiac phenotype (as discussed above) may aid in making the diagnosis. Genetic testing of the patient and family members is the gold standard; however, this testing is limited by cost. Pharmacologic provocation with epinephrine or isoproterenol is warranted in patients with a borderline presentation. The concept of this testing is that patients with Long QT syndrome have an abnormal response to sympathetic stimulation. Their ECG shows the failure of the QT interval to shorten with increased heart rates, or it may even show prolongation. In patients with LQT2, there is marked shortening with exercise, however, exaggerated lengthening of the QT interval as the heart rate declines during late recovery.

Treatment / Management

The goal of management is the prevention of lethal arrhythmias such as torsade de pointes (TdP). As described earlier, the longer the QT interval, the higher the risk is for torsade de pointes. A hemodynamically unstable patient should receive non-synchronized electrical defibrillation. Also, the first-line treatment is magnesium sulfate, and the benefit is independent of serum magnesium level. Temporary transvenous overdrive pacing should be considered for those who do not respond to magnesium sulfate. Isoproterenol and Class IB antiarrhythmic drugs, such as lidocaine and phenytoin, may also be used. [5][11][12][13]

For long-term management in congenital Long QT syndrome, beta-blockers are the first line choice, and they help prevent ventricular arrhythmias by stabilizing ventricular action potential and helping block sympathetic surges associated with arrhythmias. An implantable cardioverter defibrillator (ICD) is recommended in patients with Long QT syndrome who were resuscitated from a cardiac arrest. It is also indicated in those who have beta-blocker-resistant symptoms or have contraindications to beta-blockers. It also may be indicated in asymptomatic individuals who are suspected to be at high risk for ventricular arrhythmias.

Differential Diagnosis

The differential diagnoses for long QT syndrome include the following:

• Brugada syndrome
• Cardiac death
• Coronary artery anomalies
• Drug-induced QT prolongation
• Hypertrophic cardiomyopathy
• QT prolongation in the course of other diseases
• Seizure
• Short QT syndrome
• Sudden cardiac death
• Syncope

Enhancing Healthcare Team Outcomes

Patients with a prolonged QT interval may be first identified by the primary care provider, internist, or pharmacist. It is important to refer these patients to the cardiologist/cardiac surgeon ASAP as the management is complex.

The goal of management is the prevention of lethal arrhythmias such as torsade de pointes (TdP). The long-term management of congenital Long QT syndrome involves beta-blockers to help prevent ventricular arrhythmias by stabilizing ventricular action potential and helping block sympathetic surges associated with arrhythmias. An implantable cardioverter defibrillator (ICD) is recommended in patients with Long QT syndrome who were resuscitated from a cardiac arrest. 

The pharmacist needs to keep track of patient medications because many can cause prolonged QT syndrome. The clinicians should inform the patient to follow up with the cardiologist or primary care provider and undergo regular ECGs to ensure the condition is under control. With an interprofessional healthcare team approach, QT prolongation can be prevented or treated appropriately.

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References

1.

Zhou X, Bueno-Orovio A, Schilling RJ, Kirkby C, Denning C, Rajamohan D, Burrage K, Tinker A, Rodriguez B, Harmer SC. Investigating the Complex Arrhythmic Phenotype Caused by the Gain-of-Function Mutation KCNQ1-G229D. Front Physiol. 2019;10:259. [PMC free article: PMC6430739] [PubMed: 30967788]

2.

Dehghani-Samani A, Madreseh-Ghahfarokhi S, Dehghani-Samani A. Mutations of Voltage-Gated Ionic Channels and Risk of Severe Cardiac Arrhythmias. Acta Cardiol Sin. 2019 Mar;35(2):99-110. [PMC free article: PMC6434417] [PubMed: 30930557]

3.

Alahmadi A, Davies A, Vigo M, Jay C. Can laypeople identify a drug-induced QT interval prolongation? A psychophysical and eye-tracking experiment examining the ability of nonexperts to interpret an ECG. J Am Med Inform Assoc. 2019 May 01;26(5):404-411. [PMC free article: PMC7787352] [PubMed: 30848818]

4.

Giudicessi JR, Roden DM, Wilde AAM, Ackerman MJ. Classification and Reporting of Potentially Proarrhythmic Common Genetic Variation in Long QT Syndrome Genetic Testing. Circulation. 2018 Feb 06;137(6):619-630. [PMC free article: PMC6383807] [PubMed: 29431662]

5.

Beach SR, Celano CM, Sugrue AM, Adams C, Ackerman MJ, Noseworthy PA, Huffman JC. QT Prolongation, Torsades de Pointes, and Psychotropic Medications: A 5-Year Update. Psychosomatics. 2018 Mar-Apr;59(2):105-122. [PubMed: 29275963]

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Cohagan B, Brandis D. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): Aug 8, 2023. Torsade de Pointes. [PubMed: 29083738]

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Ebrahim MA, Williams MR, Shepard S, Perry JC. Genotype Positive Long QT Syndrome in Patients With Coexisting Congenital Heart Disease. Am J Cardiol. 2017 Jul 15;120(2):256-261. [PubMed: 28532774]

8.

Uvelin A, Pejaković J, Mijatović V. Acquired prolongation of QT interval as a risk factor for torsade de pointes ventricular tachycardia: a narrative review for the anesthesiologist and intensivist. J Anesth. 2017 Jun;31(3):413-423. [PubMed: 28229241]

9.

Hutton DM. The importance of routine QT interval measurement in rhythm interpretation. Dynamics. 2008 Fall;19(3):29-33. [PubMed: 18773713]

10.

Cavero I, Crumb W. The use of electrocardiograms in clinical trials: a public discussion of the proposed ICH E14 regulatory guidance. April 11-12, 2005, Bethesda, MD, USA. Expert Opin Drug Saf. 2005 Jul;4(4):795-9. [PubMed: 16011455]

11.

Anderson HN, Bos JM, Haugaa KH, Morlan BW, Tarrell RF, Caraballo PJ, Ackerman MJ. Prevalence and Outcome of High-Risk QT Prolongation Recorded in the Emergency Department from an Institution-Wide QT Alert System. J Emerg Med. 2018 Jan;54(1):8-15. [PubMed: 29107482]

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Gromova OA, Torshin IY, Kalacheva AG, Grishina TR. [On Synergism of Potassium and Magnesium in Maintenance of Myocardial Function]. Kardiologiia. 2016 Mar;56(3):73-80. [PubMed: 28294893]

13.

Chouchoulis K, Chiladakis J, Koutsogiannis N, Davlouros P, Kaza M, Alexopoulos D. Impact of QT interval prolongation following antiarrhythmic drug therapy on left ventricular function. Future Cardiol. 2017 Jan;13(1):13-22. [PubMed: 27990843]

Disclosure: Mohammad Al-Akchar declares no relevant financial relationships with ineligible companies.

Disclosure: Momin Siddique declares no relevant financial relationships with ineligible companies.