Rhythm, QT Prolongation

Article Author:
Mohammad Al-Akchar
Article Editor:
Momin Siddique
Updated:
5/6/2019 12:33:55 AM
PubMed Link:
Rhythm, QT Prolongation

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 be a result of electrolyte abnormalities, and/or drugs that affect those ion channels.[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 to be as many as 30% of patients in the intensive care unit.[7][8]

Pathophysiology

The duration of a QT interval is largely dependent on the duration of the ventricular action potential. This duration is largely dependent on closure and/or opening of ion channels in the heart with the influx of positive ions (potassium, calcium) causing depolarization, and the efflux of positive ions (potassium) causing repolarization. Any disturbance in these ion channels that leads to an excess of positive ions intracellularly will lead to prolongation of 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) will lead to QT prolongation. Also, certain medications affect those ion channels and lead to QT prolongation. Virtually all of the 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 are 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 to know 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 to be 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 LQT7 type (Andersen syndrome). Also, cognitive and behavioral problems and immune dysfunction may be seen in those with LQT8 type (Timothy syndrome).

Evaluation

Establishing the diagnosis of 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 is used to define the end of the T wave. A helpful tip that helps identify prolonged QT interval on 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 increased risk of torsade de pointes. While several equations exist to help correct for variation in heart rate, the most commonly used is Bazett formula ( QTC = QT / √ RR ). Though Bazett formula seems to be relatively accurate in heart rates between 60 to 100 beats/min, it tends to overcorrect with higher heart rates and undercorrect in 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, the diagnosis of Long QT syndrome is made. In those patients, it may be very helpful to obtain an electrocardiography of the patient and family members. 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 patient who is hemodynamically unstable should receive non-synchronized electrical defibrillation. Also, first-line treatment is magnesium sulfate, and the benefit is seen independent of serum magnesium level. In those who do not respond to magnesium sulfate, temporary transvenous overdrive pacing should be considered. 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 whom 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.

Enhancing Healthcare Team Outcomes

Patients with a prolonged QT interval may be first identified by the primary care provider, nurse practitioner, internist or the 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 in congenital Long QT syndrome is with beta-blockers as 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 important for the pharmacist to keep a track of patient medications because many can cause prolonged QT syndrome. The nurses should inform the patient to follow up with the cardiologist or primary care provider and undergo regular ECGs to ensure that the condition is under control.



  • (Move Mouse on Image to Enlarge)
    • Image 5672 Not availableImage 5672 Not available
      Contributed by the Southern Illinois University, Department of Internal Medicine

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. Frontiers in physiology. 2019;     [PubMed PMID: 30967788]
[2] Dehghani-Samani A,Madreseh-Ghahfarokhi S,Dehghani-Samani A, Mutations of Voltage-Gated Ionic Channels and Risk of Severe Cardiac Arrhythmias. Acta Cardiologica Sinica. 2019 Mar;     [PubMed PMID: 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. Journal of the American Medical Informatics Association : JAMIA. 2019 May 1;     [PubMed PMID: 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 6;     [PubMed PMID: 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;     [PubMed PMID: 29275963]
[6] Cohagan B,Brandis D, Torsade de Pointes 2019 Jan;     [PubMed PMID: 29083738]
[7] Ebrahim MA,Williams MR,Shepard S,Perry JC, Genotype Positive Long QT Syndrome in Patients With Coexisting Congenital Heart Disease. The American journal of cardiology. 2017 Jul 15;     [PubMed PMID: 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. Journal of anesthesia. 2017 Jun;     [PubMed PMID: 28229241]
[9] Hutton DM, The importance of routine QT interval measurement in rhythm interpretation. Dynamics (Pembroke, Ont.). 2008 Fall;     [PubMed PMID: 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 opinion on drug safety. 2005 Jul;     [PubMed PMID: 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. The Journal of emergency medicine. 2018 Jan;     [PubMed PMID: 29107482]
[12] Gromova OA,Torshin IY,Kalacheva AG,Grishina TR, [On Synergism of Potassium and Magnesium in Maintenance of Myocardial Function]. Kardiologiia. 2016 Mar;     [PubMed PMID: 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 cardiology. 2017 Jan;     [PubMed PMID: 27990843]