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.
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.
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.
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.
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).
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.
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.
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.
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.