Abnormal QT-Response to the Sudden Tachycardia Provoked by Standing in Individuals With Drug-induced Long QT Syndrome
Abnormal QT-Response to the Sudden Tachycardia Provoked by Standing in Individuals With Drug-induced Long QT Syndrome
The drug-induced long QT syndrome (diLQTS) describes a clinical entity in which
 administration of a drug produces marked prolongation of the QT interval of the
 electrocardiogram, associated with the development of a polymorphic ventricular tachycardia,
 termed torsades de pointes (TdP). The heart rate is an important variable affecting the QT
 interval. The QT interval normally shortens as the heart rate accelerates; however, the
 adaptation of the QT interval to sudden heart rate acceleration is not instantaneous.
 Interestingly, Holter studies show that the speed of response of the QT interval to sudden
 changes in heart rate (that is, the time it takes the QT interval of a given person to reach
 a new steady-state QT/RR relation) in healthy persons is highly individual and independent of
 the basic QTc. The investigators and others recently proposed the "quick standing" test as a
 simple bedside test that facilitates the diagnosis of congenital LQTS. The test takes
 advantage of the fact that as one stands up, the heart rate acceleration is abrupt while the
 associated QT-interval shortening is gradual. As the R-R interval shortens faster than the QT
 interval, the QT appears to "stretch" toward the next P wave and the corrected QT interval
 (QTc) for heart rate actually increases momentarily. The phenomenon of "QT stretching" is
 universal but is exaggerated in patients with LQTS, allowing for a simple but accurate
 diagnostic test. There is no data on the effects of quick standing on drug-associated form of
 the long QT syndrome. The investigators therefore propose the present study to better
 understand who these patients with drug-associated form of the long QT syndrome are and what
 the significance of their abnormal QT-response is.
The drug-induced long QT syndrome (diLQTS) describes a clinical entity in which
 administration of a drug produces marked prolongation of the QT interval of the
 electrocardiogram, associated with the development of a polymorphic ventricular tachycardia,
 termed torsades de pointes (TdP). Drugs that produce diLQTS block the repolarizing current,
 IKr, encoded by KCNH2, the disease gene for type 2 congenital LQTS (cLQTS) . Some risk
 factors are drug-specific (pharmacokinetic factors such as cytochrome P450 variants) whereas
 others (pharmacodynamic factors) seem more related to myocardial sensitivity across drugs.
 Studies of the congenital syndrome demonstrated variable penetrance in the congenital
 syndrome; that is, there is variability in the extent to which mutation carriers display long
 QT intervals, syncope, and sudden death. Thus, one pharmacogenetic hypothesis is that persons
 displaying diLQTS represent an atypical form of the congenital form of the syndrome. In fact,
 a minority of subjects with diLQTS have rare mutations in cLQTS disease genes and can thus be
 labeled as having the congenital syndrome. In a given family, some persons with a LQTS
 mutation have clearly long QT intervals, whereas others with the same mutation have normal QT
 intervals at baseline. In some cases, these latter people, often referred to as "latent" or
 "subclinical" LQTS, experience TdP only after therapy with a QT-prolonging drug. Indeed, when
 cLQTS disease genes have been screened, mutations are identified in about 10% of subjects
 with diLQTS. The heart rate is an important variable affecting the QT interval. The QT
 interval normally shortens as the heart rate accelerates; however, the adaptation of the QT
 interval to sudden heart rate acceleration is not instantaneous. Animal and clinical studies
 have shown that following an abrupt increase in ventricular pacing rate, it takes up to 2
 minutes until the ventricular refractory period and the QT interval shorten to a new steady
 state that is appropriate for the new (faster) rate. Interestingly, Holter studies show that
 the speed of response of the QT interval to sudden changes in heart rate (that is, the time
 it takes the QT interval of a given person to reach a new steady-state QT/RR relation) in
 healthy persons is highly individual and independent of the basic QTc. The investigators and
 others recently proposed the "quick standing" test as a simple bedside test that facilitates
 the diagnosis of congenital LQTS. The test takes advantage of the fact that as one stands up,
 the heart rate acceleration is abrupt while the associated QT-interval shortening is gradual.
 As the R-R interval shortens faster than the QT interval, the QT appears to "stretch" toward
 the next P wave and the corrected QT interval (QTc) for heart rate actually increases
 momentarily. The phenomenon of "QT stretching" is universal but is exaggerated in patients
 with LQTS, allowing for a simple but accurate diagnostic test. There is no data on the
 effects of quick standing on drug-associated form of the long QT syndrome. The investigators
 therefore propose the present study to better understand who these patients with
 drug-associated form of the long QT syndrome are and what the significance of their abnormal
 QT-response is.
Inclusion Criteria:

 -  age above 18 years

 -  were hospitalized in Tel Aviv medical center between January 2013 and June 2015 due to
 drug-induced long QT syndrome and the associated torsades de

 -  were treated with specific drugs (antibiotics) that potentially prolong the QT
 interval (only for control group)

 Exclusion Criteria:

 -  Subjects with an inability to communicate well with the investigators (i.e., language
 problem, poor mental development or impaired cerebral function).

 -  Subjects with any acute medical situation (e.g. acute infection) within 48 hours of
 study start, which is considered of significance by Investigator.

 -  Subjects who are non-cooperative or unwilling to sign consent form.