The response of the QT interval to the brief tachycardia provoked by standing: a bedside test for diagnosing long QT syndrome.
OBJECTIVE
This study was undertaken to determine whether the short-lived sinus tachycardia that occurs during standing will expose changes in the QT interval that are of diagnostic value.


BACKGROUND
The QT interval shortens during heart rate acceleration, but this response is not instantaneous. We tested whether the transient, sudden sinus tachycardia that occurs during standing would expose abnormal QT interval prolongation in patients with long QT syndrome (LQTS).


METHODS
Patients (68 with LQTS [LQT1 46%, LQT2 41%, LQT3 4%, not genotyped 9%] and 82 control subjects) underwent a baseline electrocardiogram (ECG) while resting in the supine position and were then asked to get up quickly and stand still during continuous ECG recording. The QT interval was studied at baseline and during maximal sinus tachycardia, maximal QT interval prolongation, and maximal QT interval stretching.


RESULTS
In response to brisk standing, patients and control subjects responded with similar heart rate acceleration of 28 +/- 10 beats/min (p = 0.261). However, the response of the QT interval to this tachycardia differed: on average, the QT interval of controls shortened by 21 +/- 19 ms whereas the QT interval of LQTS patients increased by 4 +/- 34 ms (p < 0.001). Since the RR interval shortened more than the QT interval, during maximal tachycardia the corrected QT interval increased by 50 +/- 30 ms in the control group and by 89 +/- 47 ms in the LQTS group (p < 0.001). Receiver-operating characteristic curves showed that the test adds diagnostic value. The response of the QT interval to brisk standing was particularly impaired in patients with LQT2.


CONCLUSIONS
Evaluation of the response of the QT interval to the brisk tachycardia induced by standing provides important information that aids in the diagnosis of LQTS.
Viskin
Sami
S
Department of Cardiology, Tel-Aviv Medical Center, Tel Aviv University, Tel Aviv, Israel. saviskin@tasmc.health.gov.il
Postema
Pieter G
PG
Bhuiyan
Zahurul A
ZA
Rosso
Raphael
R
Kalman
Jonathan M
JM
Vohra
Jitendra K
JK
Guevara-Valdivia
Milton E
ME
Marquez
Manlio F
MF
Kogan
Evgeni
E
Belhassen
Bernard
B
Glikson
Michael
M
Strasberg
Boris
B
Antzelevitch
Charles
C
Wilde
Arthur A M
AA
D000222:Adaptation, Physiological; D000328:Adult; D004562:Electrocardiography; D005260:Female; D006329:Heart Conduction System / Q000503:physiopathology*; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D008133:Long QT Syndrome / Q000175:diagnosis*; D008297:Male; D008875:Middle Aged; D019095:Point-of-Care Systems; D011187:Posture / Q000502:physiology*; D012372:ROC Curve; D013616:Tachycardia, Sinus / Q000503:physiopathology*; D055815:Young Adult