Effect of β-adrenergic stimulation on QT interval accommodation.
BACKGROUND
QT interval correction formulas are based on instantaneous heart rate (HR), but QT interval adaptation to sudden HR change occurs gradually. In humans, the QT interval has been reported to reach a new steady-state value in about 2 minutes.


OBJECTIVE
This study sought to assess β-adrenergic stimulation effects on QT interval response to HR change.


METHODS
Ten subjects (42.1 ± 15.3 years, 3 men) undergoing radiofrequency ablation of supraventricular tachycardia were studied. Atrial pacing for 5 minutes at an HR ranging from 60 to 140 beats/min was performed before and during dobutamine infusion (10 μg/kg/min). The QT response to sudden HR change was evaluated.


RESULTS
The QT response to sudden HR change consists of an immediate response (IR), followed by a gradual monoexponential course to the new steady-state value. Linear function results in inferior fit of QT adaptation course (P < .05 compared with exponential). The time constant of the exponential is approximately 1 minute (50.9 ± 11.4 seconds). The IR magnitude is approximately 3% of the RR interval change (2.97% ± 2.01%). Dobutamine shortens steady-state QT at given HR (from 301.8 ± 11.2 ms to 290.6 ± 13.2 ms at 120 beats/min; P < .001) and increases IR magnitude (to 13.28% ± 8.99% of RR change; P < .01). During sinus rhythm, QT variability and QT variability index were significantly increased by dobutamine.


CONCLUSIONS
QT adaptation has 2 distinct phases, which might have different mechanisms. The effect of β-adrenergic stimulation on IR may increase QT variability by increasing the immediate response to HR variability. These results may be important in the assessment of drug effects on repolarization and for understanding of QT variability.
Department of Medicine, Texas Tech University Health Science Center, El Paso, Texas, USA.
Srikanth
S
Seethala
Vladimir
V
Shusterman
Samir
S
Saba
Susan
S
Mularski
Jan
J
Němec
D000222:Adaptation, Physiological / Q000187:drug effects; D058671:Adrenergic beta-1 Receptor Antagonists / Q000008:administration & dosage*; D000328:Adult; D002304:Cardiac Pacing, Artificial / Q000379:methods; D017115:Catheter Ablation / Q000379:methods; D004280:Dobutamine / Q000008:administration & dosage*; D004334:Drug Administration Schedule; D004562:Electrocardiography; D005260:Female; D005500:Follow-Up Studies; D006339:Heart Rate / Q000187:drug effects; D006801:Humans; D007262:Infusions, Intravenous; D016014:Linear Models; D008133:Long QT Syndrome / Q000175:diagnosis / Q000188:drug therapy*; D008297:Male; D008875:Middle Aged; D012494:Sampling Studies; D012720:Severity of Illness Index; D013617:Tachycardia, Supraventricular / Q000175:diagnosis / Q000188:drug therapy* / Q000601:surgery; D016896:Treatment Outcome
D058671:Adrenergic beta-1 Receptor Antagonists; D004280:Dobutamine