title: Measurement of the RT interval on ECG records during electron-beam CT.
abstract: OBJECTIVE
  The R wave of the electrocardiogram is used widely as a trigger for cardiac imaging. This study was designed to determine the optimal interval between the R wave and end systole for triggering of electron-beam computed tomography (CT) in a group of patients with various heart rates who are undergoing assessment for coronary artery calcification.
  
  
  METHODS
  A total of 862 consecutive asymptomatic patients referred for screening with electron-beam CT for coronary artery calcification were enrolled in the study. Patients' R-R, RT, and PR intervals were measured by using the software of the CT console computer. Correlation coefficients were computed and linear regression analyses were performed for all intervals measured. Results were analyzed according to patient age (three subgroups), sex (two subgroups), and heart rate (nine subgroups). Separate formulas for calculating the length of RT intervals in men and in women were developed.
  
  
  RESULTS
  After correction for heart rate, a significant difference was found in mean RT and PR intervals between women and men, with the mean intervals in women being longer (P < .001). No significant difference was found in these intervals within the three age-defined subgroups (< or = 40, 41-60, and >60 years; P > .05). However, significant negative correlations were found between heart rates and the lengths of all measured intervals. The results of statistical analysis indicate that most of the variation in the R-R interval with different heart rates occurred in diastole and that the duration of systole was relatively constant.
  
  
  CONCLUSIONS
  For optimal cardiac imaging, triggering should take place in late systole, avoiding the RT interval variability that occurs in diastole.
authors.affiliation: Saint John's Cardiovascular Research Center, Research and Education Institute, Harbor-UCLA Medical Center, 1124 W Carson St, RB2, Torrance, CA 90502, USA.
authors.forename: Songshou
authors.initials: S
authors.lastname: Mao
authors.forename: Bin
authors.initials: B
authors.lastname: Lu
authors.forename: Junichiro
authors.initials: J
authors.lastname: Takasu
authors.forename: Ronald J
authors.initials: RJ
authors.lastname: Oudiz
authors.forename: Matthew J
authors.initials: MJ
authors.lastname: Budoff
mesh_terms: D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D002114:Calcinosis / Q000503:physiopathology; D003324:Coronary Artery Disease / Q000503:physiopathology; D004562:Electrocardiography; D005260:Female; D006325:Heart Atria / Q000000981:diagnostic imaging / Q000503:physiopathology; D006329:Heart Conduction System / Q000000981:diagnostic imaging / Q000503:physiopathology*; D006339:Heart Rate / Q000502:physiology; D006352:Heart Ventricles / Q000000981:diagnostic imaging / Q000503:physiopathology; D006801:Humans; D008297:Male; D008875:Middle Aged; D012189:Retrospective Studies; D012737:Sex Factors; D013223:Statistics as Topic; D013318:Stroke Volume / Q000502:physiology; D014057:Tomography, X-Ray Computed