title: Early morning reduction in ischemic threshold in patients with unstable angina and significant coronary disease.
abstract: BACKGROUND
  The objective of this study was to investigate in patients with unstable angina and significant coronary stenosis (> 70%) whether or not the morning peak of myocardial ischemia is associated with a reduction in the ischemic threshold. The morning increased incidence of ischemic episodes in stable angina appears to be attributable to a coincidence of several factors. Patients with unstable angina who remain at bed rest, however, also present a similar morning increased incidence of ischemia, but its mechanisms are not completely understood.
  
  
  RESULTS
  The ischemic threshold was assessed by atrial pacing at 7 to 8 AM and at 12 to 1 PM in 46 patients. In the 34 with a positive pacing response (ST segment shift > 1.0 mm), ischemic threshold was lower at 7 to 8 AM than at 12 to 1 PM (131 +/- 16 versus 139 +/- 15 beats per minute, P < .001), whereas in the remaining 12 patients, the pacing response was negative. Moreover, 4 patients presented ST segment elevation during pacing in the morning but only 1 at noon and at a higher threshold. Baseline heart rate and diastolic blood pressure were higher at noon than in the morning (81 +/- 16 versus 76 +/- 13 beats per minute, P < .01, and 87 +/- 11 versus 82 +/- 10 mm Hg, P < .05).
  
  
  CONCLUSIONS
  The morning lowering of ischemic threshold in the absence of increases in baseline blood pressure or heart rate suggests that a reduced coronary vasodilator capacity or an increased coronary tone may favor the increased incidence of ischemic events during this interval.
authors.affiliation: Unitat Coronària, Hospital General Vall d'Hebron, Barcelona, Spain.
authors.forename: J
authors.initials: J
authors.lastname: Figueras
authors.forename: R M
authors.initials: RM
authors.lastname: Lidón
mesh_terms: D000789:Angina, Unstable / Q000175:diagnosis / Q000503:physiopathology*; D001510:Bed Rest; D001794:Blood Pressure / Q000502:physiology; D002304:Cardiac Pacing, Artificial; D002940:Circadian Rhythm / Q000502:physiology*; D003327:Coronary Disease / Q000175:diagnosis / Q000503:physiopathology*; D003331:Coronary Vessels / Q000503:physiopathology*; D004562:Electrocardiography; D005260:Female; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D008297:Male; D008875:Middle Aged; D017202:Myocardial Ischemia / Q000503:physiopathology*; D014655:Vascular Resistance / Q000502:physiology; D014664:Vasodilation / Q000502:physiology