title: Circadian variation in ischemic threshold. A mechanism underlying the circadian variation in ischemic events.
abstract: BACKGROUND
  There is a circadian pattern in the occurrence of cardiac events in patients with coronary artery disease. Whether changes in coronary vascular tone contribute to these phenomena is unknown. We measured the ischemic threshold, defined as either the heart rate or rate-pressure product at 1-mm ST segment depression during treadmill exercise and used it as an index of the lowest coronary vascular resistance; the premise was that when ischemic threshold became lower, coronary vascular resistance was higher, and vice versa.
  
  
  RESULTS
  Fifteen patients (group A) with stable coronary artery disease underwent four identical treadmill exercise tests in 24 hours, and ischemic threshold was measured as the heart rate at the onset of 1-mm ST depression. Before each treadmill test, postischemic forearm vascular resistance was measured after 5 minutes of forearm occlusion, using strain-gauge plethysmography. Sixteen additional patients (group B) underwent two treadmill tests at 8 AM and 1 PM, and ischemic threshold was measured as the heart rate-blood pressure product at 1-mm ST depression. A circadian variation was noted: In group A, the heart rate-derived ischemic threshold was lower at 8 AM and 9 PM compared with noon and 5 PM (p less than 0.03). Also, in group B, the rate-pressure product-derived ischemic threshold was 8 +/- 2% lower at 8 AM compared with 1 PM (p = 0.008). A circadian variation parallel to the observed variation in ischemic threshold was also noted in the postischemic forearm blood flow, which was lower in the morning and at night (p less than 0.004). There was a strong correlation between postischemic forearm blood flow and ischemic threshold (p less than 0.0001), such that ischemic threshold was lower at the time of day when postischemic forearm blood flow was lower, and vice versa.
  
  
  CONCLUSIONS
  A lower ischemic threshold in the morning suggests that the ischemia-induced coronary vascular resistance is increased at this time, a finding supported by a similar variation in postischemic forearm vascular resistance. Parallel changes in forearm and coronary resistance suggest that generalized (neural or humoral factors) rather than local factors are responsible for the observed circadian changes. Increased coronary tone in the mornings may not only contribute to the higher incidence of transient ischemia but may help trigger acute cardiac events at this time.
authors.affiliation: Cardiology Branch, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Md 20892.
authors.forename: A A
authors.initials: AA
authors.lastname: Quyyumi
authors.forename: J A
authors.initials: JA
authors.lastname: Panza
authors.forename: J G
authors.initials: JG
authors.lastname: Diodati
authors.forename: E
authors.initials: E
authors.lastname: Lakatos
authors.forename: S E
authors.initials: SE
authors.lastname: Epstein
mesh_terms: D000368:Aged; D002940:Circadian Rhythm; D003327:Coronary Disease / Q000503:physiopathology*; D004056:Differential Threshold; D004562:Electrocardiography; D005260:Female; D005542:Forearm / Q000098:blood supply; D006801:Humans; D008297:Male; D008875:Middle Aged; D008991:Monitoring, Physiologic; D012039:Regional Blood Flow; D015424:Reperfusion; D014655:Vascular Resistance