Background: Various factors are implicated in the circadian pattern of cardiovascular vulnerability. The exact mechanisms involved in the peak incidence of cardiovascular events occurring during the early morning hours after awaking are not completely known. The purpose of our study was to investigate the circadian variation of timing and intensity of wave reflections in healthy individuals and to test the hypothesis that significant changes occur during the day.
Methods: Thirteen healthy non-smokers (seven women and six men, mean age 40.7 +/- 16.5 years) were examined. Aortic pulse wave analysis was performed to estimate surrogates of wave reflections intensity (augmentation index [AIx]) and timing. Twelve measurements separated by 1-h intervals were performed from 8 am to 7 pm each study day.
Results: Analysis of variance for repeated measures indicated significant changes during the 12-h period for heart rate corrected AIx (P = .033) and heart rate (P = .035). The AIx was maximal at 8 am within 1-h after awaking (17% +/- 3.6%); it was gradually diminished until 3 pm (9% +/- 4.1%) and again increased to a second (albeit lower) peak value during the late afternoon (7 pm).
Conclusions: It is possible that the increased intensity of reflected waves occurring during the early morning in combination with the rising trend of blood pressure and heart rate at the same time results in an aggravated left ventricular afterload and an increase in myocardial oxygen demand. The morning-related enhancement of wave reflections may have potential implications for the increased risk for cardiovascular events during the early morning, which remains to be clarified.