[Relationship between circadian rhythms and blood pressure and the pathogenesis of cerebrovascular insufficiency].

Cerebrovascular accidents occur more frequently during the morning, less frequently during the night, whether due to thrombosis or embolism. In cases with hypertension, however, there have been noted a more uniform distribution of these complications throughout the day. The preponderance in the morning of the number of accidents in non-embolic subjects, depends probably on the state of the autonomic nervous system during the night, which gives rise to a dilated and hypotonic microcirculation. Autoregulation is, in this way, reduced and the physiological increase in blood pressure during the morning may induce a break through of autoregulation. In embolic subjects, on the other hand, an important role may be played by heart arrhythmias which occur usually during the waking hours. In hypertensives, the microcirculatory bed is restricted and cerebro-vascular resistance relatively constant. The brain, paradoxically, is thus better defended against any moderate increase in blood pressure and fluctuations in blood pressure throughout 24 hours, which may leave cerebral autoregulation unchanged, which probably explains the more uniform distribution of cerebro-vascular accidents throughout the 24 hour period.