Adding to the complexity of the circadian timing system, increasing evidence supports the existence of two independent internal pacemakers. Under normal circumstances physiologic systems are influenced by signals from both sources. The circadian rhythms observed in each variable are the results of this dual interaction. Theoretic mathematical models of the human circadian system considering two interacting oscillators have been remarkably successful in reproducing and predicting experimental findings.
In recent years, extensive research in the field of cardiovascular diseases supports the contention that this interaction between endogenous and exogenous daily rhythms is of significant clinical relevance and could potentially modulate cardiovascular morbidity and mortality (vide infra). buy antibiotics online
The circadian variability of certain hemodynamic measurements has been long recognized. In normal individuals blood pressure levels and heart rate decline during sleep and increase during waking/rising time. Millan-Craig and associates studied normotensive and hypertensive volunteers using continuous ambulatory intra-arterial blood pressure monitoring during a 48-hour period. These authors showed similar rhythmic variations in blood pressure/heart rate in both groups of subjects, with a circadian periodicity.
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