The remarkable influence of external stimuli in modulating this cardiovascular response during the transition from sleep to an active state is underscored by the observation that individuals who arouse from sleep but remain in bed (nonambulatory) exhibit a blunted hemodynamic response. The change to an erect position and possibly the mental stress associated with initiating the daily activities is an important component of this arousal phenomenon. In addition to enhancing […]
In all individuals the rate of increase in blood pressure (dP/dT) is maximal during the waking period. Moreover, the increases in blood pressure begin approximately 1 h before waking and peak 2 h after […]
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.
The term “Zeitgeber” (a German neologism translated as “time giver”) encompasses the concept of this natures clock capable of being perceived by the human body. The timing and organization of many internal phenomena are guided by this environmental “pacemaker,” resulting in the predictable periodicity of many biorhythms. Of the many potential atmospheric Zeitgeber, the most reliable indicator of environmental time is the regular diurnal alternation of light and darkness.
In practical terms, the functional timing unit for the human appears to be approximately one day: the 24-hour cycle of alternating light and darkness. Many biorhythms exhibit a periodicity that approximates 24 hours. The […]
In addition to these major cardiovascular manifestations of coronary insufficiency, certain arrhythmias (atrioventricular junctional and idioventricular rhythms, ventricular premature beats, nonsustained ventricular tachycardia, and the ventricular rates in atrial fibrillation to mention a few) exhibit characteristic circadian variations. Establishing a direct cause-effect relationship between arrhythmic events and myocardial ischemia/necrosis is not always possible, and although many rhythm abnormalities are manifestations of impaired coronary blood supply, the incidence of primary arrhythmias […]
Due to sample size and lack of correction for waking time, no definite conclusions could be reached from those observations. A suggestion was made, however, that one of the mechanisms by which adrenergic blocking agents may protect from ischemic cardiac events is by suppressing or blunting the effects of the catecholamine surge associated with the arousal phenomenon. The absence of a circadian rhythm on weekends may reflect differences in the […]
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