Current Concepts of Pathophysiology, Circadian Patterns, and Vasoreactive Factors Associated with Myocardial Ischemia Detected by Ambulatory Electrocardiography

The circadian clock within the heart: potential influence on myocardial gene expression, metabolism, and function

It is becoming increasingly clear that the intrinsic properties of both the heart and vasculature exhibit dramatic oscillations over the course of the day. Diurnal variations in the responsiveness of the cardiovascular system to environmental stimuli are mediated by a complex interplay between extracellular (i.e., neurohumoral factors) and intracellular (i.e., circadian clock) influences. The intracellular circadian clock is composed of a series of transcriptional modulators that together allow the cell to perceive the time of day, thereby enabling preparation for an anticipated stimulus. These molecular timepieces have been characterized recently within both vascular smooth muscle cells and cardiomyocytes, giving rise to a multitude of hypotheses relating to the potential role(s) of the circadian clock as a modulator of physiological and pathophysiological cardiovascular events. For example, evidence strongly supports the hypothesis that the circadian clock within the heart modulates myocardial metabolism, which in turn facilitates anticipation of diurnal variations in workload, substrate availability, and/or the energy supply-to-demand ratio. The purpose of this review is therefore to summarize our current understanding of the molecular events governing diurnal variations in the intrinsic properties of the heart, with special emphasis on the intramyocardial circadian clock. Whether impairment of this molecular mechanism contributes toward cardiovascular disease associated with hypertension, diabetes mellitus, shift work, sleep apnea, and/or obesity will be discussed.

The Clinical Value of the ECG in Noncardiac Conditions

The ECG is an indispensable tool in the ICU for the detection and diagnosis of heart disease. ECG abnormalities however can be present in a wide variety of noncardiac conditions, complicating the differential diagnosis with primary cardiac pathology. This overview discusses the ECG abnormalities and their pathophysiologic basis in the most frequently encountered noncardiac conditions, such as electrolyte abnormalities, pulmonary embolism, CNS diseases, esophageal disorders, hypothermia, and drug-related and other conditions. Knowledge of the characteristic ECG changes may provide early clues to the presence of these disorders, the prompt recognition of which can be life saving.

Circadian rhythms in cardiac gene expression

Circadian rhythms in blood pressure, heart rate, and cardiac output have been intensely studied, largely due to the well-documented phenomenon of increased cardiovascular death in the early hours of the morning. Circadian rhythmicity in both cardiovascular physiology and pathophysiology has been attributed primarily to diurnal variations in neurohumoral factors, such as sympathetic activity. It has become increasingly apparent that the intrinsic properties of the heart (seen at the level of gene and protein expression, energy metabolism, and contractile function) show significant fluctuations during the course of the day. These changes might be due to extracardiac (eg, neurohumoral factors) and/or intracardiac (eg, circadian clocks) influences. Circadian clocks are transcriptionally based, molecular mechanisms that enable the cell to anticipate diurnal variations in environmental stimuli. The cardiac circadian clock synchronizes responsiveness of the heart to diurnal variations in its environment, and impairment of this mechanism might contribute to the pathogenesis of cardiovascular disease.