The significance of circadian rhythms and dysrhythmias in critical illness

Abstract

Many physiological and cellular processes cycle with time, with the period between one peak and the next being roughly equal to 24 h. These circadian rhythms underlie 'permissive homeostasis', whereby anticipation of periods of increased energy demand or stress may enhance the function of individual cells, organ systems or whole organisms. Many physiological variables related to survival during critical illness have a circadian rhythm, including the sleep/wake cycle, haemodynamic and respiratory indices, immunity and coagulation, but their clinical significance remains underappreciated. Critically ill patients suffer from circadian dysrhythmia, manifesting overtly as sleep disturbance and delirium, but with widespread covert effects on cellular and organ function. Environmental and pharmacological strategies that ameliorate or prevent circadian dysrhythmia have demonstrated clinical benefit. Harnessing these important biological phenomena to match metabolic supply to demand and bolster cell defenses at the apposite time may be a future therapeutic strategy in the intensive care unit.

Keywords: Chronobiology disorders; biological clocks; circadian rhythms; critical illness; physiology.

Figure 1.

Summary of different organ functions relevant to critical illness which have been found to demonstrate circadian rhythms.

Figure 2.

Schematic representation of time points (or time ranges) at which peaks and troughs of haemodynamic indices occur during a typical 24 h period. Normal degree of variation between peak and trough indicated.

Figure 3.

Morphology of circadian rhythms can be described by the amplitude (difference between the peak and trough) and the time at which the peak (acrophase) occurs within the cycle. The time period from one peak to the next is 24 h.

Figure 4.

Circadian dysrhythmia may take the form of a change in the amplitude (difference between peak and trough).

Figure 5.

Circadian dysrhythmia may take the form of a shift of the peak (acrophase) to a different time point within the 24-h period.

Figure 6.

Circadian dysrhythmia may take the form of loss of organisation of the usual pattern.