Dynamic output and control of the hypothalamic-pituitary-adrenal axis in critical illness and major surgery
The hypothalamic-pituitary-adrenal (HPA)
axis is a neuro-endocrine system that regulates circulating levels of
glucocorticoid
hormones. These hormones are vital for normal
homeostasis and play a pivotal role in the response to stress. Levels of
cortisol
fluctuate throughout the day in a diurnal rhythm,
underlying which is an ultradian rhythm of approximately hourly pulses,
and this pulsatility directly affects
transcriptional outcomes. Pulsatility is not the result of a ‘pulse
generator’, but
is inherent within the system as a result of
negative feedback. These patterns of secretion change in both acute and
chronic
illness as a result of inflammatory mediators,
splanchnic nerve output, and central nervous system control. Levels of
cortisol
in both normal and illness states are highly
dynamic and so previously used static assessment tools for diagnosing
corticosteroid
related critical illness insufficiency (CRCI) are
not likely to be useful. Therapeutic regimens have also failed so far,
to
take secretory patterns into account. In this
review we look at the dynamic control and effects of glucocorticoids and
frame
in this context the current evidence surrounding
steroid use in critical care and major surgery.
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