Cushing reflex
The Cushing reflex is a nervous system response to increased intracranial pressure. It results in Cushing's triad of widening pulse pressure, irregular breathing, and bradycardia.[1] It is usually seen in the terminal stages of acute head injury and may indicate imminent brain herniation. It can also be seen after the intravenous administration of norepinephrine and similar drugs.[citation needed] It was first described by American neurosurgeon Harvey Cushing in 1902.
Mechanism
The reflex begins when some event causes increased intracranial pressure (ICP). This increases the hydrostatic pressure of cerebrospinal fluid to the point that it meets and gradually exceeds mean arterial pressure (MAP). As the ICP exceeds the MAP, the cerebral arterioles become compressed, diminishing blood supply to the brain, a condition known as cerebral ischemia. Central chemoreceptors in the hypothalamus sense the ischemia in the form of decreased pH and increased pCO2. The central chemoreceptors respond with a swift and potent activation of the sympathetic nervous system.
The sympathetic response activates alpha-1 adrenergic receptors within the arteries, causing vasoconstriction. This constriction raises the total peripheral resistance of blood flow and elevates blood pressure causing hypertension in an attempt to restore perfusion to the ischemic brain. The sympathetic stimulation also increases heart contractility and cardiac output.
Meanwhile, baroreceptors in the carotid arteries detect the increase in blood pressure and trigger a parasympathetic response via vagal stimulation, thereby inducing bradycardia. Bradycardia may also be caused by increased ICP impinging on the vagal nerve, mechanically stimulating a parasympathetic response. An irregular respiratory pattern is typically the result of herniation or increased pressure on the brainstem.
The Cushing reflex is complex and seemingly paradoxical. Commonly, the central chemoreceptors of the brain and the baroreceptors of the carotid sinuses work together to increase or decrease blood pressure, but in the Cushing reflex, the two sensors are receiving mixed signals. Thus, even in the presence of sympathetic stimulation from the brain, which would normally produce tachycardia, there is in fact bradycardia.
See also
References
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