Cerebral blood flow monitoring: CNS vasomotor centre response
The degree of sympathetic vasoconstriction caused by cerebral ischemia is often so great in the peripheral vessels that all or nearly all the vessels are blocked.
The effective nerve control of blood pressure is due to reflexes originating at the receptor, chemoreceptors and depressurizes, all located in the peripheral circulation outside the brain. However, when the blood flow to the vasomotor centre is reduced to such an extent that malnutrition causes cerebral ischemia, the vasomotor centres and the tachycardia neurons in the vasomotor centres will respond directly to ischemia and be the strong stimulus. When this stimulation occurs, blood pressure usually rises to the highest possible level in proportion to the heart's pumping capacity. This is thought to be caused by impaired blood flow carrying CO2 away from the vasomotor centre. Elevated CO2 in the central region stimulates the area that controls the sympathetic nervous system.
Other factors, such as the formation of lactic acid, and some other acids in the vasomotor centre, are always involved to stimulate and increase blood pressure. An increase in blood pressure in response to cerebral ischemia is known as the ischemic response of the central nervous system.
Anaemia affecting vasomotor activity can cause a sudden increase in blood pressure to as high as 250 mmHg in 10 minutes. The degree of sympathetic vasoconstriction caused by cerebral ischemia is often so great in the peripheral vessels that all or nearly all of the vessels are blocked. For example, the kidneys may stop producing urine altogether because all the renal vessels are constricted. Thus, the central nervous system ischemic response strongly activates sympathetic vasoconstriction.
Importance of CNS ischemic response as a mechanism of arterial pressure regulation
The central nervous system's response to ischemia is not apparent until blood pressure drops far from normal, to 60 mmHg and below. The greatest degree of stimulation is at the blood pressure level of 15-20 mmHg. The cerebral ischemic response is therefore not a normal mechanism for arterial blood pressure regulation. The emergency blood pressure control system acts quickly and strongly to prevent a deep drop in blood pressure when blood flow to the brain is reduced to the point of death.
Cushing response to increase pressure around the brain
The Cushing response is a special type of cerebral ischemic response that results in an increase in the pressure of the cerebrospinal fluid surrounding the brain. For example, when the cerebrospinal fluid pressure equals the arterial blood pressure, it compresses the brain as well as the cerebral arteries and cuts off the blood supply to the brain. This response initiates the cerebral ischemic response, causing an increase in blood pressure. When blood pressure rises to a threshold greater than cerebrospinal fluid pressure, blood returns to the brain. Normally blood pressure reaches a new equilibrium threshold slightly higher than cerebrospinal fluid pressure, allowing blood flow to the brain. The Cushing response helps to protect the survival centre from undernutrition when the CSF pressure rises sufficiently to compress the cerebral arteries.