Structure and function anoxia medical definition

CBF = (CAP – JVP) ÷ CVR

Where CAP is carotid arterial pressure, JVP is jugular venous pressure, and CVR is cerebrovascular resistance. [how do you measure this?] intracranila pressure measured by lumbar functure???

The following factors significantly impact CBF:

● partial pressure of arterial oxygen (pao 2 ) – pao2 has its most significant effects at levels below 50 mmhg, when it causes vasodilatation in an attempt to maintain the oxygen supply to the brain.

● partial pressure of arterial carbon dioxide (paco 2 ) – hypercapnia causes cerebral vasodilatation and increased CBF, whereas hypocapnia reduces CBF. Because the response to changes in paco2 is rapid, hyperventilation is an emergency intervention as a temporizing measure before neurosurgery during the acute management of increased ICP complicated by impending transtentorial and tonsillar herniation of brain tissue.Anoxia medical definition

● autoregulation – in the intact and normal cerebral circulation, CBF is maintained at a relatively constant rate by intrinsic cerebral mechanisms referred to as autoregulation. In addition to this global phenomenon, a region of brain tissue can also bring about an acute regional change in CBF. This coupling of local blood flow to local changes in metabolic activity is determined by a number of neuro-energetic signals. A clinical illustration of this phenomenon is by the blood oxygen level-dependent (BOLD) time series used in functional magnetic resonance imaging (fmri) that can show activated regions with mental tasks.

At a global level, cerebral autoregulation adjusts CBF through changes in CVR.Anoxia medical definition in adults, changes in CVR can maintain constant CBF at mean arterial pressures of 60 to 150 mmhg. However, this plateau is under many influences including level of sympathetic activity and baseline paco2. In infants and children, the range of the plateau (with upper and lower limits) is going to differ to that identified in adults because of developmental differences in mean arterial pressure and level of sympathetic activity. Consequently, we do not know the precise lower limit in mean blood pressure at which CBF falls with declining blood pressure. Newer techniques, however, using near-infrared spectroscopy in babies with hypoxic-ischemic encephalopathy suggest that optimum mean arterial pressure can be identified.Anoxia medical definition

Outside this range, the compensatory mechanisms break down and inadequate or excessive perfusion can occur. The upper and lower limits of autoregulation are shifted to the right in patients with chronic hypertension, in whom acute reductions in blood pressure can produce ischemic symptoms ( figure 3).

• the velocity of CBF can be measured using transcranial doppler (TCD); see chapters 5 and 6 for a discussion of the doppler effect. An ultrasound probe (2 mhz, pulse wave doppler) is placed in the temporal area above the zygomatic arch, which allows insonation of the middle cerebral artery. Normal velocity in the middle cerebral artery is approximately 55 cm/sec. Velocities greater than 120 cm/sec can indicate cerebral artery vasospasm following subarachnoid hemorrhage or hyperemic blood flow.Anoxia medical definition comparison between the velocities in the extracranial internal carotid artery and the middle cerebral artery (the lindegaard ratio) can distinguish between these conditions. Middle cerebral artery velocity three times that of the velocity measured in the extracranial internal carotid artery more likely reflects cerebral artery vasospasm.

• near infrared spectroscopy was discussed in chapter 6. Decreased saturation is associated with impaired cerebral oxygen delivery, although near infrared spectroscopy primarily reflects cerebral venous oxygen saturation.

• brain tissue oximetry measures the oxygen tension in brain tissue through placement of a bolt with a clark electrode oxygen sensor. Brain tissue CO 2 tension can also be measured using a similarly placed infrared sensor.Anoxia medical definition normal brain tissue oxygen tension varies from 20-50 mm hg. Brain tissue oxygen tensions less than 20 mm hg warrant interventions, and values less than 10 mm hg are indicative of brain ischemia.

• intracerebral microdialysis can be used to measure changes in brain tissue chemistry that are indicative of ischemia and/or brain injury. Microdialysis can be used to measure cerebral lactate, neurotransmitters, markers of inflammation, and glucose concentration. Increases in the ratio of lactate/pyruvate have been associated with cerebral ischemia.

The most important extrinsic influences on CBF are respiratory gas tensions—particularly paco 2. CBF is directly proportionate to paco 2 between tensions of 20 and 80 mm hg (figure 26-3).Anoxia medical definition blood flow changes approximately 1-2 ml/100 g/min per mm hg change in paco 2. This effect is almost immediate and is thought to be secondary to changes in the ph of CSF and cerebral tissue. Because ions do not readily cross the blood-brain barrier (see below) but CO 2 does, acute changes in paco 2 but not HCO 3 -affect CBF. Thus, acute metabolic acidosis has little effect on CBF because hydrogen ions (H +) cannot readily cross the blood-brain barrier. After 24-48 hr, CSF HCO 3 – concentration adjusts to compensate for the change in paco 2, so that the effects of hypocapnia and hypercapnia are diminished. Marked hyperventilation (paco 2  20 mm hg) shifts the oxygen-hemoglobin dissociation curve to the left, and, with changes in CBF, may result in EEG changes suggestive of cerebral impairment, even in normal individuals.Anoxia medical definition

The relationship between cerebral blood flow and arterial respiratory gas tensions.