Page 77 - Encyclopedia of Nursing Research

44 n CEREBRAL ISCHEMIA

Spielmeyer first described “ischemic production of “heat” or energy. Adolf Fick
cell change” in 1922 (Spielmeyer, 1922), and (1870) defined blood flow as the quantity of
C Brierley presented the time course for neu- a substance, such as oxygen, that is taken up
ronal change during a low-flow state and by a specific organ over a unit of time (obrist,
provided evidence of the threshold for cere- 2001). The first “measures” of CBF involved
bral anoxic ischemia (Brierley, Brown, & direct and indirect observations of intra-
Meldrum, 1971; Chiang, Kowada, Ames, cranial vessels (Roy & Sherrington, 1890). It
Wright, & Majno, 1968). He observed and was not until 1945, when Kety and Schmidt
described in further detail the process of applied the Fick principle to diffusible gas,
ischemic cell change (Brierley, 1973). With the nitrous oxide, that one was able to estimate
initial decrease in blood flow, oxygen, and/ CBF (Kety, 1950; Kety & Schmidt, 1948).
or glucose to the brain, the contour of cells, Kety was the first person to measure
nucleus, and nucleolus remain unchanged. global CBF in humans using vascular transit
There is disruption of mitochondria and an time. The technique was modified by Lassen
increase in the astrocyte processes surround- and Ingvar (1972) when Xe-133, a highly dif-
ing the neurons. As the ischemic process con- fusible gas, was injected into the internal
tinues, there is neuronal shrinkage, changes carotid artery. Multiple extracranial detectors
within organelles in the cytoplasm, and the traced the transit time of the radiation from
cell is further surrounded by astrocytic pro- the Xe-133 as it flowed through the brain,
cesses. As the nucleus continues to shrink providing focal CBF measures. Diffusible
and the cytoplasm becomes more amor- tracers are now combined with tomographic
phous, incrustations begin to form. Finally, reconstruction such as computed tomog-
as the incrustations disappear and the cyto- raphy (CT), positron emission tomography,
plasm becomes increasingly homogeneous, or magnetic resonance imaging to calculate
astrocytes proliferate and lipid phagocytes vascular transit time. For example, stable
form in preparation for removal of the now xenon-enhanced CT scanning measures CBF
“ghost cell.” As the flow lowers and the mito- via conventional scanner interfaced with
chondria fail, energy sources change from computer hardware and software and directs
an aerobic to an anaerobic pathway, with a the delivery of xenon gas transit throughout
corresponding increase in lactic acid produc- brain regions. Serial CT scans are conducted
tion, metabolic derangement, and loss of ion during the inhalation of a gas mixture con-
and transmitter homeostasis. If this process taining 30% xenon, 30% to 60% oxygen, and
continues unchecked, there will be inade- room air. The serial images are stored and
quate energy to maintain the sodium potas- regional flows are calculated.
sium pump across the cell membrane (Jones CBF is also estimated from measure-
et al., 1981). Researchers have increasingly ment of cerebral blood volume. one way to
detailed the process in an attempt to identify estimate cerebral blood volume is using a
and improve the brain’s tolerance to recover gradient echo planar system on MR systems.
from an ischemic challenge. The dynamic contrast-enhanced susceptibil-
Servetus, in the sixteenth century, first ity-weighted perfusion imaging technique
presented the idea that blood flowed through involves giving a bolus of paramagnetic con-
the lungs; he was burned at the stake for his trast material (i.e., gadolinium). The contrast
efforts. William Harvey (1578–1657) sup- media are traced, and the amount of signal
ported Servetus’ findings by describing attenuation is proportional to the cerebral
the flow of blood through the body. nearly blood volume. With a series of multislice mea-
200 years later, oxygen was discovered by surements, one may generate a time–density
Priestley, and Steele and Lavoisier made the curve, and the area under the curve provides
connection that oxygen contributed to the an index of relative blood volume (Grandin,