group of patients whose cerebral blood flow falls substantially with .... Ferguson GG, Farrar JK, Meguro K, Peerless SJ, Drake CG, and. Barnett HJM (198Ia) ...
Journal of Cerebral Blood Flow and Metabolism
1:357-359 © 1981 Raven Press, New York
Editorial
Cerebral Blood Flow Measurements in Clinical Neurosurgery
Measurements of cerebral blood flow have been
of patients was treated by ligation with low mor
made by isotope clearance methods in man for more
bidity (Miller et aI., 1977).
than 15 years, but a clear role for them in the man
Although advances in intracranial microsurgery
agement of neurosurgical patients has not been es
have led to carotid ligation now being used in
tablished. Initially, concern about the potential
frequently, several lessons can be learned from
risks of studies that employed intracarotid injection
these early studies. First, blood flow was deter
may have limited their application. Less invasive
mined not only under basal conditions, but also
methods that employ inhalation or intravenous in
after a stimulus that posed some stress on the re
jection of xenon-133 have now been developed, but
serve of the cerebral circulatory system. Second,
the equipment and expertise required to measure
blood flow measurements were not used to diagnose
CBF are still to be found in only a minority of
established ischaemia, but to predict the risk of its
neurosurgical departments. As measurements of
development in the future. Third, the result of a
cerebrovascular function by more complex methods
cerebral blood flow measurement directly influ
such as emission tomography and X-ray transmis
enced clinical decisions about the management of
sion tomography begin to be applied to clinical
individual patients.
problems, it is timely to ask if measurements of ce
It is now well recognised that an isolated mea
rebral blood flow can contribute to neurosurgical
surement of cerebral blood flow provides little use
practice and to consider whether principles have
ful information about the state of the cerebral cir
already emerged that can serve to direct future
culation in any given state of health or disease. In
studies. In particular, are there circumstances in
stead, the cerebral circulation should be stressed in
which CBF measurements have broken what Las
some way, in order to measure its ability to respond
sen referred to in these columns as the "magical
to changing physiological conditions. Cerebrovas
barrier from clinical research to clinical useful
cular responsiveness can be assessed with relatively
ness."
simple, inexpensive equipment, even a single de
The earliest, and one of the clearest, illustrations
tector, and there is renewed promise that this sim
of a practical application of cerebral blood flow
ple approach can still provide clinicians with useful
measurement was in the prediction of the outcome
information. Recently, interest has returned to the
of carotid ligation. This operation was once widely
identification of
used in the treatment of aneurysms of the internal
haemorrhage whose cerebral circulatory reserve is
patients with subarachnoid
carotid artery. The problem was that some patients
inadequate. The focus now is on the identification,
were unable to compensate adequately for the re
either before or during operation, of patients liable
duction in flow that followed carotid ligation and
to develop delayed cerebral ischaemia after an in
developed a hemiplegia or other ischaemic compli
tracranial operation for a ruptured aneurysm.
cations. This often happened after a delay of 12-24 h
Intra-operative measurements of CBF with an
and was therefore not detected for 20-30 min by
intravenous xenon-133 method have identified a
the standard practice of trial ligation under local
group of patients whose cerebral blood flow falls
anaesthesia. Measurements of CBF with intraca
substantially with hypotensive anaesthesia and who
rotid xenon-l33 administration before and after trial
subsequently go on to develop delayed cerebral is
carotid clamping made it possible to identify pa
chaemic neurological deficits (Pickard et aI., 1980;
tients liable to develop ischaemia (Jennett et aI.,
Farrar et aI., 1981). These observations are in ac
1966). Permanent surgical ligation could thus be
cordance with previous anecdotal experience,
avoided in the patients at risk, and a large series
namely, that patients who did not tolerate ligation
357
358
G. TEASDALE AND D. MENDELOW
were also at risk from intracranial surgery. What
others never, and some only in selected patients. In
needs to be discovered is if blood flow mea
the third case, the results of Boysen (1973) indicate
surements performed before operation can reliably
that intra-operative cerebral blood flow mea
identify the patients at risk during intracranial
surements can indicate when a shunt is appropriate.
surgery. This information might then influence de
Although the demonstration of regional cerebral
cisions about the timing of the operation and type of
blood flow abnormalities has been reported to be a
anaesthesia used. There is also renewed interest in
useful indication that a patient is likely to benefit
the use of induced hypertension post-operatively.
from extracranial/intracranial arterial bypass
In a small group of patients this has been reported to
(Schmiedek et aI., 1976), others have found that
increase cerebral blood flow and to reverse
blood flow measurements alone do not correlate
neurological deficits associated with delayed cere
with clinical outcome after this procedure (Podreka
bral ischaemia (Symon et aI., 1978). As yet it is
et aI., 1981).
unclear if post-operative cerebral blood flow mea
Cerebral blood flow measurements can be useful
surements can provide a useful guide to the need for
in predicting the risk of future ischaemia, but they
hypertensive therapy.
have less to offer in the diagnosis of ischaemia that
The use of blood flow measurements to answer
has already occurred or is developing. Several
clinical questions has also been hindered by reser
studies have shown that cerebral function fails and
vations about the safety and practicality of testing
ischaemic brain damage develops only after blood
cerebrovascular reactivity in patients suffering from
flow falls below certain critical levels. In clinical
subarachnoid haemorrhage or other acute intracra
practice, the ischaemic episode is usually so short
nial lesions. The determination of effects of changes
lived that it is almost impossible to "capture" the
in blood pressure or blood gases might be held to
moment of ischaemia with a blood flow measure
have risks of rupturing an aneurysm, causing cere
ment. It is therefore unlikely that measurements of
bral ischaemia, or increasing cerebral oedema. Less
cerebral blood flow can ever be made sufficiently
profound stimuli, on the other hand, may not ade
frequently for the technique to be useful as a
quately demonstrate impaired cerebral blood flow
monitor of the development of cerebral ischaemia
responsiveness. There are also practical problems
outside the operating room.
in performing any type of CBF studies in patients
Greater use of cerebral blood flow measurements
with acute intracranial lesions. They often co
for per-operative monitoring will depend on the
operate poorly, particularly when a change in PaC02
availability of methods for the rapid analysis of data
is induced as part of a stress test. This results in the
so that a result is available to surgeons or
data from individual studies sometimes being in
anaesthetists within minutes of the study. A more
valid and in poor repeatability in serial studies. This
practically acceptable method for per-operative
is one reason that the stimulus applied must nor
monitoring may be in the assessment of brain func
mally result in a substantial alteration in cerebral
tion by electrophysiological techniques (elec
blood flow if the results of studies in a single patient
troencephalography, sensory evoked potentials, di
are to yield useful information. Serial studies of
rect cortical response, central conduction time),
cerebrovascular responsiveness are likely to be
Several of these can now be recorded more or less
very informative, but these are also subject to prac
continuously. Although changes usually occur only
tical limitations.
when CBF is reaching critical levels, electrophys
Cerebrovascular responsiveness has been studied
iological methods may provide a useful "last-min
extensively in patients with occlusive cerebrovas
ute" warning of impending ischaemic brain damage.
cular disease. It is well established that reactivity to
Clinicians are often faced with studying the con
CO2 is reduced in patients with either severe
sequences of an ischaemic episode from which a
stenosis or occlusion of the carotid artery, unilater
patient has survived, Some of the disenchantment
ally or bilaterally (Takagi et aI., 1979). Mea
with the determination of cerebral blood flow in
surements of cerebrovascular reactivity might
clinical practice may stem from the problems that
therefore provide a basis for selecting which pa
are inherent in measurements made when the brain
tients suspected of carotid atherosclerosis should
is already damaged. When this has happened, the
have angiography. When the investigations lead to
blood-brain partition co-efficient for the tracer may
operative treatment of a carotid stenosis, some sur
change, compartmental methods of analysis may
geons routinely utilise an intra-operative shunt,
become invalid, and for well-recognised technical
J Cereb Blood Flow Me/abol, Vol. I, No. 4, 1981
CBF MEASUREMENTS IN NEUROSURGERY
359
reasons, It IS difficult or impossible to detect lo
limitations as previous isotope-based methods but
calised regions of impaired blood flow. The com
may make it more feasible to map disturbances of
mon finding in clinical studies in patients with intra
cerebral blood flow and to correlate these with
cranial lesions such as head injury or spontaneous
other derangements in cerebrovascular function or
intracranial haemorrhage has been a reduction in
metabolism. Nevertheless, efforts should still be
the basal level of blood flow. Usually the degree of
focussed upon preventing ischaemic brain damage,
reduction in cerebral blood flow simply parallels
rather than expecting clinical returns from the in
the reduction in consciousness, and it is the oc
vestigation of patients already suffering from estab
casional divergence between clinical state and blood
lished ischaemic lesions.
flow findings that promises to be of most interest.
G. Teasdale and D. Mendelow
For example, if a patient is neurologically well after a subarachnoid haemorrhage but shows a low blood flow, this may reflect severe cerebral arterial spasm. In such patients it may be advisable to de lay intracranial surgery (Ferguson et aI., 1981a).
University Department of Neurosurgery Institute of Neurological Sciences Southern General Hospital 1345 Govan Road, Glasgow G51, Scotland
In head injury, on the other hand, there is a group of children whose blood flow is relatively high, de spite their being in profound coma with high intra cranial pressure (Bruce et aI., 1981). A major problem in the investigation of estab lished brain damage is to decide whether an ob
References Boysen G (1973) Cerebral haemodynamics in carotid surgery. Acta Neural Scand (SlIppl 52) vol 49 Bruce DA. Alavi A, Bilaniuk L, Dolinskas C, Obrist W, Uzzell B (1981) Diffuse cerebral swelling following head injuries in children: the syndrome of " malignant brain oedema." ] Nellyosurg 54:170-178
served reduction in cerebral blood flow represents a
Farrar JK, Gamache FW, Ferguson GG, Drake CG (\981) Cere
primary mechanism, i.e., denoting damage to the
bral blood flow (CBF) in profound intra-operative hypoten
brain as a result of ischaemia, or whether it is simply a response to the reduced metabolic needs of dam aged brain. The development of tomographic mea surements for mapping blood flow, oxygen utilisa tion, and oxygen extraction may allow the detection of areas in which cerebral blood flow has fallen to critical levels (" misery perfusion") (Lenzi et aI.,
1981). On the other hand, it remains to be seen how common these are, for how long they persist, and if the information can usefully influence management. There is abundant evidence that in people who die after head injury or spontaneous intracranial haemorrhage, the brain has most frequently been
sion: correlation with pre- and post-operative mea surements. .I Cereb Blood Flow Metabol I:S520-S521 Ferguson GG, Farrar JK, Meguro K, Peerless SJ, Drake CG, and Barnett HJM (198Ia) Serial measurements of CBF as a guide to surgery in patients with ruptured intracranial aneurysms,
.I Caeb Blood Flow Metabol l:S5l8-S5l9 Ferguson GG, Gamache FW, Farrar JK, Blume WT (198Ib) Physiological monitoring during carotid endarterectomy: evidence that an internal shunt is not necessary. ] Cereb Blood Flow Metabol I:S530-S531 Jennett WB, Harper AM, Gillespie FC (1966) Measurement of regional cerebral blood flow during carotid ligation. Lancet 2: 1162-1163 Lenzi GL, Frackowiak RSJ, Jones T (1981) Regional cerebral blood flow (CBF), oxygen utilisation (CMR02), and oxygen extraction ratio (OER) in acute hemispheric stroke. ] Cereb Blood Flow Metabol I:S504-S505 Millet JO, Jawad K, Jennett WB (1977) Safety of carotid ligation
damaged by a diffuse impairment of cerebral blood
and its role in intracranial aneurysm. ] Neural Neurosurg
flow. We suggest that the factor that has restricted
Psychiatry 40:64-72
the contribution of cerebral blood flow measure ment to clinical neurosurgical practice has not been the limited focal resolution of available two dimensional methods, but a failure either to ask or to obtain answers to appropriate questions. The key to improved clinical results lies in the devising of tests to identify patients whose cerebral circulatory reserve, although impaired, has not yet become critically exhausted. These patients might then be treated in ways that either avoid such stresses as hypoxia and hypotension or that increase the ca pacity of the cerebral circulation available to cope with adverse changes. The new tomographic meth ods remain subject to many of the same practical
Pickard JD, Matheson M, Patterson J, Wyper D (1980) Predic tion of late ischaemic complications after cerebral aneurysm surgery by the intra-operative measurement of cerebral blood flow. ] Neurosurg 53:305-308 Podreka I, Reisner T, Zeiler K, Brucke T, Kletter G, Heiss WD (1981) Outcome after EC/IC bypass related to pre-operative rCBF and CT findings. ] Cereb Blood Flow Metabol I:S534-S535 Schmiedek P, Gratzl 0, Spetzler R, Steiner H, Ezenbach R, Brendel W, Marguth F (1976) Selection of patients of extra cranial-intracranial arterial bypass surgery based on rCBF measurements. ] Neurosurg 44:303-312 Symon L (1978) Disordered cerebrovascular physiology in aneurysmal subarachnoid haemorrhage. Acta Neurochir 41:7-22 Takagi Y, Hata T, Ishitobi K, Kitagawa Y, Sako Y, Okada T (1979) Cerebral blood flow and CO2 reactivity before and after carotid 72:506-507
endarterectomy.
Acta
Neurol
Scand
J Cereh Blood FloI\' Metabol, Vol. I, No. 4, 1981
