(Read Only) Figali-relayout_US Neurology 16/02/2010 10:09 Page 44
Brain Trauma
However, there are also limitations of PbtO
2
monitoring that need to be monitor tissue in the penumbra of the lesion, as PbtO
2
is usually lower
considered. The catheter monitors a restricted area of brain tissue and the in these tissues.
36
Similar focal/global principles are relevant also to
determinants of PbtO
2
are still debated and require further examination. monitoring with microdialysis. PbtO
2
values require interpretation based
Although the association between low PbtO
2
and poor outcome appears on tissue being monitored, for both generalization of the results to the
to be strong, it is much less certain whether PbtO
2
-directed therapy rest of the brain and interpretation of the PbtO
2
response to intervention.
improves outcome. Lastly, while several studies of PbtO
2
monitoring in Peri-contusional (or peri-lesional) brain may demonstrate altered
adult patients have been conducted, much less is known about PbtO
2
pathophysiological responses to interventions that require a different
monitoring in children. interpretation compared with ‘non-lesioned’ brain.
6,34,37
Technical Aspects of Factors that Influence Brain Tissue Oxygen
Brain Tissue Oxygen Monitoring The best descriptor of what PbtO
2
monitoring in the brain reflects is
Three PbtO
2
devices have been produced commercially: Licox (Integra debated. Often considered a measure of the balance between supply and
Neurosciences, Plainsboro, NJ), Neurotrend (Codman, Raynham, MA), and demand of oxygen in the tissues, it has variably been associated with
Neurovent-PTO (Raumedic, Münchberg, Germany). Of these, the Licox CBF,
38–41
product of blood flow and oxygen content,
15
mean transit time of
system is most widely used, and also measures brain temperature in the blood through the brain,
42
arteriovenous difference of oxygen,
43
and end-
same catheter (IT2). The Neurotrend is no longer commercially available. capillary venous PO
2
.
21,44
In general terms, it is probably best considered a
The Neurovent-PTO is novel in that it also measures ICP, but is new on the measure of factors that affect both the perfusion and diffusion
market and few data on its clinical reliability are currently available. The characteristics of brain tissue. Some of the important practical factors that
Licox system is based on a Clarke-type polargraphic cell containing two influence PbtO
2
are discussed below.
electrodes covered by a membrane. The amount of O
2
diffusing across
this membrane depends on local tissue pO
2
and determines the electrical Brain Tissue Oxygen and
current between the two electrodes.
12
Several studies have confirmed the Arterial Partial Pressure of Oxygen
reliability of the PbtO
2
signal, in vitro accuracy, and low sensitivity and Being a measure of the partial pressure of oxygen, PbtO
2
is significantly
zero drift over time.
5,14–18
The sampling area is approximately 14–17mm
3
.
5,19
affected by the arterial partial pressure of oxygen (PaO
2
). Therefore,
Local tissue damage is minimal
15
and complications are rare.
14
The time- even in conditions where arterial blood is near full saturation and
of-good-quality-data is in the region of 99%;
14
repeat calibration is not increased PaO
2
does not change oxygen content significantly, increased
required and artifacts are unusual. Although the PbtO
2
readings are PaO
2
is followed by increased PbtO
2
.
45,46
Accordingly, the arteriovenous
usually stable within one hour of insertion, sometimes the adaptation difference of oxygen strongly influences PbtO
2
.
43
Similarly, progressive
period may take up to two hours.
14,20,21
systemic hypoxia leads to a decline in PbtO
2
and increased anerobic
metabolism.
15,47,48
A potential limitation is that the ventilator fraction of
Normal and Abnormal inspired oxygen (FiO
2
) setting may significantly influence the PbtO
2
Brain Tissue Oxygen Values reading in the absence of substantial changes in oxygen delivery. On the
Normal values in humans are not precisely known. Because the PbtO
2
other hand, dissolved oxygen may be preferentially used for tissue
value is influenced strongly by local cerebral blood flow (CBF), the value oxygenation,
49,50
and increased tissue oxygen pressure may overcome
varies widely depending on the metabolic activity and diffusion tissue barriers to diffusion
51
and may improve metabolism.
52
The relative
characteristics of the region being monitored.
22
However, variability is benefits of hyperoxia on PbtO
2
and metabolism in TBI, however, are
reduced during periods of ischemia.
21
Extrapolation from studies that currently debated.
52–55
have measured PbtO
2
in animals and human studies monitoring
relatively normal brain suggest that normal values for PbtO
2
are around Brain Tissue Oxygen and
25–30mmHg.
5,17,19,23
Studies of PbtO
2
in aneurysm surgery demonstrate Arterial Partial Pressure of Carbon Dioxide
the decline in PbtO
2
associated with ischemia due to temporary PbtO
2
varies with changes in arterial partial pressure of carbon dioxide
clipping.
24–26
Poor outcome in TBI patients is more likely when PbtO
2
falls (PaCO
2
) if CO
2
reactivity is preserved,
6,56–59
largely secondary to the
progressively below 20mmHg.
27–29
Scheufler et al.
21
demonstrated in an vasoactive effects of PaCO
2
. Therefore, hypocarbia may induce or
animal model that CBF levels below 20ml/100g/minute correlated with worsen cerebral ischemia, and relative hypercarbia may improve local
PbtO
2
levels below 10mmHg. This also appears to correlate with critical CBF and therefore local oxygenation in areas at risk for ischemia.
57
ischemic thresholds in human studies.
5,30
Low PbtO
2
values (<10mmHg) However, if hypercarbia significantly increases cerebral blood volume,
are associated with perturbations in microdialysis parameters, and therefore ICP, the reduced cerebral perfusion pressure (CPP) may
decreased mitochondrial function and impaired neuronal activity.
21,30–32
have the opposite effect on PbtO
2
. This, and variations in the strength of
CO
2
reactivity in the cerebral vessels, as well as different responses in
Choosing the Site of Monitoring abnormal tissue, may account for occasional ‘paradox’ reactions of
Because the device measures focal, not global, oxygenation, the choice PbtO
2
in response to CO
2
changes.
6,23
Moderate hyperventilation without
of the site of monitoring is important for the interpretation of the results monitoring brain oxygenation is no longer recommended.
60,61
and optimal management of the patient. When the brain is diffusely
injured or when there is a global insult, monitoring PbtO
2
in frontal white Brain Tissue Oxygen and Intracranial Pressure
matter appears to provide a useful approximation of global changes in Increased ICP may reduce PbtO
2
, either by the local tissue pressure effect
brain oxygenation.
5,19,21,33–35
If there is focal injury, many clinicians aim to or by reduction of cerebral perfusion pressure (CPP). Reports of
44
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